(521a) Conjugated Polymer and Block Copolymer Additives in Bulk Heterojunction Opvs (Invited Talk)

Bulk heterojunction organic photovoltaics (OPVs) are based on an interpenetrating blend of donor and acceptor in the active layer. Additives can be used to tune the morphology and enhance electronic properties. Here, we present work in the development of conjugated polymer and block copolymer additives to both direct morphology and enhance photovoltaic performance. The morphological impact of the additives can be predicted based on estimates of the Flory-Huggins interaction parameter, and we find good agreement with self-consistent field theory calculations. Additives can also have a surprisingly large impact on electronic properties. Interfacially active all-conjugated block copolymers with donor and acceptor polymer blocks enhance the open-circuit voltage (V_OC) at low loadings (~ 2 wt %) but have little impact on the short-circuit current. Conversely, conjugated polymer additives compatible with both donor and acceptor phases enhance the short-circuit current and have only a small effect on the V_OC. We hypothesize that the additives can form either phase-segregated â??parallel-likeâ? channels or influence charge recombination by segregating to donor-acceptor interfaces. This work demonstrates that conjugated polymer and block copolymer additives, properly tailored, can significantly enhance the properties and performance of bulk heterojunction OPVs.