Microcrystalline Silicon in a-SI:H Based Multifunction Solar Cells
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MICROCRYSTALLINE SILICON IN a-SI:H BASED MULTIJUNCTION SOLAR CELLS LIYOU YANG, L. CHEN, S. WIEDEMAN and A. CATALANO* Solarex Corporation, Thin Film Division, 826 Newtown-Yardley Road, Newtown, PA 18940 "*Presentaddress: NREL, 1617 Cole Blvd., Golden, CO 80401
ABSTRACT Materials issues central to the application of microcrystalline silicon (gtc-Si) doped layers in a-Si:H based solar cells are discussed, which include: (1) characterization of ultra-thin layers to be incorporated in the device, and (2) methods to promote nucleation of gc-Si on desired substrates within a thickness on the order of -100A. Successful application of jtc-Si in multijunction a-Si:H based solar cells are demonstrated. INTRODUCTION Amorphous hydrogenated silicon (a-Si:H) based solar cell technology is currently the leading candidate for large-area, low-cost photovoltaic applications. The basic device structure is a p-i-n junction in which all layers are traditionally amorphous and are made in a continuous plasma deposition process. The doped layers in the device play a key role in building up the strong internal electric field across the i-layer, which is the predominant force in collecting photocarriers generated in the i-layer. However, the electrical properties of amorphous doped layers are relatively poor as compared to their crystalline counterparts. For instance, the conductivities are typically only -lx10"6 (9.cm)-1 for a-Si:H p-layer and -lxlO4 (n.cm)-1 for the n-layer. This is due partly to the low carrier mobilities in a-Si:H and partly to the low doping efficiencies in the disordered material. Moreover, the extremely high densities of tail states in amorphous materials prevent the Fermi levels from being too close to the band edges. The typical conductivity activation energies for a-Si:H p-layers and n-layers are -0.4 eV and -0.2 eV, respectively, thus limiting the open circuit voltage of the a-Si:H solar cells to -0.9 V given its bandgap of -1.75 eV. Doped microcrystalline Si (ic-Si) represents a very attractive alternative for a-Si:H based solar cells not only because of its much improved electrical and optical properties but also its compatibility with the low temperature PECVD process. However, except for very few reported successes, 1-3 microcrystalline Si so far has not been widely used in a-Si:H solar cells, at least for commercial applications. The main difficulties are perhaps in making extremely thin layers of gIc-Si (< 100A), which is necessary in order to reduce the optical loss, and in alloying with carbon for raising the optical bandgap. In this paper, we will emphasize those issues related to the growth of ultra-thin microcrystalline materials which are of special concern to the application in a-Si:H based solar cells. Furthermore, we will demonstrate the use of microcrystalline Si in multijunction solar cells which has resulted in significant improvement in all aspects of device performance. MATERIALS ISSUES Thickness and Substrate Dependencies
Mat. Res. Soc. Symp. Proc. Vol. 283. 01993 Materials Research Society
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