Mixed Tin/Lead (Sn/Pb) perovskites have the potential to achieve higher performances in single junction solar cells than Pb-based compounds. Nowadays, the most used hole transport layer (HTL) for Sn/Pb based perovskite solar cells is PEDOT: PSS, even if there are many doubts on a possible detrimental role of this conductive polymer on th performance e long-term stability of the active layer.

Recently, we have shown that self-assembled monolayer [2-(3, 6-dibromo-9H-carbazol-9-yl) ethyl] phosphonic acid (Br-2PACz) can have great advantages when used as  HTL of solar cells of active layer Cs_0.25FA_0.75Sn_0.5Pb_0.5I₃ [1]. Several factors seem to determine the increased perfomance respect to devices using PEDOT: PSS as HTL, the perovskite layer deposited on SAMs show an higher crystallinity, with reduced pinhole density and larger grains but also a reduced defect density at the burried interface. In my presentation I will show devices above 25% efficiency combining this HTL and a proper surface passivation.

Tandem solar cells using all metal-halide perovskite thin films show great promise for next-generation solar cells in terms of reduced cost, flexibility, and high efficiency, an effective way to break the Shockley-Queisser limit of single-junction cells. Low-bandgap mixed tin (Sn)-lead (Pb) and wide-bandgap perovskite solar cells, as the key to make highly efficient all-perovskite tandem solar cells, have been gaining extensive interest due to their appropriate bandgaps and promising application to build efficient all-perovskite tandem cells. Growth process of perovskites plays a crucial role in the formation of high-quality perovskites. We will present perovskite crystallization regulation strategies such as universal close space annealing (CSA) and buried interface-assisted growth, etc to increases grain size, enhances crystallinity, and prolongs carrier lifetimes in low-bandgap (~1.25 eV) and wide-bandgap (~1.75-1.80 eV) perovskite films, leading to high-quality perovskite absorber layers. We will also present new self-assembly monolayer (SAM) materials for efficient wide-bandgap perovskite subcells and their mechanisms for reducing interfacial charge non-radiative recombination losses. We will also discuss the design and optimization of interconnecting layers for all-perovskite tandems.

Mixed tin-lead (Sn-Pb) halide perovskites, with tunable bandgaps ranging from 1.2 to 1.4 eV, hold significant potential for developing highly efficient all-perovskite tandem solar cells. However, achieving commercial viability and sustained high efficiency in Sn-Pb perovskite solar cells (PSCs) remains a formidable challenge. Among various optimization approaches, the use of additives has proven pivotal in regulating the crystallization of Sn-Pb halide perovskites. Despite their extensive application to enhance device performance, the underlying photophysical mechanisms are not well understood.

In this study, we investigate the role of guanidinium thiocyanate, a chaotropic agent, in the crystallization of Sn-Pb halide perovskites. Using a combination of hyperspectral imaging and real-time in-situ photoluminescence spectroscopy, we examine the crystallization dynamics. Our results demonstrate that the chaotropic agent influences the crystal growth rate during the process, leading to more uniform films with suppressed non-radiative recombination. Furthermore, we challenge the conventional view that crystallization ceases upon solvent evaporation by uncovering photoluminescence changes during the cooling phase.

The optimized films achieve a photoluminescence quantum yield of 7.28% and a charge carrier lifetime exceeding 11 µs, culminating in a device efficiency of 22.34% and a fill factor surpassing 80%. This work offers critical insights into additive-mediated crystal growth and transient cooling dynamics, paving the way for the development of high-performance, stable Sn-Pb perovskite-based optoelectronic devices.

In this talk, I will present a unified interface engineering strategy for perovskite solar devices that couples self‑assembled monolayers (SAMs) at charge‑extraction interfaces with molecular additives within the perovskite bulk. Tailored SAMs provide precise energy‑level alignment, enhanced charge selectivity, and suppressed interfacial recombination, thereby improving efficiency, operational stability, and yield. Beyond single‑junctions, these SAMs enable low‑loss interconnection layers with efficient recombination, improved current matching, and reduced voltage penalties—capabilities that are essential for tandem solar cells.

