Suppression of radical cation formation in dopant-free hole-transporting materials to inhibit iodine migration for efficient and stable perovskite solar cells

Abstract

Developing dopant-free hole-transporting materials (HTMs) with high hole mobilities is essential to achieve efficient and stable inorganic perovskite solar cells (PVSCs). Herein, two linear organic small molecules IDTT-EtCz and IDTT-PhCz with D–A–D’–A–D configuration were designed and synthesized via two high yield steps, and they were successfully employed as HTMs with effective defect passivation in all-inorganic PVSCs. Notably, the IDTT-PhCz exhibits a deeper highest occupied molecular orbital energy level comparing with that of IDTT-EtCz, along with the enhancement of antioxidant activity towards iodine. Interestingly, IDTT-PhCz with aromatized terminal groups showed significantly increased short contacts and higher hole mobilities than IDTT-EtCz. Furthermore, the IDTT-PhCz has been proven to possess effective surface passivation capability and appropriate energy level alignment at the hole-extraction interface, efficiently suppressing recombination loss and enhancing charge collection. Finally, CsPbI₃-based PVSCs with IDTT-PhCz as dopant-free HTM achieve a champion power conversion efficiency (PCE) of 21.0 %, which is one of the highest values reported thus far for all-inorganic PVSCs. The optimized unencapsulated device maintains over 90 % of the initial PCE after 500 hours in a glove box at 60°C in the dark, indicating superior thermal stability. Additionally, the CsPbI₂Br PVSC based on IDTT-PhCz exhibits an impressive PCE of 18.0 %, and a CsPbI₂Br/organic tandem solar cell based on IDTT-PhCz achieves a high PCE of 25.0 % (24.66 % certified), which is one of the highest efficiencies among the n-i-p perovskite/organic tandem solar cells to date. Overall, this work demonstrates the superiority and generalizability of the D–A–D’–A–D-type design strategy for achieving efficient PVSCs.