There is a continuing need within NASA for solar cells and arrays with very high specific power densities (1000-5000 kW/kg) for generating power in a new generation of ultralight space payloads. An emerging technology with promise to meet these ambitious goals are solar cells based on very thin films of discrete layers or interpenetrating networks (IPNs) of organic donors and acceptors. It is also recognized in the field of thin film inorganic solar cells, that it is possible to stack cells tuned to the blue, green and red portions of the solar spectrum, thus harvesting photons with less degradation of their energy. Theoretical efficiencies for multijunction solar cells are ~70% compared to ~30% for single junction devices. Indeed, these multijunction solar cells hold the current record for solar conversion efficiency. In this proposed joint STTR program between EIC Laboratories and the University of Florida, we will develop multijunction organic donor-acceptor solar cells as a means to achieve higher efficiencies than can be realized with single junction devices. Phase II will continue with development of high mobility, bandgap engineered organic donors and acceptors, as well as develop three junction devices and scaled up devices on lightweight flexible polymer substrates.