Akash Dasgupta

Studying real-time degradation in perovskite semiconductors remains an important challenge. Conventional methods often cannot separate interfacial and bulk processes or rely on destructive analysis. Here, we demonstrate multiwavelength excitation photoluminescence (PL) as a nondestructive approach to distinguish interface- and bulk-dominated recombination losses. Using phenethylammonium tetrafluoroborate (PEABF4) treatment as a model, we use PL to show that PEABF4 passivates the electron transport layer (ETL)/perovskite interface, reducing interfacial recombination losses from fullerene deposition. However, upon thermal aging, PEABF4-treated stacks degrade more rapidly, with recombination losses broadening from the ETL/perovskite interface into the absorber. Diffusion–recombination simulations support interpreting excitation-wavelength-dependent PL decay signatures consistent with changes in surface recombination velocity and bulk lifetime, providing a framework for depth-sensitive comparisons during aging. Collectively, these results further establish multiwavelength excitation PL as a platform for studying recombination losses during aging of perovskite films.

We investigate the impact of employing fullerene self-assembled monolayers (SAMs) in conjunction with SnO2 electron-transport layers (ETLs) in negative–intrinsic–positive (n–i–p) perovskite solar cells. We compare the efficacy of the fullerene-SAM surface-binding group—carboxylic or phosphonic acid—on the passivation, charge transport, and energetics of the modified-SnO2 surface. Planar n–i–p perovskite solar cells with these SAM-modified SnO2 exhibit significantly reduced hysteresis, and the steady-state maximum power point tracked efficiencies (ηMPP) of the devices improved from an average of 18.6–20.0% comparing devices with neat SnO2 to the C60-PA SAM inclusion, respectively. While C60-PA SAM improved the initial solar cell efficiency, this benefit was not maintained during light exposure at elevated temperatures as the fullerene SAM weakened adhesion at the perovskite/metal oxide interface in the aged devices. These results highlight the importance of improving the mechanical robustness of perovskite/charge-transport layer interfaces under operational aging conditions.

Improving the bulk quality of perovskite films is critical for achieving higher-performance photovoltaic devices. Chloride-containing additives, including lead chloride (PbCl₂) and methylammonium chloride (MACl)─standard additives widely adopted in perovskite photovoltaics─are effective for controlling crystallization kinetics and grain morphology. However, the distinct impacts of different forms of chloride additives on nanoscale phase uniformity and luminescence homogeneity remain underexplored. Here, we provide new insights into how the choice and combination of chloride additives influence phase transitions and spatially uniform carrier dynamics within perovskite films. We demonstrate that strategically combining MACl and PbCl₂ improves crystallinity and optoelectronic uniformity across dimensions spanning micrometers to millimeters. Leveraging these findings, we fabricated inverted (p-i-n) perovskite solar cells achieving certified quasi-steady-state efficiencies of 26.4% and 24.5% at device areas of 0.05 and 1 cm², respectively. Furthermore, these devices exhibit robust operational stability, retaining 88% of their initial performance after 1200 h of continuous maximum power point tracking at elevated temperatures (65 °C) under simulated AM1.5G illumination. Our results elucidate the mechanistic differences between chloride additive forms, providing a viable strategy for advancing large-area, high-efficiency, and thermally stable perovskite photovoltaics.

Perovskite inks play critical roles in determining film quality and device performance, and ink stability is desired to ensure high device reproducibility. Here, we reveal the instability issue of current cesium-formamidinium lead triiodide (Cs _x FA _{1− x} PbI ₃ ) inks whose aggregation and precipitation tendencies are induced by excessively strong solvent-lead-halide coordination. By modulating coordination strength between precursor salts and solvents, we identify solvent coordination-dispersion equilibrium as the governing factor for ink stability and develop a stable ink that exhibits a remarkable increase in the shelf life. It effectively tunes ink drying and film crystallization, resulting in blade-coated perovskite films with excellent uniformity and low defect density. This enhancement led to increased aperture efficiency of ambient-fabricated p-i-n perovskite modules to 23.5%. The resultant devices also exhibit high durability, and 99% of the initial PCE was retained after 1700 hours of maximum power point tracking following the ISOS-L-2 standard protocol.

Performance losses in positive–intrinsic–negative architecture perovskite solar cells are dominated by nonradiative recombination at the perovskite/organic electron transport layer interface, which is particularly problematic for wider bandgap perovskites. Large endeavours have been dedicated to the replacement of fullerenes, which are the most commonly used class of electron transport layers, with limited success thus far. In this work, we demonstrate blending the fullerene derivatives [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) and indene-C₆₀ bis-adduct (ICBA) as a thin interlayer between 1.77 eV bandgap perovskite and an evaporated C₆₀ layer. By tuning the fullerene blend to a trace 2% by mass of PCBM in ICBA, we remarkably form an interlayer which features improved energetic alignment with the perovskite and the PCBM : ICBA fullerene mixture, together with a stronger molecular ordering and an order of magnitude higher electron mobility than either neat PCBM or ICBA. Additional molecular surface passivation approaches are found to be beneficial in conjunction with this approach, resulting in devices with 19.5% steady state efficiency, a fill factor of 0.85 and an open-circuit voltage of 1.33 V, which is within 10% of the radiative limit of the latter two device parameters for this bandgap. This work highlights the complex nonlinear energetic behaviour with fullerene mixing, and how control of the energetics and crystallinity of these materials is crucial in overcoming the detrimental recombination losses that have historically limited perovskite solar cells.

