Microstructural Defects in Antimony Selenide Solar Cells

Antimony selenide (Sb2Se3) is an emerging photovoltaic material that has attracted attention not only because of its low-toxicity, earth abundant composition, but also because of its unusual one-dimensional nano-ribbon crystal structure, which has the potential to eliminate recombination losses at grain boundaries. In this project, electron microscopy analysis was carried out on three devices grown by thermal evaporation (TE) and close space sublimation (CSS) on CdS and TiO2 emitter layers, as well as a seed layer grown by CSS on TiO2, in order to better understand differences in performance between the three devices. Scanning electron microscopy and scanning transmission electron microscopy cross-sectional images of the devices showed that the device grown by CSS on CdS had voids around 300 nm thick across almost the entire width of the CdS-Sb2Se3 interface. Energy dispersive X-ray spectroscopy showed that this was likely due to Kirkendall voiding, caused by the diffusion of Se and Sb into the CdS layer. This diffusion also led to Se being substituted for S to form a Cd(S,Se) layer, which reduced external quantum efficiency, and may also form a charge transport barrier at the heterojunction. These effects would account for the low (≈1%) efficiency of this device. Analysis of electron diffraction patterns and high resolution electron micrographs allowed the orientation of the Sb2Se3 nano-ribbons relative to the film thickness direction to be measured for individual grains. For all devices the mean orientations were within one standard deviation of each other, at around 30-50°. This is consistent with X-ray diffraction patterns reported in the literature. For the device grown on TiO2 by CSS, a correlation was found between grain size and orientation: the largest grains had ribbons more normal to the substrate. This may be due to the influence of the seed layer, and may in part account for this sample having the highest efficiency (≈6%).