Fluorination of Carbazole-Based Polymeric Hole-Transporting Material Improves Device Performance of Perovskite Solar Cells with Fill Factor up to 82%

Polymeric hole-transport materials (HTMs) play a critical role in determining the device performance and stability of perovskite solar cells (PVSCs). Fluorination on the conjugated backbone has been found to have the potential to modulate the molecular planarity, electrical property, and photovoltaic performance of the resulting polymer materials. In this work, indacenodithieno[3,2-b]thiophene (IDTT) and the carbazole unit-based nonfluorinated polymer P1 and the fluorinated polymer P2 with two fluorine atoms at the 3- and 6-positions of the carbazole unit were carefully designed. Compared with the nonfluorinated polymer P1, the fluorinated polymer P2 has a deeper highest occupied molecular orbital (HOMO) energy level. Furthermore, the planar molecular structure of P2 also provides it with improved hole-extraction ability, which is favorable for reducing charge recombination at the perovskite/HTM interface. As a result, the suitable energy level together with the high hole-extraction ability of P2 endows the n-i-p PVSCs with an impressive power-conversion efficiency (PCE) of 19.4% with a high fill factor (FF) of 82%, relative to the limited 16.5% of the P1-based devices. Moreover, the smoother film surface and higher hydrophobicity properties of P2 also promoted an obviously enhanced long-term device stability of 90% of the initial PCE after 600 h of storage.