Metal oxides as buffer layers in polycrystalline CdTe thin-film solar cells
Date
2021
Authors
Pandey, Ramesh, author
Sites, James, advisor
Sampath, W.S., committee member
Ross, Kate, committee member
Harton, John, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
The optical band-gap of 1.5 eV and absorption coefficient the order of 105 cm-1 makes CdTea very attractive absorber for thin-film solar cells. This dissertation explores methods to improve both the front, or emitter, part of the cell and the back contact to the CdTe-based thin-film solar cells. The choice of an n-type emitter partner for CdTe based solar cells is crucial to the overall power conversion efficiency. In comparison to the traditional CdS emitter, metal oxides such as ZnO, MgO, and the ternary alloy MgxZn1-xO have large optical band-gaps making them transparent to most of the solar spectrum and an ideal emitter layer adjacent to light-facing side of the absorber in a superstrate configuration. The optical and electrical properties of MgxZn1-xO emitters can be modulated by varying the elemental ratio of x = Mg:(Mg + Zn) in the ternary alloy. Tracing the variation of the conversion efficiency as a function of Mg fraction in MgxZn1-xO emitter, an optimal Mg fraction of x = 0.15 was found to produce highest efficiency for the CdTe-based thin-film solar cells. Photoelectron spectroscopy demonstrated the conduction band offset at the emitter/absorber interface transitions from a cliff like -0.1 eV for x = 0.00 to a spike like 0.2 eV at the optimal x = 0.15. Photoluminescence and low-temperature current-voltage measurements showed that the interface between MgZnO and the CdSeTe is well passivated for x = 0.15. Further increase in the Mg fraction however increases the band offset between the emitter/absorber leading to distortions of J-V curves under various illumination conditions. Light soaking experiments and numerical simulations show that an insufficient density of carriers in the MgZnO due to the compensating defects causes these distortions: a failure of superposition of light and dark curves referred to as cross over, and distortion from normal current voltage behavior under spectra filtered illumination. An extrinsic doping of the emitter is critical to rectify these distortions and Ga-doped MgZnO was employed to experimentally demonstrate a cure to these J-V distortions characteristic of an undoped MgZnO emitter. It paves pathway to increase the n-type carrier density in the MgZnO emitter. The group-V doping of CdTe has shown potential to improve open circuit voltage, with level of doping in absorber the order of 1016 cm-3 and lifetimes of hundreds of ns. Numerical device simulations demonstrate that doping the emitter layer is essential and a particular challenge if the doping in the absorber is high. The results find the carrier concentration in emitter should be higher than the doping in the absorber to attain high open-circuit voltage in the highly doped CdTe-absorbers possible with arsenic doping. Various back contact metals like Ag, Co, Pt and metalloids like Te, and Se with different work functions were used to make an ohmic contact with the CdTe back surface. The use of a buffer layer behind the bare CdTe surface is found to be critical to the device performance. A thin 30-nm layer of Te have become the preferred choice of back buffer layer. Metal oxides like TeOx has been introduced as back buffer between the CdTe absorber and Te back contact to study their effect in device performance. The study finds that a double CdCl2 passivation procedure before and after the deposition of oxides is critical to the performance of these solar cells. Devices with the TeOx and the Te layer as back buffer demonstrated a power conversion efficiency in excess of 17 % without the incorporation of dopant in the absorber. Such a result is significant, as extrinsic dopants in CdTe-based absorbers often introduce defects in the absorber leading to increased recombination and degradation of cell performance particularly if the absorber is doped with Cu. Spectral and time resolved photoluminescence measurements carried out with illumination from front glass side show such cells have improved minority carrier lifetimes. The rear TRPL illumination to probe a CdTe/TeOx surface measured lifetimes of few ns indicative of the TeOx as a back buffer layer to mitigate the effects of large defects on a free CdTe surface. These results demonstrate metal oxides as a promising candidates for back buffer layers, and passivating back contact for hole selectivity in the CdTe-based solar cells.