Perovskite solar cells (PSCs) have emerged as an alternative photovoltaic technology reaching record certified efficiencies of 20.1% in only 6 years. Film morphology and crystalline structure of the perovskite are crucial factors to determine the final photovoltaic performance. In this work, the thermal transitions occurring during the crystallization of the CH3NH3PbI3-xClx perovskite were revealed by differential scanning calorimetry (DSC). Particularly, the calorimetric analysis suggest that an annealing temperature around 130 degrees C drives to better ordered perovskite structures as confirmed by X-ray diffraction (XRD) analysis. Indeed, the photovoltaic performance of the devices developed at such optimal temperature reached 8.7% average PCE which is in the state-of-the-art for the considered pin planar PSC. Our experimental observations highlight the relevance of DSC as a powerful tool to determine in situ the CH3NH3PbI3-xClx optimal crystallization conditions. This analysis provides a better understanding of the physical processes occurring during the perovskite crystallization and has the potential to be applied in the optimization of novel photovoltaic materials and structures. (C) 2015 Elsevier B.V. All rights reserved.