Formamidinium-based metal halide perovskites have emerged as leading candidates for next-generation photovoltaic technologies due to their exceptional optoelectronic properties and compositional tunability, allowing them to be applied to both single- and multijunction devices. Despite this progress, their rich structural landscape presents significant challenges, with multiple competing crystalline phases often coexisting in device-relevant thin films, where the presence of the lower symmetry polymorphs is often regarded as detrimental to device performance.

In this presentation, I will discuss our recent work exploring the relationship between crystal structure and optoelectronic behaviour in FA-based perovskites. In particular, I will demonstrate how composition and crystallisation kinetics can be used to manipulate structural complexity, providing new insight into the origin of unusual optical signatures that have recently been linked to reduced photovoltaic performance. I will also present emerging strategies for controlling phase formation across challenging compositional regimes, highlighting opportunities to both suppress performance-limiting structural motifs and exploit them to access new nanoscale optoelectronic phenomena. Overall, these investigations demonstrate how understanding and controlling polymorphism provides a powerful framework for engineering the structure–property relationships of metal halide perovskites, with potential implications extending beyond photovoltaics to emerging quantum and photonic applications.

Wide-bandgap (WBG) perovskites are key absorbers for perovskite/silicon tandem solar cells, where bandgap tuning is commonly achieved through the incorporation of bromide and cesium. However,the influence of precursor salt selection on the formation pathway and long-term stability of the absorber remains poorly understood. In particular, different precursor combinations may yield comparable final absorbers while introducing substantial differences in the chemistry of the inorganic template, ultimately resulting in markedly different operational stabilities.

Here, we investigate how the choice of cesium and bromide precursors affects the formation and degradation behaviour of two-step processed WBG perovskites. A series of absorbers with bandgaps above ~1.6 eV was prepared using different combinations of CsI, CsBr, FABr and PbBr₂. Device performance and photoluminescence measurements identified two representative high-performing systems, using either CsI or CsBr as the cesium source and FABr as the bromide source, which exhibit markedly different operational stability across many devices (Figure 1, TOC).

Despite displaying nearly identical X-ray diffraction fingerprints, the two absorbers show distinct degradation behaviour during operation at 75 °C. Ex-situ and in-situ GIWAXS measurements reveal that the precursor salt selection substantially modifies the structure of the inorganic template, influencing the formation of PbI₂ complexes and polytype intermediates during crystallization. While the annealed absorbers converge toward similar final crystalline structures, their formation pathways differ significantly up to the final annealing step.

To identify the origin of the stability differences, complementary characterization techniques including TOF-SIMS depth profiling, in-situ photoluminescence under illumination, and structural analysis upon ageing were employed. While no evidence of significant intrinsic bulk compositional inhomogeneity or light-induced halide segregation was observed, structural changes upon thermal ageing suggest that the two absorbers follow distinct degradation pathways.

These results demonstrate that precursor selection influences not only the nominal absorber composition, but also the intermediate chemistry governing crystallization and degradation. Understanding and controlling these precursor-driven processes provides an important route toward more efficient and stable WBG perovskite solar cells.

The facile fabrication of perovskites coupled with their tuneable bandgap has made perovskites an ideal semiconductor for use in tandem devices. A popular tandem configuration is a perovskite wide band gap (1.7eV) deposited on top of a silicon narrow band gap cell. These devices have exceeded 33% efficiency in lab conditions. Whilst these efficiencies are very impressive, there are outstanding questions about the stability of the perovskite layer. Typically, thin films are made with mixed A-site cation systems, mixing Formamidinium and Caesium. However, mixed A-site perovskite systems have been shown to segregate into non-photoactive phases under operation, impeding long-term operational stability. To date, it has not been possible to form pure FA wide band gap perovskites due to the empirically known ‘miscibility gap’, where the photoactive perovskite phases do not form for particular halide compositions, including the 1.7eV composition. In this work, we demonstrate the use of 2-dimensional perovskite templates as an alternative crystallisation pathway through which pure FA-perovskites can be kinetically stabilised. Concurrently, through computational modelling, we posit a theoretical explanation for the presence of the miscibility gap in FA perovskites, owing to the thermodynamic preference of the non-photoactive phases over the photoactive ones in the ‘gap’. Through incorporating this new perovskite into a device, we achieve power conversion efficiencies exceeding 17% and eliminate the need for mixed A-site cation systems, thereby removing a potential bottleneck in the long-term stability of perovskite solar cells.

