Unencapsulated small molecular solar cells with a layer of carboxylated copper phthalocyanine (CuPc*) introduced between the donor copper phthalocyanine (CuPc) and acceptor 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDI), and using ITO and Ag as the cathode and anode were investigated for 35 days. The performance and stability were compared between traditional ITO/PEDOT:PSS/CuPc/PTCDI/Ag cells and the new ITO/PEDOT:PSS/CuPc/CuPc*/PTCDI/Ag cells. The comparison shows that the modified cell has an enhanced stability compared to traditional cells. The improvement of the lifetime is likely due to the CuPc* layer working as buffer layer to prevent humidity and oxygen from diffusing into the active layer. Different thicknesses of CuPc were investigated to study the effects of CuPc layer thickness on the electrical properties of these devices. It was found that the properties of the cell with 20 nm thickness of CuPc were the best. (C) 2012 Elsevier B.V. All rights reserved.