Thin Films and Photovoltaics  
General Information
Tandem Cells

Cadmium telluride is a compound semicounductor consisting of the metal cadmium and the semiconductor tellurium, in a ratio of 1:1. The band gap of CdTe is about 1.5 eV, which is close to the optimum for conversion of sunlight to electricity. Additionally, the material is very stable both against heat and chemically. The excellent absorption characteristics of this material make it an excellent candidate for the use in photovoltaic applications. The material deposition is very easy with varying technologies and thus allows high throughput in producition.

CdTe was first used as an absorber material in solar cells in the 1960's. In 1980, the 10% mark in efficiency on lab-scale cells was broken, 2005 the still valid efficiency world record of 16.5% was established by the National Renewable Energy Laboratories (NREL) in the United States. Optical losses were minimized in that record cell. The challenge for the future is the improvement of the electronic properties for improving the cell voltage and thus the efficiency.
The annual production capacity of commercial solar modules exceeded 1 GWp in 2009. The largest producer is the U.S. company First Solar, the largest part of the production takes place in Malaysia. Its CdTe solar modules are currently the cheapest on a price per watt basis.
In our lab CdTe solar cells with efficiencies of up to 15.6% (March 2010) on glass substrates and up to 13.5% on polyimide foil could be produced. In contrast to CIGS, for all substrates the same deposition process with similar layer quality can be used. The difference in efficiency between glass and polymer substrates is caused by the lower transparency of polyimide compared to glass.
The development CdTe solar cells on flexible substrates is one of our focus points. The production process used in our lab uses temperatures of 420 °C or lower, allowing the use of polyimide as a substrate (can only be used in processes using up to 450 °C).
New back contacts are also developed in our lab. Up to now, the back contact is one of the crucial points in a CdTe solar cell. For high efficiencies, it has to be deposited as last layer, thus prohibiting the use of opaque substrates.
Doping of the semiconductor material CdTe is a new focus point in our research under a project started in December 2009. The increase of charge carrier concentration in polycrystalline CdTe is an important step for further improvements in efficiency.
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