Impact of Charge Transport Layers on the Structural and Optoelectronic Properties of Coevaporated Cu₂AgBiI₆

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.