Na+-Intercalated Two-Dimensional Vermiculite-Modified TiO2 Electron Transport Layer for Enhanced CsPbI2Br Perovskite Solar Cells

Interface engineering of the electron transport layer (ETL) is an effective strategy for enhancing the performance and stability of all-inorganic perovskite solar cells (PSCs). Herein, Na+-intercalated vermiculite nanosheets were introduced as a low-cost natural interfacial modifier for the TiO2 ETL in CsPbI2Br PSCs. The nanosheets were prepared via an aqueous-phase exfoliation process and deposited onto the TiO2 surface to regulate the buried TiO2/CsPbI2Br interface. Morphological and spectroscopic characterizations revealed that vermiculite modification reduced the surface roughness of TiO2, altered its surface chemical/electronic environment and was associated with improved interfacial energy-level alignment. As a result, the quality of the CsPbI2Br film was significantly improved, leading to larger grains, enhanced crystallinity, a lower apparent trap-state density, and more efficient interfacial charge-transfer behavior. Electrical characterizations further confirmed enhanced charge extraction, reduced carrier recombination, and an increased built-in potential in the modified devices. Consequently, the vermiculite-modified devices exhibited simultaneous improvements in power conversion efficiency (PCE), open-circuit voltage, and short-circuit current density, with the best-performing device reaching a PCE of 13.94%. In addition, the modified device retained 84.51% of its initial PCE after storage in air for 35 days, significantly higher than the 71.1% retained by the control device. These findings highlight the potential of Na+-intercalated vermiculite nanosheets as low-cost natural modifiers for TiO2/CsPbI2Br buried-interface engineering in efficient and stable all-inorganic PSCs.