Rational Design of Dopant-Free Coplanar D-π-D Hole-Transporting Materials for High-Performance Perovskite Solar Cells with Fill Factor Exceeding 80%

In this paper, two novel D-pi-D hole-transporting materials (HTM) are reported, abbreviated as BDT-PTZ and BDT-POZ, which consist of 4,8-di(hexylthio)-benzo[1,2-b:4,5-b ']dithiophene (BDT) as pi-conjugated linker, and N-(6-bromohexyl) phenothiazine (PTZ)/N-(6-bromohexyl) phenoxazine (POZ) as donor units. The above two HTMs are deployed in p-i-n perovskite solar cells (PSCs) as dopant-free HT layers, exhibiting excellent power conversion efficiencies of 18.26% and 19.16%, respectively. Particularly, BDT-POZ demonstrates a superior fill factor of 81.7%, which is consistent with its more efficient hole extraction and transport verified via steady-state/transient fluorescence spectra and space-charge-limited current technique. Single-crystal X-ray diffraction characterization implies these two molecules present diverse packing tendencies, which may account for various interfacial hole-transport ability in PSCs.