Effects of Hydrogen Dilution on a-Si:H and its Solar Cells Studied by Raman and Photoluminescence Spectroscopy
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Effects of Hydrogen Dilution on a-Si:H and its Solar Cells Studied by Raman and Photoluminescence Spectroscopy Guozhen Yue, Jessica M. Owens, Jennifer Weinberg-Wolf, and Daxing Han Department of Physics & Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255 Jeffrey Yang, Kenneth Lord, Boajie Yan, and Subhendu Guha United Solar Systems Corp., 1100 W Maple Road, Troy, Michigan 48084 ABSTRACT a-Si:H films and their n-i-p solar cells were prepared using plasma-enhanced CVD. The samples were prepared with no-, low-, standard, and high-H dilution. Raman and photoluminescence (PL) were used to characterize the i-layer. The main results are (a) Raman shows typical a-Si:H mode except for a c-Si peak in the 450 nm-thick film with high-H dilution, and (b) PL shows two regimes. (I) Below the onset of microcrystallinity characterized by x-ray diffraction, a blue-shift of the 1.4 eV PL peak energy and a decrease of the band width occur. (II) Above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4-5, and the PL peak energy is red-shifted toward 1.2 eV as the µc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells. INTRODUCTION a-Si:H films prepared at a hydrogen-to-disilane dilution ratio just below the onset of microcrystallinity have attracted a great deal of attention, because the most stable, highperformance a-Si:H solar cells were obtained by using such material.[1,2] Recent experimental results[1,3,4] indicate that an improved microstructure of the i-layer is responsible for the enhanced stability. Transmission electron microscope results suggested[1] that a-Si:H made with H dilution is a heterogeneous mixture of an amorphous matrix and more ordered linear-like objects. X-ray diffraction (XRD) demonstrated[3,4] that the width of the first diffraction peak is narrower in the most stable material, indicating improved medium-range order. On the other hand, due to the lack of sensitivity of Raman and XRD for obtaining crystalline silicon signals for very small fractions (~1%) and/or small crystallite sizes, more sensitive technique is needed. Furthermore, the structure of these transition materials was found to be sensitive not only to H dilution but also to substrate surface and film thickness.[3,4,5] In this paper, we report results on a series of films and n-i-p solar cells made by plasma-enhanced chemical vapor deposition (PECVD) with varied H dilutions and i-layer thicknesses by using PL and Raman measurements. EXPERIMENTAL RESULTS AND DISCUSSION Intrinsic a-S:H films were fabricated by PECVD on c-Si and roughed quartz substrates, and corresponding n-i-p solar cells were made on stainless steel substrates. The A9.7.1
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