Constructing vertical phase separation of polymer blends via mixed solvents to enhance their photovoltaic performance

Abstract

A polymer blend comprising poly(3-hexylthiophene) (P3HT) donor and poly[2,7-(9,9′-octyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-5′,6′-bis(hexyloxy)-2′,1′,3′-benzothiadiazole)] (PFDTBT-OC6) acceptor is used as the active layer to fabricate all-polymer solar cells. The blend morphology variance processed with pure and mixed solvents, and the related photovoltaic performance, are investigated in detail. It is found that, due to its low surface energy, a thin P3HT enrichment layer on the top surface of the active layer greatly increases bimolecular recombination and results in S-kinks of the illuminated current density-voltage curves. With the incorporation of p-xylene (a marginal solvent of P3HT) in the blend solution, the P3HT enrichment atop the active layer surface is effectively decreased because the high boiling-point p-xylene suppresses the diffusion of P3HT chains toward the top surface during the film-drying process. The bimolecular recombination was thus improved and the S-kinks of the photovoltaic curves were completely removed. The overall power conversion efficiencies of the devices are strongly boosted (from 0.88% to 1.41%) when chlorobenzene:p-xylene mixed solvent is used to replace pure chlorobenzene.