A Novel Poly-Si Solar Cell using Grain Boundary Etching Treatment and Transparent Conducting Oxide
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A Novel Poly-Si Solar Cell using Grain Boundary Etching Treatment and Transparent Conducting Oxide Dong Gun Lim, Wook Jae Lee, and Junsin Yi School of Electrical and Computer Engineering, Sungkyunkwan University 300 Chunchun-dong, Jangan-gu, Suwon, Kyunggi-do 440-746, Korea ABSTRACT This paper deals with a novel structure of polycrystalline silicon (poly-Si) solar cell for terrestrial applications. Grain boundary (GB) in poly-Si degrades a conversion efficiency of poly-Si solar cell. To reduce the GB side-effect, we investigated various parameters such as the preferential GB etch, etch time, ITO electrode, heat treatment, and emitter layer effect. Among various preferential etchants such as Sirtl, Yang, Secco, and Schimmel, a Schimmel etchant illustrated an excellent preferential etching property. We observed a 10µm deep trench along grain boundaries and randomly textured grain surface with pyramid structure. We used rf magnetron sputter grown tin doped indium oxide (ITO) film as a top contact metal. ITO films showed a resistivity of 1.14×10-4 Ω-cm and transmittance of 90.5% for the wavelength of 594 nm. ITO films served as a top electrode as well as an effective AR coating layer. ITO film properties were strongly influenced by the preparation temperatures. Substrate temperature of 400oC gave the highest conversion efficiency of poly-Si solar cell. With well-fabricated poly-Si solar cells, we were able to achieve conversion efficiency as high as 16% at the input power of 20 mW/cm2. INTRODUCTION Single crystal silicon solar cells showed disadvantages because of their high cost and small wafer size. Poly-Si solar cells have been considered as an alternative to reduce the cell cost and to increase the area of the active cell [1, 2]. However, defects and potential barriers at the grain boundaries (GBs) of poly-Si reduce the short-circuit current and the conversion efficiency of solar cells [3, 4]. To reduce these undesired effects, we developed a process to physically remove the GBs and adopted a tin-doped indium-oxide (ITO) film [5] as a top electrode. ITO films are highly conductive and transparent in the visible region whose property leads to the applications in solar cell, liquid crystal display, thermal heater, and other sensors. This paper investigated ITO films as transparent conducting films for application of poly-Si solar cells [6]. In carrier collection processes, a GB potential barrier hinders photo-generated carriers in the conventional poly-Si cells. Removing GB region and placing an ITO electrode directly on top of GB, we expect that most of the photo-generated carriers can be collected without experiencing the GB potential barrier. This paper investigated characteristics of a novel type of poly-Si solar cell. EXPERIMENT We took an experimental procedure in the order of organic cleaning, surface polishing, preferential GB etching, POCl3 doping, Al rear side metallization, ITO top electrode formation, and solar cell characterization. A25.7.1
We used poly-Si substrates with a wafer size of 10 cm × 10 cm,
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