We have studied the effect of thermal annealing on the photovoltaic properties of an organic solar cell based on an efficient donor/acceptor combination of pentacene / N,N'-Diocty1-3,4,9,10-perylenedicarboximide (PTCDI-C(8)) discrete heterojunctions. As deposited samples showed a maximum power conversion efficiency about 1.12% under standard A.M 1.5 illumination, whereas 1.60% power conversion efficiency has been achieved after annealing the samples at 100 degrees C for 5 min. External quantum efficiency (EQE) is also found to be increased from similar to 60% to similar to 70% after heat treatment. Photocurrent action spectra and the absorption profile of this donor/acceptor combination confirm the efficient light harvesting capability of the device throughout the visible region of the solar spectrum. Morphology and crystallinity of the active layers improved significantly after annealing the samples, resulting in an almost two times enhancement in the short-circuit current density and improved overall cell performances. (C) 2010 Elsevier B.V. All rights reserved.