Anchorable Polymers Enabling Ultra-Thin and Robust Hole-Transporting Layers for High-Efficiency Inverted Perovskite Solar Cells.

Currently, the development of polymeric hole-transporting materials (HTMs) lags behind that of small-molecule HTMs in inverted perovskite solar cells (PSCs). A critical challenge is that conventional polymeric HTMs are incapable of forming ultra-thin and conformal coatings like self-assembly monolayers (SAMs), especially for substrates with rough surface morphology. Herein, we address this challenge by designing anchorable polymeric HTMs (CP1 to CP5). Specifically, coordinative pyridyl groups are introduced as side-chains on poly-triarylamine (PTAA) backbone with varied contents by copolymerization method, resulting in chemical interactions between polymeric HTMs and substrates. The strong interaction allows them to be processed into ultra-thin, uniform, and robust hole-transporting layers through employing low-concentration solutions (0.1 mg mL, vs. 2.0-5.0 mg mL for conventional PTAA), greatly decreasing charge transport losses. Moreover, upon systematically tuning the pyridyl substitution ratio, the energy levels, surface wetting, solution processability, and defect passivation capability of such anchorable HTMs are simultaneously optimized. Based on the optimal CP4, we achieved highly efficient inverted PSCs with power conversion efficiencies (PCEs) up to 26.21 %, which is on par with state-of-the-art SAM-based inverted PSCs. Furthermore, these devices exhibit enhanced stabilities under repeated current-voltage scans and reverse bias ageing compared with SAM-based devices.