Formamidinium lead iodide (FAPbI₃) is a highly attractive absorber for photovoltaics, but the photoactive black α-phase is intrinsically metastable at room temperature and commonly requires compositional engineering or high-temperature processing for stabilization.[1,2] However, these strategies may introduce additional degradation pathways.[3] Here, we use pressure-assisted annealing as a physical processing route to promote phase-pure α- FAPbI₃ formation in thin films without relying on alloying. FAPbI₃ layers are crystallized under controlled temperature and uniaxial pressure. Combining heat and pressure lowers the temperature required for α-phase formation and suppresses residual PbI₂/δ- FAPbI₃ signatures, yielding compact black α- FAPbI₃ films over a broad processing window.

Structural analysis by GIWAXS and XRD shows that pressure-assisted annealing enhances the crystallite size, reduces microstrain, and induces preferential texturization. These changes indicate that pressure does not only accelerate phase conversion but also modifies the crystallization pathway and lattice relaxation of FAPbI₃. Complementary nanoscale measurements reveal a pronounced morphological improvement: rough reference films with large height variations are transformed into smooth, densely packed films with substantially reduced surface roughness. Kelvin probe force microscopy further shows a narrower contact-potential distribution after pressing, consistent with a more electronically homogeneous surface.

The structural and morphological improvements translate into enhanced optoelectronic quality. Photoluminescence imaging shows a more homogeneous emission response after pressure-assisted annealing, while spectral shifts and increased photoluminescence quantum yield indicate reduced non-radiative recombination and modified local bandgap/strain landscapes. These improvements are consistent with larger grains, fewer electronically active grain boundaries, and reduced disorder at the film surface and buried interfaces. When implemented in inverted solar cells, pressure-treated FAPbI₃ absorbers increase the open-circuit voltage and fill factor, indicating reduced recombination losses and improved charge extraction. Finally, operational testing under illumination shows improved stability for pressure-treated devices compared with reference FAPbI₃ cells under accelerated aging conditions.

Pressure-assisted annealing therefore provides a direct physical handle to control FAPbI₃ phase formation, crystallinity, surface morphology, electronic homogeneity, and device stability. This work establishes mechanical processing as a complementary strategy to chemical strain engineering and highlights pressure as a parameter for stabilizing high-quality pure- FAPbI₃ perovskite solar cells.

Stable perovskite solar cells (PSCs) under operational conditions were first demonstrated 10 years ago using multilayered 2D perovskite phases with n = 3 or 4, where n represents the number of corner-sharing octahedra along the stacking axis (Tsai, Nature 2016). While 3D perovskites currently offer superior power conversion efficiencies and are candidates for tandem solar cell architectures with silicon, their stability remains a challenge.

Beyond classical passivation approaches, fabricating thick 2D/3D bilayer stacks using solvent engineering is a strategy to improve device stability (Sidhik, Science 2022). The proper selection of the 2D layer composition and thickness (n) allows for optimizing band alignment and carrier collection in these solar cell heterostructures. Additionally, using 2D pre-crystallized seeds is essential to control the growth of the thick 2D top layer. Applying the concept of lattice matching is crucial for empirically identifying perovskite combinations and predicting the formation of thick continuous (Sidhik, Science 2022) or nanostructured (Jiang, Nature Synthesis 2026) layers. This concept was adapted in 2018 to halide perovskites, inspired by the epitaxial growth of conventional semiconductors, by introducing a linear-quadratic coupling between strain and octahedra rotations in the mechanical free energy expansion (Kepenekian, Nano Letters 2018).

