Molecular Engineering of Polymeric Hole-Transporting Materials for Efficient and Stable Perovskite Solar Cells

Perovskite solar cells (PSCs) have achieved boosted efficiency growth in the last decade by utilizing 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD). Various dopant-free small molecules and polymers were introduced as hole-transporting materials (HTMs) to revamp the drawbacks of spiro-OMeTAD such as poor stability, high cost, and sophisticated processing. Herein, a polymer showing enhanced pi-conjugation, trithiophene-benzene (TTB), was put forward through molecular engineering by further modifying the previously reported HTM [2,4-dithiobiuret (DTB)]. One more thiophene was embedded into the main chain to increase the exposure of sulfur atoms to perovskite, as well as to enhance the conjugation effect. The continuously maintained pi-pi stacking guarantees smooth hole extraction from the perovskite layer. TTB-based PSCs not only show a high power conversion efficiency (PCE) of 18.2% in high-level devices with dopant-free organic HTMs but also demonstrate outstanding ambient stability, retaining 91.1% of their initial PCE after 30 days of storage.