Prof Henry Snaith FRS

Abstract:

Carrier mobility, recombination rates and diffusion length directly govern the efficiency of hybrid lead-halide perovskites. Yet, their behavior across different carrier concentrations and the effects of microstructure remain poorly understood. Using time-resolved photoluminescence and optical pump-THz probe spectroscopy, we quantify mobility, carrier recombination rates and diffusion length for polycrystalline films of methylammonium (MA)- and formamidinium (FA)-rich lead-halide perovskites, across carrier concentrations ranging from ∼10¹⁵ to ∼10¹⁹ cm^-3. For example, at a carrier concentration of ∼10¹⁸ cm^-3, FA_0.95MA_0.05Pb(I_0.95Br_0.05)₃ exhibits a mobility of 127 ± 9 cm² V^-1 s^-1 and a diffusion length of 392 ± 85 nm, compared to 69 ± 1 cm² V^-1 s^-1 and 139 ± 1 nm for MAPbI₃. These differences in mobility and diffusion length persist across different fluences, and are captured by a fluence-dependent rate model that accounts for both carrier generation and recombination at different material depths. From scanning electron microscopy and THz time-domain spectroscopy measurements, we attribute the increased mobility and diffusion length for the FA-rich perovskite mainly to a larger average grain size, after considering possible Fröhlich-type interactions between carriers and THz-active phonon modes. Our work establishes a mechanistic link between material microstructure and ultrafast carrier dynamics, informing crucial design principles for perovskite-based photovoltaic and optoelectronic applications.

Abstract:

Long-term stability of the perovskite top cell remains a hurdle to commercializing perovskite–silicon tandem (PST) solar cells. While accelerated tests provide valuable insights into degradation kinetics, they fail in predicting real-world degradation behavior. Keeping stressors constant, accelerated tests neglect dynamic conditions in actual operational environments, like diurnal and seasonal temperature and irradiance variability. We address this challenge by integrating a degradation function into our energy yield (EY) modelling software which integrates degradation in collection efficiency (and thus photocurrent) over time due to light and heat exposure, bridging the gap between accelerated testing and in real-world stability assessment. By linking the EY model to measurable material parameters like activation energy governing degradation pathways, this approach enables physically grounded degradation modelling. Based on degradation observed under accelerated tests, the model predicts PST operational lifetimes in diverse climates, highlighting the substantial discrepancy between lifetimes measured under accelerated testing and real-world locations. Applied to a PST solar cell, we show that an operational lifetime (T90,Agg) of about 1400 h under ISOS-L2 (1 Sun, 85 °C), translates to several months in-field (about 26 months in arid Phoenix and 42 months in temperate Seattle), demonstrating strong climate dependence. We also provide a practical mapping from ISOS-L2 to real-world lifetimes and estimate the minimum stability threshold needed for deployment as around 4000 h T90,Agg under ISOS-L2, translating to more than 5 years of operation across the investigated locations. After device-specific parameterization from appropriate aging tests, this device-agnostic framework allows stability-aware EY modelling to predict real-world degradation.

Abstract:

While metal halide perovskites have yielded remarkable power conversion efficiencies in photovoltaic applications, uncertainty concerning their long-term stability remains a significant barrier to widespread deployment. Previous studies have demonstrated that trace impurities present in perovskite precursor materials can influence the crystallisation dynamics of perovskite thin-films and hence, affect crystal structure, film morphology and optoelectronic properties. However, the nature of the impurities in formamidinium iodide (FAI) and their effect(s) on film quality and device performance remain underexplored. In this work, we carry out a detailed analysis of the impurities present in commonly used commercial FAI sources, and probe their impact on the composition, structure, and optoelectronic quality of the resulting perovskite thin-films and devices. We find that while some FAI impurities can improve the optoelectronic properties of solution-processed perovskite thin-films, in vapour-processed films, their presence alters the sublimation behaviour of FAI, favouring irreversible degradation pathways which lead to the formation of sym-triazine. While sym-triazine does not directly incorporate into the perovskite films, the impurity-driven variation in sublimation behaviour results in films which can deviate from the target stoichiometry, even under otherwise optimised conditions; and thus, do not fully convert into the desired photoactive phase, eventually causing poor material stability. Our results highlight the importance of understanding and controlling impurity concentrations in perovskite precursor materials as a route to enhancing both performance and process reproducibility in perovskite solar cells.

Abstract:

Lead-halide perovskite nanocrystals (NCs) have gained attention for optoelectronics, but careful selection of the antisolvent used for purification is essential to achieve high monodispersity and yield while minimizing surface damage. Current understanding indicates that this requires lowering the relative polarity of the antisolvent, yet high-polarity antisolvents are widely used for purification, as we confirm through data mining. We show that polarity alone is insufficient for antisolvent selection by comparing ethyl acetate and acetonitrile for CsPbI3 NC purification. Despite its higher polarity, acetonitrile yields improved colloidal stability compared to ethyl acetate. Using 1H NMR, FTIR, and XPS measurements, alongside DFT calculations, we demonstrate that acetonitrile acts as a stronger Lewis base, binding to and passivating the NC surface. Coordination of acetonitrile to the perovskite NC surface enhances stability and improves their performance in light-emitting diodes. These findings establish a mechanistic framework for antisolvent selection to realize bright and stable halide perovskite NCs.