Cu(In,Ga)Se 2 Thin-Film Evolution During Growth from (In,Ga) 2 Se 3 Precursors

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Cu(In,Ga)Se2 Thin-Film Evolution During Growth from (In,Ga)2Se3 Precursors Jehad AbuShama*, Rommel Noufi, Yanfa Yan, Kim Jones, Brian Keyes, Pat Dippo, Manuel Romero, Mowafak Al-Jassim, Jeff Alleman, and Don L. Williamson* National Renewable Energy Laboratory, Golden, CO 80401 *Physics Department, Colorado School of Mines, Golden, CO 80401 ABSTRACT We examined the Cu(In,Ga)Se2 thin-film growth from (In,Ga)2Se3 precursors and found that the evolution of the microstructure and intrinsic native defects depends on the compositional changes that occur as the film transitions from being Cu rich to In(Ga) rich. INTRODUCTION Polycrystalline thin-film solar cells based on Cu(In,Ga)Se2 (CIGS) have the highest conversion efficiency for both small laboratory devices and large modules - 18.8% [1] and 12.1% [2], respectively, of any thin-film photovoltaic technology. However, the full potential of this material is yet to be realized. The four elements of this multinary polycrystalline film may form different compounds, as dictated by the phase equilibria. Even though this multiplicity makes the material complicated, CIGS nevertheless tolerates defects and impurities by selfadjusting its chemistry and microstructure. A perspective describing phase relations of the ternary Cu-In-Se system has been presented previously [3-8]. In our laboratory, we are investigating the thin-film growth mechanisms using our so-called “3-stage process” as influenced by the specific dynamics of this process. Even though the Cu2Se and In2Se3 phases may represent a pseudobinary system including the CuInSe2 phase, we realize that in the “3-stage process,” the growth kinetics, substrate temperature profile, finite diffusion geometry in thin films, and reaction time will make the outcome of local equilibria unique to the growth process. The phase and microstructure evolution of (In,Ga)2Se3 into Cu(In,Ga)Se2 with the addition of Cu are the subject of this work. However, because of the limited space, we limit our results to that segment of the film growth where it transitions from being Cu rich to In(Ga) rich. We also limit our discussion and conclusion to CIGS compositions where Ga/(In+Ga) is less than 0.3. EXPERIMENTAL The CIGS thin films were deposited by physical vapor deposition in a multi-source bell jar system. A 3-stage growth process is used for this deposition (Fig. 1). In this process, a precursor of (In,Ga)2Se3 is reacted with Cu+Se to produce Cu(In,Ga)Se2 plus Cu2-xSe as a secondary phase, followed by the addition of In+Ga+Se to adjust the composition to slightly Cu-poor. The film growth is then interrupted at predetermined points along the reaction pathway and the films are analyzed. The thin-film samples used in this study are labeled a through d and are indicated in Fig. 1. The films are characterized using a set of characterization tools, including an electron probe for micro-analysis (EPMA), transmission electron microscopy (TEM), X-ray diffraction

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Mat. Res. Soc. Symp. Proc. Vol. 668 © 2001 Materials Research Society

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