Co-electroplated Kesterite Bifacial Thin-Film Solar Cells: A Study of Sulfurization Temperature

Abstract

Earth-abundant material, kesterite Cu2ZnSnS4 (CZTS), demonstrates the tremendous potential to serve as the absorber layer for the bifacial thin-film solar cell. The exploration of appropriate sulfurization conditions including annealing temperature is significant to gain insight into the growth mechanism based on the substrates using transparent conductive oxides (TCO) and improve device performance. The kesterite solar absorbers were fabricated on ITO substrates by sulfurizing co-electroplated Cu-Zn-Sn-S precursors in argon diluted H2S atmosphere at different temperatures (475-550 °C) for 30 min. Experimental proof, including cross-section scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, UV-vis-NIR transmission spectrum, and Raman and far-infrared spectroscopy, is presented for the crystallization of CZTS on an ITO substrate and the interfacial reaction between the ITO back contact and CZTS absorber. The complete conversion of precursor into CZTS requires at least 500 °C sulfurization temperature. The aggressive interfacial reaction leading to the out-diffusion of In into CZTS to a considerable extent, formation of tin sulfides, and electrically conductive degradation of ITO back contact occurs at the sulfurization temperatures higher than 500 °C. The bifacial devices obtained by 520 °C sulfurization exhibit the best conversion efficiencies and open circuit voltages. However, the presence of non-ohmic back contact (secondary diode), the short minority lifetime, and the high interfacial recombination rates negatively limit the open circuit voltage, fill factor, and efficiency, evidenced by illumination/temperature-dependent J-V, frequency-dependent capacitance-voltage (C-V-f), time-resolved PL (TRPL), and bias-dependent external quantum efficiency (EQE) measurements.

High efficiency bifacial Cu2ZnSnSe4 thin-film solar cells on transparent conducting oxide glass substrates

“…A bifacial thin-film photovoltaic using CIGS on TCO was first investigated by Nakada et al, 7 and the concept was later applied to CIGS and CZTSSe solar cells. [8][9][10][11][12][13] Another added benefit of the bifacial design is the ability to utilize these semi-transparent thin-film solar cells in other devices such as tandem solar cells and solar windows. 14 Accordingly, bifacial CZTSSe solar cells using TCO back contacts such as indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), and fluorine-doped tin oxide (FTO) have been studied by many research groups.…”

“…14 Accordingly, bifacial CZTSSe solar cells using TCO back contacts such as indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), and fluorine-doped tin oxide (FTO) have been studied by many research groups. [10][11][12][13] To date, most reports regarding bifacial CZTSSe were based on solution processing methods. For example, Ge et al reported Cu 2 ZnSnS 4 (CZTS) solar cells prepared through co-electroplating on an ITO substrate, which exhibited only 3.4% conversion efficiency under bifacial illumination.…”

Optimization of Cu2ZnSnS4 Absorber Layers via Sulfurization of Electrodeposited Cu-Zn-Sn Precursors for Photovoltaic Applications

“…The relatively high EQE result of CZTS_Citrate is consistent with the higher Jsc of this device compared to CZTS_Pyro. However, the highest value in the EQE curves are much lower than the reported values [187,190].…”

“…The most common annealing or sulfurization temperature range is between 500 and 600 °C for CZTS based devices, where large, uniform crystal grains are obtained [125,[141][142][143].…”

“…Temperature increase is reported to change the grain morphology and enhance the crystallinity of CZTS [141][142][143]. Emrani et al reported on the influence of sulfurization temperature for sputtered Sn/Zn/Cu precursors using structure analyses and solar cell efficiency [142].…”