Thin Film Silicon Solar Cells Under Moderate Concentration
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THIN FILM SILICON SOLAR CELLS UNDER MODERATE CONCENTRATION L.M. van Dam, W.G.J.H.M. van Sark, R.E.I. Schropp Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science, Nanophotonics - Physics of Devices, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands, Tel: 030 253 3170, e-mail: [email protected] ABSTRACT There are only very few reports on the effects of concentration in thin film siliconbased solar cells. Due to the presence of midgap states, a fast decline in fill factor was observed in earlier work. However, with the advent of more stable and lower defect density protocrystalline silicon materials as well as high quality micro-/nanocrystalline silicon materials, it is worth revisiting the performance of cells with these absorber layers under moderately concentrated sunlight. We determined the behavior of the external J-V parameters of pre-stabilized substrate-type (n-i-p) amorphous and microcrystalline solar cells under moderate concentrations, between 1 sun and 21 suns, while maintaining the cell temperature at 25oC. It was found that the cell efficiency of both the amorphous and the microcrystalline cells increased with moderate concentration, showing an optimum at approximately 2 suns. Furthermore, the enhancement in efficiency for the microcrystalline cells was larger than for the amorphous cells. We show that the Voc’s up to 0.63 V can be reached in microcrystalline cells while FF’s only decrease by 9%. The effects have also been computed using the device simulator ASA, showing qualitative agreement. INTRODUCTION Recently, in the quest for higher efficiencies for thin film solar cells, much emphasis is placed on light trapping or absorption enhancement techniques, such as the use of plasmonic or diffractive back contacts [1] and luminescent concentrators [2], to concentrate the incident light in cells with a thinner absorber layer or with a smaller area. Even in less advanced schemes, merely optical concentration of light can yield higher efficiencies. There are only very few reports on the effects of concentration in thin film silicon-based solar cells [3]. Due to the presence of midgap states, a fast decline in fill factor was observed in earlier work. However, with the advent of more stable and lower defect density protocrystalline silicon materials as well as high quality micro-/nanocrystalline silicon materials, as well as the increasing concentration ratios obtained by novel light management techniques, it is worth revisiting the performance of cells with these absorber layers under moderately concentrated sunlight. Furthermore, local generation rates in thin film silicon solar cells with a plasmonic back reflector are much enhanced and the study of the effect of external optical concentration can help to optimize their design. We determined the behavior of the external J-V parameters of pre-stabilized substratetype (n-i-p) amorphous and microcrystalline solar cells under moderate concentrations, between 1 sun and 21 suns, while maintaining the cell temperature at 25oC. Main
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