Nanocrystalline Silicon Superlattice Solar cells
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1153-A10-06
Nanocrystalline Silicon Superlattice Solar cells Atul Madhavan1,2, Nayan Chakravarty1,2, and Vikram L. Dalal1,2 1 Dept. of Electrical and Computer Engineering , 2Microelectronics Research Center, Iowa State University, Ames, Iowa 50011, USA ABSTRACT We report on the growth and properties of nanocrystalline Si superlattice solar cells. The solar cells consisted of a stack of alternating layers of amorphous and nanocrystalline Si. The thickness of each of the two layers in the superlattice structure was varied independently. It was found that when the nanocrystalline layer thickness was low, increasing the thickness of the amorphous layer in the superlattice systematically reduced the grain size, while the grain size remained essentially invariant. This fact shows that by interposing an amorphous layer between two nanocrystalline layers forces the nano grains to renucleate and regrow. It was also found that when the amorphous Si layer was too thick, there were significant problems with hole transport through the device. Measurements of defect densities and effective diffusion lengths showed that there was an optimum thickness of the amorphous layer (about 10 nm) for which the defect density was the lowest and the diffusion length was the highest. We also show that the absorption coefficient in nano Si depends upon the grain size and can be increased significantly by increasing the grain size. INTRODUCTION Nanocrystalline Si is an attractive material system for solar cells [1-5]. It consists of small grains of Si, 10-20nm, whose grain boundaries are effectively passivated by H and potentially a thin layer of a-Si:H during growth[6]. As the grains grow, the smaller grains tend to agglomerate into larger conical grains, giving rise to a cauliflower-type structure which suffers from excessive recombination at the large grain boundaries [7]. To overcome this problem, many investigators use a hydrogen profiling technique where they reduce the hydrogen to silane ratio as the film grows, thereby forcing the film to remain close to the amorphous-crystalline boundary[8]. This arrangement usually results in an improvement inc ell characteristics. However, the precise dilution profile required to keep the sample just on the amorphous-nano phase change boundary is heavily dependent upon deposition parameters such as pressure, temperature, plasma power, precise reactor geometry, plasma potential etc [9]. Therefore, it would be useful to study alternative techniques whereby one can prevent the agglomeration of grains and the recombination at the large grain boundaries. We have previously shown that using a superlattice of alternating layers of amorphous and nanocrystalline Si may be such an approach [10]. In this paper, we systematically study the properties of such a superlattice cell. In particular, we systematically vary the thickness of the amorphous and nano Si layers individually, and show that the thickness of the amorphous layer plays a critical role in determining the properties of the superlattice layer. We al
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