Toxicity and instability of lead-based metal halide perovskites (MHP) have fueled explorations of new lead-free all-inorganic materials, especially in the CuI-AgI-BiI₃ phase space. In particular, Cu₂AgBiI₆ shows promising optoelectronic properties such as a high absorption coefficient of ~ 10⁵ cm^-1, a direct band gap of ~ 2 eV, a low exciton binding energy comparable to the thermal energy at room temperature, a modest mobility of 1.7 cm²V^-1s^-1 and a nanosecond charge-carrier lifetime.[1] However, the champion power conversion efficiency (PCE) of the Cu₂AgBiI₆ solar cells was only 2.39 %,[2] significantly lagging behind the PCE of the MHP counterparts. It has been shown that Cu₂AgBiI₆ exhibits ultrafast charge-carrier localization, which imposes the fundamental limits on its PCE.[3] However, such low PCEs cannot be solely attributed to the ultrafast charge-carrier localization, given that Cs₂AgBiBr₆, which also shows ultrafast charge-carrier localization and has an indirect bandgap, has achieved the champion PCE of 6.37 %.[4]

Herein, we aim to understand additional factors limiting the PCE of Cu₂AgBiI₆ beyond intrinsic ultrafast localization by investigating optoelectronic properties of charge transport layer (CTL)/Cu₂AgBiI₆ half stacks using CuI and PTAA as hole transport layers and SnO₂ and PCBM as electron transport layers. By analysing absorption spectra, X-ray diffraction (XRD) patterns and optical-pump terahertz-probe (OPTP) transients, we observe that the formation of quaternary phase Cu₂AgBiI₆ is influenced by the deposition of charge transport layers, especially by CuI and SnO₂, which modify its optoelectronic properties. Materials within the CuI-AgI-BiI3 phase space share similar band gaps and lattice parameters, making it challenging to confidently distinguish Cu₂AgBiI₆ from other impurity phases by using absorption spectra and XRD patterns alone. By extracting THz mobilities from OPTP, we confidently factor out Cu₂AgBiI₆ phase and identify impurity phases induced by the transport layer deposition. We find that deposition of CuI on Cu₂AgBiI₆ induces the formation of copper-rich quaternary Cu_xAgBiI_4+x phases near the interface and deposition of Cu₂AgBiI₆ on SnO₂ hinders the formation of the quaternary phase and instead forms ternary and binary phases, leading to the decrease in the mobility. Overall, we highlight that the deposition of CTLs can significantly affect the formation and optical properties of Cu₂AgBiI₆. Characterization of CTL/Cu₂AgBiI₆ half stacks is therefore critical to improve the device performance. Moreover, further developments are needed to suppress the formation of unwanted impurity phases upon deposition of transport layers.