Enhancement of the morphology and open circuit voltage in bilayer polymer/fullerene solar cells

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%.