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Fabrication and Characterization of Cd1-xMgxTe Thin Films and Their Application in Solar Cells

Published online by Cambridge University Press:  01 February 2011

Ramesh Dhere ,
Kannan Ramanathan ,
John Scharf ,
David Young ,
Bobby To ,
Anna Duda ,
Helio Moutinho  and
Rommel Noufi
Ramesh Dhere
Affiliation:
[email protected], NREL, 5200, 1617 Cole Blvd, Golden, CO, 80401, United States, 303-980-6169, 303-384-7600
Kannan Ramanathan
Affiliation:
[email protected], Miasole, 2590 Walsh Avenue, Santa Clara, CA, 95051, United States
John Scharf
Affiliation:
[email protected], NREL, 5200, 1617 Cole Blvd, Golden, CO, 80401, United States
David Young
Affiliation:
[email protected], NREL, 5200, 1617 Cole Blvd, Golden, CO, 80401, United States
Bobby To
Affiliation:
[email protected], NREL, 5200, 1617 Cole Blvd, Golden, CO, 80401, United States
Anna Duda
Affiliation:
[email protected], NREL, 5200, 1617 Cole Blvd, Golden, CO, 80401, United States
Helio Moutinho
Affiliation:
[email protected], NREL, 5200, 1617 Cole Blvd, Golden, CO, 80401, United States
Rommel Noufi
Affiliation:
[email protected], NREL, 5200, 1617 Cole Blvd, Golden, CO, 80401, United States
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Abstract

We present our work on development of Cd1-xMgxTe (CMT) polycrystalline thin film for solar cells for tandem cell applications. CMT thin films were fabricated by co-evaporation of CdTe and Mg at a substrate temperature of 400˚C. The spatial separation of the two sources resulted in a compositional gradient, which allowed the deposition of a wide range of compositions from fewer runs. Stable films for compositions up to x=0.73 were fabricated. The structural analysis showed that the as-deposited films have a sphalerite structure and are preferentially oriented in the <111> direction, and the lattice constant varies linearly with composition. Optical analysis of the samples shows that the optical bandgap varies linearly with composition. The optical absorption coefficient of the alloy films for the entire composition range (up to x=0.73) is over 6×104/cm-1 for energies above the bandgap, indicating a direct-gap semiconductor. We have fabricated solar cells using these films with the following structure: glass / SnO2 / CdS / CMT / back contact. Using the same processing conditions used for CdTe solar cells, the device efficiency was less than 2%. With modified contact processing, we were able to improve efficiencies to over 5%. Post-deposition chloride heat-treatment in oxygen ambient resulted in partial dissociation of the CMT alloy and loss of Mg. With modified chloride processing, we were able to reduce alloy decomposition and further improve the efficiencies. We have obtained devices with open-circuit voltages of up to 845 mV and efficiencies of 8% for CMT devices with a bandgap in the range of 1.6-1.62 eV which are highest reported values for CMT alloys.

Keywords

polycrystalthin filmphotovoltaic
Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1012: Symposium Y – Thin-Film Compound Semiconductor Photovoltaics-2007 , 2007 , 1012-Y02-02
Copyright
Copyright © Materials Research Society 2007

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References

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