Understanding processes that contribute to efficiency losses during long-term operation of perovskite solar cells is crucial for achieving operational stability. For example, photoinduced migration of halide ions and cations is known to significantly affect the device performance [1]. Under open circuit conditions, however, no current flow is allowed, therefore charge carrier recombination and charge carrier accumulation at the interfaces remain as dominating processes. Because of the possibility of charge accumulation, PSCs are susceptible to chemical transformations and undergo degradation under open circuit conditions than under MPP tracking conditions. The thermodynamic and redox properties of halide perovskites provide a strong driving force for hole trapping and oxidation of iodide species under photoirradiation.

In the case MAPbI₃/spiro-OMeTAD, hole accumulation leads to formation of I₂ and subsequent oxidation of spiro-OMeTAD. By employing in-situ absorption measurements we show that the decrease in power conversion efficiency follows the spiro-OMeTAD oxidation by I₂ while operating the solar cell device under open circuit conditions [2]. In short circuit conditions, where photogenerated charge carriers are extracted in the external circuit, the iodide induced oxidation of spiro-OMeTAD and the device instability are greatly minimized. Incorporation of Cs⁺ as A-site cation also suppresses iodine expulsion. The photoinduced iodine migration in MAPbI₃ followed by its expulsion into spiro-OMeTAD that provides new insight into the photoinstability of perovskite solar cells will be presented.

Mixed tin/lead (Sn/Pb) perovskites have demonstrated the potential to achieve higher performances in single-junction solar cells compared to lead-based compounds alone. Currently, the most commonly used hole transport layer (HTL) for Sn/Pb-based perovskite solar cells is PEDOT:PSS, despite ongoing concerns about the possible detrimental effects of this conductive polymer on the long-term stability and performance of the active layer.

Recently, we have shown that self-assembled monolayers (SAMs) of [2-(3,6-dibromo-9H-carbazol-9-yl)ethyl]phosphonic acid (Br-2PACz) offer significant advantages when used as the HTL in solar cells with the active layer Cs₀.₂₅FA₀.₇₅Sn₀.₅Pb₀.₅I₃ [1]. In this presentation, I will demonstrate how different halogenated PAC molecules influence the buried interface passivation, influencing the overal device stability. Specifically, the perovskite layer deposited on SAMs exhibits higher crystallinity, reduced pinhole density, larger grains, and a lower defect density at the buried interface, when compared with layers grown on PEDOT:PSS. I will also show devices achieving efficiencies above 25% by combining this HTL with appropriate top surface passivation. Finally, I will provide a critical understanding of the mechanism of the surface passivation.

Chloride-based additive engineering is a widely used strategy to enhance crystallinity in perovskite solar cells, typically via methylammonium chloride (MACl)[1],[2]. In FA_0.9Cs_0.1Pb(I_0.9Br_0.1)₃ films, we observed that increasing MACl concentration promotes preferred orientation along the (100) diffraction plane and grain growth, but introduces a trade-off: X-ray diffraction reveals peak splitting at the (200) and (300) planes, accompanied by a secondary photoluminescence (PL) emission at ~814 nm in addition to the primary peak around 770-780 nm. These signatures indicate segregation into lower-bandgap, halide-rich domains that act as recombination centers and lead to poor device reproducibility (CV = 37.4%).

To resolve this, we replaced MACl with methylammonium lead chloride (MAPbCl₃) precursors. This substitution eliminates both the splitting of the diffraction peaks and the 814 nm PL secondary peak, and increases the carrier lifetime eightfold, from ~180 ns to ~1.6 μs. However, MAPbCl₃ does not reproduce the grain coarsening observed with MACl, limiting further optoelectronic gains.

To simultaneously achieve phase purity, large grains, and good optoelectronic properties, we introduced pyrrolidinium-based secondary additives into the MAPbCl₃-stabilized system. Pyrrolidinium chloride (PyCl) primarily passivates electronic defects, extending carrier lifetime to ~3.8 μs without substantial morphological change, whereas pyrrolidinium thiocyanate (PySCN) enables ~5x grain growth while reducing trap density. Combining MAPbCl₃ (phase stability) with PySCN (morphology control) yields phase-pure films with a carrier lifetime of ~4.9 μs. Proof-of-concept devices using unoptimized copper thiocyanate (CuSCN) hole-transport layers deliver peak PCE of 14.9% (active area 0.2 cm²) with V_OC=1.05 V, J_SC=21.4 mAcm^-2, and FF ≈ 67%, alongside improved reproducibility (CV=6.1%). The gap between long carrier lifetime and moderate PCE indicates that interfacial/transport losses—rather than bulk recombination—currently limit device performance, highlighting a clear pathway for architectural optimization. This work establishes a protocol that decouples crystallization kinetics from defect management, providing a pathway to reliable, high-quality perovskite photovoltaics.

This study investigates the effect of molecular design in establishing structure–property–performance relationships that govern interfacial passivation at the perovskite/hole-selective layer (HSL) interface. Interfacial defects at the interface of the perovskite and charge-selective layer junction remains one of the primary bottlenecks preventing perovskite solar cells from reaching their intrinsic efficiency limits and long-term stability [1]. While molecular passivation has emerged as a powerful strategy to suppress interfacial defects and charge recombination, the fundamental relationship between molecular functionality, interfacial energetics, and charge extraction remains poorly understood.

Using structurally analogous molecules that differ only in their terminal functional groups (ammonium, carboxylic acid, and bifunctional ammonium–carboxylic acid), we isolate the role of chemical functionality in affecting the interfacial behavior. Elemental, structural and optoelectronic characterization, including X-ray Photoelectron Spectroscopy, steady-state and time-resolved photoluminescence, surface photovoltage spectroscopy, conductive atomic force microscopy, and device-level analysis, reveals that passivation effect and charge extraction are not intrinsically correlated [3]. While all molecules effectively reduce non-radiative recombination by passivating interfacial trap states, their impact on carrier extraction differs dramatically. Through a detailed investigation of the perovskite/HSL interface, we identify a correlation between the terminal functional group, the formation of quasi-2D Ruddlesden–Popper (RP) interlayers, and key photovoltaic performance metrics, including open-circuit voltage (V_OC​) and fill factor.

Single-functional group passivating agents promote efficient hole extraction, yielding enhanced V_OC​, fill factor, and overall power conversion efficiency. In contrast, the bifunctional molecule, despite exhibiting strong defect passivation, introduces an interfacial barrier that suppresses hole transport, leading to extraction-limited device performance. In particular, ammonium-functionalized molecules exhibit superior performance by selectively promoting the formation of n = 2 RP phases, thereby enabling effective surface passivation and efficient charge extraction. By decoupling defect passivation from carrier extraction, this work contributes in providing molecular design rules for next-generation interfacial materials in perovskite photovoltaics and other emerging thin-film optoelectronic devices where interfacial losses dominate performance.

Crystallization kinetics critically influence the formation of smooth, compact, and defect-free perovskite films by controlling nucleation density and crystal growth. In this study, a chelating agent is introduced either into the perovskite precursor or during antisolvent treatment, exhibiting distinct interactions with the perovskite lattice depending on the incorporation method and thereby modulating nucleation, crystal growth, and defect passivation. Systematic optimization of the chelating agent enables effective coordination with undercoordinated Pb²⁺ sites, suppressing halide vacancies and reducing nonradiative recombination. Morphological and structural analyses show that the treated films possess fewer pinholes, smoother surfaces, and larger, more uniform grains, resulting from controlled nucleation and crystallization. Surface potential measurements reveal improved interfacial energy alignment, promoting efficient charge transfer, while fluorescence quantum yield studies confirm a significant reduction in trap states and reduced radiative recombination, reflecting high film quality. Consequently, the optimized devices achieve a power conversion efficiency of ~ 24%, with an open-circuit voltage of 1.13 V and a fill factor above 82%, and unencapsulated devices retain around 90% of their initial performance after 500 hours under ambient conditions, demonstrating excellent stability. This work highlights that precise control of nucleation, crystallization, and interfacial defect passivation is a powerful strategy for producing pinhole-free, high-quality perovskite films and simultaneously enhancing the efficiency and long-term stability of inverted perovskite solar cells, offering valuable guidance for the design of next-generation photovoltaics.

Bandgap tuning in iodide perovskites for silicon–perovskite tandem applications is commonly achieved through either A-site cation alloying or halide mixing. While halide alloying enables wide bandgap tunability, it suffers from phase segregation and long-term instability. A-site alloying offers a more stable alternative that avoids halide segregation; however, conventional antisolvent-based fabrication methods face severe miscibility constraints, particularly at high cesium concentrations. Although CsPbI₃ provides an ideal wide bandgap and fully inorganic composition, its poor thermal and phase stability prevent its use as a standalone absorber, necessitating alloying with formamidinium (FA). Consequently, accessing silicon-tandem-relevant bandgaps through A-site alloying alone has remained a significant challenge.

Here, we adopt an A-site alloying strategy based on an AX intermediate phase method adapted from inorganic materials synthesis. This approach enables stable and continuous Cs–FA mixing across the entire compositional range from FAPbI₃ to CsPbI₃, achieving bandgap tuning from 1.48 to 1.72 eV without halide alloying. This previously underexplored compositional space exhibits crystallization dynamics fundamentally distinct from antisolvent processing, requiring systematic optimization of additives, charge transport layers, passivation strategies, and fabrication parameters.

Our laboratory has previously developed automation and high-throughput workflows to accelerate the screening of perovskite materials, as presented by some of us at TandemPV 2025. However, cell fabrication was a significant bottleneck with our previous methods. To overcome this limitation, we have since developed a unique high-throughput blade-coating methodology that enables the fabrication of more than 500 fully completed devices spanning a wide range of material and fabrication parameters within the timeframe of conventional device batch of 20-30 samples. Importantly, absorbers optimized using this approach are directly scalable to large-area substrates without further optimization, in contrast to conventional antisolvent-processed absorbers.

During the workshop, I will introduce the as-yet relatively unexplored A-site alloying approach that unlocks the full compositional range of Cs–FA perovskite absorbers suitable for Si-pk tandems. I will also share our novel high-throughput blade-coating methodology that successfully closes the automation loop in high-throughput experimentation, enabling rapid materials optimization and direct scalability to large-area devices.

There is a growing consensus that the exceptional optoelectronic properties of metal halide perovskites (MHPs) are largely due to the peculiar interplay between the inorganic cage lattice, composed of a labile network of corner-sharing metal halide octahedra, and the A-site cationic sublattice. This interaction significantly affects the vibrational spectrum of MHPs (phonon frequencies, linewidths, and lifetimes), resulting from the effects of lattice potential anharmonicity and/or static/dynamic disorder. Raman scattering is a suitable technique to probe phonon interactions in solids, allowing for the in-situ characterization of chemical environments, revealing the nature of lattice vibrations. In this talk, the available experimental evidence of the aforementioned interplay will be reviewed with special emphasis on understanding Raman signatures depending on whether the coupling is principally mediated by hydrogen bonding or steric hindrance. The controversy about the origin of a strong Raman background, steeply rising towards zero Raman shift and called central peak, will be specifically addressed. This background signal, which is typically observed in the temperature range of stability of cubic and tetragonal phases when the A-site cation dynamics unfold, will be shown to be mostly due to disorder-induced second-order acoustic-phonon Raman scattering. This interpretation receives support from other semiconductor systems with nanoscale structural disorder, where the central Raman peak arises either from the vertical misalignment of Ge quantum dots in multi-stack heterostructures or from the interface roughness exhibited by short-period GaAs/AlAs superlattices. In this way, a unifying picture of phonon interactions in MHPs and how they impact different Raman processes is provided, which is key to interpreting their Raman spectra.

Superflourescence (SF) represents a quantum many-body phenomena, where coherent interactions between an ensemble of emitters and radiation fields give rise to emergent spontaneously, cooperative behavior that is fundamentally different from individual emitters. The exploration of structural-functional relationships and phase transition of SF with stimulated emission (e.g., amplified spontaneous emission (ASE), lasing etc.) have garnered significant attention in both theoretical and experimental investigations. Traditional long-range ordered perovskite nanocrystal (PNC) superlattices (SL) exhibit several inherent limitations that stifled systematic investigation into SF and the interplay between SF and other stimulated emission: (i) limited ligand types that permit orderly assembly of SLs, thereby hindering detailed photophysical studies into the structure-function properties; (ii) disordered spatial distributions challenges cavity integration of SLs to allow systematic investigation of the transition between SF and lasing, (iii) dispersed SL precludes the formation of high-density films to surpass optical gain threshold of ASE.

In this work, we overcome prior limitations by harnessing long-range ordered perovskite superball (SB) superstructures as a robust and versatile platform that provides multiple tunable degrees of freedom – enabling precise control over perovskite nanocrystal (PNC) size, ligand chemistry, packing density, and cavity architecture. This platform facilitates detailed photophysical investigations across both spontaneous and stimulated emission regimes, including spontaneous emission (SE), SF, ASE, and lasing. Through systematic variation of ligand length and nanocrystal size, we identified a critical packing density (r ~0.34), above which SF is activated. The SB system also exhibits high thermal resilience, sustaining SF up to 150 K, and demonstrates over tenfold enhanced stability compared to conventional superlattices - retaining >85% of its PL intensity after 1600 hours. Notably, the interplay between SF and ASE/lasing can be modulated by tuning the emitter density or the size of the SBs.

These findings offer critical insights into the fundamental mechanisms driving both spontaneous and stimulated emissions as well as their transitions across the different realms in strongly coupled PNC systems. This understanding could pave the way for significant advancements in the development of ultrabright, coherent quantum light sources.

Interfaces play a pivotal role in governing the efficiency and long-term stability of solar cells. While device performance is typically evaluated using macroscopic electrical and photophysical measurements, these bulk characteristics emerge from complex microscopic processes occurring at buried interfaces, where structural, chemical, and electronic properties converge. Defects, carrier transport pathways, and interfacial chemical reactions—particularly under operational stimuli such as illumination, electrical bias, and ambient exposure—critically determine device functionality. This interplay is especially pronounced in metal halide perovskites, whose soft and dynamic lattice renders defect and interfacial processes highly responsive and metastable.

We have introduced Correlation Clustering Imaging (CLIM),[1] a noninvasive microscale functional imaging technique that exploits intrinsic photoluminescence (PL) dynamics under continues excitation to probe these processes in space and time. The foundation of CLIM lies in the PL blinking phenomenon observed in sub-micrometer and even larger individual perovskite crystals, first reported in 2015[2] and subsequently studied in depth.[3][4] This blinking behavior originates from so-called supertraps—metastable nonradiative recombination centers that reversibly switch between active and inactive states. Such defect metastability, or the non-persistent presence of defect states, is now recognized as a key factor underlying the remarkably low nonradiative energy losses in metal halide perovskite materials and devices. CLIM generalizes this concept to extended films and operating devices by analyzing spatial correlations in PL fluctuations using wide-field fluorescence microscopy.

Applied to high-quality perovskite thin films, CLIM visualizes the polycrystalline grain structure with high fidelity, producing images closely resembling scanning electron microscopy results while remaining entirely noninvasive. Correlative CLIM–SEM analysis reveals how structural heterogeneities manifest as spatial variations in defect-mediated emission dynamics.

Strikingly, when the same materials are integrated into complete solar cell architectures, the PL dynamics change fundamentally. Under short-circuit and operating conditions, both the amplitude of PL fluctuations and the spatial extent of correlated regions increase markedly, with correlation lengths extending up to ~10 μm compared to ~2 μm in thin films. We propose that these extended correlated regions arise from dynamically evolving charge extraction pathways at transport-layer interfaces, which act as transient PL quenching channels sensitive to applied bias.[1]

By directly linking defect metastability and interfacial dynamics to nonradiative recombination losses, CLIM provides unprecedented insight into the microstructure–function relationship in dynamic optoelectronic materials.[1][5] Owing to its simplicity and operando compatibility, CLIM offers a powerful route to monitor material evolution during fabrication and to rationally engineer interfaces for improved efficiency and stability in next-generation optoelectronic devices.

Wide-bandgap halide perovskites are essential for high-voltage and tandem photovoltaic architectures, where maximizing quasi-Fermi-level splitting is critical for minimizing voltage losses [1,2]. Transient photoluminescence (TPL) spectroscopy is widely used to probe carrier recombination dynamics in these materials. Here, we present a comprehensive numerical and analytical investigation of sub-band radiative recombination in wide-bandgap perovskites. Experimentally, sub-band emission exhibits markedly slower decay dynamics than band-edge photoluminescence [3], indicating carrier localization and delayed recombination mediated by deep trap states. To elucidate the underlying mechanisms, we develop a physics-based carrier-dynamics model that explicitly incorporates band-to-band radiative recombination, carrier trapping and emission, and a radiative trap-to-band recombination pathway.

The coupled rate equations are solved under pulsed excitation conditions [4], enabling a unified and self-consistent description of both band-edge and sub-band emission within a radiative recombination framework. This model allows clear identification of the dominant recombination channels governing the observed TPL dynamics. Model-based fitting of transient photoluminescence decays quantitatively extracts band-to-band and trap-to-band radiative recombination parameters, accurately reproducing both the power-law decay of band-edge emission and the slower sub-band photoluminescence kinetics governed by trap occupancy.

Overall, this work establishes a rigorous theoretical foundation for interpreting sub-band photoluminescence in TPL studies and provides defect-aware insights critical for voltage optimization in wide-bandgap perovskite photovoltaic devices.

Metal halide perovskites (MHPs) have emerged as promising platforms for cooperative quantum emission. Superfluorescence is particularly intriguing because it arises from spontaneous synchronization of initially incoherent emitters into a macroscopic quantum state, producing delayed, intense, ultrafast bursts of coherent light. While superfluorescence has been demonstrated in perovskite nanocrystal superlattices and thin films, the mechanisms that enable its emergence at elevated temperatures, and the boundaries that separate cooperative from non-cooperative emission regimes, remain insufficiently understood.[1],[2] Addressing these knowledge gaps is essential for advancing superfluorescence toward quantum photonic applications.[3] 

In this talk, I present how our work tackles the challenges of identifying, and controlling superfluorescence in quasi-2D perovskite thin films. We demonstrate clear evidence to distinguish superfluorescence from amplified spontaneous emission and spontaneous emission. A comprehensive experimental phase map is constructed to reveal when and how macroscopic coherence forms, how it decays, and under what conditions it transitions into non-cooperative regimes.[4]

Using quasi-2D PBA:CsPbBr₃ thin films, we demonstrate superfluorescence with the lowest threshold fluence reported at 78 K in metal halide perovskites. Temperature and fluence dependent steady state and ultrafast spectroscopies resolve cooperative emission from 78 to 180 K, including a sharper coupled emission band, a delayed emission peak, ultrafast radiative lifetimes, quadratic like intensity scaling, and Burnham-Chiao ringing. We also map a phase diagram that delineates spontaneous emission, superfluorescence, and amplified spontaneous emission, and we observe deviations from ideal superfluorescence at high fluences and elevated temperatures, reflecting competing many-body and dephasing processes.[4]

To interpret these observations, we introduce the ab initio quantum model Superfluorescence Non-Approximating Integrating Large Solver (SNAILS), which tracks emission density, inter emitter correlations, excited state populations, and trace distance under varying pumping and dephasing conditions.[4] Simulations reproduce delayed cooperative bursts when dephasing is weak and show rapid suppression of correlations and loss of delay when dephasing is stronger. Trace distance dynamics identify synchronization plateaus during cooperative emission and prolonged plateaus linked to subradiant or dark states under high dephasing, establishing a unified framework for cooperative emission and its breakdown in perovskite thin films.

Overall, this work clarifies the optical and dynamical conditions required for macroscopic coherence in perovskites. The combined insights from phase mapping, ultrafast spectroscopy, and quantum modeling highlight strategies for lowering threshold fluence, mitigating dephasing, and extending superfluorescence toward higher temperatures.

Inverted p-i-n perovskite solar cells (PSCs) are promising candidates for monolithic silicon/perovskite tandem solar cells due to their superior operational stability under various stressor conditions compared with n-i-p structures. To date, p-i-n PSCs have achieved efficiencies exceeding 27% in single-junction devices and 35% in silicon/perovskite tandem solar cells. Despite these achievements, nonradiative recombination at the perovskite/C₆₀ interface remains a major bottleneck preventing p-i-n PSCs from approaching their theoretical Shockley-Queisser limit, primarily through V_OC losses. To mitigate these losses, extensive interfacial engineering strategies have been explored, including the incorporation of alkylammonium salts and the insertion of mild dipole layers at the perovskite/C₆₀ interface. The resulting improvements in V_OC are typically attributed to more favorable energy level alignment, enhanced charge extraction, and reduced interfacial trap densities. However, the underlying physical mechanisms and their quantitative analysis remain insufficiently understood.

To address this knowledge gap, we combine widely used time-correlated single-photon counting (TCSPC) with high dynamic range gated charge-coupled device (CCD) to perform transient photoluminescence (TrPL) measurements spanning more than ten orders of magnitude in intensity on state-of-the-art Cs_0.05MA_0.10FA_0.85Pb(I_0.97Br_0.03)₃ (CsMAFA) perovskite films interfaced with different charge selective layers. The use of these complementary techniques enables cross-validation of TrPL data and minimizes potential measurements artifacts. Analysis of the differential decay time (τ_diff) as a function of Fermi-level splitting ΔE_F reveals that recombination in bare CsMAFA layer is dominated by shallow traps,^1,2 while incorporation of the hole transport layer MeO-2PACz effectively passivates these traps, resulting in prolonged decay times. In contrast, the introduction of C₆₀ significantly shortens the decay time and leads to a distinct plateau in τ_diff at high ΔE_F, followed by an exponential increase at low ΔE_F, observed in both CsMAFA/C₆₀ bilayers and complete device stacks.

These behaviors can be rationalized by a transition from surface recombination dominated decay at high ΔE_F to a shallow trap dominated regime at lower ΔE_F. To decouple the different physical mechanisms and quantitatively capture this interplay, we develop a semi-analytical model and perform drift-diffusion simulations that account for surface recombination, shallow trap states, and electron exchange (j_exc) between the perovskite and the transport layer. Beyond explaining the experimental observations, this framework provides a general platform for understanding how transport layer thickness and interfacial band offset influence carrier decay dynamics in perovskite/TL bilayer systems.

Through a remarkable series of advances in material design, the efficiency of organic solar cells has risen from 1% to over 20% within two decades, surpassing most predictions. Key to reaching this milestone has been the development of nonfullerence acceptors (NFAs) which, when paired with suitable polymer donors, yield high internal quantum efficiencies (IQEs) despite having a significantly smaller driving force for exciton splitting than is present in typical polymer:fullerene systems. There is some evidence that the best performing NFAs can generate charge separated states within single material domains, and it has been suggested that this process is what underlies their ability to efficiently generate charge pairs across a heterojunction. In this work, we address this question using an experimentally validated computational model of charge pair generation in molecular crystals. Our model can account for the significant effects of excitonic and electronic delocalisation on both excited state lifetimes and on charge and energy transfer rates. Using this model, we track the time evolution of excited states in single-component systems and demonstrate how both intramolecular parameters, such as reorganisation energy, and intermolecular parameters, such as electronic coupling, influence the yield of charge separated states. By analysing our results within a thermodynamic framework, we can identify the main loss pathways and thus identify the states which limit the overall yield of free charges. Additionally, we investigate how the same set of parameters affect charge generation efficiency in heterojunctions. Our results indicate that, although there is a correlation between the ease of charge generation in single-component and heterojunction systems, it is unlikely that charge pairs generated in the acceptor domains contribute significantly to the photocurrent in a heterojunction architecture. Finally, we apply our model to real crystal structures to understand how seemingly small morphological changes can lead to significant differences in the yield of separated charges. Taken together, our simulation results rationalise the experimentally observed differences between the ease of charge generation in different families of NFAs and suggest design criteria for future materials to further boost organic solar cell performance.

Fluorescence resonance energy transfer (FRET) is a fundamental mechanism for non-radiative electronic energy transfer (EET) widely observed in materials science [1] and biophysics [2]. However, given the challenge of balancing accuracy and computational cost in quantum calculations of large systems, it is typically modeled within the dipole-dipole approximation, using frequently unsuitable statistical methods to estimate the orientational factor (κ). To overcome the so-called Kappaphobia [3], i.e., the reluctance to determine κ adequately, we combined Molecular Dynamics (MD) simulations with Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations to properly assess the orientation factor on representative nonfullerene electron acceptors (NFAs). An MD-based solvent evaporation protocol was performed to model experimental spin-coating techniques, and the κ values were determined for 9000 pairs of molecules at thin-film conditions. As a result, the κ² parameter showed broad dispersion, with significantly higher average values of 0.949 and 0.765 for ITIC-4F and Y6 films, respectively, compared to the standard 0.476 and 2/3 statistical values. Moreover, by considering the system-specific κ² values, FRET rates became consistent with the experimental trend on exciton diffusion lengths. Finally, we assessed the limitations of the dipole-dipole approximation by employing the transition charges from electrostatic potentials (TrESP) method [4], and identified a simplified molecular descriptor that can be used as a cheap tool to extract initial insights into the orientation factor between NFA-like molecules.

A fundamental understanding of the photophysics of organic photovoltaics (OPVs) at the molecular level remains a major challenge, limiting the rational design of novel materials with enhanced performance and stability. To help bridge this knowledge gap, we developed a multi-scale methodology that integrates Quantum Mechanics (QM) calculations with Classical Molecular Dynamics (CMD) simulations in a sequential QM/CMD framework to investigate OPV photophysics under realistic conditions¹. Our approach begins with CMD simulations of macromolecules (oligomer models) in solution, followed by the simulation of film formation via solvent removal. Further CMD simulations are then conducted on the resulting films to generate statistically uncorrelated configurations for subsequent QM calculations. These calculations are carried out using density functional theory (DFT), time-dependent DFT (TD-DFT), and the wavefunction-based ADC(2) method, with environmental effects considered either explicitly with an electrostatic embedding scheme or implicitly with a polarizable embedding model. We have applied this multi-scale methodology to study (i) the PF5-Y5 polymer, serving as a model system for covalently bound donor-acceptor interfaces and (ii) Y6 and Y6-derived acceptors. Our analysis quantifies the effects of molecular dynamics and environment on electronic transitions, providing an improved description of optical absorption and redox properties. In particular, we find that the intrinsic asymmetry of Y-type acceptors induces distinct local electronic environments, which significantly influence the relative energetic positioning and character of singlet and triplet excited states. Such insight is essential, for instance, to assess triplet-mediated oxygen sensitization and material degradation, a topic of significant concern. Overall, this study highlights the critical role of disorder, dynamics, and molecular environment effects in determining the electronic properties of ground and excited states of OPV materials, offering insights for the design of next-generation photovoltaic systems.

Molecularly engineered interlayers have emerged as a powerful strategy to enhance the efficiency and stability of perovskite solar cells. [1] These interfacial layers bridge structural and electronic mismatches between the perovskite and charge transport layers (CTLs), yet the underlying mechanisms of charge transport across such interfaces remain poorly understood. In particular, it is unclear whether photogenerated electrons and holes cross the interface via tunneling or hopping mechanisms. Tunneling occurs when carriers transfer across energy barriers between the perovskite and CTL, whereas hopping can proceed stepwise through the redox-active levels of the molecular layer or directly between the perovskite and CTL. Here, we investigate charge transfer across perovskite/molecular layer/CTL interfaces using large-scale density functional theory (DFT) calculations. Our approach combines electronic-structure analysis with real-space electron density maps and projected local density of states to assess energy-level alignment and spatial charge localization at the interface. We apply this approach to experimentally realized perovskite devices [2–4] incorporating both electroactive and insulating interlayers, elucidating how molecular properties and interfacial interactions affect charge transport in the device. Our results provide a comprehensive picture of interfacial charge transfer mechanisms from a first-principles perspective, offering guidance for the rational design of molecular interlayers in perovskite photovoltaics.

The rapid increase in photoconversion efficiency (PCE) of halide perovskites cannot be separated from the degradations that alter its scaling-up to industrialization. Many instability sources need to be addressed, such as the presence of a complex sequence of phase transitions, the instability to high temperatures, the reactivity with ambient molecules. Other degradations, leading to the PCE decrease in the cell, such as the halide segregation when exposed to light sources, should be also looked at. Although the encapsulation or use of additives could help in the reduction of instability, the most pertinent pathway remains the stabilization of the intrinsic properties of perovskites. Inorganic perovskites, that reduce instabilities with ambient molecules with respect to the organic ones, should be prioritized. The stabilization strategy is focused around three main criteria: i) the stabilization of the rich sequence of phase transitions through the annihilation of soft modes, that imply structural distortions and a symmetry reduction; ii) the passivation of local (vacancies, interstitials); and iii) of more extended defects (surfaces, interfaces, etc…).