On this foundation, we report two complementary advances for narrow‑bandgap Sn–Pb perovskites. First, a dipole‑engineered, hole‑transport‑layer‑free architecture is realized by incorporating an intracrystalline dipole‑active additive into the absorber. The additive establishes an internal field for selective hole extraction, regulates crystallization to curb non‑radiative recombination, and mitigates Sn oxidation and related defects. This approach yields efficient HTL‑free solar cells with power conversion efficiencies exceeding 23% and high‑detectivity near‑infrared photodetectors operating around 890 nm, demonstrating that bulk dipole design can substitute conventional hole transport layers while preserving selectivity and stability. Second, a bio‑inspired antioxidant stabilization scheme is introduced, wherein small‑molecule antioxidants function as bulk dopants to enhance oxidation resistance and cleanse grain boundaries, while larger polyphenols assemble at the surface to block oxygen ingress and establish favorable interfacial dipoles. This synergistic bulk/surface strategy reduces voltage losses, improves shelf stability, and also delivers Sn–Pb devices with high performance. 

Finally, we integrate these low‑bandgap Sn–Pb subcells into tandem architectures. SAM‑engineered interconnection layers ensure efficient charge recombination and low resistive losses, enabling high‑quality series connection in both all‑perovskite and organic/perovskite tandems. In the latter, a low‑bandgap organic subcell serves as a benchmark to assess and compare low‑bandgap perovskites, clarifying design rules that link interfacial chemistry, bulk defect control, and optical management to durable, high‑performance tandem photovoltaics.

Tin-lead (Sn–Pb) halide perovskites hold promise as narrow-bandgap semiconductors in future solar cells. Currently, non-radiative recombination induced open-circuit voltage losses limit their full potential. Additives are commonly used to increase the performance of metal halide perovskite solar cells.

We investigated the effect of glycine hydrochloride as additive during solution processing of Sn–Pb perovskites.^[1] By combining photovoltaic performance and stability, with determining the quasi-Fermi level splitting (QFLS), time-resolved microwave conductivity, and morphological and elemental analysis a comprehensive insight is obtained. Glycine hydrochloride retards the oxidation of Sn²⁺ in the precursor solution improves the grain size distribution and crystallization of the perovskite causing a smoother and more compact layer, reducing non-radiative QFLS on perovskite layers without and with charge transport layers it is found that glycine hydrochloride primarily improves the bulk of the perovskite layer but does not contribute significantly to passivation of the interfaces of the perovskite with either the hole or electron transport layer.

QFLS measurements were also used to study the interfacial non-radiative recombination losses of Sn–Pb perovskite solar cells and the passivation strategies.^[2] The intrinsic losses in the perovskite semiconductor and at its interfaces with PEDOT:PSS and C₆₀ charge transport layers contribute significantly to the overall voltage deficit. Surface passivation with alkane-diammonium iodides or cadmium iodide mitigates the non-radiative recombination induced by the C₆₀ electron transport layer by eliminating direct contact with the perovskite semiconductor. While each of these passivation strategies are beneficial, shortcomings remain in implementing them in actual devices because effective passivation of the perovskite can limit the efficient extraction of charges.

These were largely overcome by co-depositing a mixture of alkane-diammonium diiodides and amphiphiles at the interface. This bi-molecular strategy, eliminates C₆₀-induced losses and enhances the open-circuit voltage of Sn-Pb perovskite solar cells to 90% of the detailed balance limit. Interestingly, a similar approach also reduces non-radiative recombination at the interface between the perovskite and PEDOT:PSS layer and enhances the open-circuit voltage to 0.91 V, or 93% of the detailed-balance limit. The double-sided passivation strategy enables narrow-bandgap single-junction solar cells with an efficiency of nearly 23%.

