Parallel bulk heterojunction photovoltaics based on all-conjugated block copolymer additives

Abstract

In recent studies, we demonstrated that the addition of block copolymers to binary donor–acceptor blends represents an effective approach to target equilibrium, co-continuous morphologies of interpenetrating donors and acceptors. Here, we report a study of the impact of all-conjugated poly(thieno[3,4-b]-thiophene-co-benzodithiophene)-b-polynaphthalene diimide (PTB7-b-PNDI) block copolymer additives on the electronic properties and photovoltaic performance of bulk heterojunction organic photovoltaic active layers comprised of a PTB7 donor and a phenyl-C₆₁-butyric acid methyl ester (PCBM₆₁) acceptor. We find that small amounts of BCP additives lead to improved performance due to a large increase in the device open-circuit voltage (V_OC), and the V_OC is pinned to this higher value for higher BCP additive loadings. Such results contrast prior studies of ternary blend OPVs where either a continuous change in V_OC or a value of V_OC pinned to the lowest value is observed. We hypothesize and provide evidence in the form of device and morphology analyses that the impact of V_OC is likely due to the formation of a parallel bulk heterojunction made up of isolated PCBM and PNDI acceptor domains separated by intermediate PTB7 donor domains. Altogether, this work demonstrates that all-conjugated block copolymers can be utilized as additives to both dictate morphology and modulate the electronic properties of the active layer.