SnO 2 :F with Very High Haze Value and Transmittance in Near Infrared Wavelength for Use as Front Transparent Conductive
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SnO2:F with Very High Haze Value and Transmittance in Near Infrared Wavelength for Use as Front Transparent Conductive Oxide Films in ThinFilm Silicon Solar Cells Masanobu Isshiki1,2, Yasuko Ishikawa1, Toru Ikeda1, Takuji Oyama1, Hidefumi Odaka1, Porponth Sichanugrist2 and Makoto Konagai2,3 1 Research Center, Asahi Glass Co., Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi, Kanagawa 221-8755, Japan 2 Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1, Oookayama, Meguro-ku, Tokyo 152-8552, Japan 3 Photovoltaic Research Center (PVREC), Tokyo Institute of Technology, 2-12-1, Oookayama, Meguro-ku, Tokyo 152-8552, Japan ABSTRACT Low sheet resistance (high mobility) with high transmittance in all wavelength is required for front TCO. High haze value is also required for effective light trapping. For this purpose, we have combined F-doped SnO2 (FTO) with high mobility deposited by LPCVD and reactive ion etching (RIE) processed glass substrate. However, two problems have been found. (1) The mobility of FTO on RIE substrate dropped from that on flat glass (75 to 36 cm2/Vs). To avoid this drop, thicker film is needed. (2) To keep high transmittance with thicker film, lower carrier concentration is needed. But the mobility dropped with lower carrier concentration. In order to solve these constrains, we have adopted a stacked structure using thick non-doped layer of 2700 nm and thin F-doped layer of 500 nm. With this novel approach, we have successfully achieved the high mobility (80 cm2/Vs), low carrier concentration (2.2x1019 /cm3) and high haze value (77% at wavelength of 1000 nm) at the same time. This new developed high-haze SnO2 is a new promising TCO for thin-film Si solar cells. INTRODUCTION For front transparent conductive oxide (TCO) films used in thin-film silicon solar cells, high transmittance in all wavelength, low sheet resistance and high haze value are required. To achieve high transmittance and low sheet resistance, the carrier concentration and mobility of TCO need to be low and high, respectively. Fig. 1 shows simulation results based on Drude model [1]. From Fig. 1(a), the absorption is almost the same in various carrier concentration if the mobility and the product of carrier concentration and film thickness (n*d) are fixed. From Fig. 1(b), the absorption becomes higher rapidly as mobility decreases. So, higher mobility is very important to achieve lower absorption especially in the longer wavelength. TCOs with high mobility of around 80 cm2/Vs have been reported by several groups including ours [2-4]. But all of them have flat surfaces and low haze values. Recently, Wtextured ZnO films fabricated by metal–organic chemical vapor deposition (MOCVD) on reactive ion etching (RIE) processed glass substrates are reported to have very high haze value [5]. However, the mobility of MOCVD B-doped ZnO is not high enough. Therefore, we have developed F-doped SnO2 (FTO) with higher mobility and higher haze value using RIE-etched substrate.
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n dependence ʅ=80cm2/Vs fixed, n*d fixed
ʅ depen
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