This work focuses on the optimization of the chemical composition of the complex inorganic halide perovskites AA’BB’(XX’)₃, using Density Functional Theory, in order to investigate the required conditions for a stable and high-performance inorganic perovskite. A unique home-made hybrid exchange-correlation functional is used to reproduce efficiently their structural, electronic, optic and dynamic properties, with accuracy with respect to existing experimental data. We illustrate how this approach is used to define chemical compositions, allowing to decrease the different lattice distortions by annihilating the unstable phonon modes.

Point defects are at the origin of recombination centers (causing efficiency losses), and their rate and/or their effect should thus be diminished. Although low-energy defects do not form deep traps in lead halide perovskites, we show that tuning the composition can affect both the defect formation energies and the depth of charge state energy levels. Alloying at B and X-sites can lead to significant changes in structural and electronic parameters, which can strongly increase the concentration of defects and favor the migration of halogen vacancies.

Finally, the data previously obtained, completed with other parameters (including band alignment, refractive indices…) is used as input for drift-diffusion models to predict the performance of the perovskite solar cell, with optimal material choices and layer thicknesses. We show that considering the most stable structural phase is essential to avoid inaccurate device‑performance estimations.

Hybrid perovskite photovoltaics represent a dynamic frontier in the development of next-generation solar energy technologies, offering unparalleled opportunities for tunable optoelectronic properties, low-temperature processing, and integration into multifunctional energy systems^[1]. Here, we present a holistic and co-optimized framework that unites molecular design, interface engineering, and defect passivation to overcome long-standing performance–stability–transparency trade-offs in perovskite devices. Our work highlights multimodal strategies tailored for two emerging application landscapes: building-integrated photovoltaics (BIPV) and ultra-efficient indoor light harvesting.

First, we demonstrate an integrated optical-electrical-chemical approach to semi-transparent perovskite solar cells and modules. By combining optimized transparent conductive electrodes, tailored optical management layers, and targeted molecular passivation, we achieve state-of-the-art semi-transparent devices that balance high power conversion efficiency (PCE) with exceptional light utilization efficiency (LUE)^[2]. This strategy effectively decouples the traditional transparency–efficiency compromise, paving the way for scalable, high-performance BIPV solutions that seamlessly integrate into windows, façades, and smart building envelopes.

Second, we introduce a buried-interface engineering platform based on a multifunctional polymer matrix that homogenizes electron transport layer (SnO₂) dispersion, suppresses interfacial recombination, and promotes preferential perovskite grain orientation. Devices fabricated using this approach exhibit significantly enhanced PCE with minimal hysteresis and remarkable operational stability under continuous illumination. These results underscore the pivotal role of interfacial quality and morphological control in realizing robust and efficient hybrid perovskite photovoltaics^[3].

Third, we report a novel Triple Passivation Treatment (TPT) strategy designed for wide-bandgap perovskites, which concurrently addresses bulk, grain boundary, and surface defects while modulating surface energetics from n- to p-type character. This treatment enables a record indoor PCE of 38% under 1000 lux LED illumination, alongside outstanding shelf-life and light-soaking stability. The TPT protocol highlights how synergistic passivation can unlock the full potential of perovskites for low-light and Internet-of-Things (IoT) powered environments^[4].

Collectively, these advances illustrate how cross-disciplinary innovation in materials chemistry, interface science, and device architecture can drive perovskite photovoltaics toward commercial viability^[5]. By bridging gaps between efficiency, stability, and application-specific functionality, our work enables the integration of perovskite photovoltaics into both transparent building elements and off-grid indoor energy systems. These findings establish broadly applicable design principles for hybrid photovoltaic technologies, supporting the development of efficient, stable, and application-ready solar energy systems for sustainable and decentralized energy infrastructures.

Solid-state spin qubits are a cornerstone of scalable quantum networks, traditionally requiring nuclear-spin-free hosts to maintain coherence. In this work, we present the first demonstration of halide double perovskites (HDPs) as a viable platform for solid-state spin qubits, establishing a new material library for quantum information technology. Utilizing transition-metal (TM) centers—Cr³⁺ and Fe³⁺ ions—doped into a Cs₂In(Na,Ag)Cl₆ host¹, we demonstrate that these centers exhibit long-lived electron spin coherence despite the nuclear-spin-rich environment of the perovskite lattice. We report benchmark coherence times T₂ of 29.5 µs for Cr³⁺ and 21.2 µs for Fe³⁺ at 4 K. Our findings reveal that strong spin localization at the TM sites facilitates deterministic electron-nuclear (e-N) spin rotations with neighboring ^35,37Cl and ¹³³Cs nuclear spins, limiting the number of e-N interaction and resulting in long spin coherrence in the nuclear spin-rich hosts. Furthermore, we show that Cr³⁺ centers are optically addressable through spin-selective intra-d-shell transitions, enabling clear protocols for high-fidelity optical initialization and readout. These results highlight the potential of halide perovskites to provide a chemically tunable "sandbox" for engineering advanced quantum states, such as qudits, using inexpensive and scalable solution-based synthesis. By combining superior optoelectronic properties with robust spin coherence, this platform opens new avenues for integrating quantum nodes with existing semiconductor technologies.

In this work, we investigate monolithic perovskite photoanodes derived from perovskite solar cell architecture, aiming to integrate light absorption and anodic functionality within a single device for photoelectrochemical (PEC) applications to drive solar water splitting. A glass/ITO/SnO₂/perovskite structure is adapted from conventional n–i–p photovoltaic layouts to operate as a photoanode under alkaline conditions relevant for water oxidation. This approach enables direct utilization of the high photovoltage generated by the perovskite absorber, while reducing architectural complexity compared to photovoltaic-assisted PEC configurations [1-2]. A key focus of this study is the engineering of the perovskite–interface region, which governs charge extraction, recombination losses and operational behavior under illumination. Nickel-based catalytic overlayers supported on carbon paper were introduced to promote efficient hole transfer from the perovskite absorber to the electrolyte, while preserving favorable interfacial energy alignment. From a perovskite perspective, this strategy allows decoupling of bulk optoelectronic optimization (bandgap, absorption, carrier transport) from interfacial electrochemical functionality. Photoelectrochemical measurements under simulated solar illumination, combined with structural and morphological characterization, reveal that appropriate interfacial design leads to enhanced photocurrent response and reduced onset potentials for water oxidation. These results demonstrate that architectural and interfacial engineering are essential for translating perovskite photovoltaic performance into efficient photoelectrochemical operation. Overall, this work positions halide perovskites as versatile platform materials that can bridge photovoltaic and photoelectrochemical technologies, providing insights relevant to the development of next-generation perovskite-based devices for solar fuel applications.

The two-dimensional (2D) perovskite family consists of pseudo-2D materials, crystallized into Ruddlesden-Popper, Dion-Jacobson, or alternating cation structures that are 3D materials with internal inorganic layers, and true 2D perovskites, where nanoplatelets (NPLs) belong to the latter category. 2D perovskites are of high interest in the scientific community from their higher stability in comparison to 3D perovskites and are commonly used either as stabilizing layers on top of 3D perovskites in solar cells or utilized in-themselves as tuneable emerging light emitting materials, X-ray scintillators, or within high-sensitivity photosensing/imaging [1]. A key manifestation in 2D perovskites is the systematic blue shift in emission with decreasing thickness, reflecting reduced dimensionality alongside a stronger excitonic coupling, providing a versatile platform for fundamental studies of size-dependent quantum phenomena. Understanding how dielectric anisotropy governs excitonic behavior in 2D halide perovskites is critical for predicting and engineering their optoelectronic properties. We present experimental and theoretical results for 2D Cs_(n+1)Pb_nBr_3n+1 NPLs (n = 2–5), with successively larger number of monolayers.  The interplay between dielectric confinement and anisotropic screening critically determines both their excitonic landscape [2] and their symmetry-dependent lattice dynamics [3].  The exciton binding energies show a monotonic decrease from 0.26 eV to 0.21 eV from n = 2 to 5, with 20 meV decrease per layer up to n = 4, and thereafter less change due to the strong spatial localization of excitons. Our calculated absorption spectra using a model Bethe-Salpeter equation and dielectric-dependent hybrid functionals,  capture experimental results within 0.02 eV throughout the confinement regime (n = 2-5) [2]. The effects of lattice dynamics on the dimensionally dependent dielectric response and subsequent exciton screening occurring on longer time-scales than the optical response are also analyzed, important for analysis and interpretation of exciton lifetime, diffusion, and band alignments. The symmetry of the phonon modes and their change in polarizability during displacement provide a mechanistic framework and practical toolkit to understand lattice dynamics and confined phonons in low-dimensional materials [3]. Apart from the mechanistic insights, the results show that symmetry-resolved A1g/B1g intensity ratios from cross-polarized Raman spectroscopy can be utilized as a calibrated, non-destructive thickness metrology for the number of monolayers. If time permits, we also present recent results on the electronic and vibrational dynamics in low-dimensional tin halides [4].

Emerging photovoltaics (PV), including organic PV (OPV), perovskite PV (PPV), and quantum dots PV (QDPV), have attracted significant interest for aerospace applications due to their high specific power (power-to-weight ratio), high mechanical flexibility, and low cost. For example, OPV, PPV, and QDPV exhibit specific powers of 39.3, 50, and 15.2 W/g, respectively, which are significantly higher than those of traditional PV. [1-3] High-altitude pseudo-satellites (HAPS) operating in the lower stratosphere (∼20–25 km) could be the first to deploy these emerging PV technologies. This region exposes photovoltaic systems to AM0 solar irradiance, and the environmental conditions include low-temperature cycles (+10 → –20 °C during the day and to –85 °C at night), and low ambient pressures that differ significantly from terrestrial conditions.[4] These environmental stresses can critically impact emerging PV, yet direct comparative studies across material classes are scarce. In this work, we investigate the low-temperature performance and thermal cycling stability of these three promising PV technologies under conditions mimicking HAPS environments.

For OPV, two state-of-the-art active-layer material systems have been studied and revealing acceptor-dependent performance differences at reduced temperatures. In contrast, for QDPV, different hole-transport layer configurations were studied here, showing that PbS-MPA devices maintained or slightly improved power conversion efficiency (PCE) even in an HAPS environment and down to −100 °C, and exhibited excellent thermal cycling stability, suggesting robustness under repeated stratospheric day–night cycles. Furthermore, Perovskite devices with both narrow, wide-bandgap compositions and tandem were evaluated on rigid and flexible substrates, demonstrating high PCEs under HAPS environment for tandem PV.

Interesting: the QDPV retains 100% of its initial PCE even at -100 °C, whereas the PPV and OPV retain ~80% and ~50% of their initial PCE, respectively, under similar conditions. Furthermore, the QDPV and PPV showed excellent thermal cycling stability over the range of +20 to -85 °C, with 15 cycles.

Our comparative study highlights material-dependent responses to stratospheric stresses, identifies design pathways for improved environmental resilience, and provides important initial data for the selection for next-generation lightweight power systems deployed in HAPS and related aerospace applications.

Perovskite indoor photovoltaics (IPVs) are emerging as a transformative technology for low-light intensity energy harvesting, owing to their high-power conversion efficiencies (PCEs), low-cost fabrication, solution-processability, and compositionally tunable band gaps. In this work, methylammonium-free Cs_xFA_1-xPb(I_1−yBr_y)₃ perovskite absorbers were compositionally engineered to achieve band gaps of 1.55, 1.72, and 1.88 eV, enabling spectral alignment with indoor lighting. Devices based on a scalable mesoscopic n-i-p architecture were systematically evaluated under white LED illumination across correlated color temperatures (3000–5500 K) and light intensities from 250 to 1000 lux. The 1.72 eV composition exhibited the most robust performance across different light intensities and colors, achieving PCEs of 35.04% at 1000 lux and 36.6% at 250 lux, with stable device operation over 2000 hours. While the 1.88 eV band-gap variant reached a peak PCE of 37.4% under 250 lux (5500 K), however performance trade-offs were observed in current density and consistency. Our combined experimental and theoretical optical-electrical simulations suggest that decreasing trap-assisted recombination in wide-bandgap compositions may further improve IPV performance across varying illumination conditions. In contrast, devices with a 1.55 eV band gap underperformed due to suboptimal spectral overlap. These findings establish bandgap optimization and device architecture as key design principles for high-efficiency, stable perovskite IPVs, advancing their integration into self-powered electronic systems and innovative indoor environments.

Two step deposition of lead iodide based perovskite can have many benefits as it separates the film printing from the end chemistry. This enables the formation of the film, thickness, coverage, uniformity, to be optimized separately to the end perovskite composition and morphology. The isolation of these also allows for the low-solubility lead iodide to be solubilized and deposited using relatively low toxicity solvents. In order to achieve uniform and permeable lead iodide films, DMF:DMSO is usually used. This solvent combination provides high solubility, enabling film and crystal morphology to be easily controlled using an air knife or different drying protocols.However, for scale up processes, which are often performed in open or semi-open environments, solvent extraction can become extremely difficult. As a result the toxicity, health considerations, and work exposure limits of solvents can severally hinder scale up efforts. This solvent system was identified using a combination of Hansen Solubility parameter analysis, and targeted solution trials, enabling solution stability and consistency refinement for consistent and uniform films.

              However the real benefit of being able to print a convertible lead iodide layer is that the end perovskite composition can be varied by altering the salts in the conversion step. By maintaining the broad compatibility of the first step, pivotability in composition can be conserved, as it is defined by the conversion step. As organic iodine salts are usually readily soluble in lower toxicity solvents, the entire process toxicity can be reduced. Enabling scale up investigations of multi and alternative cation perovskites without the requirement to completely reformulate, and re-optimise for printability.

              Whilst the individuality of the steps is a key take away, the interdependence is also investigated. Allowing a holistic approach to adaptable film formation. By taking this approach the scalability and variability can be adapted from spin coating to slot die coating, moving from proof of concept to roll to roll.

The translation of laboratory-scale photovoltaic devices to commercially viable manufacturing remains a critical bottleneck, particularly for organic and hybrid perovskite solar cells where vast, interdependent parameter spaces complicate optimization. We present our group's comprehensive self-driving laboratory (SDL) ecosystem for accelerating photovoltaic innovation, integrating purpose-built hardware platforms, critically evaluated machine learning software, and targeted materials development to create closed-loop autonomous discovery systems spanning the full research-to-manufacturing pipeline.

Our high-throughput hardware centres on low cost. modified 3D printer systems such as the MicroFactory platform, a roll-to-roll fabrication and characterization system producing over 11,800 organic photovoltaic devices within 24 hours across 64.8 metres of flexible substrate. This throughput transforms intractable optimization problems involving 36 manufacturing variables requiring 68.7 billion experiments through factorial design into tractable exhaustive searches. Using printing-inspired digital twins and inverse parameter generation, we achieved 1.1% absolute PCE improvement in a single optimization iteration for PBF-QxF:Y6 devices exceeding 8.5% efficiency. Recognizing that commercial automation costs restrict SDL adoption, we further demonstrate that consumer-grade 3D printers modified with custom syringe attachments can serve as automated liquid handlers, reducing costs by two orders of magnitude while enabling distributed robotic architectures for coordinated multi-station perovskite device fabrication.

Our software development critically examines assumptions underlying SDL optimization strategies. Through systematic evaluation of 11,587 devices across 25 optimization iterations, we find that Bayesian optimization achieves only marginally higher PCE (7.69%) compared to random search (7.66%), with environmental factors, particularly humidity, showing stronger performance correlation than algorithm selection. This suggests that robust process control supersedes algorithmic sophistication for manufacturing-scale systems. We extend our software capabilities through large language model integration, developing retrieval-augmented generation (RAG) chatbots for research assistance and establishing rigorous benchmarking frameworks using RAGAS evaluation metrics to assess factual correctness, context recall, and faithfulness against expert baselines. Such validation becomes essential as LLM agents increasingly support autonomous experimentation.

Materials innovation within our SDL framework addresses the stability limitations hindering perovskite commercialization. We introduce PBDF-DFC, a biomass-derived furan-based conjugated polymer enabling simplified one-pot precursor integration into hybrid perovskite solar cells. Unlike petroleum-derived thiophene polymers with limited precursor solubility, PBDF-DFC dissolves directly in perovskite solutions, streamlining fabrication while achieving 21.39% PCE, a 7.8% relative improvement over controls. Crucially, devices retain 90% initial efficiency after 1100 hours under environmental stress, compared to 52% for unmodified controls. Transmission electron microscopy reveals polymer accumulation at grain boundaries, passivating defects through oxygen-lead coordination. Complementing this work, we demonstrate non-fullerene acceptor interlayers for SnO₂ electron transport layer modification, where cyano, carbonyl, and halogen functional groups bind electrostatically to oxygen vacancies, reducing trap-state density and enhancing charge extraction in hybrid organic-perovskite architectures.

Looking forward, we position current SDLs within an evolution from basic automation toward fully autonomous discovery systems. Present implementations achieve approximately 71% recipe success rates with single-domain focus. Near-term integration phases will enable cross-domain optimization and manufacturing awareness. Ultimately, agentic discovery systems incorporating goal-directed reasoning and integrated knowledge representations will autonomously navigate from hypothesis generation through experimental validation to scale-up. Our work demonstrates that combining high-throughput hardware, rigorously validated software, and targeted materials expertise creates a powerful engine for next-generation photovoltaic development.

Organic photovoltaic (OPV) technology is rapidly emerging as a key enabler for autonomous indoor IoT devices, smart sensors, and low-power electronics. In this presentation, we report the successful transition of our printed OPV technology from laboratory-scale development to pilot production and now full industrial-scale manufacturing. Our company has implemented a fully operational production line based on scalable inkjet printing processes, enabling high-throughput fabrication of customized organic photovoltaic cells and modules.

The inkjet printing approach offers significant design flexibility, allowing tailored geometries, variable form factors, and seamless integration into constrained environments and connected objects. Our modules are specifically optimized for indoor energy harvesting, demonstrating high power conversion efficiency under low-light conditions as well as long-term operational stability.

We will detail our scale-up strategy, process architecture, and lessons learned from manufacturing, including quality control and throughput optimization. Additionally, we will highlight application-driven design capabilities, illustrating how printed OPV modules can be customized to meet the growing demand for self-powered IoT devices, wearable electronics, and smart sensor networks. This work demonstrates the industrial viability of OPV technology as a scalable, reliable, and versatile solution for emerging low-power applications.

Carbon fibre-reinforced polymers (CFRPs) combine high strength and elastic modulus with low weight, and are thus an important material within the automobile and aviation industries. Combining high performance perovskite solar cells (PSCs) with CFRP substrates presents an opportunity to create structural materials that also generate power. Here such mobile power applications require the use of devices having a high specific power in which both the efficiency and the weight of the solar cell is of critical importance. Here, we discuss the fabrication of n-i-p PSCs via ultrasonic spray coating on CFRP substrates. Our devices yield a stabilized maximum power conversion efficiency (PCE) of 14.0 %, corresponding to a specific power of 26.0 W g-1. We also spray-coat PSCs onto dome-shaped CFRP substrates having a maximum PCE of 12.3 %, equivalent to a specific power of 22.8 W g-1. Importantly, we subject devices to mechanical fatigue testing, repeatedly applying a load of 50 N, with devices retaining 100 % of their prestressed PCE after 25 stress cycles. We believe this is the first demonstration of curved, rigid, CFRP based PSCs that act as load bearing structures, making them suitable for mobile power applications.

Metal–halide perovskites have emerged as leading absorber materials for both single-junction and tandem solar cells owing to their low-temperature processing and rapidly advancing performance, with single-junction devices now exceeding 27% power conversion efficiency. Despite this progress, present devices remain below their thermodynamic efficiency limits and suffer from insufficient operational stability. Reaching and maintaining these limiting efficiencies requires a holistic understanding of the microscopic mechanisms involved in device degradation; however, conventional characterization is time consuming and low-throughput. Here, we present the Optigon Prism: a rapid, multimodal, and high-throughput optical metrology tool designed to accelerate materials characterization and production insights. Prism integrates broadband transmission/reflection, time-, and spectrally-resolved photoluminescence measurements into one device to capture holistic datasets, which are used to characterize the response of perovskite solar cells during stress testing and quickly generate device-level insight. We demonstrate that these multimodal datasets can be used to extract and track in situ changes in material parameters during stress testing, such as the non-radiative recombination rate, which can be diagnostic of overall device performance. In subsequent experiments, we show that these datasets combined with our analytical framework can be used to infer device properties without the need to fully complete devices. We measure 120 semi-fabricated devices (“half-stacks”) in 10 minutes and use this large dataset to infer the open-circuit voltage (V_OC) expected from a complete device.[1] Using this full suite of optical characterization, we quantify losses from non-radiative recombination, leading to a realistic assessment of V_OC. We find that our inferred V_OC quantitatively correlates with measured voltages on identically fabricated, fully completed sister samples, while our method also captures the effects of electron transport layer deposition on V_OC across 5 unique device architectures. Taken together, we show that non-contact optical measurements are a rapid and accurate method to access key metrics relevant to device efficiency and stability, with minimal device fabrication required. Overall, this work demonstrates that rapid and high-throughput optical characterization is invaluable in accelerating the device iteration timeline both by providing in situ material responses to stability testing and device level insight without needing fully fabricated devices.

Organic photovoltaics (OPVs) have reached above 20% Power Conversion Efficiency (PCE) in recent years in lab-scale devices using the spin coating technique. Almost all state-of-the-art devices rely on ITO as a transparent electrode, with several limitations: the need for the rare-earth element indium, high processing temperatures, limited flexibility, and incompatibility with sputtering onto organic layers, thereby restricting the use of opaque substrates. Additionally, the top evaporated metal electrodes are largely unsuitable for industry-compatible Roll-to-Roll (R2R) coating methods.

In this study, we adopted a hybrid approach that combines the advantages of R2R vacuum and solution coating methods for fabricating organic solar modules on glass and flexible polyethylene terephthalate (PET) in a top-illumination configuration, Glass or PET/Ag/ZnO/PM6:Y7-12/BM-HTL/AgNWs. In which the opaque silver electrode was developed on the substrate via R2R sputtering to achieve low sheet resistance and reduced surface roughness as a replacement for the top evaporated opaque metal electrode of a typical OPV device. As an ITO replacement, silver nanowires were coated via the slot-die method to serve as a top transparent electrode, suitable for device fabrication on many opaque substrates. The remaining layers in the devices, including the photoactive layer, the electron transport layer, and the hole transport layer, were optimized via R2R-compatible slot-die coating under ambient conditions with green solvents. The fabricated small cells for the best devices on glass substrates achieved a PCE of 13.5 %, using this ITO-free scalable OPV architecture. An equally impressive PCE of 12.5% was attained when the devices were scaled up to mini-modules with an active area of 12.8 cm² on glass substrates. Furthermore, six mini-modules on a PET substrate measuring 24 cm x 17 cm were developed, reaching PCE up to 11.5 %, the highest reported in this category. Finally, the commercial potential of this method was demonstrated by the development of a mobile charger featuring modules with an active area exceeding 100 cm², achieving a remarkable PCE of 9.9%. This research highlights the potential for developing high-performance, cost-efficient, and mechanically adaptable ITO-free devices using industry-compatible methods.

Organic photovoltaics (OPVs) have emerged as a promising alternative to conventional silicon-based solar cells for sustainable energy applications due to their low cost, lightweight nature, and compatibility with flexible substrates through solution-based processing. The detrimental impact of non-geminate recombination on high-performance organic photovoltaics has been recognised and primarily attributed to bimolecular recombination. However, the recent surge in Y-series acceptor-based systems has drawn attention to deep-trap-assisted monomolecular recombination. In this talk, we will discuss first the morphological origin of deep traps in OPV thin films, identifying isolated crystalline and amorphous Y6 domains as key contributors. The findings underscore the importance of improving inter-acceptor domain connectivity for effective trap passivation. Furthermore, we will talk about the multiscale morphology of polymer donors and its impact on charge transport and emissive properties. Last, we will discuss the morphological origin of shunt pathways, which is also related to connected domains but at a much larger scale. We will report an effective way to passivate the shunt pathways, while retaining charge transport pathways.

The property of charge transfer (CT) state determines the charge generation yield, energy losses, and charge collection efficiency, hence is the key to properly understand the operation of organic photovoltaics (OPVs). OPVs based on nonfullerene acceptors (NFAs) often possess reduced offset between local excitonic state (LE) and CT state, leading to invisible CT state emission from typical photoluminescence (PL) and electroluminescence (EL) experiments. The invisibility of CT state makes it difficult to access its physical properties in low-offset NFA OPVs, therefore hindering the further development of OPV devices to reach 20% power conversion efficiency. Here, we propose a method to probe the “invisible CT states” using a newly developed model framework coupled with PL and EL experiments. Through modelling fitting, we are able to extract the excited state properties of both LE and CT states, which allows us to draw correlations between offset and the properties of CT states. This work offers a method to access the properties of CT states in low-offset OPVs, and provides insights into the understanding of charge generation, energy losses, as well as charge collection.

All-polymer solar cells offer potential advantages in mechanical robustness and morphological stability, yet they remain less efficient than state-of-the-art small-molecule non-fullerene acceptor–based organic solar cells. In this work, we investigate the efficiency-limiting processes in all-polymer bulk heterojunction devices based on the donor polymers PBDB-T and PBDB-T-2F (PM6) blended with the polymer acceptors PYN-BDT and PYN-BDTF. Owing to π-extended naphthalene units, the acceptor polymers exhibit strong absorption extending to approximately 900 nm. A combination of steady-state spectroscopy and time-resolved techniques—including transient absorption, time-resolved photoluminescence, photoluminescence-detected magnetic resonance, and time-delayed collection field measurements—enables a quantitative analysis of charge-generation and loss processes. Kinetic parameters and process yields derived from pulsed-laser spectroscopy reproduce the measured current–voltage characteristics, confirming the consistency of the approach. The results show that moderate fill factors arise from non-geminate recombination competing with charge extraction and from field-dependent charge generation, with the relative importance of these processes depending on the polymer acceptor. The methodology provides a general framework for quantifying loss mechanisms in bulk heterojunction organic solar cells.

Organic solar cells (OSCs) offer a lightweight, flexible alternative to silicon photovoltaics, with advantages such as low environmental impact and high mechanical conformability. However, their lower efficiency and stability remain key challenges, requiring advanced characterization techniques to guide material and device optimisation.

Raman spectroscopy provides exceptional chemical specificity and is routinely used to obtain information about the various power efficiency-influencing properties of silicon, cadmium telluride, perovskite, III-V semiconductor, quantum dot, and organic cells. Combined with confocal microscopy, Raman spectroscopy enables micron-resolution imaging of material properties across a cell. Additionally, Raman microscopes can be configured for photoluminescence (PL) imaging; an industry standard for photovoltaic quality control. By further integrating  a photocurrent imaging upgrade with the Confocal Raman Microscope, we can directly correlate Raman, PL and cell performance across a device.

This presentation will demonstrate Raman, PL, and photocurrent imaging of the same area on an OSC using an Edinburgh Instruments RM5 Confocal Raman Microscope. This correlative spectral and photocurrent imaging methodology is a powerful tool in the photovoltaic industry because it allows for an enhanced understanding of the properties of solar cells, their defects and degradation over time.

Morphology-function-degradation relationships in organic and hybrid photovoltaics: Revealed using synchrotron soft X-ray microscopy

Abstract:

Understanding how nanoscale morphology, chemical heterogeneity, and ion dynamics govern performance and stability remains a major challenge across organic photovoltaics (OPV), organic semiconductor (OSC) photocatalysts, and hybrid perovskite devices. In this contribution, I demonstrate how synchrotron-based soft X-ray microscopy provides a unified and chemically sensitive framework to address these questions across different material platforms [1-6].

We combine scanning transmission X-ray microscopy (STXM) and soft X-ray ptychography (SXP) to investigate donor–acceptor OSC nanoparticles synthesized via nanoprecipitation and miniemulsion routes, model OPV-relevant blends, and vapor-deposited metal-halide perovskite thin films. In OSC nanoparticles used for photocatalytic hydrogen evolution, STXM and SXP reveal how synthesis-induced internal morphologies, ranging from highly intermixed to core-shell structures, directly control charge separation, activity, and long-term stability. Intermixed morphologies exhibit enhanced initial hydrogen evolution rates, while core-shell structures show delayed but more stable performance, highlighting a morphology-driven efficiency and stability trade-off that cannot be resolved using bulk or purely optical characterization techniques.

Soft X-ray ptychography provides access to phase contrast beyond X-ray absorption, enabling visualization of internal morphology in weakly absorbing organic systems at spatial resolutions exceeding conventional STXM under comparable or lower dose conditions. I will discuss how dose, contrast mechanisms, and spatial frequency-dependent signal-to-noise ultimately define the accessible resolution in radiation-sensitive soft matter systems.

