Solar Cells From Earth-Abundant Materials

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1032955GordonIntellectual MeritSolar photovoltaic cells (PV) can provide an abundant and sustainable supply of electricity with relatively low environmental impact. However, their present high cost and use of scarce elements prevent solar PV from delivering a significant amount of energy. This proposed research seeks to provide a scientific foundation for an inexpensive solar PV manufacturing process using earth-abundant materials, i.e. elements that are available in sufficient amounts to produce all current energy needs by solar PV. These common elements include tin, zinc, copper, aluminum, oxygen and sulfur, along with much smaller amounts of less common elements, such as germanium and antimony. These materials will be used in very thin layers, so that much less material is needed compared to crystalline silicon solar cells. The initial research approach will use atomic layer deposition (ALD) of all the layers required in a solar cell made from sufficiently abundant elements. ALD offers precise control of composition, purity and thickness, so that the potential performance of these cells can be determined. However, ALD is slow and uses expensive vacuum equipment, so it is not suitable as an inexpensive method for mass production. The key to lowering manufacturing cost are rapid and continuous production using inexpensive, non-vacuum equipment to make thin-film solar cells with integrated interconnections. Thus, in later stages of the research, chemical vapor deposition (CVD) will be used to make the layers of the solar cell. CVD can make the required layers much more quickly than ALD, and can also be operated at atmospheric pressure without the need for vacuum equipment. The proposed research plan is designed to advance the understanding of structure and properties underlying the integration of earth-abundant semiconductor materials into solar PV devices. The structures of the proposed absorber materials, such as tin sulfide, are different from those of conventional semiconductors. Electronic structures, band energies, optical properties and structures of defects will be characterized for these poorly understood materials. Particular attention will be paid to the structure and electrical properties of grain boundaries and surfaces of the absorber material. Chemical passivation will be attempted to deactivate the recombination centers that typically reside at grain boundaries and surfaces, and nano-scale analyses will be used to locate individual atoms located at the grain boundaries. High-speed growth of the thin films is a critical step in the efficient manufacture of thin-film PV. To understand the growth process, chemicals created on the surface during growth will be identified, along with the byproducts that leave the surface. The kinetics of the surface reactions that convert the vapor sources into the thin films will also be measured. Broader ImpactsThe fabrication of PV devices based on earth-abundant, environmentally benign materials is a top priority for achieving sustainability of electricity production from solar energy. The broader impacts for this proposed research emphasize technology transfer in the context of graduate student training. Specifically, efforts will be made to translate the research to manufacturing platforms used by industry that make conductive glass substrates.