A photosensitive device comprising: a first electrode and a second electrode; a series of organic photoactive layers disposed between the first electrode and the second electrode, each layer in the series being in direct contact with a next layer in the series, the series of organic photoactive layers being arranged to form at least one donor-acceptor heterojunction, the series comprising: a first organic photoactive layer comprising a first host material serving as a donor; a second organic photoactive layer comprising a second host material disposed between the first organic photoactive layer and a third organic photoactive layer; and the third organic photoactive layer comprising a third host material serving as an acceptor; wherein the first host material, the second host material, and the third host material are different, wherein the second organic photoactive layer serves as an acceptor relative to the first organic photoactive layer or as a donor relative to the third photoactive layer, and a distance from any point within the second host material of the second organic photoactive layer to a boundary of that layer is not more than one exciton diffusion length over a majority of the area of the second organic photoactive layer, and characterised in that the second organic photoactive layer is a unitary layer having openings there through, the first organic photoactive layer being in direct contact with the third organic photoactive layer through said openings. a first electrode and a second electrode; a series of organic photoactive layers disposed between the first electrode and the second electrode, each layer in the series being in direct contact with a next layer in the series, the series of organic photoactive layers being arranged to form at least one donor-acceptor heterojunction, the series comprising: a first organic photoactive layer comprising a first host material serving as a donor; a second organic photoactive layer comprising a second host material disposed between the first organic photoactive layer and a third organic photoactive layer; and the third organic photoactive layer comprising a third host material serving as an acceptor; a first organic photoactive layer comprising a first host material serving as a donor; a second organic photoactive layer comprising a second host material disposed between the first organic photoactive layer and a third organic photoactive layer; and the third organic photoactive layer comprising a third host material serving as an acceptor; wherein the first host material, the second host material, and the third host material are different, wherein the second organic photoactive layer serves as an acceptor relative to the first organic photoactive layer or as a donor relative to the third photoactive layer, and a distance from any point within the second host material of the second organic photoactive layer to a boundary of that layer is not more than one exciton diffusion length over a majority of the area of the second organic photoactive layer, and characterised in that the second organic photoactive layer is a unitary layer having openings there through, the first organic photoactive layer being in direct contact with the third organic photoactive layer through said openings.