Preferred Orientation, Grain Sizes and Grain Boundaries of Chalcopyrite-Type Thin Films
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1012-Y09-03
Preferred Orientation, Grain Sizes and Grain Boundaries of Chalcopyrite-Type Thin Films Daniel Abou-Ras, Melanie Nichterwitz, Christian A. Kaufmann, Susan Schorr, and Hans-Werner Schock Hahn-Meitner-Institut Berlin, Glienicker Strasse 100, Berlin, 14109, Germany
ABSTRACT Chalcopyrite-type thin films ñ CuInS2, CuInSe2, CuGaSe2, and Cu(In,Ga)Se2 ñ in various completed solar cells were studied in cross-section by means of electron-backscatter diffraction (EBSD). Valuable information on grain sizes, local grain orientations, film textures, and grain boundaries were extracted from the EBSD linescans and maps. The grain-size distributions from the chalcopyrite-type thin films can be represented well by lognormal distribution functions. The EBSD measurements on CuGaSe2 thin film reveal a fiber texture, in good agreement with x-ray diffraction texture analysis performed on the same sample. The EBSD maps from all samples studied exhibit considerable twinning in the chalcopyrite-type thin films. Indeed, the most frequent types of grain boundaries in these thin films are (near) Σ3 60∞- and 71∞ twins. It is shown that rotational 180∞- twins (which are symmetrically equivalent to 71∞-) are more frequently found than anion- or cation-terminated 60∞- twin boundaries. INTRODUCTION An important part of the research and development of chalcopyrite-type thin-film solar cells is the microstructural analysis of the thin films. Frequently, samples are broken, and their cross-sections studied by means of scanning electron microscopy (SEM). For grain-size determination of the chalcopyrite-type thin films, it is assumed that the samples break along the grain boundaries of the polycrystalline layers, an approach desiderating in precision and often also in good statistics. Grain sizes may in principle be determined also from peak widths of x-ray diffraction (XRD) θ-2θ patterns. However, various factors such as crystal defects and strains in the thin films affect the peak widths of XRD patterns considerably, and thus, the results on grain sizes obtained by XRD remain ambiguous. Transmission electron microscopy (TEM) and its related techniques provide very useful means for the analysis of grain sizes or grain boundaries. The disadvantage of these techniques is that the measurements are performed only on a reduced sample area. Therefore, the statistics of grain sizes and grain boundaries extracted from TEM studies on thin films are always poor. Electron backscatter diffraction (EBSD) in a scanning electron microscope represents a powerful technique to determine grain sizes, grain orientations and grain boundaries in
polycrystalline materials, providing high precision and good statistics. The information gathered on the microstructure in the thin films is of much higher quality than that of, e.g., "apparent" grain sizes from SEM images acquired on broken cross-sections. Since the current conduction in a thin-film solar cell proceeds perpendicular to the surface, it is important to study the microstructures of chalcopyrite-type thi
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