These three combined approaches (solvent engineering, seeds, and lattice matching) have also been used to incorporate nanodomains inside the FAPbI₃ perovskite matrix. This recently led to record stability for pure FAPbI₃ solar cells by properly choosing a 2D n = 2 perovskite template to block the phase reconstruction to the yellow phase of FAPbI₃ (Sidhik, Science 2024). More recently, the metastable black phase of FAPbI₃ at room temperature was definitively turned into a stable phase by bypassing the classical degradation pathway to the 2H-PbI₂ phase through chlorine incorporation (Garai, Science 2026).

Perovskite solar cells have come to the forefront of solar research in the last decade with certified efficiencies of now >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]

References

[1a] Saliba et al. Energy & 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)

In the last 15 years lead halide perovskites have raised considerable interest for photovoltaic (PV) applications, with power conversion efficiencies in a single junction now exceeding 27%. However, the presence of lead in soluble form constitutes a major obstacle for their broad deployment. More generally, the imperatives in terms of environmental responsibility require exploring lead-free alternatives. In this context, our study focuses on mixed Sn/Ge-based homovalent Pb substitution, namely compounds of type (MA,FA,Cs)Sn_xGe_1-x(I, Br)₃. The band gap values of (MA,FA,Cs)Ge(I, Br)₃ and (MA,FA,Cs)Sn(I, Br)₃ compounds, ranging from 1.2 to 3.1 eV, make them promising candidates for PV applications. Despite potential instability due to the sensitivity of Sn²⁺ and Ge²⁺ to oxidation, promising PV results have already been reported with CsSn_0.5Ge_0.5I₃. However, there is a lack of systematic experimental studies on the structural properties of (MA, FA, Cs)Sn_xGe_1-x(I, Br)₃ compounds.

Here, we investigate the temperature-dependent evolution of the structural and optical properties of selected bulk materials (A1_yA2_1-ySn_xGe_1-xI₃ with A1, A2 = FA, Cs) in order to map the phase stability, identify phase transitions, and determine the thermal expansion coefficients. We further use these results to distinguish between intrinsic compositional instability from degradation that originates from a given synthetic process or the thermal history. Based on these results, the most promising compositions are selected for thin film deposition. We will present the results of the investigation of the crystallization process of the thin films and of their resulting microstructure. Overall, the applied two-step approach strongly adds to the fundamental understanding of the Sn/Ge-based halide perovskite family and paves the way for their practical applications, thus contributing to the quest for eco-friendly solar technologies.

Halide perovskites are promising semiconductors for photovoltaics and other optoelectronic technologies, but their acute moisture sensitivity remains a major barrier to long-term operational stability. The earliest stages of water-induced degradation are especially difficult to resolve as they are confined to ultrathin surface layers and often produce disordered phases that evade diffraction-based structural probes. Here I'll discuss how we have exposed lead halide perovskites suitable for state-of-the-art perovskite photovoltaics and LEDs to ¹⁷O-enriched water vapor and used ultra-high-field solid-state ¹⁷O MAS NMR to identify the surface species formed during moisture-induced degradation. We find that water does not simply adsorb as isolated molecules but instead forms liquid-like interfacial nanofilms that act as both solvents and reactive media. These nanofilms selectively leach ions from perovskite surfaces and direct composition-dependent degradation pathways leading to hydrate, hydroxyhalide, hydroxide and oxide phase formation. These findings establish reactive water nanofilms as the key intermediate in moisture-driven degradation, map how degradation products evolve across the lead halide perovskite compositional space and provide clear design rules for moisture-stable perovskite photovoltaics and LEDs.

Low-dimensional perovskites (LDPs), consisting of single or few inorganic octahedral layers separated by bulky organic cations, have emerged as a promising class of semiconductors owing to their superior environmental and structural stability compared to conventional three-dimensional (3D) perovskites. Their exceptional robustness has enabled widespread adoption as interfacial passivation layers in high-performance perovskite solar cells. However, their direct implementation as photoactive absorbers remains largely unexplored due to their intrinsically wide bandgap, disordered morphology, and inefficient out-of-plane charge transport, which severely limit photovoltaic performance.

