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1165-M05-20

Influence of the Cu Content on Structural and Vibrational Properties in Polycrystalline CuGaSe2 Thin Films Wolfram Witte, Robert Kniese and Michael Powalla Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Industriestrasse 6, D-70565 Stuttgart, GERMANY ABSTRACT We studied CuGaSe2 (CGS) thin films with different Cu contents by means of X-ray diffraction (XRD) and micro-Raman spectroscopy. The CGS absorbers were deposited by coevaporation on Mo/glass substrates. We found a clear shift of the CGS Raman mode frequencies to lower values with increasing Cu/Ga ratio. This is in direct correlation with the increasing lattice constants a and c extracted from XRD patterns. Influence of stress on the obtained results can be neglected, because very small stress values below 50 MPa were determined with the sin2Ψ method. INTRODUCTION CuGaSe2 (CGS) thin films are a candidate for application as the top cell in tandem solar cell devices with Cu(In,Ga)Se2 (CIGS) as the bottom cell. So far, the maximum achieved conversion efficiencies of CGS thin-film solar cells [1] are too low for such an application, motivating basic research on the properties of this material. One interesting characterization method is the nondestructive technique of Raman spectroscopy, a promising tool for the in-line process monitoring of CIGS films in solar cell production. This contribution reports on the influence of the Cu content in CGS polycrystalline thin films on the Raman spectra, especially regarding changes in the frequency of the CGS modes. Lattice constants were determined from X-ray diffraction (XRD) measurements for comparison. Furthermore, the stress in the CGS films was analyzed and its influence on the Raman phonon frequencies is estimated. EXPERIMENT The polycrystalline CGS thin films were co-evaporated in-line on Mo-coated soda lime glass. The thickness of the glass is 3 mm and the sputtered Mo films are 0.5 µm thick. The CGS films have a thickness of 2 µm and typical growth temperatures were around 550-560 °C. We determined the final film composition of the samples by X-ray fluorescence analysis with an EDAX-Eagle µProbe equipped with a Rh tube. XRD spectra were recorded with a Bruker D8 Discover with a parallel beam geometry in the θ-2θ scanning mode using Cu-Kα radiation. The Euler cradle was used for the stress measurements. We recorded Raman spectra with a WITec CRM 200 confocal micro-Raman system in the back-scattering geometry at room temperature. A frequency-doubled Nd:YAG

laser with a wavelength of 532 nm was used for excitation. The penetration depth of this laser light in CGS is less than 200 nm into the absorber, respectively. We kept the laser intensity below 2.7×108 W/m2 to prevent damage of the samples, accumulated the spectra several times for spike removal and performed a background subtraction. DISCUSSION Structural characterization with XRD The XRD measurements were carried out for comparing with the Raman spectra to reveal additional phases besides the chalcopyrite compound, depen