High open circuit voltage organic photovoltaics: Minimizing energetic loss with a high band gap donor polymer and a small-molecule acceptor

Solution-processed, bulk-heterojunction Organic Photovoltaic (OPV) devices with a high V_oc (up to 1.35 V) were fabricated with a high band gap electron-donor copolymer blended with a small-molecule electron-acceptor. The copolymer (PInCZ), based on carbazole and indolocarbazole monomers, posseses a deep lying HOMO (Highest Occupied Molecular Orbital) of -5.6 eV. Coupling this polymer with the small-molecule acceptor (HPI-BT), with a relatively shallow LUMO (Lowest Occupied Molecular Orbital) near -3.6 eV, resulted in an effective electronic band gap in the donor-acceptor pair of approximately 2.0 eV. This value, which is important in determining the maximum V_oc attainable in the device, is substantially higher than the typical values found in OPV devices of ~1.0 - 1.5 eV. In the devices presented here, the thermodynamic loss in energy from the optical band gap compared to the V_oc is ~41%, which is substantially lower than that commonly found in OPV materials of 50+%.