Defect Densities, Diffusion Lengths and Device Physics in Nanocrystalline Si:H Solar Cells
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Defect Densities, Diffusion Lengths and Device Physics in Nanocrystalline Si:H Solar Cells Vikram L. Dalal*, Puneet Sharma*, David Staab, Max Noack+ and Keqin Han+ * Iowa State University, Dept. of Electrical and Computer Engr. and Microelectronics Research Center, Ames, Iowa 50011, USA + Iowa State University, Microelectronics Research Center, Ames, Iowa 50011,USA ABSTRACT We report on the properties of nanocrystalline Si:H solar cells. The solar cells were of the p nn type, with the n+ layer deposited first on a stainless steel substrates. The solar cells were prepared under high hydrogen dilution conditions using either ECR plasma deposition, or VHF diode plasma deposition processes. The deposition pressures were kept low, 5 mTorr in the ECR reactor and 50 mTorr in the VHF reactor. All the solar cells reported showed a high Raman ratio of crystalline to amorphous peaks. Properties such as dark current, deep level defects and shallow doping densities, and hole diffusion lengths were measured in these cells. It was found that the base layer was always n type, but that its doping could be changed by adding ppm levels of B during growth. A sufficient B doping even type converted the base layer to p type. It was found that there was a good one-to-one correlation between the shallow doping and deep level defects, suggesting that the same element, probably oxygen, is responsible for generating both shallow dopants and deep levels. The diffusion length of holes was measured in these cells using quantum efficiency vs. voltage techniques, and it was found that the diffusion length data could be explained very well by invoking trap-controlled recombination statistics. The dark I(V) curves could be represented by a standard diode model for highly crystalline materials, but as the degree of crystallinity was reduced, the diode factor increased. Voltage could be improved by reducing the crystallinity of the layer, but doing so resulted in a decrease in quantum efficiency in the infrared regions of the solar spectrum. +
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INTRODUCTION Nanocrystalline Si:H, with grain sizes of the order of 10-20 nm, is an attractive materials for solar energy conversion [1-5]. It is known that H is present in this material, mainly at the grain boundaries, and also, there may be a thin amorphous Si:H tissue surrounding the small grains [6]. The presence of hydrogen seems to passivate the grain boundaries, reducing recombination of minority carriers at the boundaries. Solar conversion efficiencies approaching 10% have already been achieved in this material system, and when combined with a-Si:H as a top cell, efficiencies in excess of 14% have been reported [1]. In this paper, we will investigate the device physics of nanocrystalline Si:H solar cells by measuring the fundamental properties of the materials in devices, and then correlating these properties with the expected device results. The properties measured include doping, deep level defects, hole diffusion length, and dark I(V) curves. FABRICATION OF DIAGNOSTIC DEVICES The diagnosti
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