Several thermal annealing strategies are employed to account for the formation of PCBM aggregates in a blend of P3HT/PCBM thin films. Optical absorption spectroscopy reveals dependence of PCBM cluster formation upon different annealing strategies as indicated by optical microscopic images. Using Raman image mapping of the selected locations we were able to identify PCBM rich regions. Exciton generation rate as a function of thermal annealing is in good correlation with short circuit current density. The probability of exciton dissociation as a function of thermal annealing reveals an almost static behaviour in response to the temperature rise up to 75 degrees C. The enhanced intensity of photoluminescence signal upon thermal annealing supports the increase in exciton generation rates resulting from formation of PCBM clusters and recrystallisation of P3HT domains, removing nonradiative recombination centres. Current density voltage characterisation reveals dependence of device parameters on thermal annealing strategies. (C) 2013 Elsevier B.V. All rights reserved.