We have carried out temperature dependent in-situ Raman as well as optical microscopy of poly(3-hexylthiophene) and [6,6]-phenyl-C-71 butyric acid methyl ester (P3HT:PCBM) films to investigate the structural and morphological evolution during heating and cooling cycles of thermal annealing. The full-width half-maximum of symmetric C=C stretching mode, C=C/C-C intensity ratio and Raman shift of C=C mode of P3HT measured during heating upto 130 degrees C as well as subsequent cooling revealed that: (i) molecular disorder in P3HT chains increase during heating cycle and (ii) on cooling the molecular ordering enhances rapidly at similar to 100 degrees C. However, heating to higher temperatures (> 150 degrees C) leads to the formation of PCBM aggregates. The mechanism of morphology evolution in P3HT:PCBM films envisaged through in-situ Raman and optical microscopy has been confirmed by other ex-situ techniques, such as, atomic force microscopy, scanning electron microscopy, UV-vis spectroscopy and X-ray elemental mapping. A correlation between the morphological evolution and various photovoltaic parameters of the fabricated P3HT:PCBM solar cells has been established. (C) 2013 Elsevier B.V. All rights reserved.