Extending these approaches to hybrid perovskites under electrical stress, soft X-ray STXM enables direct visualization of light-element ion migration (I, C, N) and associated chemical transformations under operando-relevant biasing conditions. These observations are complemented by hard X-ray nano-XRF measurements, which capture long-range iodine redistribution across device electrodes. Together, these multimodal measurements allow us to link nanoscale chemical evolution to macroscopic electrical degradation pathways.

Beyond individual case studies, this work highlights the broader role of synchrotron soft X-ray microscopy in disentangling morphology-function-degradation relationships across organic and hybrid photovoltaic materials. I willconclude by discussing practical considerations for applying STXM and SXP to OPV and perovskite systems, including radiation damage, acquisition strategies, and emerging opportunities enabled by next-generation synchrotron sources.

References :

1. Phd Thesis (Corentin RIEB, Univ. of Strasbourg, 16/12/2025).

2. Rieb, C., Leclerc, N., Méry, S., Hébraud, A., Swaraj, S. Journal of Physical Chemistry C., 129(41): 18537-18547. (2025).

3. Jun, H., et al. (2023) Journal of Electron Spectroscopy and Related Phenomena., 266: art.n° 147358.

4. Dindault, C., et al. (2022) RSC Advances., 12(39): 25570-25577.

5. Jun, H., et al. (2022) Scientific Reports., 12: art.n° 4520

6. PhD thesis (Jun H, Ecole Polytechnique, Palaiseau, France)

Scaling up perovskite solar cell printing is critical for commercialization, yet achieving high-quality films under industrial conditions remains challenging, particularly due to limited control over crystallization, and especially when using more sustainable green solvents. At the laboratory scale, spin coating combined with anti-solvent treatment has delivered high efficiencies by enabling precise control of nucleation and crystal growth. In contrast, large-area printing requires alternative deposition techniques, like slot-die coating, and anti-solvent treatment is often considered incompatible with these methods, leading to reliance on gas-flow-assisted drying. This approach demands changes in solvent systems, moving away from optimized lab-scale formulations and reducing control over intermediate pre-perovskite phases that can benefit high-quality crystal growth and optimal orientation.

Here, we introduce a dynamic anti-solvent bathing method for scalable perovskite printing. Results across multiple compositions, including CsFAPbI_XBr_1-X, CsFAPbI₃, MAPI₃, and CsPbI₃, show that this approach enables high-quality film formation, in most cases under ambient conditions and using green solvents. This method can be preceded by gas flow assisted drying. The anti-solvent bath can be used to facilitate targeted solvent or ions extraction, promoting effective crystallization. In situ GIWAXS and photoluminescence spectroscopy, in addition to NMR and FTIR reveal the crystallization mechanisms and suggest that pairing anti-solvent bathing with targeted additives could enhance film quality for high-performance devices.

Abstract

Inverted perovskite solar cells (PSCs) have rapidly emerged as leading candidates for next-generation photovoltaics owing to their high power conversion efficiencies (PCE) and superior stability. However, self-assembled monolayers (SAMs), which are widely used as efficient hole transport layers (HTL), still face several critical obstacles in the inverted PSC configuration. Notably, molecular disorder and suboptimal packing within the SAMs often result in poor perovskite film coverage, which increases detrimental interfacial recombination and accelerates overall device degradation.

In this work, we strategically introduce a functional passivating molecule at the interface between the SAM and the perovskite layer to address these issues and improve interfacial adhesion. This modification facilitates a more uniform nucleation process during perovskite deposition. As a result, the modified HTL significantly enhances coverage and suppresses non-radiative recombination at the buried interface, thereby increasing the open-circuit voltage ($V_{oc}$) and achieving a power conversion efficiency (PCE) exceeding 26%. These results suggest that precise molecular passivation plays a critical role in enhancing SAM structural integrity and reducing interfacial defects, consequently improving both the performance and long-term reliability of inverted PSCs.

Recent progresses on the perovskite solar cells at UCLA

Yang Yang1,2

Department of Materials Science and Engineering, UCLA

MoleculaX Inc., Pasadena, CA 91107, USA

Perovskite solar cells (PSCs) have become one of the most transformative photovoltaic technologies. In this presentation, I like to share some recent progresses at UCLA on interface engineering, PVSK crystal growth, and eventually, using the latest AI technology to create a database, the PervoskiteNet, and a LLM to guide the PSC device research.

Interface Engineering: For the NIP device, the buried SnO2/perovskite interface is a critical yet elusive origin of instability in perovskite solar cells, where light absorption and photo-induced degradation are concentrated. We demonstrate that interfacial instability drives structural and chemical decomposition of the perovskite layer via tin-ion migration. Introducing polymeric interlayers strengthens interfacial bonding and suppresses tin diffusion. These results demonstrated the buried interface as a primary degradation source and underscore interfacial polymer design as an effective route to intrinsically stable perovskite photovoltaics.

Defect management via bulk crystal growth: In parallel to our interface engineering is our intension to manage the defect formation during PVSK crystallization. Conventional intermediate phases form before perovskite nucleation and cannot heal growth-induced defects. Here, we identify a thermally activated intermediate phase emerging after perovskite formation. A sterically hindered additive enables high-temperature re-alloying with pre-formed perovskite nuclei, inducing in-situ lattice reconstruction and defect elimination. Devices achieve 26.82% efficiency and retain 90% performance after 1,300 h at 65 °C.

Interfacial disorder at the perovskite/C₆₀ contact represents a critical bottleneck for the scalability and operational stability of inverted perovskite solar cells because it induces spatially nonuniform electron extraction over large areas. While two-dimensional (2D) capping layers are widely employed for passivation, conventional symmetric spacer cations (e.g., Phenethylammonium cation) often fail to provide sufficient orientational control, resulting in poorly aligned layers that impede vertical charge transport. This work introduces a coherent molecular templating strategy utilizing an asymmetric spacer cation, which effectively suppresses unfavorable herringbone-type packing and promotes a preferential lamellar orientation at the perovskite surface. High-resolution transmission electron microscopy confirms that this interfacial order propagates through the perovskite/2D/C₆₀ organic heterojunction, guiding C₆₀ crystallization into a continuous network and enabling uniform electron extraction. Consequently, devices incorporating the asymmetric cation deliver certified power conversion efficiencies of 26.94% for small-area cells and 24.23% for 25-cm² modules (certified 23.21%), demonstrating significantly reduced cell-to-module losses. Furthermore, the templated interface enhances durability, maintaining over 93% of initial efficiency after 1,000 hours at 85 °C and achieving an operational T₈₀ exceeding 2,500 hours under continuous 1-sun illumination. These findings establish coherent molecular templating as a robust principle to integrate interfacial passivation with oriented charge-transport pathways for scalable optoelectronics.

Since the achievement of high power conversion efficiencies (PCEs) in PM6:Y6-based organic solar cells (OSCs), increased attention has been devoted to understanding the fundamental photophysical processes governing device performance, with the aim of identifying and further minimizing remaining losses. Charge recombination remains a major loss pathway limiting PCE, and multiple recombination mechanisms have been identified. The most prevalent is bimolecular recombination between charge carriers residing on donor and acceptor molecules. In addition to this pathway, triplet-charge annihilation, involving the interaction between a triplet exciton and a charge carrier, has also been reported to contribute to efficiency loss.[1]

Both recombination processes are governed by spin-selection rules due to the spins associated with each charge carrier. Electrically Detected Magnetic Resonance (EDMR) can selectively probe these spins, and the processes in which the corresponding charge carriers are involved. Controlled manipulation of spins in EDMR experiments can therefore provide detailed insights into the local environment and mutual interactions of charged states during recombination processes in fully operational, miniaturized devices.[2]

In this work, we investigated OSCs based on a range of donor:acceptor blends, including two with non-fullerene acceptors (PM6:Y6 and PBDB-T:ITIC) and two with fullerene acceptors (PM6:PCBM and PBDB-T:PCBM). The spectral signatures of the individual donor and acceptor molecules exhibit a notable degree of overlap, in particular for the non-fullerene acceptor blends. Therefore donor-only and acceptor-only devices were additionally investigated to unambiguously disentangle the different contributions. Then, we turned our focus on the nature and dynamics of the recombination processes. By selectively manipulating the spin of acceptor charge carriers while monitoring the response of donor spins, a weak spin-spin coupling was identified, consistent with bimolecular recombination. The simultaneous observation of triplet exciton signatures provided clear evidence for the additional presence of a triplet-charge annihilation process. By exploring the role of different recombination processes, this study aims to provide new insights into the main performance-limiting loss mechanisms in a range of donor:acceptor blends.

A computational model built using the QuTiP package [1] is being developed to accurately cover the dynamics of collective emission, focusing on the accuracy of computations within the master equation for systems with a small number of two-level emitters. This model introduces innovative features, such as adjustable two-dimensional system geometries and robust capabilities for the computation of key indicators of collective and coherent emission, including the collective emission flag quantified by the g⁽²⁾(0) of the first two emitted photons by the system [2,3], and the Wigner Logarithmic Negativity and purity of emitted radiation states obtained through reconstruction of the emitted photon states [4]. Ongoing efforts aim to expand the model to incorporate three-dimensional systems and to create an additional package which will provide new methods for extrapolating calculations for larger numbers of emitters.

The primary goal of this model is to facilitate the advancement of materials that demonstrate collective emission, allowing researchers to manipulate various parameters to enhance the experimental data collection process.

Beyond photovoltaics, metal halide perovskites have also emerged as promising emitters for light-emitting diodes (LEDs) due to their easily tunable bandgap, narrow emission linewidth, and good charge carrier mobility.¹ Although perovskite LEDs have recently achieved rapid increase in external quantum efficiency (EQE), the emission wavelengths of these high-efficiency LEDs remains restricted to the visible and short-wavelength near-infrared regions (<920 nm).^2–5

Environmentally friendly tin iodide perovskites, particularly all-inorganic CsSnI₃, have attracted growing interest because of their intrinsic long-wavelength near-infrared emission (>920 nm), offering distinct advantages in applications such as night vision, biological tissue analysis, biomedical imaging, sensing and optical communications.^6–8 However, realizing high-efficiency CsSnI₃ based LEDs is highly challenging. Current efforts to overcome this challenge have largely focused on improving photoluminescence quantum yields, for instance, by mitigating the oxidation of Sn²⁺ and suppressing fast crystallization.^6–9 Nevertheless, the limited success in LED efficiency highlights the urgent need for a fundamentally new strategy.

Light extraction represents another critical bottleneck in perovskite LEDs due to strong optical confinement arising from the high refractive index of perovskite films.¹⁰ Discontinuous perovskite films with submicron structures can enhance light outcoupling by extracting light trapped in waveguide modes.¹¹ While this strategy has been validated in Pb-based perovskites^11,12, its application to Sn-perovskites remains rarely explored and presents distinct challenges. For CsSnI₃, light extraction is even more challenging due to its higher refractive index.^10,13 Moreover, the fast and uncontrollable crystallization in Sn-perovskites driven by the Lewis acidity of Sn²⁺ poses significant challenges in precisely tailoring their morphological features.⁶

Here we demonstrate a morphology engineering strategy that combines a low-temperature preheating step with an organic additive to precisely regulate grain and pinhole formation in CsSnI₃-based perovskite films. This microstructural control enables fine-tunable optical structures within LED stacks, substantially enhancing light outcoupling efficiency (LOE). The resulting 960 nm-emitting LEDs achieve a record-high EQE of 9.5% in the long-wavelength near-infrared region (>920 nm). Optical simulations further demonstrate that an even wider LOE range can be realized by further manipulation of grain and pinhole sizes. Our work not only offers a practical approach for precisely tailoring morphological features of tin perovskite films but also deepens the understanding of the relationship between film morphology and LED performance.

Lead-based perovskites achieve high efficiencies but raise serious concerns due to toxicity, environmental risk, and regulatory barriers. This inspired alternate chemistries to develop Pb-free, stable alternative perovskites achieving commercially viable, environmentally sustainable, and regulation-compliant solar cells while maintaining acceptable performance. In today’s seminar, I will showcase two such emergent material class with potential for safer manufacturing, deployment, and end-of-life disposal. In particular, I will highlight how understanding structure-function links at the atomic-scale¹ utilizing multimodal x-ray techniques holds the key to drive such materials towards PV applications.  

First, we investigate the chalcogenide perovskite² BaZrS₃, which combines a direct band gap with strong light absorption and favorable transport properties. The challenge however, is adapting it to a device architecture, linked to it high formation temperature and susceptibility to surface oxidation. The talk will explore how design parameters influences optical band gap, semiconductor characteristics, transport phenomenon relating to material formation and growth, local effects in the bulk, and critical surface effects which are relevant for optimizing PV functionality (Fig.1(a)).^3-5 Based on these inputs, BaZrS₃ films with minimal surface contamination were fabricated and its chemistry, band alignment, and ambient stability are assessed for initial solar cell integration.

Second, we explore ferroelectric perovskite oxides, where large intrinsic polarization offers a pathway to above-band-gap photovoltages.⁶ Here, the main challenge is the typically large band gaps (e.g., 3.2 eV for BaTiO₃), where gap reduction by chemical doping leads to severe polarization loss. Controlled Mn-Nb co-doping in BaTiO₃ can achieve this balance,⁷ enhancing sunlight absorption via Mn³⁺ (dⁿ) doping, while largely preserving initial polarization of u-ndoped BaTiO₃ via Nb⁵⁺ (d⁰) doping (Fig. 1(b)). Element-specific disorder control is shown to enable this balance,^8-9 providing a general strategy for designing low-band-gap ferroelectric absorbers for photovoltaic applications.

Organic photovoltaic (OPV) devices have achieved high efficiency, but further progress is constrained by interfacial recombination and sub-optimal morphology, and limited stability. Here, we address these challenges by introducing CsPbI₃ perovskite quantum dots (PQDs) as an interlayer at the D18/Y6 interface, deposited via an orthogonal-solvent layer-by-layer process. Incorporation of an optimized PQDs interlayer boosts the power conversion efficiency from 16.7% in the binary device to 18.8% in the ternary architecture, corresponding to a relative increase of ~12%. The systematic comparative study of morphology, optical/optoelectronic and electrical characteristics, as well as the related charge recombination and transfer dynamics, revealed that the D18/PQDsY6 assembly provided a unique electronic structure that passivates PQDs surface defects and mitigates interdiffusion of the organic layers, enabling improved microstructure regulation, leading to enhanced fill factor. Moreover, the high dielectric constant of the PQDs appears to facilitate exciton dissociation at the D18/Y6 interface, allowing the PQDs to act as a relay that separates and stabilizes charges at the D18/PQDs/Y6 junction, thereby reducing the trap-assisted recombination. This leads to suppressed non-radiative recombination losses, resulting in higher open-circuit voltage (V_OC) and short-circuit current (J_SC). Beyond performance gains, the PQDs interlayer also improves the shelf-life stability under N₂, prolonging the stability for the pristine devices over 12 months. Hence, our findings demonstrate that introducing CsPbI₃ PQDs interlayer, implemented via orthogonal-solvent LBL processing, provides an effective route to tune interfacial morphology and energetics, offering a practical strategy to further enhance both efficiency and stability in OPV devices.

Efficient free charge generation in organic photovoltaics (OPVs) generally relies on the energetic offset at a donor (D) and an acceptor (A) interface. The interfacial area is maximised by blending the components into a bulk heterojunction (BHJ). While the BHJ morphology has undoubtedly proven effective to increase the power conversion efficiency in OPVs, the physical blending of components often leads to morphological degradation due to phase separation, resulting in poor performance stability. A promising approach to tackle the stability issue is the use of single-component macromolecular semiconductors that contain tethered D-A units [1,2]. The incorporation of Y-type acceptors into conjugated block copolymers (CBC) has recently led to significant improvements in conversion efficiency, reaching 15% in the best cases, while maintaining good performance stability [2,3,4]. Interestingly, in some cases CBC based devices have demonstrated superior performance over BHJ devices containing the same segments. This raises the question of how photocurrent and voltage generation are controlled in these materials and how constraining the heterojunction into chemically bonded D-A pairs influences the processes of charge pair generation and recombination. Factors that may influence these processes in CBCs in comparison with BHJs are the stronger coupling, more controlled domain sizes, and reduced configurational disorder at the interface.
To explore this question, we compare the PV performance of a series of CBCs with different interfacial energetics, with a series of polymer-D:small-molecule-A (PSM) BHJs and polymer-D:polymer-A (PP) BHJs constructed from the same fundamental building blocks. While the CBC series probes the impact of interface energetics, comparison with the two BHJ equivalents allow us to explore the impact of covalent bonding of the D-A heterojunction compared to a non-bonded D-A pairs, as well as the impact of the microstructure in the acceptor phase. When employing these materials in OPV devices, we find a consistent trade-off between current generation and non-radiative voltage losses across molecular architectures and across energetic offsets. Compared to the other microstructures, CBCs show low non-radiative losses, however, at the cost of reduced charge generation, which we attribute to limited exciton dissociation efficiency. We attempt to explain these trends at a molecular level, by simulating the configurational phase space of bonded and non-bonded D-A structures using molecular dynamics. We study the properties of the thermodynamic ensembles of local excited states and charge transfer (CT) state, where the latter can either occur through-space or through-bond. We uncover the relationship between the interfacial energetic offset and the CT state energy, and find that conjugated heterojunctions lead to bright, high-energy CT states that hybridise strongly with the Frenkel exciton. The landscape of excited states explains the trends in charge generation and voltage losses and allows us to propose design principles to further improve CBC photovoltaic devices.
 

Intrinsic charge generation in pristione single-component organic semiconductors is commonly believed to result from an autoionization mechanism, and hence is not expected to occur upon excitation within the lowest excited singlet state due to the large exciton binding energy. Nevertheless, unexpectedly high charge generation ratios have been found in neat films when exciting at the absorption edge of some compounds, such as the non-fullerene-acceptor Y6.[1,2] In my presentation I shall discuss the role of exciton-exciton annihilation process in the photogeneration process for both singlet and triplet states, focussing on singlet-singlet annihilation processes in Y6 and triplet-triplet annihilation processes in the carbazole-based molecule mCBP-CN. We combine solar cell measurements, thermally stimulated luminescence measurements, fs-pump-probe measurements and kintetic Monte Carlo Studies to to illustrate how efficient delocalization and thermally activated diffusion of excitons can contribute to the overall carrier yield.[3,4,5] In this context, we discuss the relationship between the activation energy for photocurrent and the binding energy for photogenerated electron-hole pairs.

Further breakthroughs in halide perovskite solar cells require advances in new compositions and underpinning materials science. Indeed, a deeper understanding of these complex hybrid materials requires atomic-scale characterization of their transport, electronic and stability behaviour. This presentation will describe recent combined modelling and experimental studies on metal halide perovskites [1,2] in two fundamental areas related to improving operational stability in optoelectronic devices: (i) atomistic insights into passivating perovskites with molecular additives including surface interactions and dynamics; here we find strong binding of certain molecular passivators at undercoordinated surface and defect sites, and thereby promoting surface passivation; (ii) iodide ion transport properties and the effects of mixed Pb-Sn compositions as there is limited understanding of the impact of Sn substitution on the ion dynamics of halide perovskites;

[1] Y.H. Lin, Vikram, H.J. Snaith et al., Science, 384, 767 (2024); T. Webb, L. Zhu, S. Haque et al., Energy Environ. Sci., 19, 605 (2026).

[2] K. Dey, S.D. Stranks et al., Energy Environ. Sci., 17, 760 (2024); A.N. Arber et al., Chem. Mater. 37, 12, 4416 (2025).

Fabricating electronic devices on extremely thin substrates enables the realization of highly flexible and lightweight electronics, which significantly decreases discomfort in wearable applications. Our research group focuses on developing electronics on ultra-flexible polymer films, approximately 1 µm in thickness, to enhance mechanical robustness against bending by minimizing the applied strain [1]. In a simplified mechanical model, film thickness is inversely proportional to strain, meaning that thinner films experience less strain for the same bending radius. This presentation will discuss the potentials of ultraflexible electronics, latest advancements in environmental and photo-stability, and the challenges in standardizing mechanical characterization.

Recently, we have made significant progress in improving both the power conversion efficiency (PCE) and the environmental stability of these devices across different material systems. We developed waterproof, high-performance ultrathin organic solar cells (OPVs) and realized rechargeable soft robot systems [2,3]. Furthermore, we achieved ultra-flexible perovskite solar cells with unprecedented stability using dual hole transport layers and Al₂O₃ moisture barriers [4]. To ensure long-term reliability under sunlight, we addressed photo-degradation by developing a 3.6 µm thick polyimide substrate with intrinsic UV-filtering properties (380 nm cutoff) [5]. This substrate effectively protects the device's active and interfacial layers, enabling ultraflexible OPVs to maintain long-term operational stability under 1-sun in ambient air while enduring up to 4,000 bending cycles.

As flexible photovoltaics approach technological maturity, standardized characterization becomes essential. We have proposed a unifying bending test protocol recommending 1% strain over 1,000 bending cycles and introduced the "flexible photovoltaic fatigue factor" F as a new figure of merit that integrates PCE retention, strain, and bending cycles [6]. Reflecting the growing importance of these methodologies, a dedicated section for evaluating the performance and mechanical reliability of flexible solar cells has been newly established within the latest comprehensive "Emerging PV Report" [7]. This integration provides the scientific community with standardized benchmarks to accelerate the development of next-generation solar cells.

The energy conversion efficiency of photovoltaic devices based on molecular materials has improved remarkably to exceed 20% for many systems. These recent advances have resulted largely from the use of fused-ring molecular acceptors that absorb light strongly and form well coupled domains in the solid state. These materials appear able to support efficient photocurrent generation with relatively small energetic offset between the ionization potential of donor and acceptor components, and some systems appear to be capable of charge pair photogeneration without a heterojunction. To better understand such materials we investigate charge generation in single-component and heterojunction devices systematically for a range of materials, and analyse their behaviour using a computational model of the generation and evolution of delocalised excited states in such systems. We consider the influence of factors such as the nature of the charge separating heterojunction, molecular packing, energy and charge transport, electron-phonon coupling and loss pathways. We explore the impact of molecular parameters and find that low exciton reorganization energy and high and isotropic electronic coupling are important for efficient photogeneration [1]. We find that although some parameters favour charge separation in neat molecular domains, the ability of currently known materials to generate photocurrent efficiently without a heterojunction is limited.  We go on to apply a similar framework to polymer materials and tethered donor-acceptor structures. The combined results allow us to consider the limits to energy conversion efficiency in such systems.

In recent years, the integration of emerging perovskite solar cells with traditional crystalline silicon (c-Si) solar cells to construct perovskite/silicon tandem devices has become a promising photovoltaic technology. However, the integration of commercial silicon cells with perovskite solar cells, particularly on textured silicon substrates featured with large pyramids (2~5μm), presents a significant challenge for achieving effective charge transfer, which is critical for highly efficient tandem solar cells. To address this challenge, this report highlights our recent research progress in molecular-scale nanotechnology. By designing the interface structure of the charge transport layer and optimizing the crystallization process of perovskite on textured silicon substrates, we have fabricated uniform, dense and high-quality perovskite films on the surface of silicon cells with a rough texture. Based on this, we developed highly efficient perovskite/silicon tandem solar cells with certified power conversion efficiency exceeding 34%. This series of research aims to systematically enhance the performance of perovskite/silicon tandem photovoltaic devices and push its industrial application.

Organic solar cells have recently exceeded 20% power-conversion efficiency, prompting a key question: how much further can we push performance? Despite rapid advances, progress is limited by photophysical loss channels that are not yet described within a unified framework. At the heart of the problem lies the intricate excited-state dynamics at the donor–acceptor interface, where excitons, charge-transfer states and fully separated charges are in constant interconversion, governed by the materials’ energetic landscape. A central challenge is to understand and mitigate loss pathways involving singlet and triplet charge-transfer states and local triplet excitons, which ultimately constrain the open-circuit voltage. 

In this talk, we share our experimental data and kinetic model that explicitly incorporates the formation and re-splitting of local triplet excitons. Fully parameterised by the interfacial energy offset, this unified framework reproduces key photovoltaic observables – such as the charge-generation efficiency, photoluminescence, electroluminescence and the Langevin reduction factor. In systems with short triplet lifetimes, triplet decay emerges as the dominant recombination pathway, reconciling long-standing experimental findings, including those in benchmark systems like PM6:Y6. In systems with long triplet lifetimes, triplets can be recycled to mitigate this loss channel. The model further offers a mechanistic explanation for the empirically observed link between energy offset, radiative singlet-exciton decay and reduced-Langevin recombination as well as a correlation, and accurately predicts the device efficiency across different material systems.

By connecting excited-state kinetics with macroscopic device metrics, our work provides a unified mechanistic picture of the photophysics in organic semiconductors.

Flexible perovskite solar cells (f-PSCs) have recently reached power conversion efficiency (PCE) above 25.09 Although still lagging behind their rigid counterparts on glass, which in very short time have achieved 27% of certified efficiency,  the use of flexible substrates opens up to a wide range of applications, from sensors for the Internet of Things, to the retrofitting of existing buildings to improve their energy efficiency (building-applied PV), to space, thanks above all to the high power/to weight ratio generated which is the range of 29.4 W/g compared to 8.31 W/g for amorphous silicon and 0.254 W/g for ultra-thin CdS / CdTe.

In this presentation an overview of the fabrication of flexible perovskite solar cells and modules and of their use in two unconventional environments (indoor wearable and space) will be reported.

In particular, for indoor applications I will report on our studies of combining perovskite solar cells and modules with energy storage systems (supercapacitors) introducing the concept of photocapacitor, a device where the energy photovoltaic (PV) generation and storage systems are combined in a single unit, offering an innovative approach to manage energy supply .

For space applications, I will report on the role of PSC in this emerging field of research, showing our studies related to the resilience of flexible perovskite solar cells to neutrons focussing on the role of the hole transporting layers. In particular, I will show how the substitution of standard spiro-OMeTAD with P3HT-modified and PTAA-modified polymers affects the overall performances of the unencapsulated devices revealing an higher resilieance of the polymer based system respect to spiro-OMeTAD under neutron irradiation.

Semiconducting polymers interfaced with electrolytes are finding wide interest across a range of energy generation and storage and sensing technologies, including as photoelectrodes for chemical energy production, electrodes for energy storage, and the active matrix in biochemical sensors. How the physicochemical nature of the interphase between soft semiconducting polymers and electrolytes change as a function of the chemistries of these components, swelling, applied electric fields, and charge-carrier injection into the polymer remain outstanding questions that need to be resolved to design materials that can achieve efficient charge and ion transport. Here we will discuss recent advances in understanding the formation and dynamics of the polymer–electrolyte interphase through the development of multiscale modeling approaches. Quantum chemical calculations are used to describe the nature of the charge carriers (i.e., polarons and bipolarons) in the semiconducting polymers, including how the dielectric environment and interactions with counterions impact charge-carrier (de)localization and the resulting optical response. We then develop and deploy molecular dynamics (MD) simulations to examine dynamic features of polymer swelling by the electrolyte as a function of applied electric fields and charge-carrier injection into the polymer.

Doping of lead-halide perovskites has always been a rather complicated topic. In the dark, lead-halide perovskites often appear intrinsic and show signs of very low static doping concentrations e.g. in Hall effect measurements.¹ Under illumination, however, features appear in data that are most easily explained by electron and hole densities being very different to each other. These features show up in comparisons of transient photoluminescence (PL) with transient absorption,² in transient PL with steady-state PL³ and in the intensity dependence of steady-state PL. These features may or may not be related to mobile ions, but their signatures could be explained by a static concentration of localized states (traps) that preferentially capture one polarity of charge carriers and thereby lead to a net charge density on those defects that is invisible to dark measurements (e.g. SCLC or capacitance-voltage) but still present, once the traps are filled by illumination. So far, the concept of photodoping has been occasionally invoked to explain data, but a systematic study of its effects on device performance and spectroscopic data is currently missing. In this study, we focus on the combination of steady-state and transient PL to explain how charge stored in traps can affect the ideality factors measured from steady-state PL on films and the shapes of the tr-PL decays that are in practice often power laws of various slopes.