In this contribution, we present recent advances in the design and implementation of low-dimensional perovskites for next-generation photovoltaic applications. First, we demonstrate how ferroelectric low-dimensional perovskites can be strategically integrated at interfaces to enhance charge extraction and suppress interfacial recombination in highly efficient perovskite solar cells. The intrinsic polarization of these materials promotes directional charge separation, resulting in improved carrier collection and device operation.

Beyond their role as interfacial modifiers, we introduce a crystallographic engineering strategy that enables low-dimensional perovskites to function as efficient active layers. By controlling nucleation and crystal growth, we induce preferential vertical alignment of the inorganic framework, effectively overcoming the major transport bottleneck associated with conventional randomly oriented LDP films. This approach establishes efficient vertical charge percolation pathways while preserving the intrinsic stability advantages of low-dimensional structures.

As a result, we achieve a record power conversion efficiency of 9.4% together with an open-circuit voltage of 1.4 V, among the highest values reported for low-dimensional perovskite solar cells. Advanced structural and optoelectronic characterization reveals a direct correlation between crystalline orientation, carrier transport dynamics, and photovoltaic performance, highlighting the critical importance of orientational control in these materials.

These findings demonstrate that low-dimensional perovskites can evolve from passive interfacial components into efficient photoactive semiconductors. The presented strategies provide a general framework for overcoming transport limitations in layered perovskites and open new opportunities for stable wide-bandgap photovoltaics targeting emerging applications including indoor energy harvesting, building-integrated photovoltaics, tandem devices, and agrivoltaic systems.

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,[1,2] 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,[2,4] 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.

Perovskite solar cells (PSCs) have shown impressive power conversion efficiencies (PCE). To become commercially successful, also challenges regarding large-scale manufacturing and stability must be addressed. In this talk we present two studies in which advanced electro-optical characterization techniques are combined to assess the influence of cell design parameters on the performance and degradation of PSCs.

In the first study, we investigate mixed self-assembled monolayers (SAMs) composed of Me-4PACz and F-4PACz and evaluate their impact on device performance and stability. The combination of the two SAMs offers the potential to combine the benefits of Me-4PACz (high PCE) with F-4PACz (good wettability), but it also poses the question of how the SAM properties are altered in the mixed state. It is found that optoelectronic characteristics of mixed SAM films and devices are primarily dominated by a variable combination of the pure SAMs properties. Interestingly, some measurements indicate a tendency toward increased variability for mixed SAMs compared to their pristine counterparts which suggests that SAM mixture do not form a homogeneous hybrid layer. This hypothesis is further analysed by a combination of device simulations (Setfos),[1] ultraviolet photoelectron spectroscopy and Kelvin probe force microscopy measurements.

In a second part, we evaluate performance and stability of a series of mesoscopic carbon-based perovskite solar cells with systematically varied thickness of the titania and the zirconia mesoporous layers. The results are consistent with previously reported decrease in collection efficiency when decreasing the mesoporous titania (m-TiO2) thickness. [2] By comparing current-voltage (JV) and impedance results with device simulations using Setfos, we infer the relative importance of bulk recombination and of recombination at the perovskite/carbon interface as a function of bias. [3,4] Next, an operational stability test (maximum power point tracking, MPPT) on such series of mesoscopic solar cells is performed under 1 sun illumination and controlled temperature using Litos Lite. While the cells show good stability at 25 °C for > 500 hours, further tests at higher temperatures reveal irreversible instability of the devices already after 100 hours. Interestingly, the extent of such a degradation is dependent on the solar cell structure, with a drop in performance in the order of 30% for cells with thicker m-TiO2 layer and an almost unchanged efficiency recorded for cells with thin m-TiO2. Different degradation scenarios are discussed and complemented with scan-rate dependent JV curves, impedance measurements as well as with photo- and electroluminescence imaging (Vitios). [2,5] The results indicate that both photo-inactive areas are formed and recombination and ionic properties of the PSC are altered during degradation. 

