Zn-Based Buffer Layer and High-Quality CIGS Films Grown by a Novel Method
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1012-Y02-06
Zn-Based Buffer Layer and High-Quality CIGS Films Grown by a Novel Method Akira Yamada1, Fanying Meng1, Yoshiyuki Chiba2, Masahiro Kawamura2, and Makoto Konagai2 1 Quantum Nanoelectronics Research Center, Tokyo Institute of Technology, 2-12-1-S9-10, Ookayama, Meguro-ku, Tokyo, 152-8552, Japan 2 Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1-S9-9, O-okayama, Meguro-ku, Tokyo, 152-8552, Japan ABSTRACT In this paper, the importance of ìall-dryî process for fabrication of a buffer layer of Cu(InGa)Se2 (CIGS) solar cells is emphasized, then both metal organic chemical vapor deposition (MOCVD) of Zn1-xMgxO (ZMO) and evaporation-based deposition of CIGS absorbers are described. For the latter the ëactive sourceí concept of growth with ionized Ga and cracked Se is described. Polycrystalline ZMO has been successfully grown by MOCVD. Bismethylcycropentadienyl-Mg (MeCp2Mg) and bis-ethylcycropentadienyl-Mg (EtCp2Mg) were used as Mg sources, and diethylzinc (DEZn) and water (H2O) were used as a Zn source and oxidant, respectively. The maximum bandgap of 3.73 eV was obtained by using MeCp2Mg, and this bandgap energy corresponded to a Mg composition of 23 %. The unfavorable rocksalt phase was easily observed in the films grown on Cu(InGa)(SSe)2 (CIGSSe) substrates at lower substrate temperatures. By optimizing ZMO growth conditions, the solar cells with a ZMO buffer layer were fabricated without any surface treatment of CIGSSe absorbers, and an efficiency of 10.2 % was obtained. CIGS films were grown with cracked selenium. The Se/metal ratio of the sample grown with Se cracking was higher than that of the sample grown without Se cracking even though the monitored Se flux for cracked Se was lower than that for uncracked Se. The grain size of the sample grown with cracking Se was about 1 µm when the film thickness was about 0.5 µm. These results demonstrated that Se cracking is believed to be effective on the growth of high quality CIGS films.
INTRODUCTION Cu-based chalcopyrite semiconductors are promising for high-efficiency thin-film solar cells, and a conversion efficiency of 19.5 % with an area of 0.41 cm2 has been achieved [1]. Several companies are now ready to produce CIS modules on an industrial level. Showa Shell Sekiyu K. K. in Japan announced production of 20 MWp/a by 2007 [2], Honda Motor Co., Ltd. also announced its plan to begin mass production of 27.5 MWp/a [3]. Wuerth Solar in Germany succeeded in 1.3 MWp/a pilot line which enabled the decision to invest in a real mass production, and they announced the capacity of 15 MWp/a should be achieved in 2007 [4]. These achievements are delightful to Cu(InGa)Se2 (CIGS) PV communities, but even greater efforts to develop highly efficient solar cells and to reduce manufacturing costs are still necessary for CIGS production.
We focus on three subjects of CIGS solar cells both to develop high-efficiency solar cells and to reduce cell fabrication costs. Firstly, we have proposed a new growth concept, which is a utilization of active sourc
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