Side chain engineering of quinoxaline-based small molecular nonfullerene acceptors for high-performance poly(3-hexylthiophene)-based organic solar cells

Abstract

Poly(3-hexylthiophene) (P3HT) is one of the most used semiconducting polymers for organic photovoltaics because it has potential for commercialization due to its easy synthesis and stability. Although the rapid development of the small molecular non-fullerene acceptors (NFAs) have largely improved the power conversion efficiency (PCE) of organic solar cells (OSCs) based on other complicated p-type polymers, the PCE of P3HT-based OSCs is still low. In addition, the design principle and structure-properties correlation for the NFAs matching well with P3HT are still unclear and need to be investigated in depth. Here we designed a series of NFAs comprised of acceptor (A) and donor (D) units with an A2-A1-D-A1-A2 configuration. These NFAs are abbreviated as Qx3 , Qx3b and Qx3c , where indaceno[1,2- b :5,6- b′ ]dithiophene (IDT), quinoxaline (Qx) and 2-(1,1-dicyanomethylene)rhodanine serve as the middle D, bridged A1 and the end group A2, respectively. By subtracting the phenyl side groups appended on both IDT and Qx skeletons, the absorption spectra, energy levels and crystallinity could be regularly modulated. When paired with P3HT, three NFAs show totally different photovoltaic performance with PCEs of 3.37% (Qx3) , 6.37% (Qx3b) and 0.03% (Qx3c) , respectively. From Qx3 to Qx3b , the removing of phenyl side chain in the middle IDT unit results in the increase of crystallinity and electron mobility. However, after subtracting all the grafted phenyl side groups on both IDT and Qx units, the final molecule Qx3c exhibits the lowest PCE of only 0.03%, which is mainly attributed to the serious phase-separation of the blend film. These results demonstrate that optimizing the substituted position of phenyl side groups for A2-A1-D-A1-A2 type NFAs is vital to regulate the optoelectronic property of molecule and morphological property of active layer for high performance P3HT-based OSCs.