Narrow bandgap lead-tin perovskites are essential components of next-generation all-perovskite multi-junction solar cells. However, their poor stability under operating conditions hinders successful implementation. In this work, we systematically investigate the underlying mechanisms of this instability under combined heat and light stress (ISOS L-2 conditions) by measuring changes in phase, conductivity, recombination and current-voltage characteristics. We find an increased impact of the redistribution of mobile ions during device operation to be the primary driver of performance loss during stressing, with further losses caused by a slower increase in non-radiative recombination and background hole density. Crucially, the dominant degradation mode changes with different hole transport materials, which we attribute to variations in iodine vacancy generation rates. By quantifying the impact of these mechanisms on device performance, we provide critical insights for improving the operational stability of lead-tin perovskite solar cells.

In this work, we demonstrate that time-resolved photoluminescence data of metal halide perovskites can be effectively evaluated by combining Bayesian inference with a Markov-chain Monte-Carlo algorithm and a physical model. This approach enables us to infer a high number of parameters that govern the performance of metal halide perovskite-based devices, alongside the probability distributions of those parameters, as well as correlations among all parameters. Via studying a set of halfstacks, comprising electron- and hole-transport materials contacting perovskite thin films, we determine surface recombination velocities at these interfaces with high precision. From the probability distributions of all inferred parameters, we can simulate intensity-dependent photoluminescence quantum efficiency and compare it to experimental data. Finally, we estimate mobility values for vertical charge-carrier transport, which is perpendicular to the plane of the substrate, for all samples using our approach. Since this mobility estimation is derived from charge-carrier diffusion over the length scale of the film thickness and in the vertical direction, it is highly relevant for transport in photovoltaic and light-emitting devices. Our approach of coupling spectroscopic measurements with advanced computational analysis will help speed up scientific research in the field of optoelectronic materials and devices and exemplifies how carefully constructed computational algorithms can derive valuable plurality of information from simple datasets. We expect that our approach can be expanded to a variety of other analysis techniques and that our method will be applicable to other semiconductors.

Multijunction photovoltaics (PVs) are gaining prominence owing to their superior capability of achieving power conversion efficiencies (PCEs) beyond the radiative limit of single-junction cells^1–8, for which improving narrow-bandgap (NBG) tin–lead perovskites is critical for thin-film devices⁹. Here, with a focus on understanding the chemistry of tin–lead perovskite precursor solutions, we find that Sn(ii) species dominate interactions with precursors and additives and uncover the exclusive role of carboxylic acid in regulating solution colloidal properties and film crystallization and ammonium in improving film optoelectronic properties. Materials that combine these two functional groups, amino acid salts, considerably improve the semiconducting quality and homogeneity of perovskite films, surpassing the effect of the individual functional groups when introduced as part of separate molecules. Our enhanced tin–lead perovskite layer allows us to fabricate solar cells with PCEs of 23.9%, 29.7% (certified 29.26%) and 28.7% for single-junction, double-junction and triple-junction devices, respectively. Our 1-cm² triple-junction devices show PCEs of 28.4% (certified 27.28%). Encapsulated triple-junction cells maintain 80% of their initial efficiencies after 860 h maximum power point tracking (MPPT) in ambient. We further fabricate quadruple-junction devices and obtain PCEs of 27.9% with the highest open-circuit voltage of 4.94 V. This work establishes a new benchmark for multijunction PVs.

This study introduces a new method to analyse mobile ion-related processes in metal halide perovskite devices, which reveals how ion conductivity determines electric field screening and provides new insights into the mixed ionic–electronic behaviour.

Obtaining micron-thick perovskite films of high quality is key to realizing efficient and stable positive (p)-intrinsic (i)-negative (n) perovskite solar cells^1,2, but it remains a critical challenge. Here, we report an effective method for producing high-quality, micron-thick formamidinium-based perovskite films by forming coherent grain boundaries, where high-Miller-index-oriented grains grow on the low-Miller-index-oriented grains in a stabilized atmosphere. The resulting micron-thick perovskite films, with enhanced grain boundaries and grains, showed stable material properties and outstanding optoelectronic performances. The small-area solar cells achieved efficiencies of 26.1%. The 1-square-centimeter devices and 5 cm × 5 cm minimodules delivered efficiencies of 24.3% and 21.4%, respectively. The devices processed in a stabilized atmosphere presented a high reproducibility across all four seasons. The encapsulated devices exhibited superior long-term stability under both light and thermal stressors in ambient air.