Analysis of Cu(InGa)Se 2 Alloy Film Optical Properties and the Effect of Cu Off-Stoichiometry
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Analysis of Cu(InGa)Se2 Alloy Film Optical Properties and the Effect of Cu Off-Stoichiometry P. D. Paulson1,2, S. H. Stephens1 and W. N. Shafarman1 1 Institute of Energy Conversion, University of Delaware, Newark, DE 19716, USA 2 Miasolé, San Jose, CA 95131, USA ABSTRACT Variable angle spectroscopic ellipsometry has been used to characterize Cu(InGa)Se2 thin films as a function of relative Ga content and to study the effects of Cu off-stoichiometry. Uniform Cu(InGa)Se2 films were deposited on Mo-coated soda lime glass substrates by elemental evaporation with a wide range of relative Cu and Ga concentrations. Optical constants of Cu(InGa)Se2 were determined over the energy range of 0.75–4.6 eV for films with 0 ≤ Ga/(In+Ga) ≤ 1 and used to determine electronic transition energies. Further, the changes in the optical constants and electronic transitions as a function of Cu off-stoichiometry were determined in Cu-In-Ga-Se films with Cu atomic concentration varying from 10 to 25 % and Ga/(In+Ga) = 0.3. Films with Cu in the range 16 – 24 % are expected to contain 2 phases so an effective medium approximation is used to model the data. This enables the relative volume fractions of the two phases, and hence composition, to be determined. Two distinctive features are observed in the optical spectra as the Cu concentration decreases. First, the fundamental bandgaps are shifted to higher energies. Second, the critical point features at higher energies become broader suggesting degradation of the crystalline quality of the material. INTRODUCTION One of the properties of Cu(InGa)Se2 that make it uniquely promising for thin film solar cells is its tolerance to deviations in stoichiometry and composition. High efficiency solar cells have been demonstrated using Cu(InGa)Se2 films with the Cu atomic concentration from 21 to 25 % and the effect of varying y ≡ Ga/(In+Ga) from CuInSe2 to CuGaSe2 has been studied extensively [1]. The compositions of Cu-In-Se thin films tend to fall along the quasibinary tieline between Cu2Se and In2Se3. Thus, the CuInSe2 film composition can be written as (Cu2Se)x(In2Se3)1-x. A section of the Cu2Se–In2Se3 tie-line taken from the comprehensive Cu-InSe phase diagram of Gödecke et al. [2] is shown in figure 1. The composition in most solar cells falls in the single phase α region or the mixed phase α + δR region where α includes stoichiometric CuInSe2 with x = 0.5 and δR includes CuIn3Se5 with x = 0.25. The δR-CIS phase is commonly referred to as an ordered defect compound since it maintains the chalcopyrite structure of the α phase with a crystallographic ordering of defects. In addition, the In2Se3 endpoint of the tie-line can be alloyed with Ga to form (In1-yGay)2Se3 and measured film compositions still fall along the tie line. Therefore, CIGS is used as shorthand for the composition described by (Cu2Se)x[(In1-yGay)2Se3]1-x. The CIGS phase diagram is assumed to follow an analogous phase diagram in the region of figure 1 although the range of the α-CIGS phase may be expanded with the inclusion of
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