Innovative Cadmium Telluride Solar Cells on Thin Film

Grant # 98-07
Principal Investigator: Vikram L. Dalal, P.I.
Organization: Iowa State University
Students: Adam Sanford and Michael Petersen
Technical Area: Renewable Energy

Background
CdTe is an attractive material for solar energy conversion. It has a bandgap, which is in the range for optimal conversion of sunlight into electricity, and also has a very high absorption coefficient for light, which means that a very thin film is needed for converting sunlight into electricity. It can be deposited in thin film form on a number of inexpensive substrates, and can be doped both p and n type, thereby allowing formation of a p-n junction photovoltaic (or solar) cell.

Recognizing these advantages of CdTe, a number of groups have developed various techniques for making CdTe solar cells. Most of these cells are made using evaporation type processes on glass substrates that are coated with a transparent conductor such as tin oxide. The standard devices consist of a heterojunction cell of the type, n CdS/pCdTe /metal contact, where the light enters from the CdS junction layer. These cells have some problems with the stability of the back metal contact, and some of them degrade over time.

Work to Date
In contrast, we are using an alternative design, pZnTe/n CdTe/ n+ CdTe, to achieve hopefully higher conversion efficiencies and intrinsic stability. The different layers are made by RF sputtering, which is a high- throughput, low- cost process. During the last year, we have successfully deposited each of the constituent layers of the cell, and have doped the materials appropriately by using different dopants. We have also made a working p-n junction solar cell. We have also succeeded in enhancing the grain size of CdTe by using a CdCl2 process. To achieve optimum efficiency, it is also necessary to design the cells appropriately. We have started to use a numerical analysis program called AMPS to understand the behavior of CdTe solar cells, and to design better cells by varying parameters such as bandgaps and device interfaces.

Conclusion
In summary, we have succeeded in making a novel type of CdTe solar cell, using a sputtering process. Future work will concentrate on improving the performance of the cell and on studying the stability of the device.