Photovoltaic laser power converters (PVLPCs) are gaining significant traction for applications in power-over-fiber (PoF) and wireless power beaming systems, with in-space applications emerging as a rapidly evolving frontier [1]. Currently, gallium arsenide (GaAs)-based PVLPCs hold the record power conversion efficiency (PCE) of 68.9 %, delivering a power density of 7.8 W/cm² and 1.1 V under 858 nm monochromatic illumination [2]. Concurrently, perovskite photovoltaic cells have emerged as promising alternatives for PVLPCs, demonstrating benchmark PCEs of 58.6 % under 785 nm illumination (50 mW/cm²) and 54.0 % under 532 nm illumination (70 mW/cm²) [3]. However, deploying PVLPCs in space environments introduces stringent operational challenges, particularly severe thermal fluctuations (thermal cycling) and intense mechanical stress. 

To address these challenges, we report the development of ultra-lightweight, fully glass-encapsulated perovskite PVLPCs optimized for monochromatic illumination within the 400 nm to 550 nm spectral range, targeting efficiencies exceeding 50 %. Wide-bandgap perovskite compositions were strategically selected to maximize the voltage output and match the target photon energies. Two distinct device architectures were investigated to balance performance and durability: a hole-transport-layer-free (HTL-free) configuration to simplify the fabrication pipeline and enhance intrinsic structural stability, and a conventional n-i-p to optimize charge extraction and achieve high power conversion efficiencies.

To ensure long-term operational stability in low Earth orbit (LEO), the perovskite LPCs were hermetically sealed with a high-durability glass frit sealant [4]. Mechanical robustness and thermal mismatch mitigation were achieved by utilizing borosilicate glass substrates, selected for their low coefficient of thermal expansion (CTE) matching the device layers. Furthermore, to minimize the weight for space qualification, the thickness of the glass substrates was reduced from 2.2 mm to 1.0 mm. Achieving a reliable, hermetic seal on these ultra-thin substrates necessitated the development and implementation of specialized, low-thermal-impact laser processing techniques.

Transparent Conducting Oxides (TCOs) sit at nearly every critical interface in perovskite–silicon tandem solar cells, governing optical coupling, charge extraction, and recombination, while introducing resistive, absorptive, and reflective losses that can each cost more than 2% absolute efficiency.

Beyond performance, TCO choice and quality can directly affect long-term device stability: microstructure and interfacial chemistry determine resistance to moisture ingress, ion migration, and damp-heat degradation, factors that are too often considered separately from efficiency optimization. In this talk, I will argue that TCOs deserve treatment as an important design parameter, alongside the carrier extraction and perovskite absorber itself, in the pursuit of tandem cells that are both highly efficient and operationally durable. I will discuss the materials physics trade-offs involved, current material limitations including reliance on scarce indium-based oxides, and emerging strategies (new dopants, multilayer architectures, and computational screening) aimed at simultaneously improving optoelectronic performance and long-term stability for sustainable deployment.

The atmosphere trapped in the cavity of a glass-glass-sealed perovskite solar cell directly shapes its long-term operational behaviour^1-3, yet the precise degradation mechanism remains poorly understood in efficient but unstable Au-based n-i-p PSCs. Here, we systematically compare ambient air, CO₂, and N₂ encapsulation in laser-sealed PSCs based on a triple-cation Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ absorber. Champion devices exceeded 19 % PCE prior to encapsulation (average 17.79 ± 1.03 %). The 65 °C laser-sealing step induced a moderate efficiency loss of ca. 7 % (16.59 ± 1.02 %), primarily from photocurrent losses, while simultaneously reducing anomalous hysteresis across all conditions: the hysteresis index dropping from 26.52 % to 7.86 % (CO₂) and 8.51 % (N₂), attributed to thermal passivation of grain boundary defects. Air-sealed devices showed a more modest improvement (23.64 %), hindered by competing oxygen passivation and early-stage moisture-driven degradation.

