Evidence for Self-healing Benign Grain Boundaries and a Highly Defective Sb₂Se₃–CdS Interfacial Layer in Sb₂Se₃ Thin-Film Photovoltaics

Abstract

The crystal structure of Sb₂Se₃ gives rise to unique properties that cannot otherwise be achieved with conventional thin-film photovoltaic materials, such as CdTe or Cu(In,Ga)Se₂. It has previously been proposed that grain boundaries can be made benign provided only the weak van der Waals forces between the (Sb₄Se₆)<i>_n</i> ribbons are disrupted. Here, it is shown that non-radiative recombination is suppressed even for grain boundaries cutting across the (Sb₄Se₆)<i>_n</i> ribbons. This is due to a remarkable self-healing process, whereby atoms at the grain boundary can relax to remove any electronic defect states within the band gap. Grain boundaries can, however, impede charge transport due to the fact that carriers have a higher mobility along the (Sb₄Se₆)<i>_n</i> ribbons. Because of the ribbon misorientation, certain grain boundaries can effectively block charge collection. Furthermore, it is shown that CdS is not a suitable emitter to partner Sb₂Se₃ due to Sb and Se interdiffusion. As a result, a highly defective Sb₂Se₃ interfacial layer is formed that potentially reduces device efficiency through interface recombination.