Effects of cathode buffer layers on the efficiency of bulk-heterojunction solar cells

The effects of cathode buffer layers on the bulk-heterojunction solar cells are investigated. Comparing with the device without buffer layer, obvious enhancements of Voc from 0.38 to 0.65 V and fill factor from 44% to 63% have been achieved by using 2 nm bathocuproine layer, which make the efficiency of the devices improved from 1.63% to 4.11%. Alternatively, lithium fluoride and/or tris(8-hydroxyquinolinato) aluminum were also introduced for clarification purpose. X-ray photoelectron spectroscopy study indicates that the degradation caused by the outer diffusion of carbon from active layers plays a crucial role in the device performance.