Incorporating bromide into metal-iodide perovskites is a commonly used approach for widening the bandgap of lead-halide perovskites. We now explored  mixing of iodide and bromide in Sn–Pb perovskites to create a mixed-metal mixed-halide perovskite composition, achieving the optimal bandgap of 1.34 eV for single-junction solar cells.^[3] Supported by in-situ absorption measurements, it is found that the delay time between starting the spin-coating of the perovskite precursor and depositing the antisolvent is key in controlling the film morphology. The optimized Sn–Pb–I–Br perovskite did not show signs of light-induced halide segregation during prolonged illumination. Applying passivation to reduce non-radiative recombination at the perovskite - electron transport layer interface and optimizing the device configuration results in a power conversion efficiency of 19.0%. This is among the highest for perovskites in the 1.3 − 1.4 eV bandgap range reported to date.

Self-assembling molecules (SAMs) are currently considered to replace PEDOT:PSS as a standard hole-transport layer (HTL) for tin-lead (Sn-Pb) narrow-bandgap (NBG) perovskite solar cells. Such NBG perovskites require precursor additives for suppression of Sn²⁺ oxidation and to balance crystallization, in order to achieve high solar cell efficiencies. Pseudo-halide precursor additives such as lead(II)-thiocyanate (Pb(SCN)₂) are commonly employed for such purposes [1-3]. We report on an unintended effect that involves Pb(SCN)₂ in SAM based NBG perovskite solar cells. Upon increasing the amount of Pb(SCN)₂ up to 6 %, SAM-based layer stacks exhibit progressively reduced non-radiative carrier recombination and very good film morphology. Photoconversion efficiency reaches a maximum of 18.5 % for 1 % Pb(SCN)₂ in the precursor and drops drastically for higher concentrations. Time resolved photoluminescence (trPL) and surface photovoltage (trSPV) measurements reveal impeded carrier extraction in Pb(SCN)₂-containing layer stacks with Pb(SCN)₂ concentrations greater than 1 %. Surprisingly, from UV- and X-ray photoelectron spectroscopy it becomes clear that the amount of SAMs is reduced at the buried interface and that SAMs appear on the top surface of the perovskite layer. We conclude that the hole-selective SAMs relocate from the ITO to the perovskite top surface in presence of Pb(SCN)₂ and subsequently form an electron blocking layer between perovskite and C₆₀. The relocated SAMs can be addressed by washing the perovskite surface, which recovers electron extraction into the electron-transport layer (ETL), however overall device performance remains impeded. This study highlights that commonly used additives that were explored for PEDOT:PSS based layer stacks have to be adapted accordingly, when introducing SAMs in NBG solar cells.

Thanks to their superior bandgap tunability and high absorption coefficient, metal halide perovskites hold great promise for the fabrication of both single- and multi-junction photovoltaics capable of delivering high power conversion efficiencies at low cost^[1].

For all-perovskite multi-junction photovoltaics, one of the major challenges lies in the low quality of narrow-bandgap (~1.25 eV) mixed tin–lead perovskite films used as rear absorbers^[2]. In the conference, we will present our recent studies on the modification of the mixed tin–lead perovskites interfaces, covering the tin metal chelating^[3], surface dipole^[4,5], and in situ surface reaction^[6] modification strategies. On the other hand, we also show our recent investigations into the solution chemistry and crystallisation behaviour of tin–lead perovskites^[7,8]. We aim to provide general material insights for improving film quality and device performance.

As a result, we have achieved efficiencies exceeding 23.9% in single-junction tin–lead perovskite solar cells, with open-circuit voltages up to 0.91 V. Building on optimisations of wide-bandgap neat-lead perovskites (spanning ~1.5 to ~2.0 eV), we further demonstrate the successful integration of improved tin–lead absorbers into double-, triple-, and first-ever quadruple-junction tandem solar cells, reaching efficiencies over 29%, 28%, and 27%, respectively. In addition, we will outline promising strategies to enhance both the light and thermal stability of these perovskite subcells, aiming to improve the reliability of efficient multi-junction photovoltaics.

