The effect of cathode buffer layer on the stability of polymer solar cells with cesium carbonate (Cs(2)CO(3)) as the cathode buffer layer was investigated. Compared with traditional devices using lithium fluoride (LiF) as buffer layer, the use of the Cs(2)CO(3) layer has enhanced not only open-circuit voltages and power conversion efficiencies of the polymer solar cells due to better contact with the cathode but also the device stability. From results of stability test, we concluded that the inserted Cs(2)CO(3) buffer layer is working as a better shielding and scavenging protector which prevents the intruding of oxygen and humidity into the active layer, thereby improving the lifetime of unpackaged devices. (C) 2010 Elsevier B.V. All rights reserved.