Decisive Role of Elevated Mobility in X55 and X60 Hole Transport Layers for High-Performance Perovskite Solar Cells

Both the transport properties in the hole transport layer (HTL) and the interfacial energetics between the HTL and perovskite in perovskite solar cells (PSCs) play crucial roles in determining the PSC performance. To prioritize their roles for the rational design of PSCs, the transport properties and interfacial energetics of two spiro[fluorene-9,9′-xanthene]-based small-molecule species termed X55 and X60 molecules on the perovskite after different doping treatments along with the performance of the corresponding PSCs were systematically investigated. Although the interfacial energetics of both molecules deposited on the CH3NH3PbI3 perovskite were determined to be favorable for hole transfer/injection by the in situ photoemission study, the PSCs with these molecular HTLs are extremely inferior due to their poor HTL conductivity/mobility. While the interfacial energetics after LiTFSI/tBP doping either barely change in X60 or even become more favorable in X55, the hole mobility in both films only improves slightly, leading to the best power conversion efficiency (PCE) of only 4-6% for their corresponding PSCs. Even though the doping effect triggered by subsequent air exposure leads to an unwanted increase of the hole injection barrier, the hole mobility in these films increases by about 1 order of magnitude, which dramatically promotes the corresponding PSC performance with champion PCEs of 19.89% for X55-based PSCs and 19.15% for X60-based PSCs, respectively. Thus, it is the significantly elevated HTL hole mobility that decisively promotes the corresponding PSC performance dramatically, which implies that the high carrier mobility in the transport layers often could be more vital over interfacial energetics for high-performance PSCs.