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

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 (V_oc) 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 V_oc and fill factor. © 2023 Wiley-VCH GmbH.