| Organic photovoltaic (OPV) cell has attracted great interests due to its low cost manufacturing method and its flexible form factor. In this thesis, we utilize physical vapor deposition to fabricate different donor-acceptor interfaces and devices. First, we study the effect of substrate work function on the energy level alignment at these interfaces by ultra-violet photoemission spectroscopy (UPS). Second, OPV cells are fabricated based on the same sets of materials used in the UPS study. Third, a theory is proposed in order to explain the observed relationship between open-circuit voltage VOC and the donor-acceptor energy offset EDA. The main findings of this thesis are: (a) the substrate work function has negligible impact on energy level alignment between donor and acceptor; (b) VOC is closely related to EDA; and (c) a new proposed theoretical model predicts that VOC reaches a maximum if EDA is equal to the difference between singlet energy and a constant which relates charge transfer state energy to EDA. |
