Graduation Year

2004

Document Type

Thesis

Degree

M.S.E.E.

Degree Granting Department

Electrical Engineering

Major Professor

Christos S. Ferekides.

Keywords

tandem solar cells, wideband gap semiconductors, thin films, CZT

Abstract

High efficiency solar cells can be attained by the development of two junctions one stacked on top of each other into tandem structures. So that, if a photon is not able to excite an electron-hole pair in the top cell can create a pair in the bottom cell, which has a smaller bandgap. For a two junction tandem device structure, the bandgap of the top cell should be 1.6-1.8eV and for the bottom cell should be 1eV to attain efficiencies in the range of 25%. Cadmium Zinc Telluride which has a tunable bandgap of 1.45- 2.2eV is a candidate for the top cell of the tandem structure. Cadmium Zinc Telluride (Cd₁-xZnxTe) films were deposited by co-sputtering of CdTe and ZnTe. Deposition of Cd₁-xZnxTe was studied in Ar and Ar/N₂ ambient.

Characterization of the films was done using transmission response, X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Secondary Electron Microscopy (SEM), current-voltage (I-V) and spectral response measurements. CZT deposited on CdS/SnO₂ substrates showed improved performance compared to other heterojunction partners. Doped graphite and copper were utilized as back contacts for CZT devices. Post deposition annealing treatments with ZnCl₂ on CZT films were done and their effect on the devices was also studied. The best combination of Voc and Jsc were 530mV and 3.66mA/cm² respectively.

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