Nontechnical The successful development of low-cost and high conversion efficiency solar cells will enable widespread use of solar electricity as an abundant source of electricity for a sustainable energy economy in the U.S. A proven method for producing the most efficient solar cells is to stack two materials in tandem such that one material absorbs the blue part of the solar spectrum and the other the red part. However, most solar cells that successfully employ this technique to date can only be produced using highly specialized single crystal materials. The complexity of growth methods and their use of expensive single crystal substrates have prevented these tandem solar cells from achieving low cost. This project proposes to explore stable and low-cost polycrystalline thin-film semiconductors, in particular one that absorbs the blue part of the solar spectrum, to enable low-cost and high conversion efficiency tandem solar cells. The integrative nature of the research and education will train and mentor graduate and undergraduate students in cross-disciplinary skills that are essential for developing innovative solutions as they enter the workplace and contribute to the U.S. leadership in the burgeoning field of electronics. The proposed project will benefit the education and research of graduate and undergraduate students and prepare them as the workforce of future energy industries.
Technical This project will develop the fundamental scientific knowledge that will lead to the fabrication of stable and efficient wide bandgap solar cells and eventually tandem cells made with two dissimilar materials. Polycrystalline thin film photovoltaic devices offer much lower production cost and complexity than epitaxial thin films, but their ultimate conversion efficiencies cannot go beyond the theoretical Shockley-Queisser limit for single-junction solar cells. A tandem cell made with two dissimilar materials, both low-cost polycrystalline thin films, would be an ideal choice for next generation low-cost and ultra-high conversion efficiency photovoltaic devices. The chief reason for failure to achieve high conversion efficiency polycrystalline thin-film tandem devices is the lack of an efficient top cell using suitable polycrystalline wide-bandgap semiconductor materials. Since the efficiency of a tandem device predominantly depends on the combined open circuit voltages of both semiconductors, a promising top cell must be able to produce high open circuit voltage. The proposed project will yield several break-through results: 1) approaches for synthesizing high-quality wide bandgap thin films will be developed; 2) fundamental defects physics of such wide bandgap semiconductors will be thoroughly understood; 3) buffer layers forming front and back junctions will be discovered and optimized; 4) Wide-bandgap top cells with high conversion efficiency and high open circuit voltage will be demonstrated; 5) If successful, it will enable the fabrication of low-cost and high conversion efficiency thin-film tandem solar cells.