Under continuous MPP tracking at 1 sun and 45 °C over 160 hours, the encapsulation atmosphere unequivocally dictated device lifespan. Air-sealed devices failed catastrophically within 10 hours: PCE reaching near-zero, with J_SC and FF collapsing below 25 % and V_OC retaining only 25 %, driven by the synergistic photo-oxidative attack of trapped O₂ and moisture. N₂ encapsulation delivered a transformative improvement: after a ca. 40-hour burn-in, PCE stabilised at 50 % while V_OC was preserved at 93 % until reach 160 h of continuous exposure, confirming that excluding oxidative species suppresses deep-level non-radiative recombination centre formation in the bulk absorber. CO₂ encapsulation offered a mechanistically distinct response: during the first 50 hours, CO₂-sealed devices matched the N₂ baseline entirely, attributed to the in-situ formation of a carbonaceous passivation layer at the perovskite surface. V_OC remained stable at ca. 90 % across 160 hours, confirming effective bulk protection. However, after 50 hours, CO₂ devices exhibited progressive FF attenuation absent in N₂ samples, causing the final PCE to stabilize at ca. 33% after 160 h of aging. This selective FF loss indicates a specific interfacial vulnerability under prolonged CO₂ exposure.

Postmortem analysis (SEM, XPS, GDOES, PL, TRPL, and EIS) of exposed perovskite surfaces from dark-stored and light-aged devices revealed atmosphere-specific degradation pathways consistent with the MPP tracking results. SEM showed that air encapsulation shrinks the pervoskite grain size by 34 % during dark storage via moisture-driven boundary degradation, while CO₂ promotes Ostwald ripening with a 22 % grain size increase. Aging induced complete morphological collapse in air-sealed films, while N₂-sealed films retained their granular structure; CO₂-sealed films, by contrast, exhibited only minimal grain size reduction, largely preserving their initial morphology. GDOES depth profiling identified the mechanistic origin of late-stage FF loss in CO₂: the degradation of the PTAA layer compromises its barrier properties, promoting aggressive iodide accumulation at the weakened interface and deep gold migration into the perovskite stack. XPS confirmed the formation of carbonaceous species through the perovskite–CO₂ reaction¹, which passivates the perovskite surface while leaving the overlying PTAA unprotected. PL and TRPL measurements corroborated severe bulk phase degradation in air, intact bulk in N₂, and increased interfacial trap density in CO₂ without changes in the bulk phase. EIS also demonstrated that recombination resistance collapsed from the megaohm range to tens of kilohms in air and CO₂ after aging, while N₂-sealed devices maintained moderate interfacial quality.

These findings establish that complete exclusion of O₂ and moisture is the primary requirement for stable encapsulation and identify CO₂-driven PTAA degradation as a previously unreported pathway for gold migration and interfacial failure in n-i-p PSCs, with critical implications for the design of hermetic encapsulation strategies in perovskite photovoltaics.

Perovskite solar cells (PSCs) have achieved over 27% efficiency, rivaling silicon cells due to the excellent optical properties of lead halide perovskites (LHPs)^[1]. As an absorbing material, LHPs possess unique and excellent optical properties, including low production cost, tunable band gap, broad absorption, low exciton binding energy, and high charge-carrier mobility^[2]. However, defects in the bulk (intrinsic) and interfaces of perovskite materials, as well as Li⁺ migration (especially in n–i–p regular architecture), caused by LiTFSI salt, used to enhance the conductivity and hole mobility of spiro-OMeTAD, can significantly impact device performance and stability^[3]. To date, numerous passivation molecules have been developed to enhance perovskite device performance by interacting with undercoordinated Pb²⁺ defects or by inhibiting Li⁺ ion migration^[4]. However, these strategies typically target only one type of degradation pathway at a time. To the best of our knowledge, a molecule capable of simultaneously passivating Pb²⁺ defects and suppressing Li⁺ ion migration has not yet been reported.

