The nucleation and growth kinetics of fullerene crystals in thin films of a polymer:fullerene bulk-heterojunction blend are investigated. We find that both processes are strongly diffusion-limited at 100-110 °C due to the proximity to the glass transition temperatures of the blend components. Whereas the growth rate exponentially increases with temperature up to 230 °C, the nucleation rate displays a broad maximum around 150-170 °C, which coincides with the highest rate of fullerene crystallisation. A time-temperature- transformation (TTT) diagram reveals that across the investigated range of temperatures the low rate of nucleation is responsible for the formation of micrometre-sized crystals, which can be detrimental for polymer solar cells. Thus, we identify the lack of sufficient nucleation, which predominantly occurs on the substrate interface, as the origin of this important degradation mechanism.