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Development of ternary oxides for use in advanced front contacts in cadmium telluride solar cells by magnetron sputtering
Meysing, Daniel M.
Meysing, Daniel M.
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2015
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2016-03-22
Abstract
Cadmium telluride (CdTe) solar cells are a commercially proven photovoltaic technology. Devices are composed of thin films which enable conversion of incident sunlight into electrical carriers and simultaneous transport of these carriers to an external load. At the front of the device, the layers should be highly transparent to minimize parasitic absorption while also supporting electron transport to the external load. In this thesis, two sputter-deposited front contact layers were investigated with the goal of improving both processing and materials science understanding. The first material investigated in this work was cadmium stannate (Cd2SnO4; CTO), a high-performance transparent conductive oxide (TCO). CTO, though used less frequently than on-line deposited fluorine-doped tin oxide, exhibits high transmission (>90%) and low resistivity (<4×10–4 Ω-cm) that are generally superior. Here, an attempt is made to demonstrate that CTO has a wide processing window, develop additional processing routes, and provide materials science insights for fabricating high-quality material. In the conventional processing scheme, as-deposited films are annealed in contact with a secondary CdS film. Films processed using this “proximity anneal” exhibited resistivities of ~2.2×10–4 Ω-cm and transmittance >90% when oxygen was present in the sputtering ambient in the range 600–700 °C. Sputtered CTO and bilayer CTO/CdS films were annealed in contact with bare glass and in an uncovered configuration. The thin CdS layer in bilayer films was adequate to maintain or reduce resistivity when using the covered anneal, while it enabled improved mobility and transmittance for the uncovered anneal. Stoichiometry adjustment to a higher cadmium/tin ratio was found to be primarily responsible for increased carrier concentration, both through the proximity anneal and the covered anneal. Next, the CdS window layer was investigated. Important structural, optical, and electronic properties were altered by varying the sputtering ambient composition (oxygen/argon). Incorporation of oxygen in the films causes the films to lose crystallinity and increases the optical band gap through a shift in conduction band energy. CdS and oxygenated CdS (CdS:O) layers were incorporated in complete devices. Maximum efficiency >14% was achieved using a CdS:O layer containing ~40 atomic % oxygen and an optical band gap of 2.8 eV. Processing was scaled up to two higher throughput systems, in which different ambient compositions were required to achieve the optimal band gap. Similar device efficiencies of 13–15% were achieved by maintaining the window layer thickness at 100 nm and the optical band gap at 2.8 eV for each system/target combination. It is notoriously difficult to characterize the window layer and electrical junction in completed devices, because (1) they are buried between the glass on one side and a “thick” ~5-µm CdTe layer on the back, and (2) the CdS and CdTe layers are chemically similar. Two techniques were developed to enable characterization of the CdS window layer and CdS/CdTe junction in completed devices. First, a chemical etch was used to selectively etch the CdTe layer. Second, a thermo-mechanical lift-off was used to cleave the device stack at the SnO2/CdS interface. Analysis of the resulting structures indicates that the high-temperature CdTe deposition and annealing steps radically alter the CdS window layer properties, including a shift in the optical band gap to ~2.2 eV, recrystallization to the hexagonal phase, and consumption of the window layer. These observations are used to explain the window layer transformations during processing and explain quantum efficiency trends observed in the previous section.
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