Mesomeric control of the optoelectronic properties of polymerized small molecule acceptors

Regioregular polymerized non-fullerene small molecule acceptors (PSMAs) are currently the most promising candidates to achieve highly efficient all-polymer solar cells. Previous studies showed a systematic correlation between polymerization sites and the properties of the PSMA, where one of the regioisomers (γ) displays more favorable optoelectronic and photovoltaic properties compared to the other (δ). However, these systematic observations have not been rationalized yet. Here, using first principles calculations, we reveal that the substituents on the cross-conjugated terminal unit of the SMA core are responsible for improving the optoelectronic properties of γ-PSMAs, by extending the conjugation length and enhancing favorable mesomeric effects. Importantly, we demonstrate that these effects may be exploited as a universal molecular design principle. We predict that functionalizing PSMAs in selected positions with electron-withdrawing groups could further improve their optoelectronic properties. Our study provides a quantitative framework for understanding and rationalizing the relationship between polymerization sites and optoelectronic properties in regioregular PSMAs; in addition, our results pave the way for a rational design principle to further improve the properties of these conjugated polymers and, in turn, the efficiency of all-polymer solar cells.