Search

Sublimated thin-film CdTe photovoltaic devices with conversion efficiencies over 18% and a fill-factor 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 12–18.7%: 1) A sputtered multilayer metal-oxide anti-reflection layer; 2) total replacement of the CdS window layer with a higher bandgap sputtered Mg x Zn 1−x O (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 very high efficiencies for CdTe polycrystalline thin-film photovoltaics using deposition processes and equipment in a university setting. [ABSTRACT FROM AUTHOR] more...

Single phase Cd0.6Zn0.4Te (CdZnTe) films of 1 μm thickness were deposited by radio frequency planar magnetron sputter deposition on commercial soda lime glass samples coated with fluorinedoped tin oxide and cadmium sulphide (CdS). The stack was then treated with cadmium chloride (CdCl2) at different temperatures using a constant treatment time. The effect of the CdCl2 treatment was studied using optical, materials, and electrical characterization of the samples and compared with the as-deposited CdZnTe film with the same stack configuration. The band gap deduced from Tauc plots on the as-deposited CdZnTe thin film was 1.72 eV. The deposited film had good crystalline quality with a preferred orientation along the {111} plane. After the CdCl2 treatment, the absorption edge shifted toward longer wavelength region and new peaks corresponding to cadmium telluride (CdTe) emerged in the x-ray diffraction pattern. This suggested loss of zinc after the CdCl2 treatment. The cross sectional transmission electron microscope images of the sample treated at 400 °C and the energy dispersive elemental maps revealed the absence of chlorine along the grain boundaries of CdZnTe and residual CdTe. The presence of chlorine in the CdTe devices plays a vital role in drastically improving the device performance which was not observed in CdZnTe samples treated with CdCl2. The loss of zinc from the surface and incomplete recrystallization of the grains together with the presence of high densities of stacking faults were observed. The surface images using scanning electron microscopy showed that the morphology of the grains changed from small spherical shape to large grains formed due to the fusion of small grains with distinct grain boundaries visible at the higher CdCl2 treatment temperatures. The absence of chlorine along the grain boundaries, incomplete recrystallization and distinct grain boundaries is understood to cause the poor performance of the fabricated devices. [ABSTRACT FROM AUTHOR] more...