Phase controlled all-polymer bulk-heterojunction photovoltaic cells with high open-circuit voltage

All-polymer bulk-heterojunction photovoltaic cells based on poly(N-vinyl carbazole) and poly(9,9-dioctylfluorene-co-benzothiadiazole) co-casting films are investigated. One of the highest reported open-circuit voltage among the all-polymer solar cells is obtained due to the large energy offset between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor. Atomic force microscopy and photoluminescence study of the active layer with varied blending ratios and annealing temperatures reveals a phase controlled working mechanism, in which the charge generation efficiency is proved to be the main limiting factor of this material system. The incident photon-to-current conversion efficiency measurement shows a unique “double peak” phenomenon with increase in the annealing temperature, indicative of asynchronous chain movement between the two polymers due to their relatively large difference in glass transition temperature. Such a phase behavior might have potential value in further optimization of film morphology towards higher device performance.