Highly efficient polymer solar cells based on novel photocrosslinkable donor-acceptor conjugated polymers are fabricated and their long-term thermal stability is reported. After 72 h of thermal annealing at 150 degrees C, a stable power conversion efficiency as high as 4.7% is maintained. The control of active layer morphology and device performance through annealing is correlated with the synthetic design of the photocrosslinkable polymer.