Single-crystalline kesterite photovoltaics: routes toward improvement in open circuit voltage
Date
2020
Authors
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Publisher
University of Delaware
Abstract
With the advent of increasing solar energy production worldwide, there exists concern over raw material availability for photovoltaic materials. The Cu2ZnSn(S,Se)4 kesterite system (CZTSSe) has achieved the highest efficiency for solar cells among inorganic, Earth-abundant thin-film materials. This achievement is undercut by a significant underperformance in open circuit voltage (VOC) caused by band tail states and high levels of poorly understood defects which have caused a bottleneck in the path towards commercialization. This dissertation focuses on single-crystalline materials to explore select performance-limiting phenomena in the absence of the convoluting secondary effects that exist in thin-films. ☐ High-purity, single-crystalline materials with millimeter size scales are obtained via a solid-state ampoule-based growth method. Precursor selection and process optimization for large, uniform crystals is discussed. Small-scale photovoltaic devices with state-of-the-art voltages are fabricated on these single-crystalline absorbers. The device response to interface processing at the pn-junction is investigated by means of chemical treatment including a chemical-mechanical polishing step unique to single-crystal absorber materials. Thermally-evaporated ZnSe passivation layers are explored as well, leading to improvements in overall device performance for crystals with particularly high copper content. ☐ Device performance as a function of crystal cation composition is explored, verifying the commonly observed negative correlation between copper content and open-circuit voltage (VOC). Low copper densities are achieved via the quasi-equilibrium growth process, enabling state-of-the-art VOC ’s near 450 mV. Voltages are further improved via thermal treatments leading to a record 506 mV open-circuit voltage. These improvements are investigated further via photoluminescence and admittance spectroscopy. ☐ Finally, cation alloying of copper with silver in CZTSe is explored as a means of improving the defect landscape, and thus overall device performance. Ag/Group I ratios ranging between 0% and 30% are investigated. Fluence and temperature dependent photoluminescence spectra taken for a range of Ag alloying exhibit a more complex luminescence response than the pure copper counterparts. Finally, single-crystalline silver alloyed CZTSe devices are analyzed, demonstrating unoptimized VOC’s near 430 mV.
Description
Keywords
CZTS, Kesterite, Photovoltaics, Renewable energy, Semiconductors, Solar energy