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Hybrid Sn–Pb perovskite solar cells are promising candidates for tandem photovoltaic applications; however, their performance is frequently constrained by pronounced photovoltage loss and limited operational stability, primarily arising from interfacial recombination and poorly controlled crystallization processes. In this talk, I will present our systematic strategy to regulate interfacial recombination and crystallization dynamics through targeted interface engineering and growth modulation. Specifically, dedoping PEDOT:PSS effectively reduces interfacial energy mismatch and suppresses non-radiative recombination, enabling more efficient hole extraction and a marked enhancement in open-circuit voltage.[1] Further modification using cPTANMe as a tailored hole transport layer improves interfacial energetics and charge selectivity, minimizing recombination losses at the perovskite/HTL interface. [2] In parallel, a fullerene derivative is introduced as an additive to modulate crystallization behavior, refine film morphology, and stabilize the perovskite lattice, thereby mitigating interfacial degradation and enhancing device durability. The synergistic control of charge-selective interfaces and crystal growth pathways not only leads to simultaneous improvements in photovoltage and operational stability, but also provides fundamental insight into the voltage–stability trade-off in hybrid Sn–Pb perovskite systems. These findings establish practical design principles for engineering high-performance and durable narrow-bandgap absorbers, paving the way toward more efficient and reliable next-generation tandem photovoltaic technologies.

Organic-inorganic metal halide perovskites have emerged as attractive materials for solar cells with power-conversion efficiencies of single-junction devices now exceeding 26%. Combinatorial optical characterization approaches are vital for probing and analysing such their electronic properties and material stability.

We demonstrate optical-pump THz-probe spectroscopy with controlled intervals of air exposure as an ideal technique to monitor air-induced degradation of optoelectronic parameters such as charge-carrier mobilities and recombination rates in low-bandgap lead-tin iodide perovskites.[1][2] We explore the best choice of A-cation in lead-tin iodide perovskites with intermediate lead-tin ratios and find that air exposure induces hole doping to a similar extent, for methylammonium (MA) formamidinium (FA), FA cesium (Cs) and FA-only cations. However, we find that MAFA-based perovskites are unstable under heat exposure owing to decomposition of MA, and FACs perovskites suffer from A-cation segregation and an accompanying non-perovskite phase formation.[2] Thus we propose that from a stability perspective, efforts should refocus on FASn_0.5Pb_0.5I₃ which minimizes all three effects while maintaining a suitable bandgap for a bottom cell and good performance.

Tin-halide perovskites currently offer the best photovoltaic performance of lead-free metal-halide semiconductors. However, their transport properties are mostly dominated by holes, owing to ubiquitous self-doping. We demonstrate a noncontact, optical spectroscopic method to determine the effective mass of the dominant hole species in FASnI₃, by investigating a series of thin films with hole densities finely tuned through either SnF₂ additive concentration or controlled exposure to air.[3] We accurately determine the plasma frequency from mid-infrared reflectance spectra by modeling changes in the vibrational response of the FA cation as the plasma edge shifts through the molecular resonance. Our approach yields a hole effective mass of 0.28me for FASnI₃ and demonstrates parabolicity within 100 meV of the valence band edge. An absence of Fano contributions further highlights insignificant coupling between the hole plasma and FA cation.[3]

We further discuss how crystalline film quality and halide segregation are critically affected by bromide fraction x in CH₃NH₃Pb(I_1−xBr_x)₃ through macrostrain and ordered-phase formation.[4] We show that the overall amplitude of phase segregation follows a broadly symmetric distribution in compositional space, maximized near x = 0.5, but the potentially ordered compositions of CH₃NH₃PbIBr₂ and CH₃NH₃PbI₂Br diverge sharply, presenting particularly stable and unstable scenarios, respectively. Notably, halide segregation is shown to occur even below the widely quoted perceived threshold of x = 0.2. Such analysis highlights promising approaches to mitigate halide segregation, through engineering of macrostrained phases and local atomistic ordering.

