We have synthesized photo-cross-linkable azide-functionalized poly(3-hexylthiophene) to explore improvements in the thermal stability of bulk heterojunction solar cells. Exposing blends of photo-cross-linkable polythiophene and [6,6]-phenyl-C-61-butyric acid methyl ester to ultraviolet light preferentially cross-linked the polythiophene without degrading its optical or electrical properties. X-ray scattering measurements showed that cross-linking slightly compacted the polythiophene chain lamellar stacking while increasing the polymer crystal coherence length by 20%. Optimized solar cells having cross-linked active blend layers retained 65% of their initial photovoltaic power conversion efficiency after 40 h of thermal annealing at 110 degrees C, while devices using un-cross-linked commercial polythiophene underwent significant phase separation and retained less than 30% of their initial efficiency after annealing.