Preparation of CuIn(S x Se 1-x ) 2 thin films with tunable band gap by controlling sulfurization temperature of CuInSe 2
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this paper, polycrystalline CuIn(SxSe1–x)2 thin films with tunable x and Eg (band gap) values were prepared by controlling the sulfurization temperature (T) of CuInSe2 thin films. X-ray diffraction indicated the CuIn(SxSe1–x)2 films exhibited a homogeneous chalcopyrite structure. When T increases from 150 to 500 C, x increases from 0 to 1, and Eg increases from 0.96 to 1.43 eV. The relations between x and Eg and the sulfurization process of CuIn(SxSe1–x)2 thin films have been discussed. This work provides an easy and low-cost technique for preparing large area absorber layers of solar cell with tunable Eg. I. INTRODUCTION
The Cu-In-Ga-Se-S multinary alloy system as the absorber layer materials of thin film photovoltaic solar cells has attracted much attention due to its excellent optical and photovoltaic properties. Alloying CuInSe2 with either Ga or S has enabled the production of quaternary Cu(In,Ga)Se2 or CuIn(S,Se)2 with band gap (Eg) varying from 1.04 to 1.68 eV1 or 1.0 to 1.55 eV,2 respectively. Moreover, alloying CuIn(S,Se)2 with Ga has been developed to produce penternary Cu(In,Ga)(S,Se)2 with Eg varying from 1.0 to 2.4 eV.3 These alloy materials with tunable Eg could improve module performance, reduce module cost, and obtain an optimum match with the solar spectrum. At present, the solar cells employing quaternary CuIn1–xGaxSe2 as absorber layers have attained a high conversion efficiency about 20%.4,5 CuIn(S,Se)2 alloy film has been considered as another promising material for solar cells with high conversion efficiency. The band gap of the CuIn(S,Se)2 alloy can be tailored to the optimum band gap value (about 1.4– 1.50 eV) for matching the solar spectrum by adjusting the S/(SþSe) ratio.6 There are a number of film growth technologies to prepare CuIn(S,Se)2 thin film, such as spray pyrolysis,7,8 solution growth technique,9 electrodeposition,10–12 and two-step growth processes.1,2,6,13–16 From the industrial point of view, the two-step sulfurization process may be one of the best methods to prepare CuIn(S,Se)2 thin films for solar cells. However, in standard two-step growth processes, many researchers use toxic H2S gas,2,15 or a complex vacuum facility to prepare precursors,1,6,13,14 which are not entirely suitable for large scale application. In addition, Izquierdo-Roca et al.16 have a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0304
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http://journals.cambridge.org
J. Mater. Res., Vol. 25, No. 12, Dec 2010 Downloaded: 23 Mar 2015
reported the preparation of CuIn(S,Se)2 films by controlling the amount of sulfur vapor. However, up to now, the x and Eg values in CuIn(SxSe1–x)2 films cannot be accurately controlled by an easy and effective method, which is still an important challenge in the preparation of CuIn(SxSe1–x)2 films. In this paper, a simple and effective method has been developed to prepare CuIn(SxSe1–x)2 films with controllable x and Eg values. When the sulfurization temperature of annealed CuInSe2 (T) increases from 150 to 500 C, x increas
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