Development of Plasma-Assisted Processing for Selenization and Sulfurization of Absorber Layers
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Development of Plasma-assisted Processing for Selenization and Sulfurization of Absorber Layers S. Kosaraju,* I. Repins,† and C. A. Wolden* * Chemical Engineering Department, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401 † ITN Energy Systems, 8130 Shaffer Parkway, Littleton, CO 80127 ABSTRACT In the synthesis of copper chalcopyrite solar absorbers the chalcogen source is always supplied in excess due to its low reactivity. This paper describes preliminary work aimed at addressing this issue through plasma processing. An inductively coupled plasma (ICP) was use to activate both sulfur and selenium vapors. First, the thermodynamic arguments for using activated chalcogens are presented. Next, this paper describes the experimental ICP setup and its characterization using optical emission spectroscopy (OES). Stable discharges have been achieved with both sulfur and selenium vapors using argon as a carrier gas. The potential of this approach was demonstrated by converting indium films into In2Se3 and InSx. The indium samples were inserted into chalcogen-containing ICP plasmas. Through X-ray diffraction it was observed that chalcogen conversion was achieved in a matter of minutes at room temperature by plasma processing. INTRODUCTION AND MOTIVATION Solar absorbers based on the copper indium diselenide (CuInSe2-CIS) material system are promising solar absorbers. The bandgap of CIS is only 1 eV. However it is may be engineered as high as 2.5 eV by either gallium substitution for indium or sulfur substitution for selenium. The former approach is commonly used, with record cell efficiencies being achieved from CIGS films with gallium constituting ~30% of the group III metal [1]. There are two basic synthesis approaches. The first is a batch process in which the metallic constituents are deposited by physical vapor deposition in the first step, followed by a post-deposition anneal in a chalcogen-containing atmosphere [2]. The choice for chalcogens is either elemental vapor sublimated from solid sources or the hydride gasses (H2Se/H2S). The second process is termed co-evaporation. In this process the metals are evaporated onto a substrate in a background of chalcogen vapor [3]. In both approaches low chalcogen reactivity limits the extent of the reaction. This is overcome by using significant overpressures of chalcogen vapor and high substrate temperatures. The excess chalcogen creates significant toxicity and maintenance concerns. The energetics involved in CuInS2 formation are illustrated in Figure 1. This diagram shows the change in Gibbs free energy for CuInS2 synthesis from various chalcogen sources [4, 5]. The formation energy of copper and indium are zero, so the reactant energy simply reflects the chalcogen source. Sulfur and selenium both sublime as dimers that may readily oligomerize into ring structures of Sn, where n varies from 2 < n < 8. The energetics of a sublimated sulfur source would be bound between the S2 and S8 values shown in Figure 1.
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350 Gibbs Energy of Formation at
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