Halide mixing provides an opportunity to tune the bandgap of perovskite semiconductors seamlessly across the visible range. However, halide segregation in APbX₃ compounds under illumination deteriorates the optoelectronic properties of these materials, placing strong limitations on the practical benefits of bandgap tunability. In our previous work, we showed that the same environmental stressor—light—that induces halide segregation can also be used to remix halides when illumination reaches sufficiently high photon fluxes. We developed a polaron-based model to explain both demixing and remixing. More recently, we have explored photodynamic effects in 2D mixed halide perovskites with the formula BAPbI₄₋ₓBrₓ. We found that the optical properties of these materials change significantly under prolonged illumination, but observed no evidence of classical halide segregation as seen in APbBr₃₋ₓIₓ compounds. Importantly, the lattice of the 2D perovskite analogues contains two distinct halide sites: equatorial (E) sites within the 2D lead halide planes, which are bonded to two neighboring Pb²⁺ ions, and axial (A) sites located above and below the planes, which are bonded to only one Pb²⁺ ion.In this work, we show that light illumination drives a redistribution of Br⁻ and I⁻ between the E and A sites, shifting the system away from thermal equilibrium. These changes are reversible and highly localized. Density functional theory calculations reveal that halide redistribution between E and A sites leads to distinct changes in the electronic properties. The light-induced structural isomerisation observed in BAPbI₄₋ₓBrₓ enables optical patterning of single-crystalline materials without any need for mass transport beyond the unit cell while fully preserving the material’s crystalline integrity. Overall, these findings highlight a fundamentally different light–matter interaction in 2D mixed halide perovskites and point to new opportunities for spatially resolved and reversible control of optoelectronic properties.

Formamidinium lead iodide (FAPbI3), which has a narrower band gap close to the Shockley-Queisser limit, offers higher power conversion efficiency (PCE) than other perovskite compositions, surpassing 27% [1]–[3]. However, under external stressors like moisture in ambient air processing conditions, its high tolerance factor value causes phase instability because of the larger ion size of FA+ [4]–[6]. Additionally, a higher annealing temperature (>390 K) is needed to reach the cubic α-phase of FAPbI3, whereas lower temperatures result in the formation of a non-photoactive δ-phase [7].

In recent years, FAPbI3 perovskite ink has been extensively incorporated with volatile methylammonium chloride (MACl) as a transitional stabilizer [8]. This substance effectively provides FAPbI3 black phase without annealing by decreasing the formation energy. However, the advantageous effects of MACl as an α-phase FAPbI3 inducer and stabilizer at room temperature are neutralized under ambient conditions and in the presence of non-volatile coordinating DMSO, which is frequently used as a co-solvent with toxic DMF to regulate the crystallization process [9]. DMSO accelerates the α-to-δ phase transition in air by displacing MACl from the intermediate film through the formation of stronger bonds with PbI2.

In this work, we use recently emerged Triethyl Phosphate (TEP) as a green solvent to dissolve FAPbI3 precursors and in-situ study its crystallization kinetics in the presence of MACl and excess PbI2 under ambient condition using transmission wide angle x-ray scattering (T-WAXS) [10] and steady-state photoluminescence (PL) techniques. Our results show that, unlike DMSO containing solvent systems, TEP with appropriate coordination ability allows for direct solvent extraction during anti-solvent quenching process avoiding intermediate phase formation. Furthermore, it has been found that the addition of excess PbI2 to the perovskite solution, along with MACl, not only regulates the pre-nucleation stage, leading to larger and more ordered crystals, as opposed to MACl alone as an additive, but also accelerates the formation of α-phase FAPbI3 at room temperature and stabilizes it under ambient condition during spin casting. This study paves the way for achieving high efficiency FAPbI3 solar cells using a non-toxic solvent system and under ambient conditions.

Hybrid halide perovskites are a promising class of materials for next-generation photovoltaics by combining high power conversion efficiencies, low-temperature processing[1,2] and compatibility with low-cost carbon electrode[3]. In particular, triple-mesoscopic perovskite solar cells based on TiO2/ZrO2/carbon architectures have attracted growing interest because they intrinsically improve device robustness by eliminating organic hole-transport layers and enabling monolithic and fully printable stacks.
However, interfacial stability at the carbon/perovskite interface remains a major challenge for industrial-scale integration, particularly in low-cost perovskite modules for solar or PEC/PV applications and is more made worse by intrinsic moisture sensibility of perovskite materials. Several encapsulation strategies have been proposed to mitigate this issue, including polymer barrier coatings, glass-glass sealing and inorganic barrier layers deposited by vacuum techniques[4]. Unfortunately, these approaches often increase processing complexity, cost or compromise scalability and compatibility with mesoporous carbon scaffolds. An alternative and interface-specific strategy consists in the electrochemical grafting of organic molecules onto the carbon electrode.
In this presentation, we will address the electrochemical surface modification of carbon scaffolds in triple-mesoscopic perovskite solar cells using diazonium salts. This controlled electrografting approach enables the formation of thin layers that induce hydrophobic to superhydrophobic properties to the carbon surface protecting the perovskite from moisture penetration within the mesoporous network. The grafting thickness and density can be finely tuned to preserve the intrinsic porosity and electrical conductivity of the carbon electrode[5]. A particular attention will be placed on the role of the electrochemical medium as the mesoporous device architecture imposes strict solvent constraints. The solvent must be compatible with the entire device stack and prevent dissolution or degradation of the perovskite. At the same time, it must guarantee adequate solubility of diazonium and electrolyte species while maintaining electronic accessibility of the carbon network. Together, these considerations define a low-temperature and scalable pathway for improving the interfacial stability and long-term durability of carbon-based triple-mesoscopic perovskite photovoltaic architectures.

Organic halide perovskites have achieved remarkable power conversion efficiencies, exceeding 35% in tandem and 27.3% in single-junction devices. However, long-term operational stability remains the primary barrier to commercialization. While macroscopic degradation trends are well documented and been presented at HOPV in Rome last year [1], the microscopic origins of instability and their relation to crystallographic structure are still insufficiently understood.

In this work, we combine macroscopic ageing studies with grain-resolved correlative atomic force microscopy and electron backscatter diffraction to investigate degradation in perovskite thin films under ambient conditions and illumination. Time-resolved AFM reveals heterogeneous surface evolution, ranging from intact grains to partially and fully degraded grains. Local RMS roughness analysis provides a quantitative measure of degradation onset and progression at the level of individual grains.

By correlating morphological changes with crystallographic orientation, we show that degradation is strongly orientation dependent. Grains oriented near {100} exhibit the fastest roughening, whereas grains oriented closer to {110} and {111} remain significantly more stable. Substrate-dependent experiments further demonstrate that interfacial properties and initial texture govern whether degradation occurs heterogeneously and slow, as on ITO, or more uniformly but considerably faster, as on glass and quartz.

These results provide direct microscopic evidence that crystallographic orientation controls the environmental stability of perovskite thin films. Suppressing the {100} texture and optimizing substrate interfaces emerge as a promising strategy to enhance the long-term operational stability of perovskite optoelectronic devices.

Hybrid vapor-solution processing offers a promising route to deposit perovskite absorbers on textured substrates, which is particularly attractive for tandem and multijunction photovoltaic applications. However, this approach has been predominantly applied to wide-bandgap compositions, while its potential for FAPbI₃-based perovskites remains underexplored. In this work, we investigate the deposition, passivation, and stability of vapor–solution-processed FAPbI₃ perovskite solar cells employing sputtered NiO_X hole-transport layers in a p–i–n architecture.

By systematically tuning the organic precursor composition and annealing conditions, we obtain uniform, large-grained perovskite films with minimal residual PbI₂. Building on this optimized baseline, we introduce a dual-interface passivation strategy using ultrathin CsBr layers deposited by thermal evaporation at both the buried NiO_X/perovskite interface and the perovskite top surface. Devices incorporating this CsBr treatment achieve power conversion efficiencies up to 23.25%, representing record performance for sputtered NiO_X-based p–i–n devices.

Beyond efficiency, the dual-interface passivation leads to exceptional thermal stability. Under ISOS-D-2I aging conditions (85 °C, dark, inert atmosphere), devices retain over 80% of their initial efficiency after more than 10,400 hours, significantly outperforming control devices. Morphological and optoelectronic analyses reveal suppressed formation of Pb-rich secondary phases, more homogeneous surface potentials, and improved preservation of carrier lifetimes upon aging.

Time-of-flight secondary ion mass spectrometry provides insight into the underlying stabilization mechanism, showing extensive redistribution of Cs and Br throughout the perovskite layer and a strong suppression of residual Cl. The presence of mobile Br appears to compensate halide loss during thermal stress, mitigating the formation of halide-deficient domains that lead to perovskite decomposition. Overall, this work demonstrates that scalable, vacuum-deposited interface engineering can simultaneously deliver high efficiency and exceptional thermal stability, highlighting a viable pathway toward durable perovskite photovoltaics compatible with industrial manufacturing.

Sequential deposition (SD) through independent processing of donor and acceptor materials, has emerged as a promising strategy to enable better control over the active layer morphology of organic solar cells. In this work [1], time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed to investigate the vertical distribution of sequentially deposited PM6/Y5 films and bulk heterojunction (BHJ) PM6:Y5 films coated from a blend solution in one-step process. The influence of thermal annealing (TA) on the vertical distribution of the components was also evaluated. The depth-profiles revealed that blend-casting samples present a donor-rich surface layer, approximately 2.4 nm wide and that TA does not affects the vertical composition. On the other hand, the depth profiles obtained for the SD-films show that SD inverts the vertical composition within the active layer, promoting an acceptor-rich surface layer. It was found that TA of the PM6 bottom layer helps to suppress Y5 diffusion, and TA of the bilayer after the acceptor deposition further promotes vertical phase separation resulting in a vertical composition gradient. Additionally, near edge X-ray absorption fine structure spectroscopy (NEXAFS) was used to investigate the molecular orientation of the donor PM6 and the acceptor Y5, in sequentially deposited and blend films. Depth-dependent molecular orientation was assessed by comparing NEXAFS spectra acquired in total electron yield (TEY) and fluorescence yield (FY). PM6:Y5 blends measured in TEY mode presented a carbon K-edge spectra dominated by the spectral features of the donor PM6, while for SD-films carbon K-edge spectra are dominated by Y5 spectral features, clearly reflecting the donor-rich and acceptor-rich surfaces, in agreement with ToF-SIMS results. Furthermore, nitrogen K-edge NEXAFS spectra were employed to selectively probe the acceptor orientation. In SD-processed samples, NEXAFS nitrogen K-edge spectra recorded in TEY and FY modes have shown that Y5 retains its face-on orientation when deposited on top of PM6, despite the combined effects of film formation dynamics and interfacial intermixing inherent to the process.

Polymer solar cells have gained considerable interest over the last decades. In recent years, photovoltaic performance has increased rapidly, with high power conversion efficiency for lab-scale devices. This work focuses on the design and synthesis of new materials, as well as the thermal stability of polymer solar cells. Thermally stable materials are important for stable solar cells, as solar cells are often exposed to elevated temperatures during fabrication and use. Our work includes device stability measurements and morphology stability studies using dynamic mechanical thermal analysis (DMA). The DMA technique is a highly sensitive method for determining the thermal transitions of the used materials.

Several completely amorphous indacenodithiophene-based polymers with varying side chains were synthesised and used in solar cells combined with the Y6 acceptor material. Low donor:acceptor (D:A) ratios are generally believed to yield lower efficiency than the more conventional 1:1.2 ratio. However, several of the solar cells exhibit a peak performance over 11% PCE at a lower D:A ratio.[1] Unexpectedly, as the polymer proportion increases, a reduced photovoltaic performance is observed. Similarly, nanoparticles made of the materials and used for photocatalytic hydrogen evolution show an analogous trend with a peak performance at a D:A ratio of 1:6.7. Importantly, our experiments also show that the thermal stability of the solar cells at 85 ^oC is significantly improved for a low donor:acceptor ratio (1:10), outperforming commercial high performance polymer systems in efficiency after 30 days stability measurements.

In recent years, the A-DA′D-A acceptor architecture, represented by Y6[1], has established a fundamental molecular design framework for low energetic disorder non-fullerene acceptors and enabled rapid efficiency advances in organic solar cells (OSCs). To date, single-junction OSCs based on this class of small-molecule acceptors have achieved power conversion efficiencies exceeding 20%. Despite these achievements, device operational stability remains one of the most critical challenges.

To address this issue, A-DA′D-A type acceptors (A) were employed as modular precursors, and two A units were covalently coupled through functionalized π-bridges to construct A–π–A type quasi-macromolecular acceptors with defined molecular weights (>3.5 kDa)[2]. Systematic molecular weight enlargement modulates the thermal and morphological properties of the materials, leading to an elevated glass transition temperature, suppressed thermodynamic relaxation in blend films, and reduced voltage losses, enabling stable OSCs with efficiencies exceeding 19% without performance degradation. Furthermore, a conjugated cyclic quasi-macromolecular acceptor (RCM) with multiple D-A interactions was constructed to suppress molecular vibrations and structural disorder. Owing to its rigid cyclic topology, RCM exhibits a thermal decomposition temperature approaching 400 °C, and devices retain over 92% of their initial efficiency after heating at 80 °C for 600 h[3,4]. Collectively, this series of quasi-macromolecular acceptors establishes a coherent molecular design strategy and material platform for achieving high-efficiency and long-term stable OSCs.

Recent progress in the molecular design and synthesis of high-performance organic photovoltaic materials has resulted in a remarkable enhancement of power conversion efficiency, surpassing the 20% benchmark in single-junction devices, largely driven by the development of non-fullerene electron acceptors [1]. Despite the success of Y6 and its derivatives, fused-ring electron acceptors continue to suffer from complex, multistep synthetic routes and low-yield ring-closure reactions, which substantially challenge organic solar cells large-scale production and commercialization [2]. To address these limitations, the advent of non-fused-ring electron acceptors has emerged as a promising alternative due to their competitive photovoltaic properties, facile synthesis, and high yields, enabling the fabrication of cost-efficient devices [3]. These materials simplify conventional fused-ring architectures by substituting rigid polycyclic frameworks with simpler three-ring or mono-/bi-cyclic units connected through single σ-bonds, typically adopting an acceptor–π–donor–π–acceptor configuration. In this work, a pair of fluorene-based acceptors, FHM-Cl and FHM-F, respectively featuring a chlorinated and a fluorinated 1,1-dicyanomethylene-3-indanone end group, π-bridged through alkyl-substituted thiophene units to a fluorene core, were synthesized through a three-step route from commercially available precursors. Organic solar cells utilizing these acceptor molecules were subsequently fabricated and optimized with various donor materials. Notably, the combinations D18:FHM-Cl and D18:FHM-F achieved PCEs of 10.7% and 7.6%, respectively. A comprehensive study involving optical and electrical characterization, along with morphological analysis, demonstrated that the FHM-Cl-based solar cells exhibited superior light absorption, enhanced solid-state packing and reduced trap-assisted recombination. Moreover, the good shelf and thermal stability of the devices further highlight the potential of these acceptors for the design of low-cost and efficient organic solar cells, with FHM-F- and FHM-Cl-based devices showing efficiency losses of only 15% and 25%, respectively, after 950 h of thermal aging at 65 °C. Owing to their favorable absorption profiles and wide bandgaps, these novel acceptor molecules also show potential for application in indoor photovoltaic systems and in ternary device architectures.

Replacing lead by less toxic elements remains a major challenge for the widespread uptake of perovskite-based technologies. Tin appears the only candidate to replace lead, due to the accidentally similar structural and electronic properties of these two elements. A major difference, however, is the stability of Sn(IV) phases, which are related to the lower oxidation potential of tin compared to lead. A related phenomenon is the stability of tin vacancies, which introduce significant p-doping in tin-halide perovskites (THPs), while their lead-based counterpart are essentially intrinsic semiconductors. Defect activity clearly controls doping and could also contribute to the instability towards Sn(IV) phases. Controlling doping and defect activity thus represents a pathway towards obtaining stable THPs with optimal optoelectronic properties. The different defect activity of tin- and lead-based materials is at the origin of their respective thermal and phot-induced degradation phenomena, including halide demixing and loss of I₂ in lead-halide perovskites. 

Here we present results of advanced modelling studies on the defect mediated degradation pathways of prototypical THPs. We show how Sn-vacancies are central in promoting both material p-doping and formation of Sn(IV) phases. Interestingly, while p-doping dominates in the bulk, Sn oxidation is only favoured at surfaces or grain boundaries. Thus achieving uniform thin films coupled with proper surface passivation strategies represent a pathway towards achieving more stable THP-based devices. Surprisingly, THPs have also received a large attention because of their superior stability in water environment compared to their lead counterparts. We further unveil the key factors determining the stability of mixed-halide THPs against photoinduced halide segregation phenomena. Molecular and ionic strategies to mitigate p-doping in THPs are also presented.

References

[1] Gatto, L.; Poli, I.; Meggiolaro, D.; Grandi, F.; Folpini, G.; Treglia, A.; Cinquanta, L.; Petrozza, A.; De Angelis, F.; Vozzi, C.; "Charge-Phonon Coupling in Tin Halide Perovskites", ACS Energy Lett. 2025, 10, 1382−1388

[2] Suo, J.; Yang B.; Mosconi, E.; Bogachuk, D.; Doherty, T.A.S.; Frohna, K.; Kubicki, D.J.; Fu, F.; Kim, Y.J.; Er-Raji, O.; Zhang, T.; Baldinelli, L.; Wagner, L.; Tiwari, A.N.; Gao, F.; Hinsch A.; Stranks, S.D.; [3] De Angelis, F.; Hagfeldt, A. "Multifunctional sulfonium-based treatment for perovskite solar cells with less than 1% efficiency loss over 4,500-h operational stability tests" Nat. Energy, 2024, 9, 172-183

Zhou Y.; Poli I.; Meggiolaro D.; De Angelis F.; Petrozza A. "Defect activity in metal halide perovskites with wide and narrow bandgap" Nat. Rev. Mater., 2021, 6, 986–1002

The commercialization of perovskite solar cells (PSCs) is still limited by common efficiency loss and long-term stability issues when scaling up solar cells.¹ On the other hand, it has been observed that chalcogenides enhance the properties of PSCs when incorporated into transport layers.² However, despite evidence that such structures enhance charge transport and perovskite growth,³ research on epitaxial heterostructures in these two materials has been limited.⁴ Recently, a CsPbBr₃/PbSe epitaxial nanocrystal was successfully synthesized, confirming its interesting decrease in hot carrier cooling.⁴ This phenomenon could provide access to high-energy photons, which are usually lost due to thermalization, potentially exceeding the Shockley-Queisser limit.⁵ My contribution aims to shed light on the physical and chemical mechanisms behind the optoelectronic properties of CsPbBr₃/PbSe epitaxial heterostructures. This will be achieved by performing accurate first-principles calculations based on density functional theory and ab initio molecular dynamics, and by analyzing the electronic and structural properties of the system. This study not only aims to characterize the CsPbBr₃/PbSe system, but also paves the way for broader research into perovskite-chalcogenide epitaxial heterostructures with different compositions, facilitating the rational design of high-quality interfaces in PSCs in the future.

A detailed understanding of photogenerated charge carriers in lead halide perovskites is essential for moving beyond empirical optimization toward the rational design of next-generation perovskite-based optoelectronic and photovoltaic devices^[1]. In this work, we investigate the photophysical origin of photoluminescence in perovskite materials.

We perform temperature-dependent steady-state and time-resolved absorption and photoluminescence measurements on a set of representative systems, including inorganic CsPbBr₃ thin films, CsPbI₃ nanoparticles, and hybrid MAPbBr₃ single crystals. The evolution of optical spectra and recombination dynamics over a wide temperature range provides direct experimental access to the nature of the photogenerated species across different perovskite material classes.

The experimental results are then compared with predictions based on the Saha model, which is commonly used to describe the thermal equilibrium between excitons and free charge carriers. While this model captures basic trends, it neglects several effects known to be significant in lead halide perovskites, such as non-parabolic band dispersions, defect states, dielectric screening, many-body interactions, and carrier–carrier and carrier–lattice coupling. Deviations between model predictions and experimental observations highlight the limitations of this simplified framework.

Finally, by analyzing the temperature-dependent evolution of spectral line shapes and recombination dynamics using numerical modeling, we extract fundamental parameters that characterize the nature of the photogenerated charge carriers and their interaction with phonons^[2-4]. This comprehensive approach provides a refined, physically grounded picture of photoluminescence in lead halide perovskites under device-relevant conditions.

Modelling non-planar perovskite solar cells (PSCs) in 1D is very challenging due to strong interfacial and geometric interactions. This affects especially mesoporous, structured tandem, phase segregated and bulk heterojunction solar cells. We present ChargeFabrica, an open source, two dimensional electro-ionic drift-diffusion simulation tool designed to address these challenges by simultaneously solving the coupled electronic and ionic transport equations across complex device geometries. Using ChargeFabrica, we successfully replicate experimentally observed thickness-dependent trends in current–voltage (JV) curves, the influence of ionic prebiasing and associated EQE, which cannot be fully captured by conventional one-dimensional models. By incorporating realistic device morphologies and experimentally demonstrated defect densities, the simulator accurately predicts performance losses, field inversion effects, and the impact of geometric and interfacial properties. ChargeFabrica thus provides a robust platform for understanding and optimizing the interplay between ion migration and charge collection in mesoporous PSCs and will aid future development of perovskite device architectures.

Long-term operational stability in metal-halide perovskite optoelectronic devices is governed by coupled processes in both the bulk and at surfaces, where defect formation, ion migration, and environmental interactions dictate device degradation. Achieving durable stability therefore requires simultaneous control of bulk defect energetics and effective surface passivation. Within the bulk, mixed-halide I/Br perovskites suffer from intrinsic halide segregation driven by ion migration, severely limiting long-term stability. Recent molecular anion additive-based strategies have shown pronounced suppression of phase segregation and enhanced operational stability. However, the mechanisms underlying this stabilization, including the influence of additives on local structure, defect energetics, and ion transport, remain poorly understood. At the surface, amino-silane-treated perovskite solar cells have recently demonstrated exceptional environmental and operational stability.[1] Understanding the atomic-scale interactions between silane molecules and perovskite surfaces is essential for establishing clear insights into their role in defect passivation and the suppression of ion migration. Here, we employ density functional theory and ab initio molecular dynamics simulations to provide atomic-scale insights into perovskite stabilization. We elucidate the mechanisms of additive-induced bulk stabilization as well as silane-based surface passivation in 3D halide perovskites. Our results are consistent with experimental observations and offer a mechanistic understanding of perovskite stability at the atomic level.

Small-perturbation measurements in the frequency domain are important tools to distinguish power loss mechanisms occurring at a given steady-state condition in perovskite solar cells. These techniques consist of impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS). However, the typical analysis methods of the measured spectra possess several limitations, leading to erroneous interpretation and parameter estimation. The biggest limitation among these is the assumption of perfect charge extraction at the collecting contact, which is very unlikely due to the low mobility of typical transport layers used in perovskite solar cells. We thus develop an extended version of the diffusion-recombination model with boundary conditions that account for charge extraction from the perovskite bulk to the electrodes, via the transport layers. This further establishes the coupling between the transport layer properties and its coupling with the bulk recombination, and their corresponding influence on the modulated photocurrent and photovoltage spectra. We further develop a figure of merit that describes the influence of absorption losses, charge collection losses both in the bulk and due to the transport layers, on the current-voltage curve. Through an analysis of the time constants from the measured spectra, we calculate this figure of merit across different steady-state conditions for state-of-the-art perovskite solar cells. 

Perovskite solar cells (PSCs) are highly printable, enabling large-volume production that is necessary to rapidly introduce this new technology to the market and help combat climate change . Flexible PSCs are typically deposited on synthetic polymer substrates such as PET and PEN—materials that suffer from poor gas barrier properties and low thermal stability [2], and whose fossil-based origin makes incineration for PSC recycling problematic. Cellulose, the backbone of paper and cotton, is a bio-based alternative to synthetic substrates, as it can form flexible and transparent films suitable for printed electronics [3]. Here, I show how cellulose can be crosslinked to improve solvent resistance [4] and how new electrode materials can be used to achieve low surface roughness while maintaining high conductivity and transparency [5], among other improvements. Transitioning to a bio-based substrate requires a combined effort from both the cellulose and perovskite communities. As we push for increased sustainability, the perovskite fabrication process must be adapted to use greener solvents [6], low-temperature deposition methods [], and more sustainable encapsulation materials []. We believe that this strategy can improve PSC recyclability and further advance the technology toward a more circular economy.

Slot-die roll-to-roll (R2R) coating represents one of the most promising scalable coating techniques for large-area flexible perovskite solar cells, however, achieving large-area films with well-controlled crystallisation kinetics and performances matching those of laboratory-scale, spin-coated films remains a significant challenge.[1] Here, we employ our custom-designed beamline-integratable roll-to-roll slot die coater combined with simultaneous, multimodal in-situ characterisation, including  X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray fluorescence (XRF) and optical spectroscopy. This integrated approach enables complementary, real-time in-situ investigation of crystallisation phase, local structure, chemical composition, lead oxidation state and electronic properties during film formation.

Using this system, we study thin film formation from 2-methoxyethanol (2ME)/N-methyl-2-pyrrolidone (NMP)-based perovskite precursor inks under varied quenching and annealing conditions. High-time-resolution (10) ms measurements during gas quenching reveal how gas quenching processes can be utilised to control crystallisation kinetics and achieve homogeneous chemical distributions, including in mixed-halide (I-Br) systems. The multimodal data reveal how solvent choice and film drying conditions impact wet-to-dry conversion, crystallisation kinetics and changes in the local environment of lead ions, establishing quantitative relationships between processing parameters and film microstructure. These studies provide a mechanistic understanding of the intricate interplay between solvents, intermediates and processing parameters on perovskite formation during R2R deposition.

These studies demonstrate how multimodal X-ray and optical in situ techniques can be leveraged to rationalise and optimise processing windows for balancing nucleation density, crystal growth rate and film uniformity toward high-throughput, large-area perovskite device manufacturing.

Metal halide perovskites have attracted attention for their exceptional optoelectronic properties, such as defect tolerance, long charge-carrier lifetimes, and extended diffusion lengths. Despite their promise for next-generation optoelectronic devices, reproducible fabrication of high-quality thin films remains challenging due to the sensitive crystallization process, especially during thermal annealing.

In situ photoluminescence (PL) has emerged as a powerful technique for real-time monitoring of perovskite film formation, providing dynamic insight into crystallization processes. Rather than relying on extensive trial-and-error parameter optimization, this approach enables a targeted investigation of the fundamental mechanisms governing annealing-induced phase evolution and grain growth. It also provides insight into defect formation, thereby informing optimal processing strategies and defect passivation. A critical challenge in this field is the phase instability of formamidinium lead iodide (FAPbI₃). While the black α-FAPbI₃ phase exhibits excellent optoelectronic properties, it is metastable at room temperature and tends to transform into the thermodynamically favoured yellow δ-FAPbI₃ phase, leading to severe performance degradation. Although high-temperature annealing is commonly employed to promote the formation of the α phase, a comprehensive understanding of the crystallization pathways is still lacking.

In this work, we systematically investigate the influence of annealing temperature on the crystallization behaviour of FAPbI₃ thin films. Using in situ PL, complemented by X-ray diffraction and photoluminescence quantum yield measurements, we identify a threshold annealing temperature that delineates two distinct material states. Films annealed at lower temperatures exhibit poor optoelectronic quality and limited stability, and certain films switch to yellow δ-FAPbI₃, detected by ex situ XRD measurements, whereas higher-temperature annealing leads to the formation of a qualitatively different material with markedly improved structural and optical properties. Even after ambient exposure, such annealed samples offer almost pristine α-FAPbI₃ (above 96%). Real-time PL measurements reveal additional features during annealing and serve as reliable indicators of the final film quality. One of the most interesting features is an additional dip in photoluminescence and a simultaneous maximum in the obtained Urbach energy; this occurs only above 140 °C. Films that don’t show such real-time PL development undergo quick degradation and have very poor optoelectronic quality. The origin of this behaviour and its implications for the reproducible fabrication of high-quality α-FAPbI₃ thin films are discussed.

These results reveal a distinct signature photoluminescence evolution, demonstrating the formation of a stable FAPbI₃ thin film. In situ photoluminescence reveals how annealing conditions govern perovskite crystallization and final film quality, providing guidance for reproducible fabrication of high-performance materials.

The envisaged breakthrough of perovskite photovoltaic technologies demands rapid advances in scalable fabrication methods. In this study, we present close-space sublimation (CSS) as a vacuum-based, industrially relevant deposition method for the fabrication of high-quality perovskite absorbers. We have used this method to prepare low (1.54 eV) and intermediate bandgap (1.63 eV) perovskites. We provide mechanistic insights in the substitution-reaction-limited CSS process for the wide bandgap perovskite thin film fabrication and reveal a new way to control the bandgap of the perovskite absorber. Single junction solar cells with power conversion efficiency reaching 19 % and exceptional stability (90% retained performance after 1000 hours of operation at 75 °C under 1 sun) are obtained with these CSS based perovskites. Monolithic integration of the perovskite top-cells onto planar, nano-, and micro-textured silicon bottom cells, revealed consistent optoelectronic and morphological properties across all configurations, without requiring adjustments of deposition parameters. This robustness is confirmed through comprehensive characterization techniques, including external quantum efficiency, Suns‑V_OC with selective illumination, scanning electron microscopy, and grazing-incidence wide-angle X‑ray scattering. The resulting tandem devices reached PCEs of up to 24.3%, with minimal variation across the different bottom cells. Our findings highlight the broad process window and versatility of CSS, positioning it as a promising deposition method for rapid and industry-suitable fabrication of efficient monolithic perovskite/silicon tandem solar cells.

