Tuning and Probing the Distribution of Cu⁺ and Cu²⁺ Trap States Responsible for Broad-Band Photoluminescence in CuInS₂ Nanocrystals

Abstract

The processes that govern radiative recombination in ternary CuInS₂ (CIS) nanocrystals (NCs) have been heavily debated, but recently, several research groups have come to the same conclusion that a photoexcited electron recombines with a localized hole on a Cu-related trap state. Furthermore, it has been observed that single CIS NCs display narrower photoluminescence (PL) line widths than the ensemble, which led to the conclusion that within the ensemble there is a distribution of Cu-related trap states responsible for PL. In this work, we probe this trap-state distribution with in situ photoluminescence spectroelectrochemistry. We find that Cu²⁺ states result in individual "dark" nanocrystals, whereas Cu⁺ states result in "bright" NCs. Furthermore, we show that we can tune the PL position, intensity, and line width in a cyclic fashion by injecting or removing electrons from the trap-state distribution, thereby converting a subset of "dark" Cu²⁺ containing NCs into "bright" Cu⁺ containing NCs and vice versa. The electrochemical injection of electrons results in brightening, broadening, and a red shift of the PL, in line with the activation of a broad distribution of "dark" NCs (Cu²⁺ states) into "bright" NCs (Cu⁺ states) and a rise of the Fermi level within the ensemble trap-state distribution. The opposite trend is observed for electrochemical oxidation of Cu⁺ states into Cu²⁺. Our work shows that there is a direct correlation between the line width of the ensemble Cu⁺/Cu²⁺ trap-state distribution and the characteristic broad-band PL feature of CIS NCs and between Cu²⁺ cations in the photoexcited state (bright) and in the electrochemically oxidized ground state (dark).