Energy Band Alignment and Interfaces in FAPbI3 Perovskite Solar Cells: A Hard X-ray Photoelectron Spectroscopy...

Interfacial phenomena critically influence both the performance and long-term stability of perovskite solar cells; hence, optimizing interfaces remains a key challenge for their commercialization. In this study, we investigate the chemical interactions and electronic structure of the buried interface between the FAPbI3 perovskite absorber and spiro-OMeTAD-based hole transport layer (HTL) in an ITO/SnO2/FAPbI3/HTL device using hard X-ray photoelectron spectroscopy. Our results indicate Pb and I ion incorporation in the spiro-OMeTAD HTL. The Pb 4f core level spectra show the formation of new nonperovskite Pb species, and the N 1s spectra exhibit an additional peak, indicating chemical modifications induced by the deposition of spiro-OMeTAD on the perovskite layer. Moreover, the spiro-OMeTAD N 1s peak exhibits a systematic shift toward lower binding energies with increasing HTL thickness, indicating a downward band bending in the spiro-OMeTAD HTL. Overall, these findings provide direct insight into the chemical and electronic interactions near the FAPbI3/HTL interface and emphasize the importance of optimizing transport layer thickness to achieve favorable energy level alignment and improved device performance.