Passivating Defects and Optimizing Interfaces to Boost the Comprehensive Performance of Carbon-Cathode Hole-Transport-Layer-Free CsPbI2Br Solar Cells via Ionic Liquid

Abstract

Carbon-cathode hole-transport-layer-free (HTL-free) CsPbI2Br solar cells (PSCs) free of expensive HTLs and noble metal electrodes have emerged as promising photovoltaics due to their low processing cost and superior stability. However, defects arising from the annealing process in CsPbI2Br result in insufficient power conversion efficiency (PCE), significantly impeding their advancement. In this study, a bifunctional ionic liquid (IL) additive, 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][OTF]), has been demonstrated to effectively reduce these defects. It is revealed that the strong electron-withdrawing effect of -CF3 in OTF− facilitates the passivation of uncoordinated Pb2+ at grain boundaries. Concurrently, the non-protonated N atoms in imidazole rings of BMIM+ passivate the interface uncoordinated Sn4+ and Pb2+ defects in SnO2 and CsPbI2Br interfaces, while the intrinsic positive charge suppresses vacancy defects of I and O. Furthermore, the hydrophobic alkyl chains create a moisture-resistant surface barrier. The anions and cations of [BMIM][OTF] improve the crystallization of CsPbI2Br, accompanied by synergistic passivation of defects, thus accelerating the extraction and transport of photogenerated carriers. Accordingly, a leading power conversion efficiency (PCE) of 15.39%, up from 11.80% for the control device with a pristine CsPbI2Br layer, which is among the highest PCEs reported for corresponding solar cells, while significantly reducing current-voltage hysteresis and improving long-term stability.