Modulating the Self‐Assembly of Coplanar D–π–D Hole‐Transporting Materials via Alkyl Chain Engineering for Efficient and Stable Inverted Perovskite Solar Cells

Abstract

Rational introduction of flexible alkyl chains into rigid conjugated molecules is a facile but effective strategy to develop advanced organic semiconductor materials for considerable enhancement of photovoltaic performance. Herein, seven hole‐transporting materials (HTMs) via attaching ethyl, hexyl, or ethylhexyl to benzodithiophene π‐linker and bromoethyl, bromobutyl, or bromohexyl to phenoxazine donor (D), respectively, named B2P2, B6P2, B8P2, B6P4, B2P6, B6P6 (N01), and B8P6, are reported. Joint differential scanning calorimetry measurements and thin‐film absorption spectra of representative HTMs, B8P6, B6P4, and B2P2 reveal that alkyl chain modulation on coplanar D–π–D HTMs enables synchronous optimization of their solution processability, molecular packing, and thermal phase transition. Consequently, benefiting from the favorable self‐assembly and surface characteristics of hole‐transporting layers and further induced superior upper perovskite‐growth, B8P6‐ and B6P4 ‐based inverted perovskite solar cells exhibit decent power conversion efficiencies of 20.67% and 20.13%, respectively, prior to that of amorphous B2P2 (19.04%). Analysis on steady‐state/transient photoluminescence spectra and light intensity‐dependent short‐circuit photocurrent and open‐circuit voltage (Voc) demonstrates that more ordered assemblies of HTMs obtained via alkyl chain engineering can facilitate efficient interfacial charge transport/extraction and inhibit detrimental charge recombination, accounting for an enhanced Voc and fill factor.