Growth and process optimization of CdTe and CdZnTe polycrystalline films for high efficiency solar cells

Polycrystalline CdTe solar cells with efficiencies of approximately 10% were achieved by metal organic chemical vapor deposition growth of CdTe on glass/SnO₂/CdS substrates. An in situ pre-heat treatment of the CdS substrate at 450 °C in an H₂ ambient was found to be essential for high performance devices because it removes oxygen-related defects on the CdS surface. This heat treatment also results in a cadmium-deficient CdS surface which may, in part, limit the CdTe cell efficiency to 10% owing to cadmium vacancy related interface defects. The CdCl₂ treatment used in CdTe cell processing was found to promote grain growth, reduce series resistance and interface state density, and change to dominant current transport mechanism from thermally assisted tunneling and recombination via interface states to recombination in the depletion region. These beneficial effects resulted in an increase in the CdTe/CdS cell efficiency from around 2% to approximately 9%. In addition to the CdTe cells, polycrystalline 1.7 eV CdZnTe films were grown by molecular beam epitaxy for tandem cell applications. CdZnTe/CdS cells processed using the standard CdTe cell fabrication procedure resulted in 4.4% efficiency, high series resistance, and a band gap shift to 1.55 eV. Formation of ZnO at and near the CdZnTe surface was found to be the source of high contact resistance. A saturated dichromate etch instead of the Br₂:CH₃OH etch prior to contact deposition was found to solve the contact resistance problem. The CdCl₂ treatment was identified to be the cause of the observed band gap shift owing to the preferred formation of ZnCl₂. A model for the band gap shift along with a possible solution using an overpressure of ZnCl₂ in the annealing ambient is proposed. Development of a sintering aid which promotes grain growth and preserves the optimum 1.7 eV band gap is shown to be the key successful wide gap CdZnTe cells.