Organic solar cells (OSCs) based on the blends of poly(3-hexylthiophene) and fullerene derivative [6, 6]-phenyl-C(61) butyric acid methyl ester have been fabricated with p-type chromium oxide film as hole-transporting layer. The distribution of temperature and heat flow in vacuum chamber was investigated by temperature probes and finite element method. It is found that the two factors have significant effect on the quality of the thermally evaporated Al cathode. The bigger momentum impinging Al particles facilitates the formation of C-Al or C-O-Al bonds at the interface between photoactive layer and Al electrode, which is propitious to the electron transfer, and the corresponding devices have higher electron mobility. The higher substrate temperature that induced by thermal radiation during Al evaporation can act as a function of thermal annealing treatment, which can eliminate some electron traps and lead to better ohmic contacts for electrons to transport at the interface. The combination of the bigger momentum impinging Al particles and the higher substrate temperature can induce the improvement of electron mobility and OSC performance. (C ) 2011 Elsevier B.V. All rights reserved.