Passivation studies on Cd0.6Zn0.4Te films using CdCl2, MgCl2 and ZnCl2 for top cell application in a multijunction solar cell
Date
2018
Authors
Shimpi, Tushar M., author
Sampath, W. S., advisor
Sites, James R., committee member
Kota, Arun K., committee member
Popat, Ketul C., committee member
Journal Title
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Abstract
Passivation treatment with the chloride compounds is an important step in the fabrication of II-VI solar cells for improving the device performance. In cadmium telluride solar cells, cadmium chloride passivation treatment incorporates chlorine along the grain boundaries and helps in recrystallization, grain growth, removal of stacking faults and doping grain boundaries as n-type. In cadmium zinc telluride solar cells, the retention of zinc after the cadmium chloride passivation treatment is one of the challenges incurred in fabricating the top cell in a multijunction solar cell. During the passivation treatment, the loss of zinc occurs in the form of volatile zinc chloride compound. The depletion or complete loss of zinc reduces the higher band gap ternary alloy into lower band gap binary compound of CdTe. This impedes the purpose of fabricating a high band gap top cell in a multijunction solar cell. The focus of this study is on passivating Cd0.6Zn0.4Te (CdZnTe) films using three different chloride compounds separately and understanding the effects by studying the material properties of the passivated films and electrical performance of the fabricated devices. In the preliminary experiments, CdZnTe films were deposited by RF sputtering from a single target. Initial characterization of CdZnTe films deposited on plain glass indicated that the films had a strong preferred orientation along {111} plane with a band gap of ~1.72eV. In the cadmium chloride passivation treatment, loss of zinc from the surface and no chlorine along the grain boundaries was observed from transmission electron microscope images and X ray diffraction measurements. No loss of zinc was observed after the magnesium chloride and zinc chloride passivation treatments. Increase in the grain size of the CdZnTe films after magnesium chloride treatment and decrease of the preferred orientation after zinc chloride treatment were the benefits of the individual passivation treatments. Modifying the test structure by adding a cadmium telluride film as a capping layer on the back of RF sputtered CdZnTe and then carrying out the cadmium chloride passivation treatment helped in retaining the zinc. Heavy diffusion of zinc into cadmium sulphide due to cadmium telluride deposition at high temperature and difficulty to isolate the photo current generated by cadmium telluride were few drawbacks of this test structure. Based on the insights gained from the preliminary experiments, two sets of experiments were conducted. In the first set, cadmium sulphide cap as a barrier was deposited on the back of RF sputtered CdZnTe and co-sublimated cadmium telluride and zinc films with a band gap of 1.72 eV. The bulk composition was maintained after the cadmium chloride passivation treatment in the films deposited by both the methods. However the device performance of co-sublimated films was better than the RF sputtered CdZnTe devices. The transmission electron image obtained from the cross section of co-sublimated film fitted with cadmium sulphide cap and then treated with cadmium chloride showed presence of chlorine along the grain boundaries. The zinc chloride passivation treatments with higher substrate temperature compared to the source were the second set of experiments. The zinc loss from RF sputtered CdZnTe films after the cadmium chloride treatment did not occur. The fabricated devices exhibited diode like behavior. The images under scanning electron microscopy showed that the grain size did not increase after the zinc chloride treatment.
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Subject
high band gap CdTe alloys
tandem solar cell
top cell
multijunction solar cells
CdCl2 treatment on II VI
thin film CdZnTe