| Organic photovoltaics, or OPVs, are an attractive potential source of low-cost renewable energy. One of the principle design considerations for OPVs is that light absorption creates a bound electron-hole pair, or exciton. To generate electricity in a device, excitons are dissociated at the interface between two dissimilar organic semiconductors, known as electron donors and electron acceptors. The efficiency of this organic heterojunction relies on the optoelectronic properties of each material as well as a nanostructured morphology between elecron donor and acceptor phase domains. Such systems have received significant research attention in the past decade. Recent advances in novel semiconducting materials to generate large open circuit voltage, Voc, and harvest a broader fraction of the sun's light have produced efficiencies as high as 8%. Further improvements require the additional development of such materials as well as an improved rational understanding of device operation.;The application of two low band gap electron donating semiconducting polymers in OPVs has been explored. These materials, poly(3-hexylthienylene vinylene) (P3HTV) and a copolymer of isothianaphthene, thiophene, and benzothiadiazole (PITN-co-ThBTD), were first optimized in bulk heterojunction blends with the electron acceptor [6,6]-phenyl C61-butyric acid methyl ester (PCBM). These materials were subsequently incorporated in a bilayer device architecture with C60 as the electron acceptor. Thermal annealing to mix the layers improves exciton dissociation and suppresses dark current, leading to an increase in Voc and improved efficiency.;Spurred by the relation between dark current and Voc, fundamental device measurements were performed to improve basic understanding of OPV device operation. The roles of heterojunction architecture, processing conditions, and chemical structure on dark current in poly(alkylthiophene) and poly(thienylene vinylene) devices were examined through temperature dependent current-voltage measurements and correlated with observed Voc under illumination. |
