Sublimated thin film CdTe photovoltaic devices with conversion efficiencies over 18% and a fillfactor greater than 79% have been repeatedly obtained using high-rate fabrication processes on commercial soda-lime glass substrates used in CdTe modules. Four major improvements to the device have enabled an increase in efficiency from a baseline of approximately 12% to 18.7%: 1) A sputtered multilayer metal-oxide anti-reflection layer; 2) total replacement of the CdS window layer with a higher bandgap sputtered MgxZn1-xO (MZO) window layer; 3) deposition of the CdTe layer at a higher thickness and substrate temperature; and 4) an evaporated tellurium back-contact. This work describes the effect of these changes on the device performance and film microstructural characteristics using various methods. Multiple devices with comparable high efficiency have been fabricated and demonstrated using methods described in this study, yielding some of the highest efficiencies for CdTe polycrystalline thin-film photovoltaics. Thin film CdTe photovoltaics have consistently demonstrated the lowest cost solar electricity generation, particularly for utility scale applications. CdTe is a p-type absorber that has a bandgap of 1.5 eV which is nearly optimal for photovoltaic conversion. Approximately 2 µm is sufficient to absorb most of the visible solar spectrum. 1,2 CdTe films are typically deposited on glass substrates using low-cost hardware and high-rate deposition processes 3,4,5 reducing production costs. Typical crystalline silicon photovoltaics require wafers that are 150-200 microns thick and use a more complex and capital-intensive fabrication process. 3 The low-cost manufacturing of thin-film CdTe PV has enabled agreement for a record low cost power purchase agreement of ¢3.8/kWh for a 100 MW field, 6 which is significantly lower than the average cost of electricity in the U.S. of ¢11/kWh. 7 With recent improvements, research-scale small devices have record efficiencies of 22.1%, 8 while modules with up to 18.6% 9 efficiency have been produced. The leading CdTe PV manufacturer, First Solar Inc., has increased average production module efficiency from 13.5% in the first quarter of 2014 10 to 16.7% in the first quarter of 2017. 11 Further improving the efficiency without substantial increase in production cost will reduce the levelized cost of energy for CdTe photovoltaics. 12,13 Maintaining the dual requirement of high efficiency and low cost requires the use of film deposition techniques suitable for mass production of millions of solar modules per year. The vapor deposition methods used for this study, including sublimation, evaporation, and sputter deposition, have been used in large scale manufacturing for solar and other industries. Commercially available 3.2-mm soda-lime glass with a fluorine-doped tin-oxide (FTO) transparent conducting layer is a standard substrate for thin-film PV manufacturing, including for CdTe, due to its sufficient strength, reliability, and low cost. Using processes suitable for large scale manuf...
As single-junction silicon solar cells approach their theoretical limits, tandems provide the primary path to higher efficiencies. CdTe alloys can be tuned with magnesium (CdMgTe) or zinc (CdZnTe) for ideal tandem pairing with silicon. A II-VI/Si tandem holds the greatest promise for inexpensive, high-efficiency top cells that can be quickly deployed in the market using existing polycrystalline CdTe manufacturing lines combined with mature silicon production lines. Currently, all high efficiency polycrystalline CdTe cells require a chloride-based passivation process to passivate grain boundaries and bulk defects. This research examines the rich chemistry and physics that has historically limited performance when extending Cl treatments to polycrystalline 1.7-eV CdMgTe and CdZnTe absorbers. A combination of transmittance, quantum efficiency, photoluminescence, transmission electron microscopy, and energy-dispersive X-ray spectroscopy clearly reveals that during passivation, Mg segregates and out-diffuses, initially at the grain boundaries but eventually throughout the bulk. CdZnTe exhibits similar Zn segregation behavior; however, the onset and progression is localized to the back of the device. After passivation, CdMgTe and CdZnTe can render a layer that is reduced to predominantly CdTe electro-optical behavior. Contact instabilities caused by inter-diffusion between the layers create additional complications. The results outline critical issues and paths for these materials to be successfully implemented in Si-based tandems and other applications.
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