The development of perovskite solar cells (PSCs) has gone from strength to strength over the last decade, enabling low-cost, flexible and high-efficiency photovoltaic devices. However, long term device stability remains a challenge for the widespread implementation of this highly promising technology. In this talk I will present our recent work this area focused on addressing the critical stability challenges associated with spiro-OMeTAD based n-i-p configuration devices and reliance on hygroscopic lithium salts. I will also discuss our recent work on identifying iodine induced degradation pathways at hole transport layer / perovskite heterojunctions as well as additive engineering approaches to mitigate the harmful effect of iodine on materials and device performance. Finally, I will present our recent work on the design and application of self-assembled molecules SAMs as hole extraction layers for tin-based perovskite solar cells. The advances reported herein highlight how the combination of chemical design / synthesis and advanced optical & optoelectronic characterization can be employed to help guide the design of perovskite heterojunctions and devices with high performance and stability.

Understanding energy-loss mechanisms in perovskite materials and their photovoltaic devices under real operating conditions is essential for advancing perovskite solar cells toward commercial viability. Optical spectroscopy, a non-contact and non-invasive technique, is a powerful tool widely used to probe charge-carrier losses in perovskite materials and devices. While many studies have focused on non-radiative recombination in neat perovskite films—correlating these losses to open-circuit voltage (V_OC)—this work emphasizes charge-extraction losses under realistic conditions, such as maximum power point and short-circuit operation, using operando photoluminescence (PL) and hyperspectral PL microscopy. [1]

Nanoscale heterogeneities, including iodine vacancies, grain boundaries, and imperfect contact layers, can induce ion migration, surface failures, and inefficient charge extraction. These defects often limit performance, causing severe non-radiative losses and accelerated degradation. Here, we demonstrate how advanced operando and hyperspectral PL techniques reveal the impact of these inhomogeneities on charge-carrier dynamics. Operando spectroscopy quantifies energy losses across all working voltages due to charge accumulation, while hyperspectral PL microscopy visualizes these losses from the nanoscale to millimeter scale.

Finally, we present successful mitigation strategies—such as precursor engineering, process optimization, and interlayer development—that significantly enhance charge-extraction efficiency and device stability.[2-4]

Perovskite solar cells have achieved significant performance improvements in recent decades. Their low-temperature, solution-processable fabrication and composition-dependent bandgap tunability make them strong candidates for fully perovskite-based photovoltaic architectures. Incorporating Tin into Lead-based perovskites extends the absorption edge into the near-infrared (NIR), but the resulting mixed Lead–Tin compositions exhibit a lower absorption coefficient than pure Lead perovskites. This reduction increases the thickness required for efficient light harvesting from roughly 500 nm to beyond 1 µm. Achieving such thicknesses through a conventional one-step spin-coating process is challenging due to precursor solubility limits, and the charge-carrier diffusion length in Lead–Tin systems is typically insufficient for very thick films.

In this study, we combine optical and synthetic strategies to enhance the effective absorption of mixed Lead–Tin perovskites while maintaining high device performance. An ultrathin self-assembled monolayer (SAM) is employed as the hole-selective contact to suppress parasitic losses and improve charge extraction. Although SAMs offer a promising low optical loss hole-transporting layer for Lead–Tin devices, their implementation often leads to inconsistent device performance due to poor substrate coverage. By examining how SAM influences perovskite crystallisation charge transport in the whole perovskite lattice, we developed an optimised treatment that promotes more uniform SAM formation. This improvement significantly enhances device reproducibility and stabilises overall performance metrics.

We further implement a plasmonic enhancement scheme together with a tailored two-step perovskite fabrication route, enabling the formation of uniform Lead–Tin perovskite layers exceeding 1 µm in thickness with improved light absorptance near the band-edge. This integrated approach allows us to systematically assess and push the light-harvesting capability of thick mixed Lead–Tin perovskite solar cells.

