A Study of the Diffusion and pn-Junction Formation in CIGS Solar Cells using EBIC and EDX Measurements
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A Study of the Diffusion and pn-Junction Formation in CIGS Solar Cells using EBIC and EDX Measurements Shogo Ishizuka, Keiichiro Sakurai, Koji Matsubara, Akimasa Yamada, Minoru Yonemura1, Shimpei Kuwamori1, Satoshi Nakamura1, Yasuyuki Kimura1, Hisayuki Nakanishi1, and Shigeru Niki National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan 1 Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan ABSTRACT Using EBIC and EDX measurements, CIGS solar cells prepared under several different conditions were observed and characterized. The results of EBIC and EDX measurements suggest that Cd plays an important role in the forming of a buried pn-junction in the CIGS layer via diffusion, and de-emphasize the possibility of the formation of the hetero pn-junction at the CdS/CIGS heterointerface. The correlation of the extent of the space charge region and the observed shift in the pn-junction location with the diffusion of the constituent elements in CIGS was investigated. INTRODUCTION Cu(In1-xGax)Se2 (CIGS) is a promising material for the fabrication of highly efficient and cost effective thin film solar cells. To achieve higher efficiencies and attain optimized cell performance for practical applications, detailed device characterization is indispensable. An understanding of the basic science with respect to junction formation and inter-diffusion mechanisms of CIGS solar cells has not jelled despite the relatively extensive CIGS research efforts reported in the literature and the high efficiencies already reported. To understand such fundamental mechanisms is essential for further insightful development, that is, the development of Cd-free new buffer layers and new architectural designs for the cell structure, which exhibit higher efficiencies. In this study, cross sectional measurements of CIGS solar cells, prepared under several different conditions, were carried out using an electron-beam induced current (EBIC) and an energy dispersive x-ray spectrometer (EDX) equipped high-resolution field-emission scanning electron microscope (FE-SEM). The correlation of the extent of the space charge region and the observed shift in the pn-junction location with the diffusion in CIGS was investigated. EXPERIMENTAL DETAILS CIGS solar cells were fabricated by the following procedure. CIGS photo-absorption layers were prepared by the three-stage process on Mo-coated soda-lime glass substrates. CdS buffer layers were then deposited by chemical bath deposition (CBD). i- and n-ZnO layers were successively deposited by radio-frequency (RF) magnetron sputtering without any intentional heating. Al grids were formed by evaporation. For the investigation of inter-diffusion mechanisms in CIGS solar cells, a 500°C vacuum annealing step after CdS deposition or a high
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temperature deposition of ZnO at 400°C was carried out to promote the inter-diffusion of constituent elements. Samples for cross sectional EBIC and EDX measurements were prepared by cleaving
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