Bilayer heterojunction polymer solar cells using unsubstituted polythiophene via oxidative chemical vapor deposition

We demonstrate the use of a vacuum-based, vapor phase technique for the deposition of a donor polymer for use in polymer solar cells. Unsubstituted polythiophene (PT), which is insoluble and infusible and thus typically difficult to process, is easily prepared by oxidative chemical vapor deposition (oCVD). The oCVD process results in a conductive PT film that is heavily doped with FeCl3, which is used as the oxidizing agent. A post-deposition methanol rinse sufficiently dedopes the film and removes spent oxidant, leaving semiconducting PT with an optical bandgap close to 2 eV. Drastic changes in the film color, absorption spectra, and film composition confirm the dedoping process. The resulting semiconducting PT is then applied as an electron donor in bilayer heterojunction solar cells with a thermally evaporated C60 electron acceptor layer, resulting in power conversion efficiencies up to 0.8%. The absorption edge of the PT at ∼620 nm closely matches the edge present in the external quantum efficiency spectra, indicating that the oCVD PT contributes to the photocurrent of the devices. This demonstrates that the oCVD technique can be used in the processing and design of polymer active layers for polymer solar cells and hybrid small molecular organic solar cells without solubility, temperature, or substrate considerations.