Role of Small Angle Grain Boundaries in CdTe Solar Cell Performance

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Abstract Title: Role of Grain Boundaries in CdTe Solar Cell Performance Non-Technical Description: Solar power is of interest to address the growing global energy demand as it is renewable, low environmental impact, and potentially low cost. Within thin film solar technology, power is generated from incident light absorbed in a microns-thin semiconducting active layer of the device, among a stack of other thin film layers on a supportive substrate to complete the device. Cadmium telluride (CdTe) solar cells are the current lowest price thin film technology, industrially mass-produced, and cost-competitive with traditional non-renewable electric power. Part of the success of this technology relies on producing high electronic quality material in large-scale processes. Industrial deposition time of the CdTe active layer is ~ 30 seconds. However, additional CdCl2 chemical treatment, required to improve material electronic quality and device performance, adds ~ 30 minutes processing time. CdTe films can be considered a composite of well-ordered crystalline grains and grain boundary materials with properties of both impacting device performance. The role grain boundary structure plays on the electronic properties of CdTe will be studied with the ultimate intention of identifying as-deposited structures that enable reduction or elimination of the time-consuming CdCl2 process to lower cost. By combining advanced material deposition to control CdTe grain boundary characteristics, theoretical modeling of these grain boundary structures, and fabrication and processing of full CdTe solar cells links between CdTe layer characteristics and solar cell device performance will be uncovered. The work has implications for energy and other technological applications of national interest. The PIs and the PhD candidates will engage in outreach activities that will contribute to the education of local K-12 students and teachers, the general public, and industry. Technical Description: CdCl2 treatment modifies CdTe by promoting crystalline grain growth, reducing the impact of defects, and changing grain boundary chemistry by replacing ~50% of the boundary Te with Cl. The role grain boundary structure plays on the electronic properties of CdTe will be studied with the ultimate intention of identifying as-deposited microstructures that enable reduction or elimination of the time-consuming CdCl2 process to lower cost. CdTe films will be sputter deposited with the substrate normal at an oblique angle relative to the deposition flux, producing material with controllable grain orientation and small angle grain boundaries. In situ real time spectroscopic ellipsometry will be used to monitor, study, and control the growth evolution of these films. Density functional theory calculations will be performed to identify energetics of crystallite orientations and grain boundaries. Controlled orientation CdTe will be incorporated into single junction solar cells as either full or partial replacement layers for normal incidence sputtered or close space sublimated CdTe. Impact of CdCl2 treatment and processing time on performance will be identified for devices with controlled orientation CdTe. Fundamental connections between grain structure and device performance will be evaluated, with grain boundary structure enabling reduced CdCl2 processing times identified