Mate, Mayank N., authorSampath, Walajabad, advisorMunshi, Amit, advisorSites, James, committee member2024-09-092024-09-092024https://hdl.handle.net/10217/239147CdTe thin-film photovoltaics (PV) is one of the most promising renewable energy technologies currently in the market. Since the inception of CdTe PV in the 1950s, the technology has come a long way to be one of the most efficient solar cell technologies on the planet. However, there are still challenges associated with this technology which limit the power conversion efficiency. Currently, the most efficient CdTe solar cell to have been fabricated is still only at ~69% of the maximum possible efficiency, given by the Shockley-Queisser (SQ) limit. Performance loss analysis has suggested that the limiting electrical parameter is the open-circuit voltage (Voc) and detailed investigations have suggested that a defect state within the bandgap of Se allowed CdTe contributes significantly to this loss of Voc, which can be observed by Photoluminescence (PL). For thin-film CdTe PV to realize its full potential, research has been ongoing to improve the Voc by doping CdTe with a group V dopant, as well as passivating the surface of CdTe which causes significant recombination of charged carriers and may inhibit voltage even further. The research in this thesis focuses on group-V doping using As as the dopant and Cd3As2 as the doping material, and the formation of TeO2 as the metal oxide on the surface of CdTe. The goal of this research was to observe any improvements in Voc and overall device performance with As doping, TeO2, and a combination of both, and whether As doping mitigates the primary defect state within the bandgap of Se alloyed CdTe. To investigate the effects of these doping and surface treatments, three key hypotheses statements were formed. The first hypothesis is that As can be incorporated into CdTe using Cd3As2 without depositing it as a separate layer. This distinction of not depositing a film of Cd3As2 is important, and it aims to avoid any complexes caused by depositing Cd3As2 on the CdTe surface, creating another interface and potentially limiting the device performance, particularly due to Cd3As2 being a semi-metal. Secondary-Ion Mass Spectroscopy (SIMS) and X-Ray Photoelectron Spectroscopy (XPS) were used as the characterization methods to confirm the incorporation of As into the CdTe film by dissociation and diffusion, without depositing a layer of Cd3As2. The second hypothesis is that exposing a CdTe film to ambient air at high temperatures can lead to the enhanced formation of TeO2. Metal oxides such as Al2O3 have long been investigated to passivate the surface of CdTe, improving carrier lifetime and Voc. Traditionally, these metal oxides have been sputtered onto the surface, while native oxides on the surface of CdTe take a long time to form, on the timescale of days to weeks. However, this hypothesis investigates a novel method of forming a native layer of TeO2 by annealing a CdTe film in vacuum at elevated temperatures (~500℃) and rapidly exposing it to atmospheric oxygen while the high temperature is maintained. XPS and PL characterization was performed on the films that had undergone this 'rapid oxidation', and the formation of TeO2 was confirmed. The effect of TeO2 was combined with As doping, as well as compared in an undoped film, and it demonstrated a significant improvement in Voc when compared to control samples that were Cu doped and undoped with no intentional formation of TeO2. The third and final hypothesis is that As doping using Cd3As2 mitigates the sub-band-gap emission in photoluminescence originating from Se alloyed CdTe. To confirm or deny this theory, absolute PL was measured with an InGaAs detector, for samples that were doped with As and for samples that had a combination As doping treatments and/or TeO2 formed by using the method described for hypothesis two. The photoluminescence results showed no mitigation of the sub-band-gap emission in Se alloyed CdTe films that were doped with As. However, the addition of TeO2 demonstrated a lowering of the sub-bandgap emission intensity and overall improvement to the band-gap PL emission. Additionally, electrical parameters of six distinct device structures were measured and compared, with Cu-doped and undoped Se-alloyed CdTe being the reference/control samples. As a result of this research, a cell measuring close to 18% power conversion efficiency was measured with As doping, the native TeO2 layer, and one third of the Cu doping that went into reference samples. Finally, this research also provides some insight into the effects of high O2 presence during the As doping process using Cd3As2, and how it inhibits the ionization of As a small P-type dopant.born digitalmasters thesesengCopyright and other restrictions may apply. User is responsible for compliance with all applicable laws. For information about copyright law, please see https://libguides.colostate.edu/copyright.characterizationphotovoltaicsthin filmsdopingcadmium telluridesurface scienceText