Isomeric Imidazole Functionalized Bithiophene-Based Hole Transporting Materials for Stable Perovskite Solar Cells and the Role of Donor Position

2,2,7,7-tetrakis(N,N-dip-methoxyphenylamine)-9,9-spirobifluorene (spiro-OMeTAD) is a popular hole-transporting material (HTM) in perovskite solar cells (PSCs), However, it suffers from high-cost and stability issues, which need to be overcome for PSC commercial-ization. Here, we report isomers of bithiophene-based HTMs func-tionalized with triarylamine and imidazole for PSCs. The planar 3-ImBT-2D (1) exhibits higher hole mobility than 5-ImBT-2D (2) via modulation of donor-group positions. The PSCs using HTM (1) deliver an excellent power conversion efficiency (PCE) of 21.73% with Li-TFSI doping and 17.79% without dopants. In addition, the Li-TFSI-free device based on 3-ImBT-2D yields a PCE of 21% after HTM surface modification with organic p-dopant dimethylanilinium tetrakis(pentafluorophenyl)borate (DPB). A molecular dynamics study shows that the isomer 3-ImBT-2D (1) folds up after deposition on perovskite films. As a result, the Li-TFSI-free devices exhibit higher stability, retaining 95.9% of the initial PCE after 800 h aging.