The advancement of all-printed carbon-based hole-transport-layer-free perovskite photovoltaics represents a major step toward scalable, sustainable and cost-effective next-generation solar power devices. Within this framework, piezoelectric drop-on-demand inkjet printing provides a powerful digital manufacturing route, enabling precise, reproducible and high-throughput deposition of functional layers under ambient-air conditions. In this work, a sustainable upscaling of carbon-based perovskite solar modules is demonstrated, using piezoelectric drop-on-demand inkjet printing as the primary deposition method for nearly all functional layers. For this case, a high-stability perovskite precursor ink from centimeter-scale single crystals and green solvents is prepared and employed to develop the perovskite active layer, while a newly engineered carbon paste is also introduced to fabricate an efficient and scalable back-contact electrode, applied via blade coating. The fabricated modules achieved efficiencies exceeding 12% on 1500 cm² (upscaling loses 8<%_reldec⁻¹, geometrical fill factor ≈70%), with the “bill of materials” and efficiency-adjusted specific costs to be estimated on the order of 30 €/m² and 0.5 €/W_p, respectively, highlighting the cost-effectiveness and scalability of the proposed manufacturing approach. Beyond performance, a comprehensive stability analysis is conducted under ISOS accelerated aging protocols, including light soaking stress, prolonged isothermal fatigue, thermal shock cycling, damp heat and bias stress. The degradation behavior was complemented by predictive modelling using a semi-analytical model developed by one of the authors, enabling accurate forecasting of the lifetime of the devices under different stress conditions, supporting their technological maturation toward real-world photovoltaic deployment.

Achieving carbon-neutral energy production requires a rapid increase of electricity production using solar energy. In order to achieve this goal, new generation organic (OPV) and perovskite photovoltaics are needed to: a) enhance the performance of existing, e.g., silicon solar PV and b) allow for the energy- and cost-efficient manufacturing of photovoltaic devices on light-weight flexible substrates, particularly suitable for applications requiring variable form factors and/or operation under low light conditions.

Both organic and perovskite photovoltaics require the industrial availability of materials with specific electronic, physical and chemical characteristics. The optimization of the properties of unfunctionalized fullerenes (especially C60), available at industrial scale for vapor deposition, will be described.

The selection and design of organic materials to be used in the active layer of high performing OPV and in the interlayers of perovskite PV p-i-n, n-i-p as well as tandem devices will be discussed. The importance of functional groups which are suitable to form self-assembled monolayers (SAM) and/or stabilize the perovskite phase will be assessed. The synthesis of fullerene derivatives, e.g., bearing phosphonic acid or amino-/ammonium-groups but also molecules such as triphenylamines and carbazoles bearing phosphonic or boronic acid with electronic properties suitable for their use as hole transport material (HTM) will be presented. The use of cross-linking to improve the stability of interface layers is explored. Device data showing the use of such innovative interface materials will be provided.

Fully optimized OPV stacks, i.e., including the electron-transport and hole transport layers (ETL and HTL), in addition, to the active layer will be shown. Device data including thermal and light soaking stability will be reported.

Formulations, containing silver nanowires (AgNW) and/or single-wall-carbon nanotubes (SWCNT), suitable for the deposition of semi-transparent top electrodes will be discussed.

For the future industrialisation of perovskite solar cells, the usage of green (non-toxic) solvents is necessary to reduce the negative impact on humans and the environment and hence to reduce costs for the enclosure of deposition machines in production lines and expensive health and safety measures.

Therefore, we deposit all layers in the cell stack (except for the vacuum-deposited electrodes) with slot die-coating and using solely green solvents. For the perovskite layer, we examined and optimised a scalable two-step deposition method. To obtain a stoichiometric perovskite layer, the precursor ratio must be carefully controlled to minimize residual lead iodide as well as excess organic cations such as formamidinium iodide. Both species are known to reduce the device performance. For instance, surplus formamidinium iodide can cause a pronounced reduction in short‑circuit current, while PbI₂ has in addition been shown to compromise long‑term operational stability of perovskite solar cells [1]. Achieving high efficiency and durability therefore requires precise compositional tuning, as deviations toward either lead‑rich or cation‑rich conditions significantly degrade device performance.

In this work, we focus on the homogeneity of solar cell efficiencies on a substrate size of 10×10 cm², with each solar cell having an active area of 0.5 cm², as this is an important prerequisite for reaching good module efficiencies. On each 10×10 cm²-substrate, we can measure up to 40 solar cells to evaluate the homogeneity over the whole area. In the next step, we prepared modules on 5×5 cm²-substrates with an area of 13 cm² containing nine monolithically interconnected cell stripes.

We investigate the influence of different parameters on solar cell characteristics and homogeneity, e.g. the used solvent for the second step, influences of additives and finally annealing temperature after second step layer. Analytical methods like x-ray diffraction (XRD), external quantum efficiency (EQE) and scanning electron microscopy (SEM) were used to further characterise the layers and evaluate their quality (e.g. complete conversion to perovskite), and to detect possible optimisation pathways.

With all slot die-coated perovskite solar cells we reached power conversion efficiencies (PCE) above 16 % on an active area of 0.5 cm² with usage of solely green solvents for all layers. For 13 cm²-modules we were able to reach an average VOC over all nine sub-cells, which is comparable to that of cells from the same experiment.

Note: We did not understand, what a TOC is and therefore provided a screenshot of the original word document, in order to make the illustrations visible.

The application of Perovskite Technology in Photovoltaics will require the transformation from laboratory research to pilot or industrial production. Numerous materials within the PVSK cell stack are shown to be compatible with thermal vacuum evaporation processes. However upscaling under industrial conditions often introduces challenges to adapt to a certain process or material. Thermal evaporation is already shown for several candidates on laboratory level and the potential for upscaling to industrial production is acknowledged. Linear evaporators of Von Ardenne are actively involved in this progress [1]. We would like to give an overview on the actual state of this component. This update considers stable operation under or close to industrial conditions as well as results on new materials like the inorganic PVSK precursors.

The basic design of a linear evaporator is illustrated in figure1. It is quite different from a point source and consists of:

• a crucible comprising the evaporation material
• a vapor guiding tube with
• the vapor emitting nozzles
• a rate nozzle serving as vapor source for a Quartz Crystal Microbalance (QCM) and
• a substrate shutter

The QCM is used for deposition rate monitoring. The heating system of linear evaporator can be operated in power, temperature (measured by attached thermocouples) or rate controlled mode. Reproducibility and process stability are realized by a Programmable Logic Controller (PLC). Since halogens are known to be corrosive, crucible and nozzle tube are manufactured from inert material.

During operation, crucible and tube are heated up and the material to be coated starts to sublimate or evaporate. The vapor is guided along the nozzle tube and emitted via the nozzles to the substrate, which is moving across the line of nozzles. This principle allows for a bottom-up as well as top-down coating geometry, the latter being impossible for a point source. Moreover, even arrangements for coevaporation of up to four materials are possible. Each linear evaporator is equipped with a shutter in front of the nozzles. Shutters are useful to protect the surrounding chamber parts from undesired coating during heating up, conditioning/soaking or in standby mode. Another application is the on/off switching of the vapor stream to easily realize different film compositions in coevaporation mode.

The linear evaporators can be operated a temperatures up to 700 °C. An adjusted configuration for temperatures 100°C to 250°C is also available. This temperature range covers a wide range of materials used in PVSK and other technologies. A summary of the already evaporated materials is illustrated in figure 2. As expected, the dynamic deposition rates (DDR) increase exponentially with temperature, pointing out the importance of a precise temperature control.

The contribution will give an overview on possible arrangements (parallel and coevaporation units, bottom-up, top-down), challenges and solutions for stable operation as well as uniformity data for different materials.

Roll-to-roll (R2R) manufacturing is central to the industrialization of perovskite photovoltaics, yet module-level integration remains one of the least resolved challenges. While continuous coating of perovskite device stacks has advanced rapidly, scalable interconnection strategies compatible with uninterrupted web processing are still largely challenging. Although effective at laboratory and pilot scales, laser-based patterning introduces fundamental incompatibilities with continuous R2R manufacturing, including thermal damage, debris generation, alignment sensitivity, and cumulative yield loss over long web lengths.

Here, we report the first demonstration of a fully R2R perovskite solar module in which serial interconnections are formed without any laser processing. Instead, interconnection and isolation are defined additively through deliberate lateral alignment during sequential slot-die coating. A fully R2R-compatible architecture is first established at the single-cell level on flexible substrates, providing a reproducible foundation for module integration. This architecture is then extended to a multi-stripe module design, where controlled layer alignment replaces post-deposition material removal as the defining interconnection mechanism. Using this alignment-driven strategy, four-stripe perovskite modules with an overall area of 10 × 10 cm² are fabricated in a continuous coating sequence. The resulting modules exhibit stable diode behavior, uniform electroluminescence across all stripes, and reproducible performance over active areas up to 40 cm². Progressive optimization of coating conditions, ink rheology, and alignment offsets leads to stabilized PCEs exceeding 5%, with predictable scaling trends governed by lateral charge transport rather than process variability. Importantly, the absence of laser scribing eliminates common sources of edge damage and debris-related degradation.

To assess the intrinsic performance potential of this laser-free architecture, distributed electrical modeling is employed to decouple geometric and material limitations. The analysis reveals that module efficiency is currently constrained primarily by the conductivity of the transparent electrode and the carbon contact, rather than by the interconnection geometry itself. Simulations show that efficiencies exceeding 9% are achievable solely through realistic improvements to the transparent conducting substrate and carbon electrode, without altering the module layout or coating sequence. This finding demonstrates that the proposed alignment-defined interconnection strategy has not yet reached its intrinsic performance ceiling.

As interconnections become embedded directly within the coating sequence, maintaining alignment fidelity over long web lengths becomes a critical determinant of yield. In this context, inline metrology and real-time process monitoring emerge as key enablers for stabilizing alignment and detecting drift during production. This study establishes laser-free, alignment-based interconnection as a viable route toward truly continuous “ink-to-module” roll-to-roll manufacturing of perovskite solar modules.

Organic photovoltaic (OPV) technology has experienced substantial growth over the past decades [1,2], primarily driven by the design of innovative donor polymer and non-fullerene acceptor materials [3], enabling certified power conversion efficiencies (PCEs) exceeding 20% in single-junction devices [4]. Nevertheless, the performance of single-junction OPVs remains fundamentally limited by thermalization, spectral mismatch, and recombination losses, as defined by the Shockley–Queisser constraint. Tandem solar cell configurations provide an effective approach to mitigate these intrinsic losses by integrating multiple sub-cells with complementary light absorption, thereby enhancing overall utilization of the solar spectrum [5]. In this context, we present our recent work carried out as a joint collaboration between CHOSE labs at the University of Rome Tor Vergata and CAPE center at Southern Denmark University, which focuses on the study and optimization of Interconnecting Layer (ICL) for two-terminal organic-organic tandem devices (2T-TSCs), in inverted architecture. Compared with four-terminal configurations, 2T‑TSC devices offer lower fabrication costs and are compatible with scalable, large-area, roll-to-roll processing, making them industrially relevant [6,7]. However, they require careful current matching and solvent compatibility to prevent interlayer mixing and damage to the active layers. The front and rear sub-cells were independently optimized through the careful selection of donor and non-fullerene acceptor materials exhibiting complementary absorption in the visible and near-infrared light spectral regions. Wide-bandgap and narrow-bandgap bulk heterojunctions were designed to maximize photocurrent generation while minimizing series resistance and recombination losses. The tandem architecture was subsequently realized, through spin coating technique, using a fully solution-processable ICL consisting of n-type and p-type materials sequentially deposited. A systematic investigation of ZnO nanoparticles-based electron transport layer, processed from different solvents provided by different suppliers, was conducted to address challenges associated with interlayer intermixing, film morphology, and voltage losses. This approach enabled the identification of an efficient ohmic n‑type/p‑type interface, resulting in tandem devices with a maximum PCE of 14.9%, that consistently overcome the performance of their corresponding single‑junction sub‑cells. Based on these results, the optimized ICL architecture was successfully transferred to scalable fabrication methods via slot‑die coating, yielding to a maximum PCE of 13.6%. Large‑area tandem slot-die coated modules with an active area of 13.8 cm² were fabricated, exhibiting an open-circuit voltage of 10.8 V, a fill factor of 67.4%, and a PCE of 12.9%. These results highlight the critical role of fully solution‑processable interconnecting layers and demonstrate a viable pathway toward scalable, high-efficiency organic tandem photovoltaic technologies.

Photochromic solar cells are an emerging class of photovoltaic devices exhibiting dynamic optical and electrical properties, making them a promising option for integration into building façades and smart glazing systems. When exposed to sunlight, these devices can reversibly modulate their colour, transparency, and photovoltaic performance in real time.

In 2020, we demonstrated an effective strategy for developing this class of light-responsive solar cells, which is based on push–pull photochromic photosensitisers incorporating a diphenylnaphthopyran core within dye-sensitised solar cell (DSSC) architectures. This approach enabled excellent reversibility between coloured and bleached states while maintaining power conversion efficiencies of around 4% at the cell level and a maximum power output of around 32 mW in semi-transparent mini-modules [1].

In this contribution, we show that targeted structural modifications of these photochromic dyes allow fine control over key device parameters, including colour modulation, decolourisation kinetics, absorption spectra, and photovoltaic performance. We describe the synthetic strategies employed to access these materials and provide a comprehensive investigation of their optoelectronic properties and structure–property relationships [2–4].

We further demonstrate that rational molecular engineering enables the fabrication of semi-transparent mini-modules with average visible transmittance values ranging from 50 to 66%, while preserving an excellent colour rendering index over 94 in both coloured and uncoloured states [5]. Finally, we will present innovative strategies to optimise the optical properties of photochromic dyes and solar cells, paving the way for the next generation of smart photovoltaic devices. [6-7]

In recent years, remarkable progress has been made in the field of metal halide perovskite solar cells, resulting in power conversion efficiencies (PCE) that surpass 27%. As the vast majority of research efforts are dedicated to their processing from solution, the use of the highly toxic, yet ubiquitously used, dimethylformamide (DMF) for the processing of perovskite represents a major hurdle for their adaptation in industrial applications on the large-scale. In the first part of this talk, I will propose a strategy for selecting green solvent alternatives to DMF that enable a more sustainable processing of perovskite based solar cells. To validate the proposed selection strategy, I will demonstrate the ability of multiple green solvents to be used to fabricate efficient perovskite solar cells. In the second part of the talk, I will introduce novel strategies for a solvent-free fabrication of perovskites by vapor deposition, focusing on addressing the current challenges of this deposition method.

Semi-transparent OSCs (STOSCs), capable of generating electricity and transmitting light simultaneously, are a highly promising new energy technology with potential applications in building-integrated photovoltaics for windows and roofs, as well as in agricultural greenhouses and transportation vehicles. However, the balance between the power conversion efficiency (PCE) and average visible light transmittance (AVT) of STOSCs remains a challenge. In this presentation, I will introduce the high performance STOSCs developed in my lab from multiple aspects, including the molecular design of narrow band gap electron acceptors, the device and optical engineering, etc. By synthesizing the organic electron acceptors with absorption edge extended into 1000 nm, we successfully increased the light utilization efficiency of STOSC, as well as improved the PCE and the AVT related to the energy conversion ability and the visible light transmittance ability, respectively. The diverse applications of STOSCs were demonstrated, such as smart greenhouse, thermal-insulating organic photovoltaics, flexibility, etc..

The inverse design of tailored organic molecules for specific optoelectronic devices of high complexity holds an enormous potential but has not yet been realized. Current models rely on large data sets that generally do not exist for specialized research fields. We demonstrate a closed-loop workflow that combines high-throughput synthesis of organic semiconductors to create large data sets and Bayesian optimization to discover new hole-transporting materials with tailored properties for solar cell applications. The predictive models were based on molecular descriptors that allowed us to link the structure of these materials to their performance. A series of high-performance molecules were identified from minimal suggestions and achieved up to 26.2% (certified 25.9%) power conversion efficiency in perovskite solar cells. Recent works went beyond 27 % single junction perovskites. But, more important is the general applicability of the workflow, that already shows success in discovering radiation hard materials, photooxidative stable materials or transparent semiconductors.

That milestone underlines the feasibility of developing autonomous research strategies that discover materials tailored for specific applications with highly interconnected workflows including synthesis, purification, characterization and device optimization. However, before such workflows are capable of solving real world problems, optimization has to be thought in a much broader sense, going beyond single-objective optimization (such as efficiency) to multi-objective optimization (such as efficiency, lifetime, toxicity, …). Such workflows could specifically develop fully optimized solar cells, LEDs, photodetectors or X-Ray detectors that can be directly transferred from the lab to the fab. The outlook will summarize the advantages but also the limitations of data driven methods and will give first examples of such campaigns.

The commercialization of perovskite solar cells faces three critical challenges: reliance on the use of toxic solvents, large variation in perovskite film quality across large-area substrate, and limited operational reliability. Here, we address these challenges firstly by developing a non-hazardous solvent system for large-scale manufacturing of void-free and defect-minimized perovskite films under ambient air conditions. We further improve the film uniformity by introducing a solvent confinement edge-protection strategy that mitigates the rapid solvent evaporation over wet perovskite films and at substrate edges during slot-die coating. We recently further improve the efficiency by adopting stable surface passivation. These approaches enable the production of 7,200 cm2 commercial perovskite photovoltaic modules with a certified total-area efficiency above 22%. The commercial-scale modules meet all reliability criteria of the IEC 61215 standards, as independently certified by TÜV Rheinland, demonstrating their readiness for real-world deployment.

Perovskite solar cells have come to the forefront of solar research in the last decade with
certified efficiencies of now &gt;26%. This is approaching rapidly the Shockley-Queisser limit for
single-junction solar cells, implying that the main breakthroughs for perovskites were
achieved with relatively narrow bandgaps.[1a,1b] Less progress, however, was made for
wider bandgap perovskites, which are of interest for multijunction photovoltaics, detector
applications, or water splitting. These wide bandgap perovskites are often comprised of fully
inorganic components, which are hard to dissolve in conventional solvent systems and
require more sophisticated synthesis as well as crystallisation techniques.
In this talk, I will discuss strategies to address these challenges by providing a library of
hitherto unexplored wider bandgap perovskites using combinatorics. Mechanosynthesis is
then studied to attain otherwise inaccessible liquid precursors permitting the realization,
e.g., of “triple cation” wide bandgap perovskites.[2]
Unfortunately, the newly formulated liquid precursors often exhibit complex crystallization
behaviour struggling to expel the typically used DMSO solvent. To delay the crystallization
time, two strategies are proposed to remove the strongly complexating DMSO molecules
through a) modified processing of the liquid thin-film[3] and b) a coordination solvent with a
high donicity and a low vapor-pressure[4] leading to a marked improvement in the overall
film quality.
Lastly, interface manipulation, especially on top of the formed perovskite, is becoming a
central topic to advance further. Typically, this involves chemical surface treatments with a
complex interaction. Here, light annealing is introduced as a universal, non-chemical
approach to modify the perovskite surface resulting in a reduced surface recombination.[5]
[1a] Saliba et al. Energy &amp; Environmental Science (2016), [1b] Turren-Cruz, Hagfeldt, Saliba; Science (2018)
[2] Ferdowsi,…, Saliba ; Chemistry of Materials (2021)
[3] Byranvand,…, Saliba; One‐Step Thermal Gradient‐and Antisolvent‐Free Crystallization of All‐Inorganic
Perovskites for Highly Efficient and Thermally Stable Solar Cells, Advanced Science (2022)
[4] Zuo,…, Saliba; Coordination Chemistry as a Universal Strategy for a Controlled Perovskite Crystallization,
Advanced Materials (2023)
[5] Kedia,…, Saliba; Light Makes Right: Laser Polishing for Surface Modification of Perovskite Solar Cells, ACS
Energy Letters (2023)

Perovskite solar cells (PSCs) have emerged as a leading candidate for next-generation photovoltaics, primarily due to their potential for fabrication via solution-based coating processes. Over the past decade, significant improvements in power conversion efficiency (PCE) have been driven by fundamental advances in materials chemistry. Key breakthroughs include refined film formation methods for metal halide perovskite semiconductors, the development of sophisticated surface passivation techniques, and the engineering of novel materials for efficient charge extraction from the perovskite layer.

Our research group has been engaged in the development of PSCs since the field’s early stages, focusing on these material chemistry perspectives to enhance device performance. In 2018, leveraging our accumulated expertise, we co-founded EneCoat Technologies Co., Ltd.—a Kyoto University-spinoff startup—to accelerate the practical application and commercialization of this technology.^1,2

In this presentation, I will outline our strategic research approach and highlight representative achievements to date, including:

• High-purity precursor materials: Development of proprietary precursors that enable the highly reproducible fabrication of high-quality perovskite thin films.^3,4
• Photophysical mechanism clarification: Utilization of advanced spectroscopic techniques to elucidate the unique optical properties and charge-generation mechanisms inherent to metal halide perovskites.^5,6
• Charge-collecting material engineering: Design of multipodal pi-conjugated molecules (e.g., PATAT) to precisely control energy levels and molecular orientation on transparent electrodes (ITO).^7-9
• Interface and process optimization: Development of surface-modified materials with dipole moments and a unique passivation method for both the top and bottom interfaces of the perovskite layer, leading to significantly higher open-circuit voltages.^10-17

Perovskite solar cells (PSCs), due to their outstanding photovoltaic properties, simple and low-cost solution fabrication process, abundant precursors, and skyrocketing power conversion efficiency (PCE), have attracted tremendous attention and show great promise for scale-up and future commercialization. While the unprecedentedly high efficiency using perovskites is an astonishing achievement, issues relating to the long-term durability against atmospheric moisture and oxygen, heat, and light still raise concerns for the successful commercial application of PSC technology. The most effective devices employ acidic self-assembled monolayers (SAMs) of organic small molecules that are capable of binding metal oxide contacts, resulting in inverted PSCs with improved efficiency. Such molecularly thin SAMs offer numerous potential advantages, such as the need for tiny amounts of material, and tunable chemistry to obtain cheap but efficient semiconductors that enable additional functionalities and further device performance improvements. However, further spread of such materials is hindered by the rather poor coverage, dewetting of the perovskite precursor solution, due to the nonpolar SAM surface, and stability issues due to acidic SAM-favored corrosion of contacts.

Despite the central role of SAMs in PSCs, their molecular design paradigm has remained essentially unchanged since it was discovered. Nearly all reported SAMs rely on intrinsically acidic anchoring groups, resulting in interfacial inhomogeneity, insufficient charge extraction, and poor operational stability—issues that fundamentally constrain the long-term performance and manufacturability of PSCs.

This work disrupts this long-standing design bottleneck. We introduce an entirely new class of alkali metal–based ionic phosphonate salt SAMs (2PACz-M)—the first demonstration of SAM formation directly from ionic salt solutions. Through targeted head-group neutralization of the benchmark 2PACz molecule, we transform acidic SAMs into non-acidic, highly delocalized, electronically active ionic interfaces. This departure from conventional SAM chemistry offers several breakthrough advantages: non-acidic, neutral SAMs for PSCs eliminate acid-induced corrosion and dramatically improve interfacial uniformity and electronic coupling; the first demonstration of ionic phosphonate SAMs assembled from aqueous-compatible salt solutions enables green-solvent processing and vastly improves compatibility with perovskite inks. These innovations translate directly into record-level device performance: 1.55 eV PSCs achieve a certified PCE of 26.88% and a FF of 86.57%, and a 29.7 cm² module delivers 23.32%, the highest among SAM-based modules.

Beyond performance, the conceptual shift—from acidic molecular anchoring to ionic, electronically delocalized SAMs—establishes a new direction for interfacial engineering in perovskite photovoltaics. This molecular design framework is generic, scalable, and applicable across diverse bandgaps, device architectures, and fabrication environments.

Hole-transporting materials (HTMs) play a pivotal role in achieving high efficiency and long-term stability in perovskite solar cells (PSCs), yet the limited operational stability and scalability of state-of-the-art HTMs remain major challenges. Our research group has been actively developing and engineering novel molecular HTMs and hybrid strategies aimed at simultaneously enhancing device performance, durability, and large-area applicability. In this context, we first reported two doped HTMs based on electron-rich spiranic scaffolds, namely spiro-POZ and spiro-PTZ. These materials deliver photovoltaic performance comparable to the benchmark spiro-OMeTAD while exhibiting markedly enhanced long-term stability, retaining performance for more than 300 days under ambient conditions and over 1200 h under continuous 1-sun illumination.^[1] Building on these promising results, we designed a new family of four spiro-PTZ derivatives functionalized with asymmetric diphenylamine units. When integrated into PSCs, these materials enable power conversion efficiencies (PCEs) as high as 25.75%, clearly surpassing both the efficiency and stability of spiro-OMeTAD-based devices. Notably, large-area mini-modules (25 cm²) fabricated using these HTMs exhibit outstanding PCEs exceeding 22%, positioning spiro-PTZ derivatives among the most efficient HTMs reported to date.^[2]

Beyond spiranic systems, our group has also explored alternative molecular frameworks and hybrid approaches. Chemically modified ullazine-based HTMs were successfully integrated into PSCs, showing excellent long-term stability under room ambient conditions in the dark, with operational lifetimes exceeding six months, underscoring the robustness of this molecular platform. Additionally, we developed a hybrid HTM approach by incorporating p-type-doped carbon nanostructures (zinc-metalated porphyrin (ZnP)-SWCNT) as additives in spiro-OMeTAD. This strategy enhances hole transport, improves device stability, and increases the photovoltaic performance from 18% to 19.8% compared to pristine devices.^[3]

Overall, our collective work highlights that rational molecular design, chemical modification, and hybrid material engineering are effective routes to simultaneously enhance efficiency, stability, and scalability in perovskite solar cells, paving the way toward highly efficient, stable, and scalable photovoltaic technologies.

References

[1] Urieta-Mora, J.; García-Benito, I.; Illicachi, L. A.; Calbo, J.; Aragó, J.; Molina-Ontoria, A.; Ortí, E.; Martín, N.; Nazeeruddin, M. K. Improving the Long-Term Stability of Doped Spiro-Type Hole-Transporting Materials in Planar Perovskite Solar Cells. Sol. RRL 2021, 5 (12), 2100650.

[2] Urieta-Mora, J.; Choi, S. J.; Jeong, J.; Orecchio, S.; García-Benito, I.; Pérez-Escribano, M.; Calbo, J.; Zheng, L.; Byun, M.; Song, S.; et al. Spiro-Phenothiazine Hole-Transporting Materials: Unlocking Stability and Scalability in Perovskite Solar Cells. Adv. Mater. 2025, e05475.

[3] Khan, A. A.; Uceta, H.; Urieta-Mora, J.; Pérez-Escribano, M.; Abdollahzadeh, S.; Barrejón, M.; Calbo, J.; Ortí, E.; Langa, F.; Martín, N. Integration of p-type-doped carbon nanostructures as additives for boosting spiro-OMeTAD performance in perovskite solar cells. J. Mater. Chem. A 2025, 13 (47), 41260-41273.

Third-generation photovoltaic technologies, such as organic and perovskite solar cells, are attracting strong interest thanks to their tunable absorption, competitive efficiencies, and low-temperature, cost-effective fabrication. These advantages make them particularly promising for building-integrated photovoltaics (BIPV), where aesthetics and transparency are as important as energy generation. In this work, we present the development of a semi-transparent tandem photovoltaic module designed for window integration, combining a perovskite top cell with an organic bottom cell. The main challenge addressed is the optimization of optical and electrical coupling between the two sub-cells to maximize power conversion efficiency while preserving high transparency in the visible region. To improve light harvesting in the near-infrared range without compromising visible transmittance, a light management approach was implemented by integrating a near-infrared distributed Bragg reflector (DBR) on the organic bottom cell. Optical simulations were used to guide the design and integration strategy, resulting in a significant increase in the tandem current density. The tandem architecture was scaled to a 5 × 5 cm² device by connecting six organic cells in series for the bottom module and three perovskite cells in series for the top module, achieving excellent voltage matching with a mismatch below 1%. The resulting semi-transparent tandem solar module delivered a maximum power conversion efficiency exceeding 12%, with an average visible transmittance of 30%. In addition, it showed a light utilization efficiency (LUE) of 3.65 and a color rendering index (CRI) of 77, confirming its suitability for window-integrated applications. Finally, the transparent tandem modules were successfully integrated into a standard window, demonstrating a functional BIPV prototype.

