Low gap Amorphous (Si,Ge) Solar Cells
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Low gap Amorphous (Si,Ge) Solar Cells Vikram L. Dalal and Zhiyang Zhou Iowa State University, Dept. of Electrical and Computer Engineering, Ames, Iowa 50011 ABSTRACT We report on the growth and fabrication of low-gap a-(Si,Ge) solar cells, including cells made from a-Ge:H. The cells were deposited from silane and germane, or germane alone, using a high dilution of hydrogen in a remote ECR plasma reactor. We have been able to achieve a Tauc gap of as low as 1.15 eV with good device properties. The fill factors were in the 50%60% range, and the Urbach energies measured in devices were in the range of 45 meV. The devices were of the p/i/n type with light entering the p layer. The devices were fabricated on stainless steel substrates. We also report on experiments to improve graded gap solar cells in a(Si,Ge) using a novel combination of ppm-boron dopant and bandgap grading. It is shown that this grading significantly improves the device performance, both voltage and fill factor. INTRODUCTION Amorphous (Si,Ge) alloys have been used for some time now to improve the performance of a-Si based solar cells.[1,2,3]. These alloys are typically fabricated using various plasma-CVD processes. In general, high dilution with hydrogen is required to produce good cells and films.[4-6]. Typically, it is found that the quality of the material and the device degrades significantly as the Ge content increases, with alloys having >50% Ge content ( Tauc gap of
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