Quasi-type II CuInS₂/CdS core/shell quantum dots

Ternary chalcopyrite CuInS₂ 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 CuInS₂ QDs. In this work, using ultrafast spectroscopy, we showed that both conduction band (CB) edge electrons and copper vacancy (V_Cu) localized holes were susceptible to surface trappings in CuInS₂ QDs. These trap states could be effectively passivated by forming quasi-type II CuInS₂/CdS core/shell QDs, leading to a single-exciton (with electrons delocalized among CuInS₂/CdS CB and holes localized in V_Cu) 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 CuInS₂/CdS QDs from 10 ps in CuInS₂ QDs. These demonstrated advantages, including passivated trap states, long single and multiple exciton lifetimes, suggest that quasi-type II CuInS₂/CdS QDs are promising materials for photovoltaic and photocatalytic applications.