Accelerated degradation of polymer:fullerene solar cells at elevated temperatures due to phase separation is one of the main issues affecting their lifetime. On page 1209, F. Piersimoni et al. report a strong reduction of the polymer:fullerene phase separation upon exposure to light. The photo-stabilization is quite independent of the polymer and is attributed to the dimerization of fullerene induced via photochemical 2+2 cycloaddition reaction. The cover picture shows crystals of the fullerene PCBM in a matrix of rr-P3HT, observed in films heated at 110 °C without exposure to light.