EPEEnergy and Power Engineering1949-243XScientific Research Publishing10.4236/epe.2013.52A003EPE-30600ArticlesEngineering Microstructure Analysis and Properties of Anti-Reflection Thin Films for Spherical Silicon Solar Cells asatoKanayama1TakeoOku1*TsuyoshiAkiyama1YouichiKanamori2SatoshiSeo2JunTakami2YoshimasaOhnishi2YoshikazuOhtani2MikioMurozono2Clean Venture 21 Co. Ltd., Kyoto, JapanDepartment of Materials Science, The University of Shiga Prefecture, Shiga, Japan* E-mail:oku@mat.usp.ac.jp(TO);2904201305021822February 13, 2013March 10, 2013 March 27, 2013© Copyright 2014 by authors and Scientific Research Publishing Inc. 2014This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

Structure and properties of anti-reflection thin films of spherical silicon solar cells were investigated and discussed. Conversion efficiencies of spherical Si solar cells coated with F-doped SnO2 anti-reflection films were improved by annealing. Optical absorption and fluorescence of the solar cells increased after annealing. Lattice constants of F-doped SnO2 anti-reflection layers, which were investigated by X-ray diffraction, decreased after annealing. A mechanism of atomic diffusion of F in SnO2 was discussed. The present work indicated a guideline for spherical silicon solar cells with higher efficiencies.

Solar Cells; Spherical Silicon; Anti-Reflection Film; FTO; SnO21. Introduction

Solar cells are expected to be clean energy devices instead of fossil fuels. They are clean energy devices which discharge no greenhouse gas. However, reduction of the cost of solar cells is an important issue. Spherical silicon (Si) solar cells [1-3] are the technology that can reduce the consumption of Si compared with conventional crystal Si solar cells [4-6]. Flexible solar cells are also manufactured by silicon spheres with a diameter of ~1 mm with p-n junction [7]. To Improve the efficiencies of the Si solar cells [8], formation of a texture structure on Si surface [9-11], optimization of structures of reflectors [12-14], reduction in resistance and high transmission of anti-reflection films [15-17] are also needed.

The purpose of the present work is to investigate spherical Si solar cells with anti-reflection (AR) SnO2:F thin films.

2. Experimental

The spherical silicon used in the present experiment was supplied by Clean Venture 21 Co., Ltd. [18,19]. The surface is covered with anti-reflection films of SnO2:F. The SnO2:F anti-reflection films were prepared by spraying hydrolyzed SnF4, and annealed at 650˚C for 4 h to form the SnO2:F crystal structure [20-23]. Figure 1 shows a schematic illustration of spherical Si solar cell. Reflectors gather lights to improve conversion efficiency. Optical absorption of the solar cells was measured by photo spectroscopy (Jasco V-670). Fluorescence spectra of the samples were measured by fluorescence spectrophotometer (Hitachi F-4500), and excitation wavelength was 250 nm. The microstructures of SnO2:F films were investigated by X-ray diffraction (XRD, Philips X’PertMPD System). Thermodynamical calculation of reactions of SnO2 was performed by HSC Chemistry 5.

3. Results and Discussions

Figure 2(a) shows measured optical absorption of spherical Si before and after annealing. The spherical Si absorbs the light in the range of 300 to 1200 nm. The optical absorption of the spherical Si increased after annealing at 650˚C. Figure 2(b) shows optical absorption spectra of SnO2:F, which was an enlarged spectrum of Figure 2(a). Since the absorption range was shifted to lower energy, a structural change would occur after annealing. The refractive index of anti-reflection SnO2:F films changed from 1.8 to 1.9 after annealing. Refractive indices of the substrate and anti-reflection films would influence reflectance and optical absorption. Since refractive indices are different for wavelengths of light, the effect of the reflectance reduction depends on the wave-

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