Excitonic solar cells based on single heterojunctions between electron donors, like copper phthalocyanine (CuPc), and electron acceptors, like C60, routinely obtain power conversion efficiencies of at least 1-2% (AM 1.5). Open-circuit photovoltages (VOC) are determined, in part, by the differences between the electron affinities (EA) and ionization potentials (IP) of the two contacting phases minus the exciton binding energy. For optimized TiOPc/C60 OPV devices, VOC increases by at least 0.1 volts, coupled with better near-IR responses than divalent metal phthalocyanine OPVs providing for the potential to obtain high efficiency solar cells in a single donor/acceptor heterojunction device. In all OPV devices, however, undesirable parasitic processes such as poor diode quality factors, or high series and low shunt resistances, detract significantly from device efficiency. The optimization of these devices must be preceded by a clear understanding of the correlation between the physical and chemical properties of each interface and the equivalent circuit parameters which control both the dark and photoelectrical behaviour. We have recently focused our attention on the way in which the composition of the bottom contact electrode (ITO) affects the deposition of the Pc layer and thereby the series and shunt resistances (Rs and Rp) as well as the reverse saturation current density and diode quality factor (Jo and n). Correlations are drawn to obtain a clear understanding between device chemistry and the factors of device performance. We show here how interface optimization can lead to an OPV device with the J/V characteristics of only one diode.