Dopant-free Polymeric Hole Transport Materials for Efficient CsPbI2Br Perovskite Cells with a Fill Factor Exceeding 84%

Inorganic perovskite solar cells (PSCs) have attracted extensive attentions in recent years due to the excellent thermal stability. The CsPbI2Br PSCs, combining the merits of stable CsPbBr3 and efficient CsPbI3, demonstrate tremendous application potential. In this work, n-i-p structured inorganic CsPbI2Br PSCs are fabricated using ZnO as electron transport materials (ETMs) and doping-free PBDB-T and its derivatives as hole transport materials (HTMs). Alkylsilyl-substituted PBDB-T (i.e. PBDB-T-Si) based devices demonstrate the best efficiency of 15.60% (compared to 14.20% of the PBDB-T based reference device) with an ultrahigh fill factor over 84% due to the deep highest occupied molecular orbital energy levels, superior hole mobility and quasi-Ohmic contact characteristics. However, fluorine- and chlorine- substituted PBDB-T (i.e. PBDB-T-2F, PBDB-T-2Cl) based devices exhibit enhanced open circuit voltage but decreased short circuit current density due to the unbalanced hole extraction and large leakage current at the interface, giving moderate efficiency of 14.87 and 14.03% respectively. Our work provides a sophisticated analysis of various optoelectronic properties of polymeric HTMs, and points out that the quasi-Ohmic/Ohmic contact plays a critical role in determining device performance.