Constructing D-π-A Type Polymers as Dopant-Free Hole Transport Materials for High-Performance CsPbI2Br Perovskite Solar Cells

Hole-transporting materials (HTMs) play a major role in efficient and stable perovskite solar cells (PSCs), especially for CsPbI2Br inorganic PSC. Among them, dopant-free conjugated polymers attract more attention because of the advantages of high hole mobility and high stability. However, the relationship between the polymer structure and the photovoltaic performance is rarely investigated. In this work, we choose three similar D-π-A-type polymers, where the D unit and π-bridge are fixed into benzodithiophene and thiophene, respectively. By changing the A units from classic benzodithiophene-4,8-dione and benzotriazole to quinoxaline, three polymers PBDB-T, J52, and PE61 are utilized as dopant-free HTMs for CsPbI2Br PSCs. The energy levels, hole mobility, and molecular stacking of the three HTMs, as well as charge transfer between CsPbI2Br/HTMs, are fully investigated. Finally, the device based on PE61 HTM obtains the champion power conversion efficiency of 16.72%, obviously higher than PBDB-T (15.13%) and J52 (15.52%). In addition, the device based on PE61 HTM displays the best long-term stability. Those results demonstrate that quinoxaline is also an effective A unit to construct D-π-A-type polymers as HTMs and improve the photovoltaic performance of PSCs.