CdCl2 passivation of polycrystalline CdMgTe and CdZnTe absorbers for tandem photovoltaic cells

Abstract

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.

Predicted Power Output of Silicon-Based Bifacial Tandem Photovoltaic Systems

“…[23][24][25] Yet, with such alloys, ternary-element (Mg in CdMgTe, Zn in CdZnTe) out-diffusion occurs during the cadmium chloride (CdCl 2 ) passivation treatmentwhich is required for high efficiencies in polycrystalline CdTe-based devicesrendering films with optical properties closer to 1.5-eV-bandgap CdTe. 26 One way to relax the wide-bandgap constraint is to move to three-or four-terminal tandems. By electrically decoupling the top and bottom cells, the efficiency of c-Si-based tandem devices becomes considerably less sensitive to the top-cell bandgap, such that deviations from the optimum do not result in dramatic degradation of the tandem efficiency.…”

CdS barrier to minimize Zn loss during CdCl2 treatment of Cd-Zn-Te absorbers

“…The Cd-Zn-Te film was polycrystalline and void free after the CdCl2 treatment. The as-deposited Cd-Zn-Te grains from the co-sublimation process are columnar [29]. After the CdCl2 treatment, the morphology of the grains changed to a more granular shape.…”

Possible top cells for next-generation Si-based tandem solar cells

“…To date, singlecrystal CdZnTe and CdMgTe solar cells with a bandgap of 1.7 eV have demonstrated 16.4% and 11.2% device efficiencies, respectively [27,28,37]. Nevertheless, polycrystalline CdZnTe and CdMgTe thin-film solar cells have shown much lower performance owing to alloy Osaka University, 2T [31] degradation during CdCl 2 passivation [38]. Developing a more practicable passivation method could result in highefficiency CdZnTe and CdMgTe thin-film solar cells, thus decreasing the manufacturing costs of Si-based tandem devices.…”