We have studied a range of organic photovoltaic devices (OPVs) based on a thin-film blend of P3HT and PCBM and show that, by comparing device studies with a characterization of the thermal transitions of the blend, we can provide a mechanistic description of the optimum annealing temperatures necessary to improve the operational efficiency of a device. For as-cast P3HT:PCBM blend thin-films we evidence two glass transition temperatures corresponding to the existence of two compositionally different amorphous states; an observation that is unreported to date. We demonstrate that an improvement in device efficiency only occurs once the film has been heated above the upper apparent glass transition temperature of the blend. If annealing is performed above the optimum temperature, excessive phase-separation and a partial reduction in film optical density leads to a general decrease in device efficiency. Both of these characteristic temperatures are dependent upon the composition of the blend. The temperature-dependent competition between such processes therefore opens a “window” within which device efficiency can be optimized and provides an opportunity to design effective annealing protocols for future polymer:fullerene blend OPVs.