Quasi-type II CuInS/CdS core/shell quantum dots.

Ternary chalcopyrite CuInS quantum dots (QDs) have been extensively studied in recent years as an alternative to conventional QDs for solar energy conversion applications. However, compared with the well-established photophysics in prototypical CdSe QDs, much less is known about the excited properties of CuInS QDs. In this work, using ultrafast spectroscopy, we showed that both conduction band (CB) edge electrons and copper vacancy (V) localized holes were susceptible to surface trappings in CuInS QDs. These trap states could be effectively passivated by forming quasi-type II CuInS/CdS core/shell QDs, leading to a single-exciton (with electrons delocalized among CuInS/CdS CB and holes localized in V) half lifetime of as long as 450 ns. Because of reduced electron-hole overlap in quasi-type II QDs, Auger recombination of multiple excitons was also suppressed and the bi-exciton lifetime was prolonged to 42 ps in CuInS/CdS QDs from 10 ps in CuInS QDs. These demonstrated advantages, including passivated trap states, long single and multiple exciton lifetimes, suggest that quasi-type II CuInS/CdS QDs are promising materials for photovoltaic and photocatalytic applications.