Suppressing intermolecular charge recombination in photovoltaics through conjugated block copolymer architectures

Block copolymers have the potential to control the interfacial and mesoscopic morphology of the active layer of organic photovoltaics and consequently enhance device performance. For example, the self-assembly of conjugated block copolymers into periodic microstructures with nanometer length scales could facilitate exciton dissociation by creating large amounts of donor-acceptor interfaces. Furthermore, the interfacial structure may strongly affect charge transfer processes. Using Density Functional Theory, we have examined charge transfer rates in model interfaces of poly(3-hexylthiophene)$-$block$-$poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2$\prime $,2?-diyl) donor-acceptor block copolymers which yield 3{\%} efficient devices when incorporated into solar cells. 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. Furthermore, we compare intramolecular and intermolecular charge transfer rates in donor-acceptor block copolymers through Constrained Density Functional Theory calculations.