Microstructural Defects in Antimony Selenide Solar Cells

Abstract

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%).