Perovskite/CIGS tandem solar cells provide a compelling all–thin-film route toward high-efficiency, lightweight, and flexible photovoltaics. Despite rapid progress, their practical realization remains limited by complex interfacial optical losses, the intrinsic instability of wide-bandgap perovskites, and strain-induced degradation arising from heterogeneous tandem stacks.

In this talk, I will present recent advances from our laboratory that address these challenges through interface engineering, strain regulation, and multiphysics-guided device design. For four-terminal tandems, we identify parasitic absorption in transparent electrodes and interfacial reflection as dominant optical loss pathways, and demonstrate systematic mitigation strategies that enable highly efficient perovskite/CIGS tandems with minimal optical penalties. For monolithic two-terminal tandems, we establish a molecular-level understanding of passivation failure in wide-bandgap perovskites and develop thermodynamically and kinetically stable passivation strategies that suppress cation desorption and under-coordinated Pb defects. Furthermore, we reveal how the surface roughness of the CIGS bottom cell governs strain formation and defect evolution in the perovskite top cell, enabling strain-relieved tandem devices with certified stabilized efficiencies above 28%. Together, these findings establish general, physics-based design principles for efficient, stable, and scalable perovskite/CIGS tandem photovoltaics.

Significant advancements in research have been made in recent years, with single-junction organic solar cells achieving efficiencies exceeding 20%. However, scaling up laboratory prototypes to large-area commercial modules remains challenging due to the absence of high-quality thin-film deposition techniques, particularly for ultra-thin interfacial layers^[1]. Here, a fully vacuum-processed approach utilizing InCl₃ as a hole contact and C₆₀/BCP as an electron-contact interlayer, respectively, which act as dense and uniform charge transporting layers, while also ensuring consistent batch-to-batch reproducibility of module performance (PCE = 17.3%@15.6 cm²) ^[2]. To broaden the processing window of active layer for large-area modules, a seed crystal strategy by incorporating oligo (ethylene glycol)-modified asymmetric BDTF-CA2O molecule donors was proposed to optimize the nucleation and crystallization of PM6:BTP-eC9 blend, leading to an active area module efficiency of 17.7%^[3,4]. Further enhancement was achieved via the integration of a homogeneously bonded InCl₃ SAM monolayer, resulting in a record active‑area module PCE of 18.8%. When consider semi-transparent modules for energy-generating windows, NIR-absorbing PCE10 polymer is used instead of PM6 for achieving high visible transparency over 40%. By integrating strategies to reduce non-radiative recombination losses[5], a cost-effective double-layered nano-photonic structure, and a high-quality 12-nm-thin Ag top electrode, semi-transparent modules achieve 9.4% with AVT > 40%, and demonstrate excellent reproducibility^[6,7].

Wide-bandgap perovskite solar cells (PSCs) based on Br-rich and Br/Cl compositions enable the high photovoltages required for tandem photovoltaics, unassisted solar-driven water splitting, and emerging applications such as indoor photovoltaics (IPV) and building-integrated photovoltaics (BIPV). Full-bromide FAPbBr₃ represents a benchmark system, combining a large bandgap of 2.28 eV with compositional simplicity and intrinsic halide stability. Extending this class to mixed Br/Cl perovskites allows further bandgap widening but introduces challenges related to crystallization kinetics, optoelectronic losses, and halide redistribution. Across both systems, suppressing nonradiative recombination and interfacial energy losses is essential for high-performance devices.

We first report on full-bromide FAPbBr₃ PSCs developed for unassisted photovoltaic-electrochemical (PV-EC) water splitting. To meet the high voltage requirements imposed by thermodynamic and kinetic overpotentials, nonradiative recombination is minimized through dual passivation: formamidinium thiocyanate (FASCN) for bulk passivation and 1,3-propane diammonium iodide (PDAI₂) for surface passivation. This promotes larger grain growth, reduces interfacial recombination, and improves charge extraction. Further optimization of the electron-transport layer using a ternary fullerene blend (PCBM:CMC:ICBA) improves energetic alignment and suppresses interfacial losses, increasing open-circuit voltage (V_OC) from 1.41 to 1.60 V and yielding a power conversion efficiency of 9.4% in small-area devices. Scaled to a 1.0 cm² active area and integrated into a PV-EC system with Pt and RuO₂ catalysts, the device achieves continuous solar-driven water splitting with a solar-to-hydrogen (STH) efficiency of 6.5%, the highest reported for a single-absorber system.^[1]

Starting from full-bromide FAPbBr₃, we investigate mixed Br/Cl perovskites with chloride incorporation up to 80%. Structural and optical measurements confirm full chloride incorporation, resulting in lattice contraction and bandgap widening up to 2.85 eV, along with shifts in the valence and conduction band edges and altered film morphology. In situ UV-vis spectroscopy shows that higher chloride content retards crystallization, consistent with stronger Pb–Cl bonding. Mixed Br/Cl perovskites exhibit light-induced halide segregation, with remixing after dark storage. Time-resolved and absolute photoluminescence measurements indicate similar carrier lifetimes, but higher bandgaps yield increased quasi-Fermi level splitting alongside larger nonradiative voltage losses. Focusing on a 20% chloride composition, targeted bulk and surface engineering with small cesium incorporation and F-PEACl passivation effectively suppresses nonradiative recombination, hysteresis, and halide segregation, improving performance and stability. This approach is extended to 40% and 60% chloride devices.

Single-junction perovskite solar cells (PSCs) are nearing the maximum efficiency attainable with a single absorber layer. To exceed this limit, tandem cells are utilized. These architectures necessitate low bandgap materials, achieved by partially substituting lead(II) (Pb²⁺) with tin(II) (Sn²⁺) in the perovskite structure. In this study, we present the sequential thermal evaporation (sTE) of the low bandgap formamidinium lead-tin iodide (FAPb_0.5Sn_0.5I₃).[1] An alloy of SnI₂ and PbI₂ is prepared by heating and evaporated in vacuum, followed by the deposition of FAI. This layer-by-layer technique yields highly oriented, compact, and crystalline thin films with continuous grains averaging over 1 mm in size throughout the film thickness. Photoconductance measurements reveal mobilities exceeding 60 cm²/(Vs) and lifetimes surpassing 2 micros. Most interestingly structural analysis indicates that precursor interdiffusion readily occurs at room temperature, resulting in a mixed amorphous material. Complete crystallisation into the perovskite phase requires annealing at 200°C. In contrast to findings with pure lead perovskites, sTE of mixed lead tin perovskites allows the fabrication of 750 nm thick films in a single cycle. A comparison between the sTE films and spin-coated samples of the same composition shows the superior photoconductance of the sTE films without the need for any additives such as SnF₂. PSCs produced using this method, with the architecture ITO/PEDOT:PSS/FAPb_0.5Sn_0.5I₃/C60/BCP/Ag, reach efficiencies over 10%. Overall, this study highlights the potential of sTE in producing high-quality low bandgap perovskite materials and solar devices.

There is not yet a consensus on the most effective interface passivation technique to suppress non-radiative recombination and unwanted phase transitions in perovskite solar cells. A range of approaches have been demonstrated, including 2D/3D heterojunctions, organic cations, and molecular layers. However, identifying the most successful technique experimentally has proven challenging, given the difficulty in characterising the underlying atomistic mechanisms driving improvements. Herein, we exploit computational modelling techniques at a range of length-scales to overcome this barrier.

In this work, we apply finite-element device simulations to quantify the interfacial losses present in an industrially relevant perovskite-on-TOPCon architecture. We have developed a state-of-the-art perovskite-silicon tandem device model including transfer matrix method and raytracing optics, carrier tunnelling, and ion migration, coupled with a widely used drift-diffusion solver[1] to do so. We then move down the length-scales to ab-initio modelling of an experimentally demonstrated novel passivation scheme. This elucidates the atomistic mechanisms driving observed improvements in device performance and stability. Finally, we introduce the established passivation mechanism back into the device simulation to quantify the maximum possible performance improvement, setting a final target for the fully optimised interface passivation. A clear strategy for ameliorating the most critical interface identified by the device model hence emerges. Our methodology uniquely provides deep physical insight, whilst remaining broadly applicable to a range of device architectures.

Motivation: A central question in perovskite solar cell (PSC) stability research is whether bias-sweep instability can be explained solely by ion migration, or whether additional intrinsic mechanisms contribute under realistic operating conditions. We demonstrate that the coupled interplay between ion migration, self-heating, and the internal electric field in PSCs opens a distinct non-isothermal instability signature, which we term thermal hysteresis [1]. Relaxing the quasi-isothermal assumption commonly adopted in mixed electronic-ionic transport models can change not only the magnitude of hysteresis, but also its mechanistic origin and timescale.

Methodology: We develop a self-consistent optical-electrical-ionic-thermal multiphysics framework that resolves layer- and interface-resolved heat-generation mechanisms across the full device stack [1]. We track spatiotemporal hotspot footprints during bias sweeps, capturing where dissipation localizes and how ionic screening redistributes it. Moving beyond conventional J–V analysis, we introduce temperature–voltage (T–V) and heat–voltage (P–V) diagnostics that capture scan-rate-dependent thermal hysteresis and identify its dominant physical contributors.

Results: PSCs develop internal thermal inertia on timescales comparable to ionic relaxation during bias sweeps. At intermediate scan rates, coupled thermo-electro-ionic interactions produce non-monotonic temperature evolution, characterized by dual-peak forward-scan profiles and pronounced T–V hysteresis. This behavior is consistent with experimental evidence showing that temperature gradients can trigger ion-transport-mediated chemical instability in halide perovskites [2]. The dual-peak profile arises from a bulk-to-interface dissipation crossover, while rapid sweeps show efficient heat dissipation because of short thermal dwell time. Neglecting this interaction underestimates transient temperature rises by more than 10 K and misidentifies the scan-rate window associated with S-shaped distortions. Scan-rate-resolved analysis further reveals that ionic screening redistributes internally generated heat toward power-dissipation regions, concentrating losses near interfaces and within transport layers. This localization leads to hotspot formation even under uniform illumination, indicating that ion migration simultaneously screens the internal electric field and modulates the self-heating effects. As a result, Joule heating becomes strongly confined to transport-layer Debye regions adjacent to the perovskite absorber, forming nanometer-scale hotspots and making a nominally bulk loss channel effectively interfacial.

Conclusion and significance: Thermal hysteresis is an operando fingerprint of thermo-electro-ionic dynamics that cannot be inferred from conventional J–V characteristics. The study provides design guidance for interface and transport-layer engineering aimed at mitigating hotspot formations and enhancing the operational stability of PSCs. More broadly, it motivates the explicit inclusion of T–V alongside J–V characteristics as operando stability diagnostics that distinguish ionic-driven hysteresis from thermo-ionic instability pathways. Finally, it provides an initial roadmap for operando experiments to directly measure thermal hysteresis and hotspot precursors.

Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is increasingly used in perovskite (PVK) solar cell research for its high chemical specificity and ability to probe buried interfaces without invasive sample preparation [1]. However, we show that acquisition and interpretation of ToF-SIMS data from multilayered PVK solar cells should not be conflated with that of its thin films [2,3]. Here, we present a rigorously substantiated ToF-SIMS methodology that separates true ion migration from artifact-induced signals, revealing widely underappreciated measurement artifacts. Through systematic comparison of thin films and full devices of archetypal MAPbI₃ (1.6 eV) and compositionally complex (FA₇₅Cs₂₅)(Pb₆₀Sn₄₀)I₃ (1.25 eV), we show that spurious ion gradients arise exclusively in multilayer stacks, driven by top-layer interactions. We confirm their measurement-induced origin using a controlled peeling protocol designed to isolate these effects. We further introduce a novel fluence-matched acquisition protocol and the first statistically grounded workflow for replicate-based analysis—demonstrating that single-profile measurements can lead to contradictory interpretations, particularly in studies targeting subtle interfacial phenomena by trace passivation. Applying this framework, we enable reliable detection and aging analysis of buried self-assembled monolayers (SAMs), whose ultrathin (~1 nm), localized nature presents unique analytical challenges and growing importance in the design of molecular selective layers for high-efficiency, stable devices. Together, these findings establish a reproducible, artifact-aware framework for high-fidelity ToF-SIMS depth profiling in PVK solar cells and provide a reference standard for reliable chemical analysis of hybrid multilayer semiconductors. This work advances best practices in nanoscale characterization and in doing so supports precision interface engineering across next-generation energy technologies and beyond.

Keywords: Perovskite solar cells; ToF-SIMS; Depth profiling; Interface characterization, Ion migration; Self-assembled monolayers; Measurement artifacts; Energy materials.

Solar cells are among the most promising renewable energy technologies today. Therefore, a detailed understanding of the dynamics and energetics of photon-to-electron conversion within the solar cell is essential for developing higher-efficiency devices. Since these processes occur on ultrafast timescales, time-resolved measurements are required to directly study the underlying mechanisms within the solar cell to identify factors that might limit device efficiency.

In this presentation, I will show how we employ time-resolved X-ray photoelectron spectroscopy (PES) to probe the ultrafast dynamics of charge-carrier generation and recombination. The method is based on a pump–probe scheme in which a visible laser pulse, operating at a lower repetition rate, excites the sample, while synchronized X-ray pulses at a higher repetition rate probe the resulting excited states. By systematically varying the time delay between the laser and X-ray pulses, the rapid generation of charge carriers (photovoltage) can be resolved with picosecond resolution. In addition, the combination of a high X-ray and low laser repetition rate enables sampling over longer timescales, allowing both the rise and decay of the photovoltage to be measured within a single experiment.[1,2]

This approach allows investigation of dynamics spanning from picoseconds to microseconds. By modifying the sample architecture, we can directly study how individual layers influence charge transport within the device.

We have applied this technique to AgBiS₂ quantum dot solar cells[3] to investigate how different ligand choices and combinations affect charge transport. Four samples with different active-layer structures were studied: (I) three quantum-dot layers using EDT as the ligand, (II) three layers using TBAI, (III) one EDT-treated layer followed by two TBAI-treated layers, and (IV) the reverse sequence, with two TBAI-treated layers followed by one EDT-treated layer. The results show pronounced differences in charge-carrier separation and recombination dynamics, leading to varying photovoltage dynamics depending on the architecture. These findings provide insight into how (and why) ligand selection affects device efficiency.

More generally, this measurement technique is applicable to a wide range of device types, provided the samples are stable under illumination, offering a powerful tool for gaining a deeper understanding of ultrafast processes in different solar-cell technologies.

The lab-scale power conversion efficiency of organic solar cells has recently breached the 20% mark thanks to the introduction of Y-series acceptors. Key to these advancements is the efficient exciton separation at low energy level offsets between acceptor exciton (S_1,A) and interfacial charge transfer state (CT) as well as barrierless separation of the interfacial CT states.[1],[2] While the origin of the efficient charge generation has been widely investigated, a mechanistic understanding of the free charge recombination kinetics and energy level offset related voltage losses remains elusive. For example, previous studies concluded that despite the low energy offset between S_1,A and CT, and the masking of the CT emission and absorption spectra by the S_1,A, the majority of free charge recombination proceeds through interfacial CT states.[3] Yet the CT state properties and non-radiative recombination mechanisms are difficult to assess from the optical spectra complicating the voltage loss analysis.

Here, I show transient optoelectronic (charge extraction and transient photovoltage) results that enable us to study the lifetime of free charges as a function of the charge carrier density and quasi-Fermi level splitting. Such an approach is particularly useful for low-offset Y-series blends where the CT properties are difficult to assess experimentally. In an interfacial CT state mediated recombination model, the lifetime is expected to crucially depend on the interfacial area[4] and energetics that govern the thermal equilibrium between CT states and free charges[5]. Contrary to that, I present evidence that the lifetime of free charges in polymer:Y-acceptor blends is independent of the interfacial area, charge carrier density or CT state energy. Independent of these factors, the free carrier lifetime is solely determined by the quasi-Fermi level splitting. I highlight the difference to the expected CT recombination model by comparison of different planar and bulk heterojunction systems,⁶ morphology dependent electrostatic energy level shifts,⁷ and variation of the interfacial energetics. To rationalise our experimental findings, an analytical model reproducing experimental trends will be presented. The model suggests that free charge recombination in high efficiency polymer:Y-acceptor blends is limited by the back hole transfer to the Y-acceptor and subsequent recombination through acceptor excitonic states – that we assign to dark triplet states due to the low emission yields.  Since I conclude that acceptor mediated – rather than CT state mediated – presents the bottleneck of free charge recombination in these polymer:Y-acceptor blends, addressing molecular properties of the Y-acceptor such as the (triplet) exciton energy and lifetime are key to reducing the free charge recombination related voltage losses in high performance, low-offset polymer:Y-series blends to realise their full potential.

Over the last years, metal chalcogenide-based nanocrystalline compounds were shown to be promising candidates for various optoelectronic application and solar cells in particular. They provide the unique opportunity to tune the band gap and other properties of a specific material through changes in the particle size or through surface treatment and passivation. This adaptability can result in making compounds available or further optimizing them for certain applications they would otherwise be less suitable for.[1] Two prominent examples of this material class that have been successfully utilised as solar cell absorber layers are PbS and AgBiS₂, resulting in device efficiencies exceeding 10% and 8%, respectively.[2,3]

Nevertheless, developing new material platforms around non-toxic and earth-abundant materials is still crucial for optimal resource management. With that in mind, NaBiS₂ nanocrystals have been identified as an interesting and promising candidate due to its high absorption coefficient, stability and a suitable band gap.[4] However, its implementation into solar cell devices up to this point has proven to be challenging, but at the same time still remains a largely unexplored topic.[5,6]

Therefore, the main focus herein is to investigate potential reasons preventing NaBiS₂ nanocrystals from being applied more successfully in solar cells thus far, as well as to explore strategies to overcome these issues. We have found that the conductivity of the nanocrystal film and its passivation remains a limiting factor, properties that are closely tied to the ligands employed in the nanocrystalline system. Results from X-ray photoelectron spectroscopy indicate that ligand exchange with commonly employed passivation agents, such as TBAI, does not occur as readily, compared to similar systems like AgBiS₂. Further testing revealed that for NaBiS₂ nanocrystals, it is rather the presence of a suitable inorganic cationic passivation component that is crucial in determining the resulting surface and overall chemistry of this system after the ligand exchange. Consequently, functional devices could be fabricated, while the new found insights regarding passivation and chemical modification in this material system will hopefully provide a starting point for further optimisation and improvement.

Achieving efficient and stable solution-processed infrared absorbers is critical for next-generation photovoltaic technologies targeting tandem solar energy harvesting. III-V semiconductor colloidal quantum dots (CQDs) offer a pathway toward heavy-metal-free infrared photovoltaics. With its narrow bandgap and large exciton Bohr radius, Indium antimonide (InSb) is an ideal material to facilitate bandgap tuning in the short-wave infrared region. However, controlled synthesis has been hindered by poorly managed precursor reactivity, resulting in broad size distributions and inefficient light harvesting diodes. In this work, we discuss our approaches to regulate the reactivity of the In and Sb precursors together with ligand choice and organo-metallic additives can lead to size distributions of less than 8% and corresponding well-defined excitonic absorption features in the range of 0.9-1.5 um.

We further show that controlled surface passivation is critical for forming electronically homogeneous CQD solids with suppressed trap densities and balanced charge transport. We found that as-synthesized CQDs contain oxide-like species that complicate ligand exchange and device integration; to address this, we apply thiol-based resurfacing to effectively remove native ligands and oxide species, facilitating subsequent ligand exchange to short organic and inorganic ligands. Metal-halide-exchanged CQD films exhibited low trap-state densities as a result of the reduced surface oxides. Photodiode structure exhibited peak external quantum efficiencies in excess of 30 % in the SWIR region. Devices robust operational stability, retaining the majority of initial performance after extended bias and illumination [1,2].

This work highlights that tailored surface chemistries can concurrently reconstruct and passivate CQD surfaces, addressing long-standing challenges in surface chemistry, and advancing heavy-metal-free CQDs toward stable, high-efficiency SWIR photovoltaics.

Motivated by the development of solar cells based on non-toxic materials, AgBiS₂ has emerged as a promising absorber for next-generation sustainable photovoltaics. Although AgBiS₂ nanocrystal (NCs) based devices have shown significant progress, their fabrication routes typically relies on solvent-intensive and multistep processing routes [1,2,3]. These approaches involve (i) the synthesis of colloidal nanocrystals with well-controlled sizes below 10 nm, (ii) deposition of nanocrystals thin film, and (iii) repeated ligand-exchange treatments to promote electron coupling between nanocrystals. Such a complex procedure generates significant chemical waste, time, and limits scalability, reproducibility, and manufacturing compatibility.

In this presentation, I will introduce perovskite inspired, one-step solution-processing strategy for the fabrication of AgBiS₂ thin films that avoids complex nanocrystal routes and reduces the complexity and waste. I will first discuss the materials design and crystallization mechanism, followed by the structural, optical, and electronic properties of the resulting films.

This method consists of a mixed precursor solution of silver nitrate AgNO₃, bismuth nitrate Bi(NO₃)₃, and Thiourea in dimethylformamide, which is deposited onto the SnO₂ layer by spin-coating and annealed at 240 ^oC for 2 min [4]. This process yields ~280 nm thick film composed of AgBiS₂ with well-connected grains. When integrated into SnO₂/AgBiS₂/P3HT architecture solar cell and with a sustainable surface treatment, the devices deliver efficiency of 6.3 % with a high photocurrent density of 39.2 mA cm^-2, representing the highest reported to date for AgBiS₂ films prepared via a simple solution-based process. Additionally, both the AgBiS₂ films and devices exhibit a good structural and operational stability, with no detectable phase degradation confirmed by XRD, and stable performances under maximum power point tracking for both short-term (40 min) and extended (24 hours) continuous illumination. These results highlight the potential of AgBiS₂ as a sustainable, solution-processable absorber for environmentally friendly solar cells.

A major challenge for the practical application of perovskite solar cells (PSCs) is their limited operational stability. In n–i–p device architectures, all state-of-the-art PSCs with high power conversion efficiencies (PCEs) currently rely on the benchmark hole transport layer (HTL) Spiro-OMeTAD, which is conventionally doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (tBP). However, these dopants substantially compromise device stability. Furthermore, the complex in situ oxidation processes associated with conventional Spiro-OMeTAD doping obscure the underlying mechanisms, thereby hindering the rational design of stable, high-efficiency HTLs.

Here, we introduce a clean, post-oxidation-free doping strategy for Spiro-OMeTAD based on stable organic radicals as dopants and ionic salts as dopant modulators, termed ion-modulated (IM) radical doping. In this approach, the organic radicals directly generate hole polarons, resulting in an immediate enhancement of conductivity and work function, while the ionic salts further tune the work function by modulating the energetics of the hole polarons. Previously, PSCs employing IM radical-doped Spiro-OMeTAD achieved high PCEs with excellent stability, exhibiting T80 lifetimes of approximately 1200 h under 70 ± 5% relative humidity and 800 h at 70 ± 3 °C without encapsulation, effectively mitigating the trade-off between efficiency and stability. By further optimizing the dopant system, we demonstrate a significant enhancement in the thermal stability of the Spiro-OMeTAD layer, which remains stable at temperatures up to 85 °C. Moreover, the resulting HTL effectively suppresses Au migration into the perovskite layer, further contributing to improved device stability.

Although stability of perovskite solar cells (PSCs) has improved remarkably in recent years, with several studies reporting 1000-2000 hours of stability [1,2] under continuous operation, we still see cases that suffer a significant reversible drop in performance. We believe this phenomenon is also observed by others. In general, we found that perovskite of same composition (FAPbI₃) but different thickness shows a dramatic difference in the stability, with the thin (~400 nm) perovskite films showing way slower decay than the thick (~700 nm) films. The stability was also influenced by the annealing temperature (100oC or 150oC) which alters the preferred orientation in the FAPbI3 films.  In all cases, photocarriers directly influence the performance decay, evident from the stability tested under light of different intensities. We understand that the main cause of such performance decay is unbalanced carrier extraction between holes and electrons, depending on the electron and hole extraction materials, that leads to screening of field in perovskite which, in turn, results in poor conduction of carriers in perovskite. Electron beam induced current (EBIC) measured on fresh and stressed devices showed weakening of built-in field (depletion zone) in the cells that were operated at maximum power point for several hours. Although it is believed that ion migration to contacts/interfaces generate an ion-induced field that screens the built-in field and thus slows down the carrier extraction causing the performance loss, elemental mapping on devices before and after continuous operation in our case do not show any clear evidence of ion migration causing such fast and reversible performance decay. Instead, such reversible degradation of performance was found to have a strong correlation with carrier density and conduction in perovskite, which depends on the process by which it is made. More details on how the processing condition affects the perovskite composition and its interfaces and eventually its stability will be discussed during the talk.

Advancing Stability in Perovskite Solar Cells through Mechanistic Spectroscopy and Charge-Transport Engineering

Alexander R. Uhl*
Laboratory for Solar Energy & Fuels, University of British Columbia, Kelowna, BC, Canada
*alexander.uhl@ubca.ca

Operational instability in perovskite solar cells (PSCs) remains a central challenge for their technological deployment, with mobile ions playing a pivotal role in both performance losses and long-term degradation. In this contribution, I present two complementary studies from my group that address ion-related instabilities from both a fundamental diagnostic perspective and a materials and device engineering approach.

In the first work, we investigate ion-induced degradation beyond conventional recombination losses, focusing on charge collection failures that lead to reduced short-circuit current density, pronounced current–voltage hysteresis, and anomalous low-frequency features in impedance spectra. We introduce a physical modelling framework that differentiates between degradation pathways driven by mobile ions and enables a unified interpretation of electrical ageing phenomena. A central outcome is the identification of a double inductor response in the low-frequency impedance region, which emerges as a distinctive fingerprint of coupled recombination and charge-collection limitations. Analysis of the associated time constants reveals how their interplay governs the evolution of impedance responses and current–voltage characteristics during ageing, providing a robust spectroscopic marker of dominant instability mechanisms in PSCs. ^[1] 

In the second work, we address ion-related interfacial instabilities through the development of a low-temperature, ink-based electron transport layers (ETL) based on ultra-small SnO₂ quantum dots synthesized under ambient conditions. By optimizing the chemical composition and formulation of SnO₂ quantum dot inks, we achieve controlled thin films with favorable electronic properties and improved interfacial contact to the perovskite absorber. PSCs incorporating the optimized SnO₂ quantum dot ETLs exhibit enhanced power conversion efficiency and significantly improved operational stability compared to devices employing conventional SnO₂-based transport layers. ^[2]

Together, these two studies demonstrate how advanced electrical diagnostics can identify ion-driven failure modes, while tailored interface engineering can mitigate their impact. This combined approach provides both mechanistic insight and practical strategies for improving the performance, reliability, and scalability of next-generation perovskite solar cells.

References:

^[1] E. Ghahremani Rad, E.H. Balaguera, A.T. Gidey, J. Bisquert, A.R. Uhl, Tracing the Path of Ion-Induced Degradation with Combined Action in Recombination and Charge Collection for Perovskite Solar Cells, 2026, under review. 
^[2] A.T. Gidey, E. Ghahremani Rad, S. Suresh, A.R. Uhl, Room-Temperature Synthesis of Ultra-Small Sulfur-Free SnO₂ Quantum Dots as Electron Transport Layer in Perovskite Solar Cells, 2026, under review.

Transitioning global energy production away from carbon-emitting sources toward renewable technologies is one of the foremost issues faced by the scientific community. Given the vast amount of solar energy incident on the planet, photovoltaic technologies will need to play a crucial role in decarbonization. To this end, metal halide perovskite solar cells (PSCs) have emerged as promising candidates for next-generation commercial photovoltaics, primarily thanks to the exceptional power conversion efficiencies (PCE) achieved (>27%).

Despite impressive performance improvements, several key challenges remain in the field of PSC, spanning both regular (n-i-p) and inverted (p-i-n) architectures, with further efficiency gains still achievable through interfacial and bulk defect management. Additionally, stability remains a key concern for the commercialization of this technology.

To address outstanding challenges in the field, an international multidisciplinary collaborative effort between Imperial College London and City University of Hong Kong has, during the past several years, spearheaded the development of highly tunable ferrocene-based organometallic compounds for PSC applications. The oral presentation proposed herein will focus on the directed compound design, from chemistry to device applications, of organometallic species for solar cell applications.

Our work on the n-type interface in inverted architectures began with a landmark paper in Science, exploiting a substituted ferrocene core (FcTc₂) to achieve record-breaking efficiencies and performance. This work was followed up by highly efficient and scalable multi-ferrocene Fc₂Tc₂ and Fc₃Tc₂ structures (JACS) and, more recently, by varying the side arms on the ferrocene core (Angewandte). This has allowed us to draw structure-function-efficiency relationships from the chemistry to the device application in these systems.

