All of the current efficient organic photovoltaic devices have a bulk heterojunction (BHJ) structure. Regular BHJ films, formed from randomly mixed donors and acceptors in solution, inevitably have many breaks and dead ends, and have uniform distribution of donors and acceptors along the vertical direction which does not match the gradient electron and hole current across the films. The nonideal morphologies inevitably cause the charge recombination both in the BHJ films and at the metal/organic interface, in forms of geminate recombination, or bimolecular recombination. Compositionally graded BHJ films, donor enriched at the anode and acceptor enriched at the cathode side, were proposed to facilitate the charge extraction and to reduce charge recombination in several highly efficient OPVs.
However, the graded BHJs with preferred composition profiles were only observed in a few high efficiency material systems which require special substrate surfaces and suitable surface energy differences between donors and acceptors for its formation. Here we report a universal method to form the compositionally graded BHJ using a simple solvent-fluxing treatment of the wet BHJ films, in which the fluxing solvent brings the additives, as well as fullerene-derivatives, toward the film surface. The graded BHJ significantly reduces bimolecular charge recombination. The fluxing process also resulted in more uniform nanodomain distribution in the plane of the films and higher charge carrier mobilities. An efficiency enhancement of 15~50%, with highest efficiency of 8.6% for poly[4,8-bis-(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-alt-[2-(2′-ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl] (PBDTTT-C-T):C71-butyric acid methyl ester (PC70BM) system, was achieved for all of the solution-processed BHJ systems studied compared to those devices with regular BHJ films.