Surface Passivation of CsPbI ₃ Films for Efficient and Stable Hole-Transporting Layer-Free Carbon-Based Perovskite Solar Cells

Hole-transporting layer (HTL)-free CsPbI 3 carbon-based perovskite solar cells (C-PSCs) are regarded as a promising photovoltaic candidate due to their low cost and enhanced device stability. However, the imperfect perovskite/carbon interface, including surface defects of CsPbI 3 films, unmatched energy level alignment, etc., leads to a low power conversion efficiency (PCE) and thus hampers its further development for commercialization. Herein, a multifunctional interface modifier octylammonium iodide (OAI) is introduced into the CsPbI 3 /carbon interface, which can not only reduce the amount of residual PbI 2 at grain boundaries by converting PbI 2 to the (OA) 2 PbI 4 two-dimensional (2D) phase but also passivate defects located at the surface and grain boundaries of CsPbI 3 films. Consequently, greatly reduced defect density of CsPbI 3 films as well as matched energy level alignment of the CsPbI 3 /carbon interface are achieved, which significantly boost the PCE of CsPbI 3 C-PSCs from 12.97 to 14.64%. Moreover, due to the reduced amount of PbI 2 at grain boundaries and the hydrophobic property of long-chain alkyl in OAI, the unencapsulated CsPbI 3 C-PSCs demonstrate excellent long-term ambient stability, which can retain 91% of its initial PCE after 30 days of storage in air.