Significant Compositional Changes and Formation of a Ga-O Phase after Oxygen-annealing of Ga-rich CuGaSe 2 Films
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Significant Compositional Changes and Formation of a Ga-O Phase after Oxygen-annealing of Ga-rich CuGaSe2 Films Akihiko Nishio1, Akimasa Yamada, Paul. J. Fons, Ralf Hunger, Koji Matsubara, Kakuya Iwata, Shigeru Niki and Hisayuki Nakanishi1 National Institute of Advanced Industrial Science and Technology, Energy Electronics Institute, Thin Film Solar Cells Group, Tsukuba, Ibaraki, Japan 1 Science Univ. of Tokyo, Faculty of Science and Technology, Noda, Chiba, Japan ABSTRACT CuGaSe2 (CGS) is a promising material for high efficiency thin film solar cells though predicted device performance has not been realized. Understanding the difference in the chemical nature between CuInSe2 (CIS) and CGS is critical for improving Cu (In, Ga) Se2 solar cells with high Ga concentrations. In this work, we have investigated the effects of oxygen-annealing on Ga-rich CGS epitaxial films focusing on compositional changes and secondary phase formations. The photoluminescence (PL) spectrum of Ga-rich films after oxygen-annealing was observed to always change into a spectrum characteristic of CGS grown under Cu-excess conditions. Electron probe micro-analysis (EPMA) measurements indicate the formation of Ga-O after oxygen-annealing. Selective etching of the Ga-O phase showed the composition of the CGS phase became close to stoichiometric. The oxygen-annealed films showed multiple pits ~ 100 nm in depth and ~ 2.5 µm in width. The Ga-O phase is founded in a layer formed on the surface of the CGS phase and in a columnar form rising from the bottom of the pits to the film/substrate interface. The above results suggest that excess Ga in Ga-rich CGS tends to react with oxygen to form Ga-O, thus the composition of the remaining CGS approaches stoichiometry consistent with the changes observed in PL. INTRODUCTION The ternary chalcopyrites of Cu-III-VI2-type are promising materials for high efficiency thin film solar cells absorber layers [1]. ZnO:Al/ZnO/CdS/CuIn1-xGaxSe2 (CIGS)/Mo/glass thin film solar cell have reached efficiencies up to 18.8 % [2]. In theory, the optimum bandgap energy of solar cells absorber is about 1.5 eV. Therefore, the optimum Ga-content x of CuIn1-xGaxSe2 is about 0.75. The bandgap energy of CIGS and the open circuit voltage (Voc) of CIGS-based cells should increase linearly with increasing Ga-content in CIGS. However, the slope of Voc with respect to Ga-content decrease for Ga content x > 0.3 [3]; the efficiencies of CuGaSe2 (CGS)-based solar cells have been limited to 9.3% [4] in spite of the theorical prediction that such cells should have efficiencies ~ 22 %. The reasons behind the decrease in Voc for high Ga-content CIGS-based solar cells can be attributed to intrinsic defects in the absorber layer and at the interface of the absorber layer and the buffer layer. Therefore, to realize high efficiency optimum Ga-content CIGS-based solar cells, it is essential to fabricate high efficiency CGS-based solar cells. It is well known that the properties of CGS and CuInSe2 (CIS) films are strongly dependent on composition as we
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