CsBr-Buffered Self-Assembled Monolayers for Efficient and Stable Inverted Wide-Bandgap Cs _0.7 FA _0.3 PbI ₂ Br Perovskite Solar Cells

Recently, the Cs-rich Cs 0.7 FA 0.3 PbI 2 Br has shown good light and UV stability, and a suitable bandgap (1.8 eV) for tandem devices, with a theoretical power conversion efficiency (PCE) of 43.5%. To improve the performance of inverted Cs 0.7 FA 0.3 PbI 2 Br solar cells, self-assembled monolayers (SAMs) can be used to optimize band alignment and enhance interfacial properties. However, the poor wettability of SAMs greatly limits their application in Cs-rich perovskite solar cells. Here, a SAM layer, Me-4PACz, was first introduced into Cs-rich inverted Cs 0.7 FA 0.3 PbI 2 Br as a hole transport layer. By modifying the Me-4PACz with a 7 nm evaporated CsBr buffer layer, which is difficult to dissolve in DMF/DMSO mixed solution, we optimized the surface wettability and energy-level alignment at the Me-4PACz/perovskite interface. These modifications passivated interfacial defects and ultimately improved the device performance. Consequently, the PCE of the Cs-rich, wide-bandgap PSCs increased from 14.87% to 16.45%, which is the highest reported for Cs-rich Cs-FA devices. Furthermore, Cs 0.7 FA 0.3 PbI 2 Br exhibits outstanding photo and UV stability. When irradiated under AM 1.5G illumination for 1000 h in a nitrogen atmosphere, the PCE retained 90.30% of its initial value. The devices also showed negligible degradation after exposure to UV radiation with a total dose of 21 kW·h/m 2 . This highly stable, wide-bandgap perovskite presents a promising candidate for the top cell in tandem solar cell configurations.