High-quality and stable perovskite absorbers are crucial for a wide range of photovoltaic and optoelectronic applications, spanning single-junction solar cells, multi-junction architectures such as perovskite–silicon and all-perovskite tandems, as well as light-emitting devices and photodetectors [1, 2]. However, achieving high-quality perovskite films remains challenging due to such problems as incomplete crystallization, non-uniform halide incorporation, phase stability, and high defect densities, which often arise from the chosen deposition method and are closely linked to the targeted composition and bandgap. In this presentation, we introduce a potential multiple-deposition strategy for modulating the perovskite formation process, thereby enhancing crystallization kinetics and film uniformity, with potential broad applicability across a wide bandgap range.

By employing diverse deposition routes, including fully vacuum thermal evaporation and sequential hybrid that combine vacuum-deposited inorganic scaffolds with solution-based organic deposition, it becomes possible to control perovskite crystallization and tailor compositions and optoelectronic properties for targeted applications [3-5]. Coupled with additive and passivation strategies that improve crystallization and suppress defects, the precise coating and compositional control of vacuum-based methods, together with the flexibility of solution-based organic deposition, enable these complementary techniques to collectively advance perovskite deposition for specific optoelectronic properties [6], offering broad compatibility across diverse surfaces and providing a highly versatile and scalable platform for efficient materials screening and rapid optimization.

References:

1.         White, L.R.W., et al., ACS Energy Letters, 2024. 9(9): p. 4450-4458.

2.         Chin, X.Y., E. Albanesi, and A. Bruno. Springer Nature Switzerland: Cham. p. 1-24.

3.         Kosasih, F.U., et al., Joule, 2022. 6(12): p. 2692-2734.

4.         Dewi, H.A., et al., ACS Applied Energy Materials, 2025. 8(12): p. 7769-7779.

5.         De Luca, D., et al.,  ACS Energy Letters, 2025. 10(5): p. 2236-2240.

6.         White, L.R.W., et al., ACS Energy Letters 9 (3), 835-842

The development of efficient, stable, and sustainable perovskite solar cells (PSCs) continues to drive significant innovation in materials research and interface design. Recent advances in multicomponent Sn–Pb perovskite systems demonstrate how precise control over precursor chemistry and carefully engineered interfaces can substantially improve both device performance and long-term durability. By integrating molecular passivation with strategically optimized interface structures, researchers have produced methylammonium-free Sn–Pb perovskite devices that show markedly reduced non-radiative recombination, enhanced operational stability, and increased power conversion efficiency. Special emphasis is placed on understanding how the bulk composition of these materials influences interfacial behavior in inverted device architectures, revealing key mechanisms through which charge recombination can be effectively suppressed. Additional engineering strategies further strengthen these benefits, offering new routes to robust performance under realistic operating conditions. Collectively, these findings expand the range of viable material compositions for PSCs and support the development of scalable, environmentally responsible solar technologies capable of meeting future energy demands.

Despite the continuously increasing power conversion efficiency of perovskite photovoltaics, crystal damage and defect accumulation induced by coupled thermo-photonic stress under realistic outdoor operating conditions remain key bottlenecks for long-term device stability and large-scale deployment. To address this challenge, we have carried out a series of systematic studies on thermal cycling fatigue and self-healing regulation in perovskites, and progressively established a “crystal surface/interface synergistic regulation” paradigm that integrates dynamic self-healing, structural adaptivity, and contact optimization. Through interfacial chemical engineering, this body of work achieves multi-site cooperative defect passivation, ordered energy-level reconstruction, and effective mitigation and redistribution of local stress, thereby markedly suppressing non-radiative recombination and thermal-cycling-induced crystal fatigue degradation. In representative devices, a certified power conversion efficiency exceeding 27% has been realized. More importantly, after 200 stringent thermal cycles (–40–85 °C) and 2000 h of thermal aging at 85 °C, the devices retain over 90% of their initial performance, demonstrating excellent fatigue tolerance and operational reliability. Taken together, these studies elucidate surface/interface-dominated stabilization mechanisms from atomic to mesoscopic length scales and establish a generally applicable framework for perovskite surface/interface regulation, offering new materials and engineering pathways to mitigate fatigue failure and enable long-term stable outdoor operation.