Optical Properties of Strained Polycrystalline CuInS 2 Layers
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1012-Y12-25
Optical Properties of Strained Polycrystalline CuInS2 Layers Jens Eberhardt1, Heinrich Metzner1, R¸diger Goldhahn2, Florian Hudert2, Kristian Schulz1, Udo Reislˆhner1, Thomas Hahn1, Janko Cieslak1, and Wolfgang Witthuhn1 1 Institut f¸r Festkˆrperphysik, Friedrich-Schiller-Universit‰t Jena, Max-Wien-Platz 1, Jena, 07743, Germany 2 Institut f¸r Physik, Technische Universit‰t Ilmenau, PF 100565, Ilmenau, 98684, Germany
ABSTRACT Using molecular beams, polycrystalline thin CuInS2 (CIS) films of different thicknesses were grown on Si substrates covered with a sputtered Mo-buffer layer. Systematic photoluminescence and photoreflectance measurements were performed to investigate the influence of strain - introduced during growth - on the optical properties. The transition energy of the free A-exciton (FXA) decreases with increasing tensile strain caused by (i) increasing thickness of the Mo buffer layer and (ii) decreasing thickness of the CIS layer. Furthermore, the energetic splittings between FXA, FXB, and FXC increase with increasing tensile strain. When combined with X-ray diffraction data, the oscillator strengths of the excitonic transitions yield information on the strain distribution within the films. INTRODUCTION Polycrystalline absorber layers of CuInS2 (CIS) and Cu(In,Ga)Se2 (CIGSe) are utilized successfully in thin-film photovoltaic devices. Due to its optimum band gap, CIS promises higher efficiencies accompanied by high open-circuit voltages and should, in principle, be more favorable for thin-film solar cells than CIGSe. However, the conversion efficiencies for CIS solar cells are presently below 13% [1] while 19,5% have been achieved for CIGSe photovoltaic devices [2]. In the CIGSe system, the solar-cell material does not show excitonic emissions in photoluminescence but rather a broad emission only [3]. Thus, a unique feature of the CIS system among the chalcopyrites is the prominent appearance of excitonic emissions in the solarcell material. In a thin-film solar cell which is a multi-layer structure, strain is a very common phenomenon and will thus also effect the light absorber layer as the most crucial part of the solar cell. Investigations of strain induced phenomena in CIS solar-cell material have not been reported to date. Some effects of lattice strain have been studied in epitaxial films of Cu(Al,Ga)(S,Se)2 grown on GaAs and GaP substrates with different surface orientations [4,5]. In this paper, we present an approach to detect the effects of lattice strain on the optical properties of polycrystalline CIS solar-cell material. Different amounts of strain are imposed to these layers by a thick silicon substrate with or without a molybdenum buffer layer of various thickness. Additionally, the internal strain of the CIS layers depends on the CIS film thickness. Without taking into account the occurring orientations of the CIS crystallites, we assume a tensile strain within each crystallite of our CIS layers. Our investigations show that the tensile strain reduces the band gap and widens
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