Band alignment at CdS/wide-band-gap Cu(In,Ga)Se 2 hetero-junction by using PES/IPES
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Band alignment at CdS/wide-band-gap Cu(In,Ga)Se2 hetero-junction by using PES/IPES S.H. Kong1, H. kashiwabara2, K. Ohki2, K. Itoh2, T. Okuda2, S. Niki3, K. Sakurai3, A. Yamada3, S. Ishizuka3, and N. Terada2,3 1 Kagoshima University, Venture Business Laboratory, 1-21-40 Koorimoto, Kagoshima, 890-0065, Japan 2. Kagoshima University, Department of Nano Structure and Advanced Materials, 1-21-40 Koorimoto, Kagoshima, 890-0065, Japan 3. National Institute of Industrial Science and Technology, 1-1-1 Umezono Tsukuba, 305-8568, Japan ABSTRACT Direct characterization of band alignment at chemical bath deposition (CBD)-CdS/Cu0.93 (In1-xGax)Se2 has been carried out by photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). Ar ion beam etching at the condition of the low ion kinetic energy of 350 eV yields a removal of surface contamination as well as successful measurement of the intrinsic properties of each layer and the interfaces. Especially interior regions of the wide gap CIGS layers with a band gap of 1.4 ∼ 1.6 eV were successfully exposed. IPES spectra revealed that the conduction band offset (CBO) at the interface region of the wide gap CIGS with x = 0.60 and 0.75 was negative, where the conduction band minimum of CdS was lower than that of CIGS. It was also observed that the energy spacing between conduction band minimum (CBM) of CdS layer and valence band maximum (VBM) of Cu0.93(In0.25Ga0.75)Se2 layer at interface region was no wider than that of the interface over the Cu0.93(In0.60Ga0.40)Se2 layer. INTRODUCTION Cu(In1-xGax)Se2 (CIGS) thin films have attracted great interest as absorber layers in solar cells with CdS buffer layer owing to tunable band gap by varying the ratio of Ga substitution [1-4]. Several model-calculations have indicated a potential of a high conversion efficiency as high as 25 % in the solar cells based on CIGS with a wide band gap of around 1.4 eV corresponding to Ga/(In+Ga)=0.7 ∼ 0.8 [5]. In previous reports, the efficiency of the CIGS based cells takes a maximum of 19.2 % at a condition of the band gap around 1.2 eV [6]. A serious degradation of the efficiency was observed in the wide gap CIGS based cells. One of the origins of this discrepancy between the expectation and the experiments is a saturation of open circuit voltage (Voc) in the wide-gap region, which is correlated with bulk recombination and an unfavorable band alignment at p-n junction in the cell structure [7-9]. Determination of the band alignments has been attempted by various techniques [10-12]. In our previous study of the interfaces of CIGS with a Ga substitution ratio x up to 40 % fabricated by three stage
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co-evaporation, a decrease of the CBO at the interface as a function of x has been observed. For the samples with a low x of 20 %, the conduction band minimum (CBM) of CBD-CdS was higher than that of CIGS by 0.3 eV (positive conduction band offset (CBO)), whereas the CBO fell below 0.1 eV in the sample with x of 40 % [13]. For a further improvement of the cell performance, the
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