Effect of solvent additive on active layer morphologies and photovoltaic performance of polymer solar cells based on PBDTTT-C-T/PC71BM

Abstract

Photovoltaic properties of polymer solar cells (PSCs) are strongly affected by surface and bulk morphologies of their active layers. Herein, three solvent additives with different boiling points (BP) and asymmetric solvencies with respect to the polymer donor and fullerene-based acceptor were used to control the surface and bulk morphologies of the blend active layers based on PBDTTT-C-T and PC₇₁BM. Based on the detailed results obtained from photovoltaic measurements and morphological characterization, the correlations between the nature of the solvent additives, the surface and bulk morphologies of the blend films and the photovoltaic performance of the devices are rationally demonstrated. We found that the V_oc is determined by the contact surface potential, which is affected by the donor/acceptor composition at the top surface, while the J_sc and FF are heavily influenced by the sizes and the relative compositional fluctuations of the aggregations in the blends. It was also found that the relative solvencies of the additives to the solutes and the large difference of saturated vapor pressure (P^o) values between the host solvent (o-dichlorobenzene, o-DCB) and the guest solvent (additives with high BP) play the key roles in affecting surface and bulk morphologies of the PBDTTT-C-T : PC₇₁BM blend films. Overall, this work provides informative and useful guidance to select the best solvent additive for morphology control of polymer solar cells.