A new approach is presented in order to improve the thermal stability of polymer: [6-6]-phenyl C-61 butyric acid methyl ester (PCBM) bulk heterojunction solar cells. The central idea in this approach is the use of a polymer with high glass transition temperature (T-g), well above the normal operating temperatures of the devices. In this paper, a PPV-derivative with a T-g of 150 degrees C was used as an electron donor and the thermal stability of the obtained solar cells was compared with solar cells based on the reference material poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with a T-g of 45 degrees C. The use of the material with higher glass transition temperature resulted in a significant improvement of the thermal stability of the photovoltaic parameters. Furthermore, a systematic transmission electron microscope (TEM) study demonstrates that the better thermal stability of performance coincides with a more stable active layer morphology. Both improvements are attributed to the reduced free movement of the electron donor material (PCBM) within the active layer of the solar cell. (c) 2006 Elsevier B.V. All rights reserved.