Quantifying the role of interfacial width on intermolecular charge recombination in block copolymer photovoltaics

Block copolymers have the potential to control the interfacial and mesoscopic structure in the active layer of organic photovoltaics and consequently enhance device performance beyond systems which rely on physical mixtures. When utilized as the active layer, poly(3-hexylthiophene-2,5-diyl)-block-poly((9,9-bis-(2-octyldodecyl)fluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5′,5-diyl) donor-acceptor block copolymers have recently demonstrated 3% power conversion efficiencies in devices. Nevertheless, the role of the interfacial structure on charge transfer processes remains unclear. Using density functional theory, we examined charge transfer rate constants in model interfaces of donor-acceptor block copolymers. Our results demonstrate that intermolecular charge recombination can depend on the interfacial breadth, where sharp interfaces (ca. 1 nm) suppress intermolecular charge recombination by orders of magnitude.