Effect of electron-withdrawing terminal group on BDT-based donor materials for organic solar cells: a theoretical investigation

Abstract

Rational end group modification has been found as an effective strategy to improve power conversion efficiencies (PCEs) for photovoltaic materials. However, due to different electronic processes competition, various interaction factors must be taken into account to make materials design. Through density functional theory (DFT) and time-dependent DFT (TD-DFT), the effect of electron-withdrawing substitution on benzodithiophene-based donor materials from the open circuit voltage ( $$ V_{\text{OC}} $$ ), light absorption, exciton dissociation to charge transport in bulk materials has been investigated. The results point to that strong electron-withdrawing end group remarkably (1) enhances $$ V_{\text{OC}} $$ due to lowered HOMO energy level; (2) induces photon absorption redshift due to narrow optical gap (Eg); (3) facilitates exciton dissociation because of enhanced intramolecular charge transfer character. However, there is no direct correlation between electron-withdrawing ability and charge transport properties, since steric hindrance, noncovalent interaction and electrostatic interaction altogether have large impact on intermolecular stacking and then charge mobility. Comprehensive factors should be considered to improve PCEs for photovoltaic materials. Impressively, the designed molecule SM8 with dicyanovinyl-capped reveals excellent optical-electron properties, which may be a promising donor for high performance SM-OSCs.