| This thesis is an investigation of the fabrication, characterization, and performances of organic photovoltaic (OPV) devices with fullerenes as the primary light absorber. Various device architectures have been examined, including Schottky-barrier structure, bilayer heterojunction, bulk heterojunction, and tandem structures. Major accomplishments are summarized as follows.;The performance of indium tin oxide/molybdenum oxide/fullerene ( ITO/MoOx/C60) Schottky-barrier photovoltaic cells is highly sensitive to the method of depositing MoOx film. The highest open-circuit voltage (Voc) and short-circuit current density (Jsc) are obtained using in-situ thermally evaporated MoOx. XPS and UPS analyses indicate that pristine thermally evaporated MoOx has a high work function of 6.8 eV and Mo+6 oxidation state. In contrast, cells with MoO x prepared by sputtering and exposed to air produce lower efficiencies due to reduced work function.;The effect of HOMO energy level offset between a donor and an acceptor in a heterojunction, ΔEHOMO, on OPV cells has been investigated using a Donor/C60 structure, where C60 is the acceptor and the donor is chosen from a set of amines with various HOMO energy levels. The distinct features of the photovoltaic cells are: the Voc decreases with ΔEHOMO; the Jsc and fill factor increase with ΔEHOMO, reaching a maximum around ∼ 0.7 eV; and all three photovoltaic parameters decrease with further increase in ΔEHOMO.;Current-voltage (I-V) characteristics of bilayer heterojunction OPV cells based on a Donor/C60 structure are investigated. Through variation of the layer thickness and composition, specifically chemical doping donor with MoOx, we show that the hole-transport limitation in the donor layer is the determining factor in shaping the I-V characteristics of Donor/C60 cells.;Highly efficient OPV cells have been obtained using a unique bulk donor/acceptor heterojunction (BHJ) structure where the donor component is present in low concentration (< 10%) and with MoOx serving as a high work function Schottky barrier contact. With C60 or C70 as the acceptor, Voc (∼ 0.9 V) has been universally achieved with a wide variety of donor materials, including the prototypical donor, TAPC. Enhancement in efficiency has been achieved in tandem OPV cells using identical BHJ subcells based on TAPC doped C60 in series connection. |
