Microcharacterization of polycrystalline semiconductor thin films for photovoltaic applications

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Abstract
We have investigated the properties of individual grains in polycrystalline thin films of CuInSe2 and CdTe by transmission electron microscopy (TEM), transmission electron diffraction (TED), scanning electron microscopy (SEM), and energy dispersive x‐ray spectroscopy (EDS) in a Scanning Transmission Microscope (STEM). TED experiments showed that most grains possess the chalcopyrite structure, which is expected for CuInSe2. In some cases, however, a complex arrangement of different phases was found within a single grain, allowing a glimpse at the kinetics of the grain boundaries, EDS spectra were recorded in a SEM. The spectra show unambiguously the presence of grains with disparate composition. To assess the compositional changes within single grains, EDS spectra were taken at various locations on the grains in a STEM. Using the total photon count as a measure of the local grain thickness and the ratio of Se Kα/Se Lα intensities to correct for absorption losses, we can analyze the relative concentrations of Se, In, and Cu independently of grain thickness. CdTe films, grown on a CdS/SnO2/glass and examined by SEM showed that the grain size depends primarily on the film thickness. TEM experiments revealed the type, density, and distribution of the structural defects in this material. Threading dislocations, stacking faults, and twinned grains were frequently observed. However, their densities differed markedly from one grain to the next. Some grains exhibited defect densities in excess of 109 cm−2, while adjacent grains were virtually defect free. Cathodoluminescence experiments in the SEM showed a similar difference in the density of nonradiative recombination centers. The grain boundaries were found to be sites of enhanced nonradiative recombination.

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