Study of Band Alignment at the Interface between CBD-CdS and CIGS grown by H 2 O-introduced co-evaporation
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1012-Y07-04
Study of Band Alignment at the Interface between CBD-CdS and CIGS Grown by H2O-Introduced Co-Evaporation Norio Terada1,2, Hirotake Kashiwabara1, Kazuya Kikunaga1, Shimpei Teshima1, Tetsuji Okuda1, Shigeru Niki2, Keiichiro Sakurai2, Akimasa Yamada2, Koji Matsubara2, and Shogo Ishizuka2 1 Kagoshima University, 1-21-40 Korimoto, Kagoshima, 890-0065, Japan 2 Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, 305-8568, Japan ABSTRACT For understanding the origin of the improvements of properties of CIGS-based cells, in which the CIGS absorber has been fabricated by H2O-introduced co-evaporation [CIGS-H2O], the band alignment at the interfaces between chemical bath deposited CdS and CIGS-H2O with Ga substitution ratio ~ 40 % has been studied by photoemission and inverse photoemission spectroscopy. The CdS layer over the CIGS-H2O showed an identical electronic structure to that of CdS on the conventionally grown CIGS; band gap energy of 2.3 ~ 2.4 and the location of the conduction band minimum (CBM) and valence band maximum (VBM) relative to Fermi level were + 0.75 eV and -1.6 ~ -1.7 eV, respectively. In the interface region, decreases of CBM and a rise of VBM were observed. Total amount of the decrease of CBM over the interface was about 0.3 eV. Binding-energy shifts of the core-level signals over the interface showed a band bending correction for band offset of -0.1 eV. Consequently, the conduction band offset (CBO) and valence band offset (VBO) at the CBD-interface above the CIGS-H2O (Ga~40%) are about +0.2, and 0.8 ~ 0.9 eV, respectively. This positive CBO is in contrast with the almost zero or slightly negative CBO at the interface between CBD-CdS/conventionally grown CIGS (also with Ga ~ 40 % as measured previously. These results indicate that the H2O introduction is effective in extending the upper limit of the Ga substitution ratio where the Type-I conduction band alignment is maintained. The observed band alignments are consistent with the rise of Voc and efficiency in the CIGS-H2O based cells. INTRODUCTION Cu(In1-xGax)Se2 (CIGS) film is one of the key-materials for high performance solar cells, because of tunable band gap by varying the Ga substitution ratio [1-3]. Several simulations have indicated a potential of high conversion efficiency above 25 % in the cells based on CIGS with a wide band gap of around 1.4 eV and CdS buffer layer grown by chemical bath deposition (CBD) [4]. For pursuing higher performances in the CBD-CdS/CIGS based cells, it is one of the major trends to utilize wide-gap CIGS with a saved In content. For efficiency of the wide gap CIGS-based solar cells, however, there is a serious discrepancy between theoretical expectation and experiments. The main origin of it is a saturation of open circuit voltage which is correlated with the band alignment at the p-n junction in the wide gap cells [5-7]. Therefore, determination of the band alignments has been intensively attempted by various techniques [8-10]. In o
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