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A standardized process used for fabrication of CdTe solar cells was varied by increasing the substrate temperature during CdTe layer nucleation from approximately 460°C to 610°C and by increasing the CdTe sublimation vapor source temperature. Higher substrate temperatures increase device efficiency, but cause significant CdS re-sublimation. This effect was eliminated by using a Mg 1−x Zn x O window layer that also has higher transparency. Elevated CdTe source temperatures were found to increase contamination in the deposition system but did not further improve device efficiency. The improvement using high substrate temperatures is attributed to larger CdTe grains and better crystalline quality. TEM cross section analysis, X-ray diffraction measurements and device results are presented.

In this study Cd 1−x Mg x Te thin films are investigated for use as an absorber layer for future use in top cells of a tandem device. An essential step in the development of this new absorber layer is the passivation process. An experimental passivation process using a mixture of MgCl 2 and CdCl 2 was conducted. Localized magnesium loss during the passivation process is shown to be along grain boundaries as well as in the formation of an oxide layer at the back surface. Devices which underwent the experimental passivation process showed significant improvement in device performance. A next generation n-type window layer is used in junction with a Cd( 1−x )Mg (x) Te absorber, resulting in higher performing cells than CdS.

Atomic-scale defects limit the open circuit Voltage, and the conversion efficiency of thin film polycrystalline cadmium telluride solar cells. Using state of the art aberration-corrected high resolution transmission electron microscopy, the type, density and atomic structure of intragranular defects present in cadmium chloride treated and untreated CdTe has been established. The cadmium chloride activation process dramatically reduces defect density but faults do remain. Characterizing the defects in both materials is an essential first step to determining their potential electrical effects, and to understanding how the cadmium chloride treatment reduces their density. Improving our knowledge of the mechanisms involved can lead to further process improvements.

Different organic compounds are evaluated to form back contact for CdTe photovoltaics (PV) and to study the effect of band alignment. CdTe devices with CdS n-type window layer were fabricated using closed space sublimation (CSS) and organic back contact layers were deposited using dip coating. In order to prepare photovoltaic devices, the appropriate deposition parameters and thicknesses of organic compounds were selected experimentally. The best results were obtained with dip coated PEDOT-PSS aqueous dispersion with high electrical conductivity. The performance of devices with organic back contacts are compared to those with baseline CdTe devices with reduced copper doping.

CdTe solar-cell efficiency at Colorado State has now been independently verified above 18% on commercial glass. The parameters for the highest-efficiency cell are 863 mV for V OC , 26.8 mA/cm2 for J SC , and 79.2% for fill-factor, combining for 18.3% efficiency. The cell features that allowed the increases include higher-temperature pre-heating before CdTe deposition, a Te layer to facilitate the back contact, MgZnO for the buffer layer, and an anti-reflective coating. The current and fill-factor have achieved large fractions of their ideal values for the CdTe band gap, but as is typical for CdTe cells, the voltage has not. Two general strategies to address the voltage deficit are described. One of these, electron reflection with fully-depleted CdTe, has achieved voltage increases of 60 mV with 1-μ m CdTe and higher with sub-micron absorbers.