Recently, we have explored several new avenues exploiting the chemical tunability, electrochemical properties, and interchangeable oxidation states of ferrocene materials for applications in self-assembled monolayers, organic photovoltaics, and organic semiconductor doping, respectively. In n-i-p perovskite solar cells, convenient and highly tunable ferrocenium compounds were synthesized and employed as Spiro-OMeTAD dopants to achieve state-of-the-art power conversion efficiencies and stabilities (Joule, EES). Clear design guidelines for next-generation organic semiconductor dopants were drawn to inspire future development in the field. In lead-free tin-based perovskite solar cells, we developed a novel two-step on-surface self-assembled monolayer synthesis using conjugated molecular wire contacts. In organic solar cells, we exploited the redox-potential tunability of ferrocene centres to dope common n-type electron transport materials, clearly analysing the impact of ferrocene electrochemistry on performance, ultimately achieving power conversion efficiencies >20%.

In this talk, the story of organometallic compound development for solar cell applications will be outlined, and key translatable structure-property-performance relationships drawn from the wealth of work conducted will be provided.

Accepted Unpublished:

F. Vanin, .... Zonglong Zhu, Saif Haque, Nicholas J. Long, Joule, 2026

F. Fang, .... F. Vanin, Saif Haque, Maxie Roessler

In recent years, perovskite solar cells have attracted considerable interest due to their high efficiencies (over 27%), lightweight and independence from light quality[1]. However, these cells are sensitive to external conditions such as moisture and oxygen[2], which limits the deployment of this technology on a larger scale. To prevent this degradation, some research has focused on charge transport layers in order to improve their interface with perovskite and act as a protective layer for the latter, thereby improving its stability [3].

In this context, our project aims to developp cross-linkable polymer-based electron transport layers in order to improve the stability and the lifetime of perovskite solar cells. To do so, we designed different molecules based on an alternation of perylene diimide or naphthalene diimide structures (strong electron acceptor units) with a non-conjugated spacer within the chain backbone, instead of lateral chains, to improve the solubility and limit the visible absorbance of our materials.

These materials present good thermal resistance and energy levels that are compatible with those of the active layer. We will show that these materials can be deposited onto the perovskite layer and how the acceptor unit and the chain length influence their physico-chemical properties and photovoltaic characteristics of the developed devices. Interestingly, this new class of materials presents a high structuration leading to decent conductivity. Finally, the integration of reactive end-chains to cross-link these materials to make them more resistant to external conditions while maintaining good electron extraction capacity, will be presented.

Hole-collecting monolayers (HCMs) for inverted perovskite solar cells have attracted significant attention because they offer superior stability, conductivity, and transparency compared with conventional polymer-based hole-collecting materials such as PTAA and PEDOT:PSS. By incorporating HCMs, a power conversion efficiency (PCE) of 26.9% has been achieved for single-junction solar cells[1]. To further improve HCM performance, elucidating the energy-level alignment at the electrode/HCM/perovskite interfaces is crucial. Efficient hole collection cannot be realized without proper energy-level alignment, even when other requirements—such as film uniformity and defect-passivation capability—are satisfied. Several models, including vacuum-level alignment, Fermi-level alignment, and Schottky-type models, have been proposed to explain and predict energy-level alignment; however, they are often applied selectively to rationalize one’s own photovoltaic performance, and no consensus model has established within the community. Therefore, developing an appropriate model remains essential.

In this study, we propose an energy-level alignment model for electrode/HCM/perovskite interfaces. We find that the model is consistent with HCM-dependent photovoltaic performance in terms of hole-collection efficiency and electron-blocking capability. In our framework, the electrode/HCM/perovskite interface is separated into two interfaces: electrode/HCM and HCM/perovskite. The electrode/HCM interface is treated in terms of interface dipole formation, whereas the HCM/perovskite interface is described by a semiconductor heterojunction model[2].

To demonstrate the validity and generality of the proposed model, we first tested it using three representative carbazole-derived HCMs—2PACz, MeO-2PACz, and 3PATAT-C3[3]. We precisely determined the energy parameters in the solid states of these HCMs and a mixed-cation perovskite by ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoelectron spectroscopy (LEIPS). Reliable energy parameters obtained under controlled conditions enable rigorous validation of the model. We then assessed the model by comparing photovoltaic performance with hole-collection efficiency and electron-blocking capability predicted from the derived energy-level alignment. This comparison shows that our model for electrode/HCM/perovskite interfaces explains the experimentally measured photovoltaic performance, whereas the three earlier models could not. To further test the model’s universal applicability, we evaluated a broad range of HCMs, including Me-PhpPACz, Br-2EPT, MeO-BTBT, 4PADCB, ID-Cz, Py3, MPA-BT-XA, and 4-XPBA, using photovoltaic and energy parameters reported in the literature. These results confirm the model’s universality for electrode/HCM/perovskite interfaces.

This study enables prediction of HCM photovoltaic performance in terms of energy-level alignment and provides guidelines for the design and selection of HCMs for high-efficiency perovskite solar cells.

Inverted (p-i-n) perovskite solar cells (PSCs) employing carbon-based electrodes offer a promising pathway toward scalable, low-cost, and operationally stable photovoltaics; however, their performance is frequently constrained by interfacial losses at the electron-selective contact. Here, we report high-efficiency for such devices, enabled by an engineered PCBM/SnOₓ interface combined with carbon electrode lamination, fully compatible with low-temperature and vacuum-free top-contact processing. A thin interlayer of ethoxylated polyethylenimine (PEIE) is introduced on top of PCBM as a permanent interfacial dipole, facilitating uniform SnOₓ nucleation during atomic layer deposition (ALD) and enhancing charge extraction at the perovskite/ETL interface.

A systematic comparison of different ALD SnOₓ processes reveals that devices fabricated without PEIE exhibit pronounced S-shaped current–voltage characteristics and limited fill factors, indicative of non-ideal interfacial energetics and injection barriers. Incorporation of PEIE effectively suppresses this, resulting in improved charge carrier extraction, reduced interfacial recombination, and enhanced electron selectivity. Under simulated AM 1.5G illumination (1 sun), optimized devices achieve power conversion efficiencies (PCE) exceeding 20.5%, with high open-circuit voltages and fill factors reproducibly maintained across multiple fabricated cell batches. This performance approaches that of silver-based reference devices (PCE ≈ 22%), where the remaining efficiency difference is primarily attributed to a higher short-circuit current density due to enhanced reflection by the silver layer rather than inferior interfacial quality in the carbon-based architecture.

Notably, the same device design exhibits excellent performance under low-light conditions relevant for indoor photovoltaics. At an illumination level of 500 lux, the power conversion efficiency reaches 32.4%, accompanied by negligible shunt leakage. In contrast, the PSC reference employing a metal electrode achieves a PCE of only 27.8%, primarily due to pronounced losses in open-circuit voltage (V_OC) and fill factor (FF). The enhanced fill factor of CPSC under low-light operation is directly linked to the PEIE/ALD SnO_X combination, which improves the ETL layer uniformity, lowers the effective electron extraction barrier and suppresses space-charge-limited transport at reduced carrier densities, thereby stabilizing diode behavior under weak illumination. The combination of dipole-assisted interfacial engineering, ALD-compatible processing, and laminated carbon electrodes establishes a robust strategy for high-performance, low-light-optimized carbon-based p-i-n perovskite solar cells.

Metal halide perovskite solar cells (PSCs) are rapidly progressing toward commercialization. Among the various device configurations, the p–i–n (inverted) architecture has emerged as particularly promising due to the very high record efficiencies achieved with this design[1], its compatibility with conventional silicon solar cells for tandem applications, and the elimination of expensive and relatively unstable doped hole transport layers such as Spiro-OMeTAD[2]. The charge transport layers, which represent the primary distinction between the normal and inverted architectures, offer significant potential for further improvement and optimization owing to the wide range of semiconducting materials available. In this work, we investigate both charge transport layers.

The benchmark electron transport material (ETM) for inverted PSCs is C₆₀/PCBM, a fullerenic carbon allotrope or its soluble derivative. While these materials are reliable and enable highly efficient devices, non-fullerene alternatives may offer enhanced stability due to closer molecular packing, which can provide improved barrier properties. Naphthalene diimide (NDI) has been widely used as a building block in organic semiconductors and represents a promising ETM for PSCs owing to its strong electron-accepting characteristics, high tunability—with or without bandgap modification—and relatively low synthesis cost[3]. We synthesized several NDI derivatives with different solubilities via one-step condensation reactions, characterized them electrochemically and thermally, and fabricated PSCs employing these non-fullerene ETMs.

In addition, the selection and deposition of the hole transport material (HTM) in inverted PSCs are strongly influenced by the choice of transparent conductive substrate. For large-scale commercialization, fluorine-doped tin oxide (FTO) offers a key advantage over indium tin oxide by avoiding reliance on the critical and costly resource indium. However, the deposition of self-assembled monolayers (SAMs) as HTMs on FTO is challenging due to the high surface roughness[4] and the absence of indium required for the formation of In–O–P bonds with common SAM anchoring groups[5]. To address these challenges, we optimized both the SAM deposition process and molecular selection for FTO substrates, and compared the device performance of SAM-only HTMs with that of combined SAM and nickel oxide layers.

Next-generation photovoltaics (PV) systems provide key opportunities for manufacture and commercialization. Hybrid lead halide perovskite solar cells (PSCs) represent an attractive system as they combine high efficiency with inexpensive, abundant materials and low-temperature and solution-based processing methods. Among the various perovskite solar cell architectures, the carbon-based, triple-stack perovskite solar cell (C-PSC) is a promising candidate for upscaling and commercialization, employing a fully printable architecture, and allowing manufacture at low capital cost using screen-printing technology.

One of the most attractive aspects of this technology is its inherent better stability against moisture related to the hydrophobic character of the carbon top layer [1]. However, until recently [2] the record efficiency has been much lower that conventional PSCs, specifically due to a much lower open circuit photovoltage and generally lower fill factor. Several reasons include the non-selectivity of the carbon top layer, interface recombination related to the nanostructured morphology, and the complexity of infiltration of optimized perovskite precursors.

In our group, we are performing research on several aspects of materials design, device optimization and scale-up: (i) design and synthesis of new materials based on green chemistry principles to improve sustainability; (ii) enhancing performance by implementation of improvement strategies, included interfacial modifications at both the electron and hole extraction contacts; (iii) advanced characterization methods including impedance spectroscopy and light intensity-modulated photocurrent and photovoltage spectroscopy (IMPS/IMVS), and optimization strategies using machine learning; (iv) up-scaling to mini-module size (200 cm²); and (v) long-term module monitoring under outdoor conditions.

In this presentation, four aspects of recent work will be presented: (i) effect of humidity on the performance of the TiO₂ electron extraction layer using anatase, brookite and rutile-based solar cells [3]; (ii) increase of the solar cell efficiency by incorporating a hole-selective NiCo₂O₄ printed layer between the zirconia and (non-selective) carbon layer [4]; and (iii) analysis of solar module performance under harsh, tropical outdoor conditions as a function of climatological parameters (manuscript submitted).

Solar photovoltaics (PV), coupled with large-scale energy storage, will be central to achieving global net-zero targets. With global PV deployment projected to grow from ~1 TW (2022) to ~75 TW by 2050, next-generation technologies must deliver higher efficiency, lower cost, and reduced carbon footprint to meet climate goals aligned with the Paris Agreement.

Metal-halide perovskites offer tunable bandgaps and exceptional optoelectronic properties, enabling tandem architectures with theoretical efficiencies beyond 45%. Industrial R&D has already demonstrated 34.9% efficiency at 1 cm² and >30% on commercial wafer sizes. At NCPRE, IIT Bombay, we have achieved 30.2% efficiency at lab scale and are actively scaling toward commercial wafer formats through our venture, ART-PV India. This talk will discuss the scientific foundations, device engineering strategies, and scalable pathways enabling high-efficiency tandem photovoltaics for energy transition.

Wide-bandgap perovskite solar cells have attracted increasing attention for high-voltage photovoltaic applications, with methylammonium lead trichloride (MAPbCl₃) as a promising candidate. Nevertheless, the performance of MAPbCl₃-based solar cells remains limited by poor film quality, high defect densities, and severe non-radiative recombination losses, which limit voltage output, reproducibility, and long-term stability. While previous approaches, such as methylammonium chloride (MACl) vapor annealing, have demonstrated improved crystallinity and defect reduction, their reliance on prolonged thermal treatment raises concerns regarding scalability and process reproducibility. (1-2)
In this work, we present a phenylethylammonium chloride (PEACl) surface passivation strategy as an efficient and scalable alternative for defect passivation in MAPbCl₃ perovskite solar cells. Unlike vapor-assisted annealing methods, the PEACl treatment is implemented through a rapid, solution-based process, enabling precise interface control without extended annealing steps. This approach leads to substantial improvements in film morphology, including enhanced grain growth, stabilized grain boundaries, and a notable reduction in surface and interfacial defect states.
Comprehensive structural, optical, and electrical characterizations reveal that PEACl passivation effectively suppresses non-radiative recombination pathways and extends charge-carrier lifetimes, while maintaining the integrity of the MAPbCl₃ crystal lattice. In particular, the treatment minimizes metallic lead species at the surface and reduces halide vacancy-related trap states, resulting in improved charge extraction and reduced voltage losses. Statistical evaluation across multiple devices demonstrates that PEACl-treated solar cells exhibit significantly improved reproducibility compared to both pristine and MACl-treated counterparts, with simultaneous enhancements in open-circuit voltage, short-circuit current density, fill factor, and overall power conversion efficiency.
The performance gains are most evident in the voltage characteristics, where devices incorporating PEACl-treated MAPbCl₃ layers consistently deliver open-circuit voltages approaching 1.7 V, underscoring the effectiveness of this passivation strategy in suppressing recombination-induced voltage losses. Although MACl-treated devices can reach similar peak voltages, their response is notably less consistent, with increased variability that highlights the advantage of the PEACl-based approach. Beyond efficiency, PEACl passivation plays a decisive role in improving device stability: under ambient storage and operational conditions, PEACl-treated devices maintain stable voltage output with minimal fluctuation, while pristine and MACl-treated counterparts show pronounced instability and frequent voltage collapse, indicative of interfacial degradation. Steady-state measurements further confirm that PEACl-treated devices sustain high voltage output over extended operation times, reflecting enhanced resistance to degradation pathways. Surface wettability test supports these observations, revealing a marked increase in hydrophobicity after PEACl passivation, which implies reduced surface energy and improved resistance to moisture ingress, directly contributing to the enhanced operational stability and emphasizing the importance of surface chemistry control in wide-bandgap perovskite devices.
Overall, this study establishes PEACl surface passivation as a practical and reproducible strategy for overcoming key limitations of MAPbCl₃ perovskite solar cells. By simultaneously improving voltage performance, device uniformity, and environmental stability, this approach strengthens the prospects of wide-bandgap perovskites for next-generation photovoltaic technologies, including tandem architectures, building-integrated photovoltaics, and low-power energy applications.

Reducing non-radiative recombination at the interfaces between metal–halide perovskite absorbers and adjacent charge-selective transport layers is critical for improving the open-circuit voltage (Voc) of perovskite solar cells. In this work, a mixed passivation–insulating interlayer is introduced at the perovskite/electron-transport interface, forming a multifunctional interface that suppresses interface-induced non-radiative recombination and enhances Voc in narrow-bandgap tin–lead perovskite solar cells. In addition, a similar interfacial treatment is shown to reduce non-radiative recombination at the perovskite/hole-transport interface. This double-sided interfacial passivation strategy significantly reduces voltage losses, achieving a voltage deficit as low as 355 mV and increasing Voc to 91.5% of the detailed balance limit. As a result, narrow-bandgap devices reach a power conversion efficiency of 22.6%. Furthermore, all-perovskite tandem solar cells incorporating the modified interfaces achieve a Voc of 2.16 V and a power conversion efficiency of 26.1%. These results demonstrate that engineering mixed passivation–insulating interlayers at perovskite/transport layer interfaces is an effective approach to minimising voltage losses and advancing the performance of perovskite solar cells.

Mitigation of Current Mismatch Limitations and Spectral Variation Losses Using Three-Terminal Perovskite–Silicon Tandem Solar Cells

Mohammad Gholipoor^1,2, Michael Rienaecker³, Xuzheng Liu^1,2, Seyedamir Orooji^1,2, Lingyi Fang^1,2, Paul Fassl^1,2, Renjun Guo^1,2*, Uli Lemmer^1,2, Robby Peibst³*, and Ulrich Wilhelm Paetzold^1,2*

¹Institute of Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT), Germany

²Light Technology Institute (LTI), Karlsruhe Institute of Technology (KIT), Germany

³Institute for Solar Energy Research in Hamelin (ISFH), Germany

mohammad.gholipoor@kit.edu

ABSTRACT

Two-terminal (2T) Perovskite/silicon tandem solar cells (PSTSCs) have emerged as a leading candidate for next-generation high-efficiency photovoltaics [1,2]. However, challenges such as instability and current mismatch under realistic operation conditions restrict their commercial development [3]. Although four-terminal (4T) PSTSCs can alleviate the above limitations, they have disadvantages such as higher cost, complexity of electrical interconnection, inactive areas due to laser scribing [4], and significant parasitic absorption losses occurring in the thick transparent conductive oxide layer [5].

Three-terminal (3T) tandem architectures offer potential solutions to these limitations [6]. Here, we demonstrate that three-terminal (3T) perovskite/silicon tandem solar cells effectively mitigate weather-dependent performance variations and current mismatch under real-world operating conditions. We report a power conversion efficiency (PCE) of 30.1% in 3T tandem cells using a front-side textured, interdigitated back contact (IBC), and passivated POLysilicon-on-silicon Oxide (POLO) silicon bottom cell. Through a comparative analysis of two-terminal (2T) and three-terminal (3T) device configurations using an iterative measurement technique, we demonstrate that the 3T architecture is independent of the perovskite bandgap. While the PCE of 2T PSTSCs decreases by about 7% as the perovskite bandgap drops from 1.72 to 1.52 eV, the PCE of 3T solar cells remains relatively stable. Such flexibility overcomes the limitations of fixed-bandgap halide perovskites, allowing the use of more stable bandgaps.

Furthermore, 3T devices appear to be more robust to variations in the solar spectrum, particularly when the tandem cell’s short-circuit current density is limited by the top cell. Numerical simulations show that 3T solar cells deliver higher annual energy yield (EY) than 2T devices under various climates, confirming their greater robustness to spectral variations. These findings highlight the potential of 3T architectures to address key challenges in tandem photovoltaic technology, facilitating their widespread adoption in industrial and real-world applications. The full results of this study have recently been published in Advanced Science [7].

Tandem solar cells based on thin-film photovoltaics offer substrate flexibility and a potentially cost-effective alternative to crystalline silicon. In particular, monolithic two-terminal tandems combining a wide-bandgap perovskite top cell with a Cu(In,Ga)Se 2 2 ​ (CIGS) bottom cell are a compelling route to high-efficiency thin-film modules. Optical simulations have proposed suitable perovskite bandgap windows for current matching, yet the bandgap dependence has rarely been examined experimentally in a controlled, like-for-like device platform. Here we systematically fabricate and compare perovskite top cells with three bandgaps and integrate them monolithically onto CIGS bottom cells to identify practical bandgap–architecture combinations for high performance. Guided by optical–electrical co-design, we implement bandgap-resolved absorber compositions together with interface and surface-passivation strategies that expose the trade-offs among voltage, current and fill factor in the tandem configuration. Monolithic perovskite–CIGS tandems built from the selected top‑cell architecture demonstrated power‑conversion efficiencies reaching 25% on 1 cm² active area, placing the performance close to the state of the art efficiency reported on the same area scale. Beyond a single champion, the comparative dataset clarifies when the tandem becomes top-limited and shows that higher-bandgap top cells can partially compensate reduced current through improved fill factor and voltage. By explicitly testing multiple perovskite bandgaps rather than assuming a single target composition, we provide experimentally grounded design rules for top-cell selection and interface engineering in monolithic perovskite–CIGS tandems.  

Scaling perovskite photovoltaics from lab cells to reliable, large area devices hinges on solvent-free, high-throughput, and compositionally precise film formation. Spin-coated absorbers (central to many record efficiencies) are difficult to industrialise and typically rely on hazardous solvents; conversely, multisource co-evaporation is solvent-free but complex, throughput-limited, and sensitive to cross-contamination. Spin-coated absorbers (central to many record efficiencies) are difficult to industrialise and typically rely on hazardous solvents; conversely, multisource co-evaporation is solvent-free but complex, throughput-limited, and sensitive to cross-contamination. The growth of perovskite films using vapor-phase techniques is therefore emerging as a practical commercial pathway because it offers uniformity, conformality on textured surfaces, and precise control over composition and interfaces [1,2]. Within this context, we introduce a one-step chemical vapor deposition (CVD) route that converts lead precursors directly into metal halide perovskites, offering a simplified, scalable alternative to solution and co-evaporation routes [1].

Our low-vacuum, multiparameter CVD platform reacts methylammonium vapour with either PbI₂ seed layers or ultrathin metallic Pb to form phase-pure with either PbI₂ seed layers or ultrathin metallic Pb to form phase pure MAPbI₃ thin films. The reactor architecture enables control of partial pressures and temperature gradients to promote rapid halide transport and interfacial reaction while remaining fully solvent free. Starting from PbI₂, we obtain highly uniform MAPbI₃ across large areas; X-ray diffraction (XRD) shows attenuation of PbI₂ reflections with concomitant emergence of perovskite peaks, confirming complete conversion. UV–vis data demonstrate tunability of the optical bandgap via mixed PbI₂–PbBr₂ precursors and separate MABr / MAI sublimation sources, providing a straightforward route to higher bandgap absorbers for tandems. Critically, we also convert metallic Pb directly to perovskite: a continuous ~20 nm Pb layer evolves, after MAI exposure, into a compact crystalline perovskite, with serial XRD across successive CVD cycles revealing a mechanistic Pb → PbI₂ → MAPbI₃ sequence. Together, SEM/XRD/optical datasets establish uniform, phase pure films from either PbI₂ or Pb precursors via a single, scalable vapor-phase operation.

From a manufacturing perspective, the one‑step Pb‑to‑perovskite CVD process eliminates toxic solvents, reduces material wastage and tool complexity relative to co‑evaporation, and supports bandgap tailoring (Br/I mixing) and conformal coating on textured Si; key for monolithic perovskite‑on‑silicon tandems and robust single‑junction modules. Ongoing work extends the process to larger substrates and benchmarks device performance, conformality and phase purity under ambient/operational stress, aligning with recent community progress on vapor‑phase scale‑up and process control [1,2].

This program builds on our group’s contributions to perovskite materials chemistry and device physics. In this study, we earlier observed film delamination (likely arising from macro film strain). Tthe next phase to improve optoelectronic performance will be attempting to link microstructural strain to non‑radiative losses and defect formation, providing a framework for defining the CVD process window, defect management and ultimately stability testing [3].

This talk will focus on: (i) the reactor concept and process window for one‑step Pb‑to‑perovskite conversion; (ii) comparative pathways from PbI₂ versus metallic Pb seeds and the kinetic evolution captured by SEM/XRD/UV–vis across CVD cycles; (iii) uniformity maps and phase‑purity metrics at scale; (iv) bandgap tuning via mixed‑halide precursors for single‑junction and tandem targets; and (v) manufacturability benefits including solvent‑free operation, waste minimisation, and conformality relative to other established and reported deposition processes.

Many of the key properties of photoactive and semiconducting perovskite devices have origins in the microstructure of the perovskite absorber layer, and obtaining analytical results from this layer is therefore a key aim in the further development of perovskite devices. While it is possible to create perovskite films with grain sizes in the micrometres, most high-performance perovskite devices rely on films with grain sizes of an order of a few hundred nanometres, well below the diffraction limit of most conventional visible light-based microscopes and associated techniques. As such, it has been necessary to develop other microscopy techniques that are able to collect spatial and analytical information with nanometre precision. This includes various electron microscopy techniques, scanning probes, like atomic force microscopy (AFM) and associated techniques, such as Kelvin probe force microscopy (KPFM), conductive AFM (c-AFM), and infrared AFM (IR-AFM), as well as high-resolution confocal scanning microscopes.

In this talk, I will show applications of various microscopy techniques for the study of metal halide perovskites. This includes how electron microscopy can distinguish phases that other diffraction-based techniques cannot,[1] and how changes in the atomic structure caused by altering the precursor chemistry leads to changes in the surface potential, which can be observed using KPFM. The distribution of various polymer additives can also be observed through a combination of c-AFM, KPFM, IR-AFM, and TEM,[2] showing that most polymer additives tend to aggregate at grain boundaries, where they change the surface potential of the material, effectively inducing barriers for ion migration at the grain boundaries. Finally, I will show how highly localised IV-curves can be obtained using conductive AFM, which enables the study of phase segregation and its inhibition with nanometre resolution, using a technique which can be generalised to study any IV-related property of a device under various operating conditions.

Metal-halide perovskites (MHPs) have emerged as a versatile platform for optoelectronic and quantum devices, combining a tunable electronic structure, strong light–matter interaction, and compatibility with multiple fabrication routes.¹–² Among these, thermal evaporation provides exceptional control over film thickness, stoichiometry, and interfaces, enabling stress-free growth and scalable device integration.

In this talk, I will present our progress in overcoming key scalability and reproducibility challenges in perovskite solar cells (PSCs). We demonstrate a sixfold increase in deposition rate without compromising film quality or power conversion efficiency, enabling an annealing-free, high-throughput co-evaporation process.³ In parallel, the development of soft sputtering processes for transparent conductive oxides enables fully vacuum-processed solar cells with total thicknesses of only a few tens of nanometers, while retaining high performance and operational stability.⁴

Leveraging the nanometer-level thickness control and exceptional uniformity enabled by thermal evaporation, we further extend this approach to the realization of perovskite-based Multiple Quantum Wells (MQWs).⁵ These engineered heterostructures allow precise tuning of carrier confinement, excitonic coupling, and quantum interference effects, enabling modified carrier dynamics, tunable emission, and access to quantum-confined regimes not attainable in bulk perovskites. MQWs thus offer a powerful platform for tailoring band structure and realizing unconventional photonic and quantum optoelectronic functionalities.⁶

Overall, these results highlight how thermally evaporated perovskites can simultaneously address manufacturability challenges in photovoltaics and enable new quantum-confined optoelectronic architectures, reinforcing their potential for next-generation energy and photonic technologies.

References

1) Min, H., et al., Nature, 2021. 598, 444.; Yoo, J.J., et al., Nature, 2021. 590 587.

2) J. Li et al., Joule 2020, 4, 1035; H.A. Dewi et al., Adv. Funct. Mater. 2021, 11, 2100557; J. Li et al., Adv. Funct. Mater. 2021, 11, 2103252;

3) Dewi et al. ACS Energy Lett. 2024, 9, 4319−4322; Dewi et al, ACS Energy Materials 2025

4)L. White etc, unpublished results

5) Advanced Materials 2021, 33, 2005166; L. White et al. ACS Energy Lett. 2024, 9, 83;

6) L. White, ACS Energy Lett. 2024, 9, 4450. L. White, ACS Energy Lett. 2024, 9, 4450.

The highest expectations for future photovoltaics are undoubtedly associated with halide perovskites. This is because halide perovskite solar cells are made of abundant and low-cost materials, yet they show high solar conversion efficiencies.

Large-scale applications such as photovoltaics require scalable and well-controllable deposition methods for halide perovskite thin films. The currently used methods are simple and low-cost but are difficult to scale up for industrial mass production. Atomic layer deposition (ALD) is well known for its unique controllability and excellent scalability and can therefore become a key method in halide perovskite photovoltaics. Indeed, ALD is being widely studied for deposition of charge transfer and protective layers for halide perovskite solar cells.

In contrast, reports on ALD of halide perovskite thin films are limited to those published by our team. Our main contributions are ALD-based processes for CsPbI₃ [1] and CsSnI₃ [2]. Our approach to deposit halide perovskites relies on combining ALD processes of the binary iodides CsI [1], PbI₂ [3], and SnI₂ [2]. CH₃NH₃PbI₃ can be prepared too, by exposing ALD-PbI₂ to CH₃NH₃I vapor [3], but an actual ALD process for this essential compound has been missing until now.

In this work we report the world’s first ALD processes for CH₃NH₃I and CH₃NH₃PbI₃. The ALD process of CH₃NH₃I is a key advancement since it enables, for the first time, ALD deposition of CH₃NH₃PbI₃. This opens up new possibilities for compositional tuning of halide perovskites with ALD.  

The CH₃NH₃I films were deposited at 20 – 80 °C using methylamine and anhydrous HI gas generated on site with a self-designed setup. Crystalline CH₃NH₃I films were formed at all deposition temperatures. The films consisted of grains that were tens of nanometers in size and agglomerated into larger clusters.

