Unravelling the Correlation of Electronic Structure and Carrier Dynamics in CuInS₂ Nanoparticles

Abstract

In this work, we report the direct correlation of photoinduced carrier dynamics and electronic structure of CuInS2 (CIS) nanoparticles (NPs) using the combination of multiple spectroscopic techniques including steadystate X-ray absorption spectroscopy (XAS), optical transient absorption (OTA), and X-ray transient (XTA) absorption spectroscopy. XAS results show that CIS NPs contain a large amount of surface Cu atoms with ≪fourcoordination, which is more severe in CIS NPs with shorter nucleation times, indicating the presence of more Cu defect states in CIS NPs with smaller size particles. Using the combination of OTA and XTA spectroscopy, we show that electrons are trapped at states with mainly In or S nature while holes are trapped in sites characteristic of Cu. While there is no direct correlation of ultrafast trapping dynamics with NP nucleation time, charge recombination is significantly inhibited in CIS NPs with larger particles. These results suggest the key roles that Cu defect sites play in carrier dynamics and imply the possibility to control the carrier dynamics by controlling the surface structure at the Cu site in CIS NPs.

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

“…From this, we attempt to correlate the observed quenching/brightening of the characteristic PL feature of CIS NCs under oxidizing and reducing conditions to the presence of Cu 2+ cations (“dark”) and Cu + cations (“bright”). Our simulations show that the so-called “white line”, which is the main peak observed in the Cu K-edge XANES spectrum, decreases when a small cluster is simulated compared to the bulk CIS spectrum (Figure a), in line with a decrease of the ratio between internal and surface Cu atoms . Interestingly, when additional positive charge is added to the simulated cluster, the “white line” increases more drastically (Figure a).…”

“…Our in situ PL spectroelectrochemistry experiments can be very well explained by oxidation of Cu + into Cu 2+ , which results in “dark” NCs, as was suggested in Figure 1 c. To verify that electrochemical oxidation of CIS NCs indeed results in an increase in Cu 2+ cations, we measured the Cu K-edge X-ray absorption near-edge spectroscopy (XANES) spectra upon application of a reducing and oxidizing potential by in situ X-ray absorption measurements 41 − 44 and analyzed the observed features in the XANES pattern by simulating the spectra with standard FEFF and FDMES programs (Figures 4 and S11–13 ). From this, we attempt to correlate the observed quenching/brightening of the characteristic PL feature of CIS NCs under oxidizing and reducing conditions to the presence of Cu 2+ cations (“dark”) and Cu + cations (“bright”).…”

“…Our simulations show that the so-called "white line", which is the main peak observed in the Cu K-edge XANES spectrum, decreases when a small cluster is simulated compared to the bulk CIS spectrum (Figure 4a), in line with a decrease of the ratio between internal and surface Cu atoms. 44 Interestingly, when additional positive charge is added to the simulated cluster, the "white line" increases more drastically (Figure 4a). In general, a more-intense "white line" is associated with an increase of positive charge on the absorber atom 45,46 and thus infers an increase in the average oxidation state of the absorber atoms.…”

Optoelectronic Properties of Ternary I–III–VI2 Semiconductor Nanocrystals: Bright Prospects with Elusive Origins

“…109 This study also showed the presence of a significant fraction of undercoordinated Cu + (i.e., coordination number <4), which were attributed to surface Cu + sites, because their concentration increased with decreasing NC size. 109…”

“…48 We note that this assignment is consistent with the conclusions derived from the XANES measurements under laser excitation carried out by Ludwig et al on similarly sized CuInS 2 NCs. 109 A weak photoinduced absorption feature was later observed in copper-doped Cu + :CdSe NCs and interpreted similarly. 47 These observations support the mechanism proposed by Knowles et al 54 in which Cu 2+ is created only after localization of a photogenerated valence band hole (eq 2).…”