Prof Henry Snaith FRS

Abstract:

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.

Abstract:

The copper-silver-bismuth-iodide compound Cu₂AgBiI₆ has emerged as a promising lead-free and environmentally friendly alternative to wide-bandgap lead-halide perovskites for applications in multijunction solar cells. Despite its promising optoelectronic properties, the efficiency of Cu₂AgBiI₆ is still severely limited by poor charge collection. Here, we investigate the impact of commonly used charge transport layers (CTLs), including poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), CuI, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and SnO₂, on the structural and optoelectronic properties of coevaporated Cu₂AgBiI₆ thin films. We reveal that while organic transport layers, such as PTAA and PCBM, form a relatively benign interface, inorganic transport layers, such as CuI and SnO₂, induce the formation of unintended impurity phases within the CuI-AgI-BiI₃ solid solution space, significantly influencing structural and optoelectronic properties. We demonstrate that identification of these impurity phases requires careful cross-validation combining absorption, X-ray diffraction and THz photoconductivity spectroscopy because their structural and optoelectronic properties are very similar to those of Cu₂AgBiI₆. Our findings highlight the critical role of CTLs in determining the structural and optoelectronic properties of coevaporated copper-silver-bismuth-iodide thin films and underscore the need for advanced interface engineering to optimize device efficiency and reproducibility.

Abstract:

We show that the use of 1,3-diaminopropane (DAP) as a chemical modifier at the perovskite/electron-transport layer (ETL) interface enhances the power conversion efficiency (PCE) of 1.7 eV band gap mixed-halide perovskite containing formamidinium and Cs single-junction cells, primarily by increasing the open-circuit voltage (VOC) from 1.06 to 1.15 V. We find that adding a postprocessing annealing step after C60 evaporation further improves device performance. Specifically, the fill factor (FF) increases by 20% in the DAP + postannealing devices compared to the control. Using hyperspectral photoluminescence microscopy, we demonstrate that annealing helps improve compositional homogeneity at the electron-transport layer (ETL) and hole-transport layer (HTL) interfaces of the solar cell, which prevents detrimental band gap pinning in the devices and improves C60 adhesion. Using time-of-flight secondary ion mass spectrometry, we show that DAP reacts with formamidinium (FA+) present at the surface of the perovskite structure to form a larger molecular cation, 1,4,5,6-tetrahydropyrimidinium (THP+), which remains at the interface. Combining the use of DAP and annealing the C60 interface, we fabricate Si-perovskite tandems with a PCE of 25.29%, compared to 23.26% for control devices. Our study underscores the critical role of the chemical reactivity of diamines at the surface and the thermal postprocessing of the C60/Lewis-base passivator interface in minimizing device losses and enhancing solar-cell performance of wide-band-gap mixed-cation mixed-halide perovskites for tandem applications.