Three-dimensional π-conjugated compounds as non-fullerene acceptors in organic photovoltaics: the influence of acceptor unit orientation at phase interfaces on photocurrent generation efficiency

Abstract

Recently, organic photovoltaics (OPVs) using electron-accepting π-conjugated systems as non-fullerene acceptors have been extensively studied. The fine-tuning of donor–acceptor (D–A) interfaces in bulk-heterojunction (BHJ) structures is crucial for accomplishing high power conversion efficiencies (PCEs); however, the rational design of non-fullerene acceptors for control over the film morphology is still unclear. To investigate the influence of structural modification on D–A interfaces, we synthesized a series of three-dimensional (3D) π-conjugated acceptors that contain perylene bis(dicarboximide) (PDI) units. These compounds showed little difference in the molecular properties. However, OPVs containing a blend of the poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) donor and an acceptor showed different PCEs, ranging from 0.02 to 2.02%, and originating from the differences in the short-circuit current densities (J_SC). By investigating the blended film properties, we found that the degree of charge-separation mainly influences the photovoltaic characteristics of the OPVs. Furthermore, the J_SC of OPVs and the London dispersion (γ^d) components of the surface free energy of the 3D acceptors are correlated. Consequently, increasing the interfacial exposure of the π-conjugated framework increases the value of γ^d orienting the PDI π-planes toward the D–A interfaces, which is desirable for the efficient charge separation into free carriers. This study highlights the importance of γ^d for the molecular design of non-fullerene acceptors for BHJ-type OPVs.