An ever increasing need for energy in conjunction with an ever decreasing source pool of conventional energy reserves has impelled the scientific community towards the development of renewable energy resources. Amongst them, solar energy offers a great potential that could be tapped in due to the vast sunlight that the earth receives. Despite that, conventional Si based solar cells have not been in widespread usage because of high manufacturing and installation costs. The energy expended in fabrication of these Si based devices is another bottleneck. In recent years an alternate approach has attracted attention, making a solar cell with the photoactive layer of fully organic materials. These organic solar cells can be manufactured at very low costs and can be used in a number of ways to be integrated in the existing circuitry. However organic solar cells deliver efficiencies quite lower than conventional Si-based solar cells. A number of studies have shown the efficiency to improve with annealing. Therefore the influence of crystallinity, annealing and phase boundaries on the device performance has become the focus of attention. We have investigated organic solar cells consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric methyl ester (PCBM) as the active layer. Our results show that annealing at temperature of 150°C leads to crystallisation of PCBM while P3HT remains amorphous.