We describe the influence of thermal annealing on the performance of polymer bilayer solar cell devices of regioregular poly(3-hexylthiophene), regiorandom poly(3-hexylthiophene), or poly(3-hexyl-2,5-thienylene vinylene) (P3HTV), with C 60 as the electron acceptor. In all cases, charge collection improves with heat treatment as the donor/acceptor layers intermix. Open circuit voltage, V oc, also improves with annealing temperature, trending consistently with a reduction in the dark forward current. With extreme temperatures, C 60 completely penetrates the polymer film, causing a reduction in V oc. This C 60 penetration in annealed devices can be prevented by inclusion of a pentacene electron-blocking layer at the anode. For P3HTV/C 60 solar cells, these treatments lead to peak power conversion efficiencies of 1.19%.