EAPSI: Moving Toward High-Efficiency, Cost-Effective Tandem Solar Cells through Real-time Study of Novel Substrate Preparation

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To meet the needs of a growing society, the clean, abundant power produced by solar energy holds tremendous promise. To scale up this powerful energy source, solar conversion efficiency must be maximized while manufacturing cost must be minimized. Optimization of tandem solar cell technology has the ability to cause a beneficial disruption in current commercial-scale solar energy research and production, resulting in higher efficiency, lower cost power. Tandem solar cells feature a stack of semiconductor materials. This unique configuration enables the absorption of more sunlight than one just material alone. Chemical surface pretreatment techniques will aim to reduce the thermal and time budget of the preparation of Silicon (Si) substrate surfaces in order to grow Indium Gallium Phosphide/Silicon (InGaP/Si) tandem solar cells with an optimized and cost-effective light-collecting configuration. Silicon is already a relatively cheap and very manufacturable semiconductor material which is efficient at collecting longer wavelength sunlight. Simplifying the growth on top of Si of high-quality indium gallium phosphide (which collects higher wavelength light) will make this tandem solar cell technology more scalable to industrial applications. This research will be conducted in collaboration with Dr. Masakazu Sugiyama of the Integrated Photonics Institute at the University of Tokyo.Metalorganic vapor phase epitaxy (MOVPE) growth of Gallium Phosphide (GaP) on silicon has been studied for several decades with the goal of providing a suitable surface with which to grow high-quality indium gallium phosphide (InGaP) in order to achieve InGaP/Si tandem solar cells with a 1.7 eV/ 1.1 eV configuration for optimized sunlight absorption. Few researchers have been able to produce GaP films with low dislocation densities, notably device-efficiency degrading anti-phase boundary (APB) defects, on silicon substrates. Silicon surface preparation involving vapor etching and surface passivation are non-equilibrium processes and are thus very dependent upon time. Without real-time analysis, important dynamic processes can be misunderstood and overlooked. Recent advancements in Si(100) surface preparation for APB-free GaP thin film growth has been greatly facilitated by real-time data provided by reflectance anisotropy spectroscopy (RAS) techniques. This study will be an in-depth look at RAS as it enables in-situ, real-time surface-sensitive analysis of the Si(100) dimerized surface undergoing preparation and subsequent GaP layer-by-layer epitaxial growth. This award under the East Asia and Pacific Summer Institutes program supports summer research by a U.S. graduate student and is jointly funded by NSF and the Japan Society for the Promotion of Science (JSPS).