Hg 1-x Cd x Te as the Bottom Cell Material in Tandem II-VI Solar Cells
- PDF / 1,145,867 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 83 Downloads / 190 Views
1012-Y12-37
Hg1-xCdxTe as the Bottom Cell Material in Tandem II-VI Solar Cells Viral Parikh, Jie Chen, Sylvain Marsillac, Robert W Collins, and Alvin D Compaan Physics and Astronomy, Wright Center for Photovoltaic Innovation and Commercialization, University of Toledo, 2801 W. Bancroft Street, Mail Stop #111, Toledo, OH, 43606 ABSTRACT We have measured the electrical, optical and morphological properties of as-grown Hg1-xCdxTe films prepared by r.f. sputtering. The Hg1-xCdxTe films were grown at substrate temperatures ranging from 25 °C to 150 °C. Films grown at temperatures lower than or equal to 70 °C were highly resistive (≥ 105 Ω-cm) and were not measurable by our Hall apparatus. Optical transmission data show that the band gap of these films ranged from 0.8 eV-1.5 eV, satisfying the optimum band gap criteria. Plan view and cross sectional SEM studies indicate that the films grown at 85 °C and 100 °C have larger grains with compact grain boundaries and these films typically yield the best cell performance. Spectroscopic ellipsometry studies are being used to estimate the band-gap as a function of substrate temperature. We have also studied the effect of various back contacts and have fabricated complete solar cells. We found that Cu-Au serves as the best ohmic back contact to CdS/HgCdTe solar cell. Our preliminary results of J-V analyses on the complete solar cell show that the efficiency is mainly limited by the short-circuit current. Electrical-bias-dependent QE measurements indicate voltage-dependent current collection mainly in the long wavelength region. Further optimization of growth parameters and CdCl2 treatment needs to be carried out to improve the cell performance. INTRODUCTION Coutts et al [1] have shown that the optimum band gap of a bottom cell in a two terminal tandem device lies in the range 0.85 < Eg < 1.3 eV. The band gap of Hg1-xCdxTe (MCT) can easily be tailored in this range depending on the substrate temperature and ratio of Cd/Hg in the film. We have shown that HgCdTe can readily be sputtered at temperatures low enough not to degrade the properties of a CdS/CdTe solar cell already deposited. Thus MCT becomes a potential candidate bottom cell material. In this paper we discuss the compositional, electrical, optical and morphological properties of these films and their dependence on sputter deposition parameters. We also report on a study of back-contact materials. EXPERIMENT HgCdTe films are deposited by r.f. magnetron sputtering on pre-cleaned 1 mm thick sodalime glass, using a sputtering target 2 inch in diameter and º inch thick supplied by Plasmaterials Inc., containing 40% Hg by weight. Films were grown at different temperatures, viz. room temperature, 44 °C, 70 °C, 85 °C, 97 °C, 113 °C and 153 °C. The nominal thickness of these films is 2.0 µm. Electrical characterization of these films was done by the hot-probe technique and by Hall measurements. We have also done plan-view SEM as well as crosssectional SEM to study the grain size dependence on substrate temperature and type of grai
Data Loading...
