CdTe Quantum Dots in Ionic Liquid: Stability and Hole Scavenging in the Presence of a Sulfide Salt

The light-harvesting properties of both CdSe and CdTe nanocrystals are ideally suited for their use in quantum dot (QD)-sensitized solar cells. However, corrosion of the CdTe QD in an aqueous environment in the presence of sulfide/polysulfide electrolyte renders it unsuitable despite its better electron injection ability (compared to CdSe QD) to a large band-gap semiconductor like TiO₂. In this work, we explore the stability of a CdTe QD, which we have developed exclusively for its use in ionic liquids, in 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid in the presence of S^2– and investigate the hole transfer process from this photoexcited QD to S^2–. We not only demonstrate that an appropriate capping of the CdTe QD and use of an ionic liquid in place of the aqueous medium enhances the stability of the QD significantly in the presence of S^2– but also provide evidence of hole transfer from a photoexcited QD to the sulfide salt using steady-state and time-resolved emission and ultrafast transient absorption measurements