The inventions relates to a device comprising an anode, a cathode, a plurality of stacked organic photoactive regions, disposed between and electrically connected to the anode and the cathode, each organic photoactive region comprising an organic acceptor material and an organic donor material, and a plurality of exciton block layers, wherein for at least one of the organic photoactive regions, an exciton blocking layer of said plurality of exciton blocking layers is disposed adjacent to and in direct physical contact with the organic acceptor material of the respective organic photoactive region, wherein a LUMO of each exciton blocking layer other than the exciton blocking layer closest to the cathode is not more than about 0.3 eV greater than a LUMO of the adjacent acceptor material.