Origin of the efficiency improvement in all-polymer solar cells upon annealing

The origin of the enhanced efficiency upon annealing in solar cells based on blends of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] and poly{9,9-dioctylfluorene-2,7-diyl-alt-1,4-bis[2-(5-thienyl)-1-cyanovinyl]-2-methoxy-5-(3,7-dimethyl-octyloxy)benzene} is investigated. Current-voltage measurements on solar cells and single-carrier diodes reveal that the electron and hole transport is not improved for thermally treated devices. From the analysis of the photocurrent, the authors show that the dissociation efficiency of the bound electron-hole pairs increases upon annealing, due to an increase of the initial electron-hole separation distance.