A low-cost processing of CuIn(Se x Si 1-x ) 2 films: Using sulfides nanoparticle precursors
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CuIn(SexS1x)2 (CISS) thin films are deposited via conventional two-stage process. However, a significant problem related to the conventional two-stage process is the separation of CuInSe2 and CuInS2 phases. In this article, single-phase CISS thin films have been successfully prepared by selenizing sulfides of copper and indium. The mixed sulfides of Cu–In precursors were synthesized by coprecipitation method and then partly reduced. The inks containing partly reduced powders and organic binders were deposited onto glass substrate using a spin-coating technique. After coating, the precursor films were selenized to get CISS. X-ray diffraction and energy dispersive x-ray spectroscopy data show that the single (112) peak position changed with the variation of Se/S ratio. The absorption energy Eg changes linearly with Se/(Se 1 S) calculated by ultraviolet–vis absorption spectra. Those results confirm the formation of single-phase CISS with homogenous composition.
I. INTRODUCTION
In recent years, CuInSe2 and related chalcopyrite compounds have attracted great attention in photovoltaic materials and devices.1–6 To achieve greater success and match the optimum solar absorption spectrum, it is necessary to widen the band gap of CuInSe2 with doping gallium (Ga) or sulfur (S) element. However, the cells produced by the selenization process contain gallium only toward the back of the cells because of the principal difficulty to incorporate this element homogeneously in the film during this process.7 For photovoltaic devices, CuIn(SexS1x)2 (CISS) thin solid film has emerged to be the best film due to its tunable band gap via changing S/(Se 1 S) ratio.8 The unit cell parameters decrease when S takes place of Se element, which leads to the increase of band gap.9–14 However, this theoretical concept of “band gap engineering” is extremely complicated. When metal alloys are exposed to Se and/or S atmosphere, this conventional two-stage growth process is often changed to the formation of two distinct and stable CuInSe2 and CuInS2 phases.15 Devices produced from these double-phase absorbers exhibit inferior device characteristics. To resolve this problem, the present industrial processes include a postsulfurization step,16,17 but this method improved a little of open-circuit voltage. The coevaporation method has been utilized to prepare the single-phase quaternary CISS film.18,19 Using partial selenization of Cu–In alloy and then sulfurization, Alberts et al.14,20 got single-phase CISS thin film by sputtering. Yan et al.21 applied a two-step a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.393 J. Mater. Res., Vol. 27, No. 24, Dec 28, 2012
method consisting of partly selenizing Cu–In precursors and then exposing the partly selenized films to H2S/Ar. But these methods need vacuum equipments, thus nonvacuum methods offer an attractive alternative. One fast and simple technique is via spray pyrolysis route to synthesize CISS film.22 Yang et al.23 reported a low-temperature solutionprocessed CISS with h
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