Grain boundary strain, and anisotropic charge transport in thin-film Sb2Se3 photovoltaics

Abstract

Quasi-one-dimensional materials, such as Sb2Se3, are emerging photovoltaic materials that have attracted considerable attention due to their optoelectronic properties, reasonable efficiencies at low cost, and the potential to eliminate recombination losses at grain boundaries.
In this work, high resolution electron microscopy imaging of multiple Sb2Se3 grain boundaries was performed in order to find empirical evidence of the theoretically proposed ‘self-healing’ mechanism of Sb2Se3, which can potentially passivate band gap defect states at grain boundaries, therefore improving the efficiency of Sb2Se3 thin-film photovoltaic devices. Strain was found to be an anisotropic effect. For grains terminated along the (010) plane, the strain was found to be within the error of the measurement, while for grains terminating along a non-benign plane, such as (110) and (120), strains up to 5.3% and 9.7% respectively, were found. A possible link between structural relaxation induced strain and stoichiometric changes at the grain boundaries was found using energy dispersive X-ray spectroscopy. Additionally, a simulation study about the effect of ribbon growth misorientation from the ideal (001) substrate normal direction was carried out, using a novel 2D simulation approach developed by deriving analytical solutions for charge transport along the [001] ribbon direction. Deviating from the ideal growth orientation to a (221) grain texture commonly found in Sb2Se3 thin-film photovoltaic devices was found to decrease the efficiency by 3.2%. For this misorientation angle of approximately 44◦, passivating a symmetric twin grain boundary was found to increase the efficiency by only 0.2%, indicating the overwhelming effect of ideal ribbon growth orientation on the overall device efficiency.