Influence of Amorphous Layers on Performance of Nanocrystalline/Amorphous Superlattice Si Solar Cells
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0989-A18-02
Influence of Amorphous Layers on Performance of Nanocrystalline/Amorphous Superlattice Si Solar Cells Atul Madhavan1, Debju Ghosh2, Max Noack3, and Vikram Dalal4 1 Electrical and Computer Engr, Iowa State University, Ames, IA, 50011 2 Elec. and Computer Engr, Iowa State University, Ames, IA, 50011 3 Microelectronics Research Center, Iowa State University, Ames, IA, 50011 4 Electrical and Computer Engineering, Iowa State University, Ames, IA, 50011 ABSTRACT Nanocrystalline Si:H is an important material for solar cells. The electronic properties of the material depend critically upon the degree of crystallinity, the efficacy of passivation of the grain boundaries and concentration of impurities, particularly oxygen, in the material. In this paper, we examine different degrees of passivation of grain boundaries by amorphous Si, by deliberately introducing various thicknesses of amorphous tissue layers at the grain boundaries. The device structure consisted of a p+nn+ cell on stainless steel.The base n layer was fabricated using alternating layers of amorphous (a-Si) and crystalline (nc-Si) phases, creating a superlattice structure. The thicknesses of the amorphous and crystalline phases were varied to study their influence on structural and electrical properties such as grain size and diffusion length. We find that grains continued to be nucleate independently of the thickness of the amorphous layer, but the size of grains decreased when the thickness of the tissue layer became very large. We also find that as the thickness of the amorphous tissue layer increased, the quantum efficiency at 800 nm decreased and the open circuit voltage increased. For significant thickness of amorphous layer (>10nm), the transport properties degrade dramatically, causing an inflection in the I-V curve, probably because of difficulty of holes tunneling across the barriers. INTRODUCTION Nanocrystalline Si:H is an increasingly important material for solar cells, thin film transistors, integrated sensors etc. [1-6]. It consists of small nano-sized grains of Si surrounded by a thin amorphous Si:H tissue. Numerical simulations show that H is bonded at the grain boundary [7], and there may also be amorphous tissue surrounding the grain. The electronic properties can be expected to depend critically upon recombination at the grain boundaries and transport through the surrounding amorphous tissue. In particular, since there is expected to be a considerable mismatch between the amorphous and crystalline valence bands, holes photogenerated within the crystalline grain may have difficulty tunneling through the amorphous phase if it is too thick. In addition to this effect, one may expect that the crystalline grain structure itself may change as we increase the thickness of the amorphous tissue layer. This effect may arise because the growth of thermodynamically preferred grains depends upon the template effect related to thickness and crystallinity of the preceding layers, whereas those grains which nucleate randomly do not need a templ
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