Correlation of Hydrogenated Nanocrystalline Silicon Microstructure and Solar Cell Performance
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A9.53.1
Correlation of Hydrogenated Nanocrystalline Silicon Microstructure and Solar Cell Performance Keda Wang, Anthony Canning, J.R.Weinberg-Wolf, E.C.T. Harley, and Daxing Han Department of Physics & Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3255 Baojie Yan, Guozhen Yue, Jeffrey Yang, and Subhendu Guha United Solar Ovonic Corporation., 1100 W Maple Road, Troy, MI 48084 ABSTRACT We used Raman and photoluminescence (PL) spectroscopy to study the relationship between the material properties and the solar cell performance of hydrogenated nanocrystalline silicon (nc-Si:H). The crystalline volume fraction (fc) was deduced from the Raman spectrum. Generally, a high fc leads to a high short circuit current density and a low open circuit voltage. PL spectra were measured using 632.8-nm and 442-nm laser lines. There are two distinguished PL peaks at 80 K, one at ~1.4 eV originating from the amorphous region, while the other at ≤ 0.9 eV from the nanocrystalline grain boundary regions. Generally, the intensity fraction of this low energy PL peak, IPLc/(IPLa+IPLc), was larger for 442-nm than 632.8-nm excitation, indicating an increase in crystallinity along the growth direction. However, for the best initial performance cells obtained by H2 dilution profiling and the i/p buffer layer, the intensity fraction IPLc/(IPLa+IPLc) decreased from the bulk to the top i/p interface. The Raman and PL results give insight into the correlation between the microstructures and the cell performance, and verified that properly-controlled crystallinity in the intrinsic layer and buffer layer at the i/p interface layer are important for optimizing nc-Si:H solar cells. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) solar panels have been used for both terrestrial and space applications. One limitation of such panels is the light-induced degradation caused by the metastability of a-Si:H. Although much effort has been made to optimize solar cell structures, such as using a spectrum splitting triple-junction structure [1,2], the light-induced degradation still exists in a-Si:H based solar cells. In order to make high efficiency and high stable thin film solar cell, hydrogenated nanocrystalline silicon (nc-Si:H) solar cell has received a great deal of attention in the last decade due to its lower light-induced degradation and higher current density than a-Si:H cell [3-6]. Initial efficiencies of over 10% for a single-junction nc-Si:H solar cell and over 14% for a-Si:H/nc-Si:H double-junction solar cell have been reported [5]. At United Solar, several techniques have been used to increase the deposition rate and improve cell performance [6]. It is well accepted that a high quality material is essential for achieving high efficiency solar cells. Therefore, it is very important to find a direct correlation between material properties and device performance for improving cell efficiency. In previous studies [7-12], Raman and photoluminescence (PL) spectroscopy have been used to characterize the microstructure a
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