High open-circuit voltage polymer/polymer blend solar cells with a polyfluorene copolymer as the electron acceptor

Abstract

Polymer/polymer blend solar cell with a high open-circuit voltage (V_OC) have been fabricated using a blend of poly(3-hexylthiophene) (P3HT) 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) as the active layer. The influence of thermal annealing on the early time excited state dynamics, the morphology of the P3HT:PFDTBT-OC6 blend films and the resulting photovoltaic performance have been investigated in detail. The thermally annealed P3HT:PFDTBT-OC6 blend solar cell demonstrates a power conversion efficiency of 1.80% with a V_OC of 1.36 V, much higher than the as-cast solar cell (0.02%). Transient absorption experiments disclosed that thermal annealing does not show a significant influence on the early time excited state dynamics of the P3HT:PFDTBT-OC6 blend. The severe geminate recombination of the charge pairs shortly after the charge transfer in the P3HT:PFDTBT-OC6 blend film still exists even if the blend film is subjected to thermal annealing, possibly due to the intrinsic donor/acceptor interfacial nature and the impure donor/acceptor phase separation. The enhanced photovoltaic performance after thermal annealing can be mainly attributed to the formation of interconnected networks of P3HT crystallites within the PFDTBT-OC6 matrix, which helps to improve the collection of photo-generated charges. The results implicate that restraining the dominant geminate recombination in the P3HT:PFDTBT-OC6 blend films is a key issue to boost the photovoltaic performance of polymer/polymer blend solar cells further.