Versatile Processability by Breaking the Symmetrical Chemical Structure of Nonfullerene Acceptors

Abstract

Despite the outstanding photovoltaic performance of symmetrical Y6, its use is limited in that only chloroform (CF), a highly volatile processing solvent, can induce favorable morphology. This dependence on a particular solvent poses an obstacle to mass production. Here, we investigate the effect of symmetry-breaking of nonfullerene acceptors (NFAs) on device performance and processability by employing chlorobenzene (CB) or CF processing solvents. In organic solar cells (OSCs) based on a symmetrical Y6 acceptor, a significant difference in the power conversion efficiency occurs between the OSCs fabricated using CB and those fabricated using CF. However, in OSCs based on IPC-BEH-IC2F with asymmetrical structure, no difference in photovoltaic performance occurs between the two OSCs. Grazing-incidence wide-angle X-ray scattering measurements indicate that IPC-BEH-IC2F exhibits nearly identical diffraction features in both the CB- and CF-processed photoactive films, whereas Y6 shows markedly different stacking structures. Because of these morphological features, OSCs based on Y6 are associated with a relatively large energy loss difference in CB and CF, whereas OSCs based on IPC-BEH-IC2F show no significant difference in energy loss. The introduction of an asymmetrical structure can therefore be an important strategy to enhance the versatile processability of OSCs based on NFAs for future mass production.