Properties of Cu(In,Ga)Se2:Fe Thin Films For Solar Cells
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Properties of Cu(In,Ga)Se2:Fe Thin Films For Solar Cells Keiichiro Sakurai1, Hajime Shibata1, Satoshi Nakamura2, Minoru Yonemura2, Shinpei Kuwamori2, Yasuyuki Kimura2, Shogo Ishizuka1, Akimasa Yamada1, Koji Matsubara1, Hisayuki Nakanishi2, Shigeru Niki1 1 Research Center for Photovoltaics, AIST, #2-24412, 1-1-1 Umezono, Tsukuba, 305-8568 Japan 2 Electrical Engineering, TUS, Noda, 278-8510 Japan ABSTRACT We have fabricated CIGS:Fe polycrystalline thin films using a standard three-stage method, and investigated the effects of Fe doping on cell performances. The Ga / (In+Ga) ratio was varied between 0.3 ~ 1.0 (= CGS), and the Fe concentration was varied between 0.0 ~ 1.2 mol%. The films were characterized by various means, including the cell performance. Increment of the grain size with higher Fe content was observed. Redshift with higher Fe content was observed in the absorbance spectra. The spectral response of the fabricated solar cells deteriorated with higher Fe content, from the long wavelength side. INTRODUCTION Cu(In,Ga)Se2 (CIGS) , a member of the "chalcopyrites", is a promising material for absorber layers of future thin-film polycrystalline solar cells [1]. Recently, due to the greatly increased market demand for ITO, there are some fears for the future price of indium, which may affect the cost performance of CIGS. Thus, it is meaningful to search for alternative materials that can reduce the use of indium. One possible candidate material for the replacement of indium is iron, which is a component element of the original "chalcopyrite". It has been reported that the bandgap of bulk CIGS decreases with increasing Fe content [2], suggesting that the use of iron would help reducing the use of indium. However, to the best of our knowledge, no reports on fabrication of thin film CIGS:Fe films, or the fabrication of actual solar cell devices, have been made. In this work, we report the effects of Fe doping into the CIGS absorber layer of the solar cells. EXPERIMENTAL We have deposited CIGS:Fe films on soda-lime glass (SLG) substrates coated with RF-sputtered Mo. Unless otherwise noted, we have used our standard deposition process [3] for all of the CIGS films described in this paper; the Cu / (Ga+In+Fe) ratio at the end of stage 2 was 1.3, the final Cu / (Ga+In+Fe) ratio was 0.93, the final thickness was 1700nm, the substrate temperature during stage 1 was 315oC, and the maximum substrate temperature was about 570oC. 5N Fe was evaporated from a K-cell using an alumina crucible, and irradiated to the substrate during stage 1 and 3 (together with In, Ga). The Ga / (Ga+In+Fe) ratio was varied between 0.3 ~ 1.0 (= CGS), and the Fe concentration was varied between 0.0 ~ 1.2 mol%. The film composition was measured by electron probe micro analysis (EPMA). The films were evaluated
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by secondary ion mass spectroscopy (SIMS), X-ray diffraction (XRD), scanning electron microscopy (SEM), absorbance spectrum and cell performance. The absorbance spectrum was measured for a film directly deposited on SLG, a
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