Herein, we report a rationally designed meso-crowned porphyrin derivative ([12]-C-4POR) featuring dual macrocyclic binding sites, i.e., a porphyrin core for undercoordinated Pb²⁺ and a crown ether unit selective for Li⁺ to suppress surface defects and mitigate lithium-ion migration simultaneously^[5]. We found that the porphyrin core is strongly bonded with Pb²⁺ while the ether part captures the Li⁺ and suppresses its migration. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) shows that [12]-C-4POR based device showed much lower levels of Li⁺ migration, with only limited diffusion compared to the control device. The incorporation of [12]-C-4POR into perovskite films significantly reduced the trap-state density and suppressed non-radiative recombination, leading to improved charge-carrier dynamics. Devices treated with [12]-C-4POR delivered a champion PCE of 23.14%, surpassing the control device (21.6%), along with enhanced open-circuit voltage (V_OC) and fill factor (FF). More importantly, the passivated devices retained ∼95% of their initial PCE after 800 h of continuous operation, compared to ∼55% for the control. Furthermore, [12]-C-4POR-treated device showed significantly better thermal and environmental stabilities compared to the control cell. This study demonstrates a dual-site host–guest passivation strategy as an effective route to improve both efficiency and operational stability of PSCs.

Dimensional heterojunction perovskites, characterized by the stacking of three-dimensional and low-dimensional (3D/LD) heterojunctions, have the potential to reduce interfacial recombination losses and enhance the structural stability of perovskite solar cells. However, performance variations due to fabrication techniques and ligand design remain a challenge. Specifically, the design of cation ligands and selection of anions can influence the quality, coverage, dimensionality, energy levels, and stability of LD perovskites. Our work aims to identify suitable ligand and anion combinations that enhance the stability and uniformity of LD layers in large-area modules. Additionally, our fabrication engineering, which employs a hybrid deposition method, can modulate the properties of LD perovskites. Consequently, inverted perovskite devices have achieved efficiencies exceeding 27% for small-area devices and over 21% for a 30 × 30 cm² module with an active area exceeding 700 cm². Our dimensional heterojunction strategy has also demonstrated stable perovskite devices with less than 5% relative efficiency loss under continuous 1-sun operation at 85°C for over 1000 h, equivalent to more than 10,000 h, by controlling the phase purity and ensuring strong ligand attachments in LD perovskites. Finally, we hope that our method will contribute to meeting the industrial stability criteria for perovskite photovoltaic modules.

Perovskite solar module upscaling relies on P1-P2-P3 laser scribing process to achieve efficient interconnection  and efficient devices. However, the laser can introduce local damage and accelerate degradation in the already sensitive perovskite absorbers. Consequently, the scribe regions themselves may become critical sites governing long-term module stability.

In this work, laser-scribed regions in p-i-n perovskite mini-modules were investigated before and after 337h of ISOS-L3 ageing under maximum power point tracking (MPPT). The electrical performances (IV curves) of the cells constituting the module as well as  SEM morphological studies and spatially-resolved photoluminescence (PL) spectra are measured. Indeed, the energy positions and the intensities of the PL peaks are intimately related to the composition and the crystalline quality of the perovskites layers, giving information about their degradation. Some EDX mapping measurements have been performed to have more detailed insights in the degradation mechanisms.

From the analysis of the PL measurements at the three different scribes, P1-P2-P3 and their interscribe area,  the P1 scribing seems to be a  major source of degradation.  After ageing the whole P1 scribe area displayed PL peaks which were red-shifted compared the fresh (non-aged) module, and a ten-fold increase of the PL response has been observed. The increase of PL signal could potentially be explained by the 1 μm hill that emerged on the side of the P1 scribe after ageing. The hill was analyzed by EDX where asymmetric concentration changes were observed at the different sides of the hill. Moreover, a SEM cross section of the P1 scribing revealed that the perovskite had expanded from a densely packed layer to a more porous type with internal cavities. This could support the theory of potential gaseous I₂ in the P1 due to poor crystallization that then is more prone to defects under operation [1]

The P1 PL-response did also have a direct correlation to the other scribe PL signals where secondary peaks emerge behind red-shifted P1 scribes towards P2-P3, indicating a potential correlation between the P1 PL-shift and the P2-P3 degradation.

In conclusion, the P1 scribe seems to be majorly affected from the ageing process comparing to the P2 and P3 ones, due to poor crystallization. The correlation between P1 peak shift and following change of P2-P3 seems to indicate that the P1 impacts the remaining scribe degradation.