The CH₃NH₃PbI₃ thin films were deposited by a two-step approach: first, a PbI₂ film was deposited at 65 °C, followed by deposition of CH₃NH₃I on top of the PbI₂ without breaking the vacuum. The PbI₂ films were deposited using lead(II)bis[bis(trimethylsilyl)amide] (Pb(btsa)₂) and anhydrous HI gas. The PbI₂ film reacts with the CH₃NH₃I being deposited on it, forming CH₃NH₃PbI₃. A sufficient number of CH₃NH₃I cycles results in full conversion of PbI₂ to CH₃NH₃PbI₃. The conversion of PbI₂ to CH₃NH₃PbI₃ causes the film grains to grow and merge to form a continuous network. Lowering the conversion temperature results in films with larger grains and fewer pinholes.

Nickel oxide (NiOₓ) has emerged as a leading inorganic hole-transport layer (HTL) for inverted perovskite solar cells because it combines a wide optical band gap (3.6–4.0 eV) and high transparency with a high work function enabling favorable valence-band alignment and strong electron-blocking selectivity that suppresses interfacial recombination [1]. Its chemically and structurally stable oxide lattice supports improved thermal stability and photostability compared to common organic HTLs, while offering low-cost materials and industrially relevant processing routes. Nevertheless, the key properties for device performance, such as hole conductivity, interfacial recombination and energetic alignment, are highly sensitive to the defect chemistry and surface termination of NiOₓ. Since NiOₓ relies on nickel-vacancy driven self-doping and associated Ni³⁺ formation to generate holes, the attainable hole density and conductivity in sputtered films often remain insufficient for low-loss charge extraction [2]. Relating to those challenges, surface engineering via plasma offers a direct route to tune the defect chemistry and surface termination of NiOₓ, enabling improved hole extraction and reduced interfacial recombination.

We introduce a set of plasma-enabled process routes that turn sputter-deposited NiOₓ into a tunable and scalable HTL for perovskite solar cells, targeting improved reproducibility and stability. Plasma is leveraged as a controllable source of radicals, ions and UV photons that can modify oxides without introducing wet-chemical residues, while allowing independent control over chemical reactivity and ion energy. For example, a dielectric-barrier-discharge activation prepares the substrate by removing adventitious contamination and increasing surface energy, which stabilizes NiOₓ growth and improves film continuity and adhesion at technologically relevant thicknesses. After deposition, localized atmospheric plasma-jet processing provides spatially resolved control over near-surface stoichiometry and the Ni³⁺/Ni²⁺ balance, allowing for adjustment of band gap and interfacial energetics while reducing recombination-active sites at the NiOₓ/perovskite junction. To address bulk transport limitations without compromising optical transmission, HiPIMS-driven plasma ion implantation is discussed as a route to introduce controlled transition-metal dopants with tunable ion energy and pulse conditions, enabling deliberate steering of dopant depth profiles, charge state, and defect formation across 5 ‑ 40 nm NiO_x.

Ultimately, our work aims to identify which plasma species and regimes most effectively suppress interfacial losses while enhancing hole transport, providing a mechanistic blueprint for NiOₓ contacts that remain effective through perovskite deposition and device operation.

Printed non-fullerene acceptor (NFA) organic photovoltaics (OPVs) are rapidly approaching record power conversion efficiencies (PCEs), but further progress towards manufacturing-relevant modules requires simultaneous control of interfacial losses, process tolerance, and operational stability. A major bottleneck resides at the transparent electrode/active-layer interface, where non-ideal energy-level alignment, interfacial disorder, and defect-mediated recombination reduce fill factor (FF) and accelerate degradation, especially when devices are translated from small-area lab cells to printed, larger-area architectures. Carbazole–phosphonic acid self-assembled monolayers (SAMs) have emerged as an attractive route to create ultrathin hole-selective contacts on ITO, because they can tune the ITO work function, improve wetting/film formation of the organic stack, and be compatible with low-temperature processing. However, single-component SAMs may form quasi-monolayers with incomplete coverage and microscopic defects; in addition, the molecular geometry of the anchoring group, spacer, and substituents can impose a trade-off between dense packing (coverage/passivation) and charge transport (tunneling distance and series resistance). These issues become increasingly consequential for printed OPVs, where coating dynamics and large-area uniformity amplify sensitivity to interfacial imperfections.

Here we summarize and connect three complementary studies that establish a molecular design–assembly framework for dense, defect-tolerant carbazole SAM interfaces in high-efficiency NFA OPVs, with a focus on scalability relevant to Symposium L. First, systematic monosubstituent engineering in 1X-2PACz (X = F, Cl, Br, I, CF3) reveals that modest chemical modifications can strongly influence packing, interface dipoles, and ultimately device performance. In representative NFA systems, Cl/Br/I substitutions favor improved packing and more favorable interfacial energetics, enabling PCEs up to 19.03% in PM6:L8-BO and 20.12% in D18:L8-BO devices, while F and CF3 tend to increase interfacial disorder and reduce performance. These results highlight that “near-identical” SAMs can behave very differently at buried interfaces, emphasizing the need for chemistry-informed selection when targeting robust printed fabrication.

Second, spacer-length engineering in chlorinated nPACz derivatives (2Cl-nPACz, n = 2–5) decouples interfacial coverage/passivation from charge-transport limitations by explicitly controlling tunneling distance. Structural and spectroscopic analysis indicates that shorter spacers strengthen intermolecular interactions and increase ITO coverage while reducing series resistance; conversely, longer spacers increase the hole-tunneling barrier and progressively degrade FF and Voc. In this series, 2Cl-2PACz yields the most favorable balance and achieves up to 18.95% PCE. Together, these two studies provide quantitative guidance: dense packing and defect passivation are crucial, but they must be achieved without introducing excessive transport resistance at the buried contact.

Guided by these insights, our third contribution introduces a co-assembled multilayer SAM (coSAMu) strategy designed for scalable processing. Instead of relying on a single molecule to simultaneously set the work function, maximize coverage, and maintain low resistance, we combine 2PACz and a longer-spacer chlorinated analogue (2Cl-4PACz) using blend casting and sequential casting to control interfacial composition. The resulting layered interface comprises a 2PACz-rich chemisorbed bottom layer that primarily sets the ITO work function and an upper region enriched in 2Cl-4PACz that fills voids and passivates defects. This defect-filling architecture suppresses trap-assisted recombination, improves charge extraction, and delivers 20.1% efficiency (0.042 cm2) with FF above 81%. Importantly for printed and manufacturing-relevant OPVs, the interfacial strategy translates to scale: a 17.14 cm2 six-subcell mini-module reaches 17.0% efficiency, compared to 12.2% for the 2PACz control. Overall, substituent tuning, spacer engineering, and multilayer co-assembly provide a general pathway to dense, defect-tolerant buried contacts that support high-efficiency, printed, and potentially more stable NFA OPVs by reducing the sensitivity to interfacial disorder and coverage gaps.

Organic Photovoltaics (OPVs) are being investigated for various applications due to their multifaceted properties[1], such as variable transparency, tunable absorption, flexibility, lightweight properties, and a low energy payback time. In this work, we focused on characterising OPV for use in urban environments, where the irradiance and spectrum can differ from standard test conditions. Given the various illumination conditions, ranging from UV to Visible to infrared, we have chosen suitable photoactive layer materials and blended them in different ratios to achieve panchromatic absorption. We then used a high-throughput methodology[2] to evaluate the best device parameters to maximise power conversion efficiency (PCE). After optimising for the PCE under standard testing conditions, major considerations for incident light on the OPV in urban environments, such as diffused and low-intensity light[3], angular dependence[4], street LED lighting[5], and vulnerabilities, such as fallen leaves on OPV modules, are tested and discussed. A ternary blend of PTQ10:DTY6:PC61BM (1:1:0.25), which exhibits broad absorption, achieves the best power conversion efficiency (PCE) of 14.07%, surpassing the 12.80% observed in the binary blend of PTQ10:DTY6 (1:1.5). We then up-scaled the devices via complete solution processing to 4.07 cm² modules, yielding a PCE of 12.22%. Our results indicate that OPVs are well-suited for deployment in urban settings. We recommend testing OPVs under various currently non-standard conditions to properly evaluate PV systems and, in addition, to develop standardised protocols for such measurements that provide a realistic picture of PV performance under varying spectral inputs.

Organic solar cells (OSCs), which have recently achieved 20% efficiency, have drawn growing attention from the research community due to their potential in optoelectronic device applications with advantages of flexibility, transparency, low weight, ease of manufacturing, and so on. The advance stems from the development of non-fullerene acceptors, particularly the Y-series acceptors, and high-performance polymer donors like PM6. While efficiency improvements have been substantial, the underlying charge carrier dynamics remain a topic of debate. This uncertainty hinders a deeper understanding of performance gains and limits broader applications of organic photovoltaics, such as in transparent solar technologies.

In this work, to understand the working principle of OSCs, including charge generation, exciton dissociation, charge transfer (CT) state, and charge recombination, a series of devices based on PM6:Y6 were fabricated with a wide range of donor-to-acceptor (D:A) ratios, from 5% to 95%. This variation allows for systematic control of domain sizes, producing either donor- or acceptor-rich regions and enabling precise tuning of the donor–acceptor interface, from sparse, isolated contacts at extreme ratios to well-mixed morphologies near a 1:1 ratio.

As the D:A ratio varies, the absorbance spectra reveal a gradual shift in both peak intensity and position. Correspondingly, the photoluminescence (PL) spectra show varying degrees of quenching for both donor and acceptor components. Notably, a dilution effect is observed for the acceptor, evidenced by a gradual blue shift in its PL peak with increasing donor content. Time-resolved photoluminescence (TrPL) measurements provide exciton lifetimes for each component. The photoluminescence quenching efficiency (PLQE) of the donor remains consistently high—exceeding 90% across all D:A ratios—indicating efficient energy transfer. In contrast, the PLQE of the acceptor follows an asymmetric parabolic trend: it increases from 26% to 92% with rising donor content, then slightly declines to 87% at higher donor concentrations.

Turning to the device performance, Y6 with 5% PM6 can work decently with an efficiency of 4%, whereas PM6 with 5% Y6 can hardly function as a solar cell. Short-circuit current displays an asymmetric parabolic trend corresponding to the exciton splitting efficiency. A similar trend is also observed in the fill factor, which also indicates the change in charge recombination. While open-circuit voltage shows a linear change. Detailed characterization of charge carriers dynamics using ultrafast spectroscopy has been conducted from fs-ps to ns-µs. Preliminary results reveal distinct charge generation and recombination behaviors depending on the donor–acceptor ratio. This work helps to understand the processes from exciton to CT, CT to free carriers, and charge recombination, with a focus on the role of interfacial dimensions.

Solution-processed bulk heterojunction (BHJ) organic solar cells (OSCs) have emerged as a promising next-generation photovoltaic technology. A rapidly growing strategy in this field is the use of solid additives (SAs) to precisely tailor BHJ morphology and unlock the full potential of OSCs. SA engineering provides several advantages for commercialization, including: i) tuning film-forming kinetics to accelerate high-throughput manufacturing; ii) exploiting weak noncovalent interactions between SAs and active-layer materials to enhance device efficiency and stability; and iii) simplifying processing steps to enable cost-effective, scalable fabrication. These benefits position SA engineering as a key driver for advancing OSC technologies. However, the fundamental principles governing the discovery of high-performance SAs remain unclear. In this presentation, I will introduce a closed-loop workflow that integrates high-throughput experimentation—with the capacity to generate large and diverse datasets—with Bayesian optimization to discover promising SAs for high-efficiency OSCs. By building predictive models based on molecular descriptors, we established clear correlations between SA molecular structure and device performance. Through this data-driven approach, we successfully identified a series of high-performance SAs from minimal iterative suggestions, enabling the realization of highly efficient OSCs.

Indium tin oxide (ITO)–free, solution-processable transparent electrodes are increasingly recognized as crucial components for producing organic optoelectronic devices at low cost and on a large scale. Among the many alternatives to ITO, silver nanowires (AgNWs) have emerged as a highly promising option because they combine excellent optical transmittance with low sheet resistance, making them suitable for use in high-performance inverted organic photovoltaic (OPV) architectures. However, to realize their full potential particularly for roll-to-roll fabrication, AgNW networks typically require embedding within a multilayer electrode structure. Additionally, efficient carrier-selective interface layers that can be applied from solution are needed to ensure proper charge extraction and device stability. In this study, we introduce a simple yet highly effective strategy for improving AgNW-based transparent electrodes by incorporating a solution-processed pyridine interlayer. This pyridine layer acts as an electron-selective contact when deposited on top of ZnO and is positioned above both the AgNW network and the upper ZnO capping layer in ZnO/AgNW/ZnO (ZAZ) electrode structures. The incorporation of pyridine leads to a remarkable enhancement in the optoelectronic properties of the resulting ZAZ electrodes. Specifically, pyridine-modified ZAZ films achieve optical transmittances of 88.27% and 88.50% at 550 nm, paired with sheet resistances of 13.24 Ω/sq and 17.30 Ω/sq, respectively. These values correspond to figures of merit (FoM) of 21.69 and 17.00 Ω⁻¹, surpassing the performance of standard ITO electrodes. Beyond improving conductivity and transparency, the pyridine coating also effectively smooths the surface of the AgNW network, mitigating roughness-related issues that often hinder device performance.

Notably, this work represents the first demonstration of using pyridine functionalization to tune the surface potential and modify the work function of AgNW-based transparent electrodes. These interfacial modifications facilitate more efficient electron extraction while maintaining compatibility with fully solution-processed device fabrication. The pyridine-treated ZAZ electrodes also display strong stability, they retain their performance for up to three months under ambient environmental conditions and withstand thermal aging at 90 °C for up to three weeks without significant degradation. An additional advantage of the pyridine-coated ZAZ structure is its ability to function as a dual-purpose transparent electrode. Through a carefully engineered multi-step coating process, the electrode simultaneously supports efficient electron transport and effective electron collection, reducing interfacial losses and improving charge extraction in thick-film devices.

When integrated into fully solution-processed OPV devices, these dual-function electrodes enable a significant boost in photovoltaic performance. Devices constructed with pyridine-modified ZAZ electrodes reach a power conversion efficiency (PCE) of 9.95% and a fill factor (FF) of 63.34%, outperforming reference devices based on conventional ITO electrodes, which exhibit a PCE of 8.84% and an FF of 59.19% under comparable conditions (250 nm active-layer thickness and 0.150 cm² active area). Furthermore, the resulting organic solar cells demonstrate promising operational stability when tested under continuous illumination. Overall, this work highlights a practical and scalable approach to engineering high-performance, ITO-free transparent electrodes for organic solar cells. By utilizing a simple pyridine functionalization strategy, it provides valuable insights into interface engineering, work-function tuning, and device stability—paving the way for efficient, low-cost, and manufacturable organic optoelectronic technologies.

Reducing voltage losses is essential for advancing the efficiency of organic solar cells (OSCs), yet state-of-the-art non-fullerene acceptors (NFAs) still exhibit substantially larger non-radiative losses than their inorganic counterparts. Here, we identify intermolecular charge-transfer (iCT) states as a key intrinsic factor limiting radiative recombination in Y-series NFAs. Using an oligomer-based model strategy, we demonstrate that the formation of an iCT state, strongly hybridized with the local exciton, suppresses the radiative decay rate without increasing the non-radiative decay rate and reduces the oscillator strength of the lowest excited state. This hybridization leads to a diminished photoluminescence quantum yield (PLQY) and prolonged emission lifetimes, consistent with the emergence of partially dark excitonic states. Guided by this mechanistic insight, we develop a molecular design strategy based on side-to-tail coupling to promote J-type excitonic interactions while suppressing dark iCT formation. Devices employing this design achieve a voltage loss less than the 0.45 eV, among the lowest reported for OSCs to date. These results establish controlled excitonic coupling as a powerful design principle for minimizing voltage losses and advancing next-generation high-efficiency OSCs.

The commercialization of mixed-halide perovskite solar cells (PSCs) is currently impeded by intrinsic instability arising from light-induced ion migration and phase segregation. While various composition engineering strategies have been proposed to mitigate these issues, establishing a quantitative link between microscopic ion transport kinetics and macroscopic device degradation remains a challenge. In this work, we demonstrate how operando Electrochemical Impedance Spectroscopy (EIS) serves as a critical diagnostic bridge to decouple electronic and ionic dynamics, thereby elucidating the impact of material design on photovoltaic performance.

We first investigated the synergetic effect of heat and light on inorganic perovskites. Through temperature-dependent EIS analysis, we utilize an equivalent circuit model comprising bulk/interfacial recombination resistances and a Warburg diffusion element. We reveal that elevated temperatures facilitate ion migration, manifesting as a significant increase in Warburg admittance and a simultaneous collapse in recombination resistance. This analysis quantitatively explains the rapid degradation of power conversion efficiency (PCE) and the formation of iodide-rich domains, establishing a baseline for ion-migration-induced failure.

Building on this mechanism, we apply EIS to validate a "steric control" strategy for stabilizing the perovskite lattice. By systematically tuning the A-site cation size, from DMA-doped mixed-cation compositions, we increase the Goldschmidt tolerance factor to impose steric hindrance against halide motion. Operando EIS results show that increasing the A-site steric bulk significantly suppresses the Warburg admittance under illumination. Notably, Arrhenius analysis of the EIS data allows us to extract the activation energy (E_a) for ion migration, which rises from 0.037 eV in the reference device to 0.199 eV in the sterically optimized device.

This increase in activation energy, quantified solely through impedance measurements, directly correlates with a 54% improvement in operational stability and suppressed photocurrent degradation under continuous illumination. Furthermore, we corroborate these electrical findings with optical characterizations, where "probe-set-probe" photoluminescence mapping visualizes the suppression of phase segregation.

In conclusion, our work highlights that EIS is not merely a characterization tool but a powerful framework for material design. By quantifying the activation energy of ion migration and distinguishing it from carrier recombination processes, EIS provides a predictive metric for assessing the operational stability of perovskite solar cells, guiding the development of robust, phase-stable photovoltaic materials.

Manipulating ionic configurations in perovskite solar cells

Metal–halide perovskites combine high photovoltaic efficiency with low-temperature processing, positioning this material class as a promising thin-film solar technology. A major open question concerns the role of mobile ionic species in setting device performance and stability under operating conditions. Ionic reconfiguration under internal and external fields alters the internal potential landscapes experienced by charge carriers, leading to interfacial band bending, hysteresis, and operational losses due to ion-induced field screening. These ionic effects can influence steady-state current collection, fill factor, and open-circuit voltage. Understanding how specific ionic distributions affect device operation may support strategies to mitigate losses and improve efficiency.

Under dark conditions the mobile ions respond to the net potential drop across the perovskite layer and redistribute to screen the electric field. At room temperature, ionic distributions under different bias conditions are difficult to probe because of high ionic mobility. Lowering the temperature reduces ionic motion. Here, ionic distributions are created by applying an electrical bias, a light bias, or both, and then “frozen in” by cooling the device under the applied conditions. The cells’ current–voltage behavior and absolute photoluminescence (APL) are measured at discrete temperatures after polarization. The quasi-Fermi level splitting (QFLS) is determined from APL as an internal voltage metric and compared with the open-circuit voltage (Voc). From the mismatch between QFLS and Voc, an effective potential barrier associated with the frozen ionic distribution is measured as a function of the polarization bias.

Below 180K, the performance of a polarized Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃  perovskite solar cell remains stable over time, suggesting a frozen ionic regime. Between 180K and 255K, the Voc, fill factor, and hysteresis suffer due to the low ion mobility. Above 255K, the ions are sufficiently mobile to respond to standard voltage sweeps at 0.25 V/s, indicating an increased mobility region. A general trend is observed when cells are polarized under a higher electric field, their low-temperature device performance improves significantly, with power conversion efficiency rising from about 5% to approximately 26% at 170 K for polarization biases of −1 V and 2 V. Cooling is accompanied by a higher integrated APL spectrum and increased QFLS, consistent with reduced non-radiative recombination. At cryogenic temperatures, the QFLS of various half-stacks and full devices even approaches the radiative limit. At room temperature, introducing a C60 layer on the perovskite results in an ~80 mV reduction in QFLS. This reduction diminishes at lower temperatures, where the QFLS of a complete device converges to the QFLS of an intrinsic perovskite, indicating a significant reduction in non-radiative losses at the interface. In these measurements, the QFLS at low temperatures appears largely independent of the polarization bias used.

Mobile ions in metal halide perovskites are found to degrade perovskite solar cells (PSCs). Therefore, characterizing their density and mobility is crucial for improving the long-term performance of PSCs. A common method to characterize PSCs is impedance spectroscopy.

In the literature, the low-frequency (LF) feature of the PSC impedance response is related to ion dynamics and the high-frequency feature to electronic processes. Yet, why these features can vary over orders of magnitude in terms of frequency and impedance is unclear.

In this work, we identify the specific ion dynamics that drive the LF feature. We derive two seperate analytical expressions that directly relate the frequency and magnitude of the LF feature to the ion density and ion mobility, respectively. The validity of both analytical expressions are confirmed through extensive drift-diffusion simulations, varying over 35 parameters to ensure that they are applicable to a wide range of perovskite solar cells.

Alternative formulas from the literature are also tested, but are found to be suboptimal. After the validation, we experimentally determine the ion density and mobility of a methylammonium lead iodide PSC. They are 2 × 10²² m^-3 and 4 × 10^-10 m² V^-1 s^-1, respectively.

This new method, which depends on the low-frequency feature of the impedance spectrum, facilitates the precise and straightforward determination of the ion density and ion mobility in PSCs.

Mixed-halide perovskite materials are promising candidates for multijunction solar cells aiming to surpass the detailed-balance limit. However, perovskite solar cells (PSCs) still suffer performance losses due to nonradiative charge recombination, primarily caused by electronic defects (traps). Efficient extraction of charge carriers from the absorber layer to the electrodes is also crucial for achieving high device performance. Under operational conditions, recombination and extraction processes compete, and understanding their relative contributions is essential for improving PSC efficiency.

Photoluminescence (PL) spectroscopy is a widely used technique to probe nonradiative losses in semiconductors. In particular, transient PL (tr-PL) measurements enable time-resolved analysis of recombination dynamics. In metal-halide perovskites, tr-PL decays are often governed by trapping and de-trapping at shallow defects, leading to long decay times.^[1] Differentiating between charge recombination and extraction mechanisms typically requires steady-state or voltage-dependent PL measurements.^[2] Notably, under short-circuit conditions, PSCs often exhibit strong PL emission, suggesting impeded charge extraction due to poorly optimized transport layers or ion-induced screening effects.^[3]

In this work, we introduce voltage-dependent transient PL spectroscopy (tr-PL(V)) to disentangle charge recombination and extraction processes in PSCs. By comparing the tr-PL decay under open-circuit and short-circuit conditions, we show that charge extraction dominates after a specific laser delay time, resulting in fast quenching of the PL signal. We demonstrate the applicability of the tr-PL(V) method by intentionally modifying the charge extraction rate by tuning the C₆₀ layer thickness and inserting passivation layers at the perovskite/C₆₀ interface. We identify that ion-induced screening affects the PL decay, changing the recombination dynamics. Hence, we apply a pre-bias before switching to short-circuit during tr-PL(V) measurements to enforce a homogeneous mobile ion distribution across the perovskite layer. This pre-biasing leads to enhanced charge extraction and confirms that mobile ions affect (transient) PL(V) measurements under short-circuit operation, hindering extraction.

Finally, we reveal that mobile ions also affect regular tr-PL measurements under open-circuit conditions in complete devices, whereas in half-stacks (without electrodes), their influence is significantly reduced.

Our results demonstrate that tr-PL(V) spectroscopy is a powerful tool to distinguish between charge recombination and extraction in PSCs. Moreover, applying a pre-bias enables the characterization of ion-free charge extraction dynamics, revealing the true extraction velocity.

Determining charge carrier trap densities in perovskite solar cells (PSC) is challenging, with different experimental techniques often producing inconsistent results. Space-charge-limited current (SCLC) measurements on single carrier devices are a commonly used technique to estimate charged trap densities in PSCs. However, a major limitation of this method is that the measured current is frequently dominated by charges injected by the contacts rather than by charges on trap states within the perovskite layer, leading to reported trap densities on the order of 10¹⁶ cm⁻³ for 500 nm thin films. [1]

To overcome this limitation, we employ a lateral contact geometry of charge selective contacts on perovskite thin films. The trap resolution limit due to contact-induced space charge depends on the lateral contact spacing rather than on the film thickness. We further highlight that, for a given contact distance, an upper limit of the resolvable trap density is imposed by the maximum voltage of the measurement setup. A maximum applied voltage of 100 V limits the accessible trap density range to approximately two orders of magnitude above the resolution limit. By employing lateral contact spacings between 400 and 1100 µm, we expand the measurable window of charged trap densities, covering a range from 10⁹ to 10¹² cm⁻³.

Using this approach, we determined an electron trap density of approximately 5 × 10¹¹ cm⁻³ in perovskite thin films, significantly lower than previously reported values from SCLC measurements for polycrystalline thin films. Drift–diffusion simulations are used to rationalize our experimental findings and to reconcile the discrepancies between different trap characterization methods. Our results indicate that shallow traps in perovskite solar cells are not detectable by SCLC measurements and other electrical measurements in the dark, while their signatures are clearly observable in optical techniques such as transient photoluminescence.

Lead halide perovskites have shown tremendous potential within photovoltaic applications due to their multifunctional properties, and the interest to study these has therefore grown at a quick pace the last decade. However, these materials have also shown to succumb to structural instability and degradation when exposed to external stress, such as heat, moisture, illumination etc. Thus, for these materials to be applicable in large scale production of e.g. solar cells, these challenges and efficiency limiting factors need to be addressed [1].

In order to study these degradation processes, we visit synchrotrons and use a technique known as X-ray Photoelectron Spectroscopy (PES), where incident x-ray photons excite electrons in the perovskite under ultra-high vacuum conditions, thereby ejecting them from the surface of the material. By measuring the kinetic energy of the ejected electron, this technique can give element specific resolution of chemical and structural changes within the material.

By using an x-ray beam with low enough flux, we could ascertain that the x-rays themselves were not inducing any structural changes to the material. Thus, we could perform PES while additionally applying a 515 nm laser, the power of which could be tunable, in order to see how visible light affects the material.

Studying MAPbI₃, MAPbBr₃, MAPbBr_3-xI_x and MAPbBr_3-yI_y single crystals (where x and y indicate different ratios of the two halides) using the above-mentioned setup, we saw that the laser induces ion migration of the two halides over time, indicating that bromide migrates toward the surface of the material, while iodide moves toward the bulk. This process seems to be reversible to a certain extent when the laser is subsequently turned off, implicating a level of self-healing within the perovskite.

The conversion of low energy into higher energy photons provides an attractive opportunity to improve solar energy technologies. Several types of thin film materials which can transform two or more low energy excited states into a higher energy one, have been previously developed. Due to the non-linear nature of such optical processes, up-conversion is most efficient at high light intensities, typically well above sunlight intensities (100 mW/cm²). Here, we propose an innovative stacked diode approach relying on novel, low-cost organic- semiconductor materials. The thin-film stack comprises a series of organic NIR photovoltaic stacks, providing sufficient photovoltage to drive an organic light-emitting layer deposited on top. We combine state-of-the-art, vacuum-processable absorbing and emitting systems with careful, simulation-assisted stack engineering. Converting photons from NIR (≤835 nm) to green (530 nm), the stack achieves an external upconversion efficiency (EUE) of 1.9%. Importantly, the EUE stays constant over more than 3 orders of magnitude in intensity, down to less than 1 mW/cm². This presentation will focus on efficiency limiting processes within such layer stacks, efficiency limits as well as the potential of this approach for photovoltaic and photocatalytic conversion.

Photovoltaic cells using molecular dyes, semiconductor quantum dots or perovskite pigments as light harvesters have emerged as credible contenders to conventional devices. Dye sensitized solar cells (DSCs) use a three-dimensional nanostructured junction for photovoltaic electricity production and reach currently a power conversion efficiency (PCE) of over 15 % and 36 % in full sunlight and ambient daylight respectively. They possess unique practical advantages, in particular bifacial light harvesting, ease of manufacturing, flexibility and transparency, aesthetic appeal and environmental compatibility, which have fostered industrial production. Several commercial products are now on the market which have found widespread consumer approval. They served as a launch pad for perovskite solar cells (PSCs) which are being intensively investigated as the most promising future PV technology. The PCE of solution processed laboratory cells having currently reached 27.3% and turnkey plants for pilot production of modules are available on the market. Present research focusses on ascertaining their long-term operational stability and on their use in tandem devices with silicon cells which have reached a PCE of 34.9 %. My lecture will cover our most recent findings in these revolutionary photovoltaic domains.