Regulating Bulk-Heterojunction Molecular Orientations through Surface Free Energy Control of Hole-Transporting Layers for High-Performance Organic Solar Cells

Interface properties are of critical importance for high-performance bulk-heterojunction (BHJ) organic solar cells (OSCs). Here, a universal interface approach to tune the surface free energy (gamma(S)) of hole-transporting layers (HTLs) in a wide range through introducing poly(styrene sulfonic acid) sodium salts or nickel formate dihydrate into poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is reported. Based on the optimal gamma(S) of HTLs and thus improved face-on molecular ordering in BHJs, enhanced fill factor and power conversion efficiencies in both fullerene and nonfullerene OSCs are achieved, which is attributed to the increased charge carrier mobility and sweepout with reduced recombination. It is found that the face-on orientation-preferred BHJs (PBDB-TF:PC71BM, PBDB-T:PC71BM, and PBDB-TF:IT-4F) favor HTLs with higher gamma(S) while the edge-on orientation-preferred BHJs (PDCDT:PC71BM, P3HT:PC71BM and PDCBT:ITIC) are partial to HTLs with lower gamma(S). Based on the surface property-morphology-device performance correlations, a suggestion to select a suitable HTL in terms of gamma(S) for a specific BHJ with favored molecular arrangement is provided. This work enriches the fundamental understandings on the interface characteristics and morphological control toward high-efficiency OSCs based on a wide range of BHJ materials.