An Electron Paramagnetic Resonance and Photoelectron Spectroscopy Study on the Native Oxidation of CuGaSe 2
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An Electron Paramagnetic Resonance and Photoelectron Spectroscopy Study on the Native Oxidation of CuGaSe2 R. Würz1, A. Meeder, D. Fuertes Marrón, Th. Schedel-Niedrig, K. Lips2 Hahn-Meitner-Institut, Abteilung SE2, Glienicker Strasse 100, D-14109 Berlin, Germany 1 Institut für Physikalische Elektronik, Pfaffenwaldring 47, D-70569 Stuttgart, Germany 2 Hahn-Meitner-Institut, Abteilung SE1, Kekuléstrasse 5, D-12489 Berlin, Germany ABSTRACT The complementary techniques of electron paramagnetic resonance (EPR) and photoelectron spectroscopy (PES) have been used to study the native oxidation of CuGaSe2 crystals and polycrystalline thin films. After storage of specimen under ambient conditions for few months, an EPR signal occurred which is assigned to Cu2+ and is found independently from the sample morphology. This signal is due to the formation of a copper hydroxide surface phase, Cu(OH)2, observed only after long term oxidation. Chemical etching in KCN removes and thermal reduction by annealing in vacuum at 200 °C reduces this Cu(OH)2 surface phase as proved by EPR and PES. Implications for solar cell device performance will be discussed. INTRODUCTION The semiconductor CuGaSe2 belongs to the class of I-III-VI2 chalcopyrite and is a promising candidate for absorber applications in thin-film solar cells due to its high band gap of Eg = 1.68 eV at room temperature and optical absorption coefficient larger than 104 cm-1 for photon energies hν > 1.7 eV. Up to now solar energy conversion efficiencies of 9.7% and 9.5% have already been achieved for single-crystal [1] and thin-film cells [2], respectively. The major limiting factors of the solar cell performance are intrinsic and extrinsic defects in the absorber or at the interfaces. Interface related recombination is increased if the p-type CuGaSe2 thin-film absorber layer is exposed to ambient conditions before depositing the n-CdS emitter [3]. Exposure of Cu(In,Ga)Se2 films to humid air accelerates this degradation process strongly [4]. Intrinsic and extrinsic defects of solar cell absorbers can be highly sensitively identified by using Electron paramagnetic resonance (EPR). In the past, defects in “bulky” powder samples and crystals of CuGaSe2 [5-8] have been studied by EPR. After exposing CuGaSe2 powder samples to air a Cu2+ EPR signal was observed [5] which was strongly enhanced when the relative humidity of the air was high [6]. Additionally, the g tensor of the signal in CuGaSe2 shows no axial symmetry (gxx ≠ gyy) [5,6] indicating an aging effect associated with the oxidation of non paramagnetic Cu+ to paramagnetic Cu2+ at the surface of the powder samples [5,6]. Since the development of Cu2+ signal in powder samples occurred on the same time scale as the degradation of solar cells, Birkholz et al. [6] suggested, that both processes are associated. However, for device-grade thin-film material as used in solar cell devices this proposed oxidation mechanism was neither experimentally verified nor observed. The native oxide formation of lumps, powder and thin-fil
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