We report on the synthesis of new thermally cross-linkable all-conjugated diblock copolymers composed of a poly(3-hexylthiophene) (P3HT) block and an acrylate-functionalized polythiophene block. These copolymers are then used in bulk heterojunction (BHJ) solar cells with [6,6]-phenyl C61-butyric acid methyl ester (PCBM), and their photovoltaic performances are compared with standard P3HT/PCBM devices. Thermal cross-linking of the functional copolymers/PCBM blends is performed to improve the thermal stability of the active layer. BHJ photovoltaic cells with cross-linkable copolymers and PCBM show initial power conversion efficiencies slightly lower than that of P3HT devices. However, solar cells with cross-linkable copolymers retain more than 85% of their initial power efficiency value after 165 h of thermal annealing (accelerated aging test), whereas the same devices with P3HT retain less than 65% of their initial power efficiency. This improvement of the thermal stability of BHJ photovoltaic cells is the...