L.C.Palilis, M.Vasilopoulou, A.M.Douvas, D.G.Georgiadou, S.Kennou, N.A.Stathopoulos, V.Constantoudis, P.Argitis “Solution processable tungsten polyoxometalate as highly effective cathode interlayer for improved efficiency and stability polymer solar cells” Solar Energy Materials & Solar Cells 114, 205–213, (2013)