Cu(In,Ga)Se 2 Prepared from Electrodeposited CuGaSe 2 /CuInSe 2 Bilayer for Solar Cell Applications
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1012-Y12-39
Cu(In,Ga)Se2 Prepared from Electrodeposited CuGaSe2/CuInSe2 Bilayer for Solar Cell Applications Yusuke Oda, Takashi Minemoto, Hideyuki Takakura, and Yoshihiro Hamakawa College of Science and Engineering, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan
ABSTRACT We report the preparation of Cu(In,Ga)Se2 (CIGS) thin-films using the electrodeposited (ED) CuGaSe2 (CGS) / CuInSe2 (CIS) bilayers. CGS/CIS bilayers were prepared on soda-lime glass /Mo substrates to realize the controlled Ga / (In+Ga) ratios and smooth layers of ED-CIGS thinfilms. It was found that the composition and morphology of CGS films was highly dependent on the composition of the bath. Crack-free and morphological CGS thin-films were obtained by the addition of supporting electrolyte and brightener. For ED-CIGS films, the best of electro charges of ED-CGS and ED-CIS films were 1.0 C and 6.0 C respectively and the films were annealed at 600oC for 60 min. However, the interface of an ED-CGS / CIS film had some voids and interdiffusions of In and Ga didinít take place by annealing. The CIGS solar cell using EDCGS/CIS films as an absorber exhibited diode behavior. INTRODUCTION Polycrystalline Cu(In,Ga)Se2 (CIGS) thin-films have exhibited promising performance for application to solar cells. A CIGS solar cell has achieved the highest conversion efficiency of 19.5 % on a 0.4 cm2 ZnO / CdS / CIGS laboratory cell [l] with high vacuum techniques. Electrodeposition (ED) has many advantages of a high utilization of materials, a low temperature processing, a large area deposition and a good control of film thickness. Several research groups have prepared CuInSe2 (CIS) and CIGS by electrodeposition [2-7]. Bhattacharya et al. reported thin-film photovoltaic devices prepared by simultaneous ED-CIGS materials reaching the conversion efficiency of 15.4 % in one-step process [2] but the composition of the films was adjusted and optimized by physical vapor deposition. Kampmann et al. [3] reported about 9.0 % efficiency with precursors electrodeposited on Cu and stainless steel substrates following sequential plating of Cu, In and Ga followed by evaporation of Se. Recently reported results about one-step ED-CIGS based solar cells indicate a conversion efficiency of 4.5 % achieved on ED-CIGS film with Ga / III (=In+Ga) ratios of 0.23 [4]. Ga / III ratios for high efficiency CIGS thin-film solar cells range from 0.2 to 0.3. It is difficult to obtain ED-CIGS thinfilms with the controlled Ga / III ratios from a single-step electrodeposition because the deposition potential of Ga is different from other elements and Ga precipitate preferentially in the Ga2O3 oxide form [5]. The performance of ED-CIGS thin-film solar cells with Ga / III ratios which were from 0.2 to 0.3 was inferior to the performance of ED-CIS thin-film solar cells [6]. In this study, CIGS thin-films were grown using a new two-step ED-process. CuGaSe2 (CGS) / CIS bilayers were prepared on a soda-lime glass (SLG) / Mo substrates to realize the controlled Ga / III ratios
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