Mobile ions play a crucial role in metal halide perovskites, shaping the device physics of perovskite-based devices such as solar cells or memristors. Although the importance of mobile ions in metal halide perovskites has long been recognised, a comprehensive understanding of ion migration, its driving forces, and its impact on perovskite solar cell performance is still lacking.

In this talk, I will discuss how mobile ions influence the device physics of perovskite solar cells, and impact device performance. I will demonstrate how field screening due to mobile ions leads to inefficient charge extraction, which eventually causes current losses [1]. Thereafter, I will discuss ageing and recovery of perovskite solar cells, identifying an increase in mobile ion density as a dominant factor in the early performance degradation of perovskite solar cells [2], and demonstrating that this process may be (partially) reversible [3].

Finally, I will explore how different device parameters, such as transport layer properties, interfaces, and device architecture influence ionic losses, offering a perspective on pathways towards engineering more stable perovskite solar cells.

Metal halide perovskite photovoltaics have rapidly approached the performance of established photovoltaic technologies; however, long-term operational stability and scalable manufacturing remain the major barriers to commercialization. Addressing these challenges requires a holistic approach that combines materials engineering, interface optimization, and industrially compatible fabrication processes.

This presentation will discuss recent advances in the development of fully wet-processed perovskite photovoltaic devices fabricated under ambient conditions, with particular emphasis on strategies that simultaneously improve stability, manufacturability, and scalability. The optimization of the perovskite deposition process, guided by nucleation and crystal-growth control, enables the formation of homogeneous large-grain absorber layers suitable for large-area fabrication. Particular attention will be devoted to interface engineering in carbon-based architectures, where replacing conventional metal electrodes with low-temperature-processed carbon electrodes offers significant advantages in chemical stability, reduced degradation pathways, lower cost, and improved sustainability. The role of hole-selective and passivating interlayers in suppressing interfacial recombination, improving charge extraction, and enhancing long-term operational stability will be discussed.

Beyond cell-level optimization, the presentation will address the transition toward photovoltaic modules by considering laser patterning, interconnection losses, screen-printing processes, and industrial encapsulation approaches. Recent results demonstrate encouraging operational stability under continuous maximum power point tracking, damp-heat conditions, and outdoor testing, highlighting the potential of fully printed carbon-based perovskite photovoltaics for real-world applications. The scalability of the proposed approach is further demonstrated by the fabrication of a 600 cm² fully printed perovskite photovoltaic demonstrator, consisting of four carbon-based modules connected in series and manufactured using industrially compatible printing and lamination processes.

The results highlight how the combination of materials design, interface engineering, and scalable manufacturing technologies provides a viable pathway toward stable metal halide perovskite photovoltaics compatible with high-throughput industrial production. These advances help bridge the gap between laboratory-scale devices and commercially relevant perovskite photovoltaic modules for both outdoor and emerging indoor energy-harvesting applications.

Very high efficiencies for Perovskite-based single junction and tandem solar cells have been reached by research institutes and industry, shifting the research focus to long-term operational stability. Our longest data series for a single-junction Perovskite solar cell, currently 5.5 years long, shows pronounced seasonal variations in device performance. We investigated how this seasonal behavior can be attributed to changes in the solar spectrum, unconventional temperature-dependent behavior of aged perovskite-based devices and material metastability effects (specifically the light-soaking effect).

We show that these factors affect the two-terminal perovskite-based tandem solar cells (produced in-house) as well. However, tandem solar cells are more sensitive to changes in solar spectrum. The current mismatch between the top and bottom cell can reduce the device performance due to series connection of the sub-cells. We use indoor measurements under controlled conditions using different light spectra and operating temperatures to support the analysis.

Finally, we present the results from a one-year long dataset for six MPP-tracked industrial-size Perovskite-Silicon tandem modules (installed at an optimal 35° inclination, facing South) acquired at HZB’s test field in Berlin, Germany. Data will be presented together with monitoring results of a Silicon module as a reference, quantifying the possible gain in annual energy yield. All of the modules operated stably throughout the monitoring period, with no abnormal performance changes. Seasonal variations in relative performance were observed in the tandem modules. These changes are consistent with the changes in irradiance and module temperature.