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.