A simulation study of the role of anisotropic charge transport and grain boundary recombination in thin-film Sb2Se3 photovoltaics

Abstract

The crystal structure of Sb2Se3, which consists of [001] oriented ribbons held together by weak van der Waals forces, gives rise to unique absorber layer properties different from other more conventional photovoltaic materials, such as Si or CdTe. Charge carrier transport occurs preferentially along the ribbon direction, and grain boundary recombination is minimal provided the ribbons are oriented parallel to the grain boundary plane. For optimum performance the Sb2Se3 absorber layer must have a (001) orientation, although in practice (211) and (221) growth textures are more common. The effect of this non-ideal orientation on anisotropic charge carrier transport and grain boundary recombination has however not been quantified. Here we derive analytical expressions for charge transport along a 1D ribbon under different boundary conditions. The local device properties for a given microstructural feature (e.g. grain boundary) can then be simulated by superposition of all individual ribbon contributions. It is found that anisotropic charge transport has the largest impact on device properties, even in the presence of electrically active grain boundaries (107 cm/s recombination velocity). For example, a 44° ribbon misorientation (i.e. (221) growth texture) resulted in a ∼3% efficiency loss compared to the ideal (001) orientation. This highlights the importance of ribbon misorientation for achieving the highest efficiency Sb2Se3 devices.