Raman microspectroscopy provides the means to obtain local orientations on polycrystalline materials at the submicrometer level. The present work demonstrates how orientation-distribution maps composed of Raman intensity distributions can be acquired on large areas of several hundreds of square micrometers. A polycrystalline CuInSe2 thin film was used as a model system. The orientation distributions are evidenced by corresponding measurements using electron backscatter diffraction (EBSD) on the same identical specimen positions. The quantitative, local orientation information obtained by means of EBSD was used to calculate the theoretical Raman intensities for specific grain orientations, which agree well with the experimental values. The presented approach establishes new horizons for Raman microspectroscopy as a tool for quantitative, microstructural analysis at submicrometer resolution.
Inhomogeneities in Cu(In,Ga)Se2 thin films have been reported to lead to band‐gap or electrostatic potential fluctuations, which may reduce the photovoltaic performance of the corresponding solar cells via enhanced recombination. The issue of where these inhomogeneities occur in the Cu(In,Ga)Se2 absorber has so far not been discussed in detail in literature. The present work gives an overview of spatial variations in composition, net doping, and lifetime on various scales, also with respect to their occurrences at interfaces and extended structural defects. Impacts of these spatial variations on the device performance of the corresponding solar cells are discussed. It can be shown that compositional inhomogeneities possibly affecting the device performance are only present at (partial) dislocations and at the Cu(In,Ga)Se2/buffer interface, and that inhomogeneous distributions of excess charges at line/planar defects as well as of net doping concentrations affect considerably the potential landscape within Cu(In,Ga)Se2 thin films.
While presence of Na is essential for the performance of high-efficiency Cu(In,Ga)Se2 thin film solar cells, the reasons why addition of Na by post-deposition treatment is superior to pre-deposition Na supply—particularly at low growth temperatures—are not yet fully understood. Here, we show by X-ray diffraction and electron microscopy that Na impedes annihilation of stacking faults during the Cu-poor/Cu-rich transition of low temperature 3-stage co-evaporation and prevents Cu homogeneity on a microscopic level. Lower charge carrier mobilities are found by optical pump terahertz probe spectroscopy for samples with remaining high stacking fault density, indicating a detrimental effect on electronic properties if Na is present during growth.
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