Inhomogeneous microscopic carrier transport is difficult to study, but important in many condensed-matter applications. For example, the role of grain boundaries (GBs) in polycrystalline semiconductors has been controversial for 20 years. In cadmium telluride (CdTe) solar cells, electron-beam-induced current (EBIC) measurements consistently demonstrate enhanced current collection along GBs, which is argued as evidence for interpenetrating CdTe p-n current-collection networks critical to high efficiency. Conversely, cathodoluminescence (CL) measurements consistently indicate that GBs are deleterious low-lifetime regions. Here, we apply transport imaging (TI) in conjunction with spatially correlated EBIC, CL, and scanning Kelvin probe force microscopy measurements to understand carrier drift, diffusion, and recombination in polycrystalline CdTe. We simultaneously observe GB potential wells, reduced carrier lifetime at GBs, and seemingly contradictory enhanced GB current collection, and then describe their coexistence with microscopic TI and physical arguments. The results provide visualization of inhomogeneous transport that is critical to understanding and engineering polycrystalline solar technology.

Inhomogeneous local minority-carrier transport in thin-film solar cells is a critical aspect for the device operation. In this work, we applied a transport imaging (TI) technique to the intra- and inter-grain carrier-transport properties of cadmium telluride (CdTe) solar cell materials. We compared the TI results with cathodoluminescence and electron backscatter diffraction (EBSD) on the same CdTe thin film. The diffusion lengths of two distinct grain interiors were measured directly, and the values are significantly different. Three types of grain boundaries (GBs) determined by EBSD were studied by TI, and they have different decays, suggesting that different GB structure can be responsible for carrier-transport properties. Further, we did a 2D analytical simulation of carrier diffusion across GBs with varying GB lifetimes. The results suggest that GB lifetimes affect the carrier transport. Comparison of multiple technique results, together with numerical simulation, provides a deeper understanding of carrier-transport properties.

Nonuniformity in charge transport properties is a limiting factor in energy resolution of radiation detectors. In this paper, we investigate variations in the low temperature ambipolar diffusion length and the mobility-lifetime (μτ) product in bulk doped TlBr using cathodoluminescence (CL) and transport imaging. One TlBr crystal was doped with sodium (Na), aluminum (Al), and silver (Ag). A second TlBr crystal was doped with copper (Cu), iron (Fe), and zinc (Zn). We report the first low temperature high resolution CL spectroscopy and mapping in bulk doped TlBr, showing spatial variation in recombination luminescence on a scale of ~10 μm. Transport imaging is applied to quantify these variations in TlBr at 5 K. Ambipolar diffusion lengths and μτ products, dominated by the transport of holes, are mapped across a 40 μm segment of TlBr at a resolution of 2 μm. Ambipolar diffusion lengths are found to vary between 4.6 μm and 11.2 μm, on a spatial scale comparable to the variation observed in the CL map.

A contact-free optical technique is developed to enable a spatially resolved measurement of minority carrier diffusion length and the associated mobility-lifetime (μτ) product in bulk semiconductor materials. A scanning electron microscope is used in combination with an internal optical microscope and imaging charge-coupled device (CCD) to image the bulk luminescence from minority carrier recombination associated with one-dimensional excess carrier generation. Using a Green's function to model steady-state minority carrier diffusion in a three-dimensional half space, non-linear least squares analysis is then applied to extract values of carrier diffusion length and surface recombination velocity. The approach enables measurement of spatial variations in the μτ product with a high degree of spatial resolution.