Wide-gap CIGS solar cells with Zn 1-y Mg y O transparent conducting film
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Wide-gap CIGS solar cells with Zn1-yMgyO transparent conducting film K. Matsubara, A. Yamada, S. Ishizuka, K. Sakurai, H. Tampo, Y. Kimura１, S. Nakamura１, M. Yonemura１, H. Nakanishi１, and S. Niki National Institutes of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, 305-8568, Japan １ Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan ABSTRACT Zn1-yMgyO bandgap controllable transparent conducting films were used for the wide-gap Cu(In1-xGax)Se2 thin film solar cells. Undoped Zn1-yMgyO and Al doped Zn1-yMgyO films were deposited by co-sputtering using a carousel type sputtering apparatus. Zn1-yMgyO films with Mg content y of up to 0.10 were examined. For Cu(In1-xGax)Se2 with band gap energy ~ 1.38 eV, the cell performance was slightly improved by using Zn1-yMgyO and Al doped Zn1-yMgyO instead of ZnO and Al doped ZnO. An unexpected improvement of short circuit current density was observed.
INTRODUCTION Cu(In1-xGax)Se2 (CIGS) thin film solar cells are expected as cost effective, high performance solar energy source. The band gap energy Eg of CIGS can be controlled between 1.00 eV and 1.68 eV by varying the Ga composition x. It is theoretically predicted that the highest efficiency of the solar cells would be obtained at Eg of 1.4 ~ 1.5 eV. Therefore the CIGS is the promising material for high performance solar cell. So far, the highest conversion efficiency was however obtained by the CIGS absorber with Ga composition x ~ 0.3, which corresponds to an Eg of about 1.12 eV [1,2]. Gloeckler and Sites [3] calculated Voc of wide gap CIGS cells and showed that Voc was saturated around 0.8 V regardless of the increase in the band gap of CIGS; their results agree well with the experimental facts. They suggested that the alternative window layer, which had proper conduction band offset (CBO) against the wide gap CIGS absorber, could improve Voc. Minemoto et al. [4] reported CIGS solar cells with i-Zn1-yMgyO (ZMO) window layer. They aimed to control the CBO of window layer/CdS/CIGS by using i-ZMO instead of i-ZnO. In their study, ITO was used as transparent conducting oxide (TCO) layers as their regular process. We however think not only CBO between the window layer and CIGS but also CBO
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between TCO and the window layer is also important; the conduction band minimum (CBM) of TCO layer should be smaller than that of window layer [5]. Degenerate ZMO TCO is reasonable to use with ZMO window layer. In this report we have fabricated wide-gap CIGS solar cells with i-ZMO and degenerate Al doped Zn1-yMgyO (AZMO) transparent conducting film.
EXPERIMENTS Cell fabrication process was follows; Mo back contact layers were deposited on soda lime glass (SLG) substrates by RF sputtering; CIGS absorber layers were deposited by the three-stage process using a molecular beam epitaxy (MBE) apparatus [6]; CdS layers were deposited by chemical bath deposition; Undoped ZMO and AZMO layers with the same Mg content were successively deposited; Finally Al grids were formed by evaporation.; No a
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