Chemical and Electronic Investigation of Buried NiO_1−δ, PCBM, and PTAA/MAPbI_3–xCl_x Interfaces Using Hard X-ray Photoelectron Spectroscopy and Transmission Electron Microscopy

Identification and profiling of molecular fragments generated over the lifespan of halide perovskite solar cells are needed to overcome the stability issues associated with these devices. Herein, we report the characterization of buried CH₃NH₃PbI_3-xCl_x (HaP)-transport layer (TL) interfaces. By using hard X-ray photoelectron spectroscopy in conjunction with transmission electron microscopy, we reveal that the chemical decomposition of HaP is TL-dependent. With NiO_1-δ, phenyl-C₆₁-butyric acid methyl ester (PCBM), or poly(bis(4-phenyl) (2,4,6-trimethylphenyl)amine) (PTAA) as TLs, probing depth analysis shows that the degradation takes place at the interface (HaP/TL) rather than the HaP bulk area. From core-level data analysis, we identified iodine migration toward the PCBM- and PTAA-TLs. Unexpected diffusion of nitrogen inside NiO_1-δ-TL was also found for the HaP/NiO_1-δ sample. With a HaP/PCBM junction, HaP is dissociated to PbI₂, whereas HaP/PTAA contact favored the formation of CH₃I. The low stability of HaP solar cells in the PTAA-TL system is attributed to the formation of CH₃I and iodide ion vacancies. Improved stability observed with NiO_1-δ-TL is related to weak dissociation of stoichiometric HaP. Here, we provide a new insight to further distinguish different mechanisms of degradation to improve the long-term stability and performance of HaP solar cells.