Evaluation of Polycrystalline Silicon for Solar Cells by Small p-n Diode Array
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0974-CC01-04
Evaluation of Polycrystalline Silicon for Solar Cells by Small p-n Diode Array Satoshi Tanaka1, Keita Imai1, Atsushi Ogura1, Yoshio Ohshita2, Koji Arafune2, Hideaki Kawai2, Futoshi Kusuoka2, Michio Tajima3, and Masaaki Inoue1,3 1 School of science and technology, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki, Kanagawa, 214-8571, Japan 2 Toyota Technological Institute, 2-12-1 Hisakata, Tenpaku, Nagoya, Aichi, 468-8511, Japan 3 The Institute of Space and Astronautical Science (ISAS)/ Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Sagamihara, Kanagawa, 229-8510, Japan
ABSTRACT A small p-n diode array was fabricated on a polycrystalline Si substrate and the electrical characteristics were measured for each small diode to evaluate the distribution of energy conversion efficiency in the substrate. The crystal qualities in conjunction with the electrical characteristics were also evaluated. We found large variations in measuring the current-voltage (I-V) characteristics of the p-n diode. We also observed variations in quality even in diodes without any grain boundaries at the p-n junction. Therefore, we evaluated crystalline quality using various techniques to compare the diode characteristics. We found clear evidence in photoluminescence (PL) mapping, where grains, including degraded diodes, were darker in the mapping, implying lower PL intensities than the others. The PL spectra obtained from the “dark grains” included D-lines indicating the existence of dislocations. We could conclude that the electrical characteristics of p-n diodes were not only affected by grain boundaries but also by crystalline defects evaluation such as dislocations. We observed a surface after Secco-etching [1] for crystalline defects evaluation using an optical microscope. The origins of etch pits were also determined by transmission electron microscope (TEM) and three different types of defects were confirmed.
INTRODUCTION Silicon is one of the most important materials in the semiconductor industry. It is also very useful as a substrate material for solar cells [2,3]. Polycrystalline silicon (poly-Si), in particular, is widely used because its production costs are lower than those for single-crystalline silicon (sc-Si). However, the energy-conversion efficiency of solar cells fabricated on poly-Si substrates has not been as high as that of sc-Si cells. This may be because poly-Si substrates have many grain boundaries, defects, and impurities, which deteriorate their minority carrier lifetimes [4-6]. Solar cells essentially consist of semiconductor p-n junctions. There are several parameters that indicate the quality of p-n junctions, such as the ideality factor (n) and reverse bias leakage current. These parameters can be determined by measuring the I-V characteristics of the diode. As diode quality should significantly affect conversion efficiency, we could estimate
the quality of solar cells by measuring the diode characteristics. The quality of p-n junction also depends on that of substrate. We fabric
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