Spectroscopic and Magneto-Optical Signatures of Cu¹⁺ and Cu²⁺ Defects in Copper Indium Sulfide Quantum Dots

Journal Article · Mon Jan 13 00:00:00 EST 2020 · ACS Nano

DOI:https://doi.org/10.1021/acsnano.9b09181· OSTI ID:1601424

Fuhr, Addis ^[1]; Yun, Hyeong Jin ^[2]; Crooker, Scott A.  ^[2]; Klimov, Victor I.  ^[2]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Univ. of California, Los Angeles, CA (United States)
2. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

Colloidal quantum dots (QDs) of I–III–VI ternary compounds such as copper indium sulfide (CIS) and copper indium selenide (CISe) have been under intense investigation due to both their unusual photophysical properties and considerable technological utility. These materials feature a toxic-element-free composition, a tunable bandgap that covers near-infrared and visible spectral energies, and a highly efficient photoluminescence (PL) whose spectrum is located in the reabsorption-free intragap region. These properties make them attractive for light-emission and light-harvesting applications including photovoltaics and luminescent solar concentrators. Despite a large body of literature on device-related studies of CISe(S) QDs, the understanding of their fundamental photophysical properties is surprisingly poor. Two particular subjects that are still heavily debated in the literature include the mechanism(s) for strong intragap emission and the reason(s) for a poorly defined (featureless) absorption edge, which often “tails” below the nominal bandgap. Here, we address these questions by conducting comprehensive spectroscopic studies of CIS QD samples with varied Cu-to-In ratios using resonant PL and PL excitation, femtosecond transient absorption, and magnetic circular dichroism measurements. These studies reveal a strong effect of stoichiometry on the concentration of Cu¹⁺ vs Cu²⁺ defects (occurring as Cu_In^'' and Cu_Cu^• species, respectively), and their effects on QD optical properties. In particular, we demonstrate that the increase in the relative amount of Cu²⁺ vs Cu¹⁺ centers suppresses intragap absorption associated with Cu¹⁺ states and sharpens band-edge absorption. In addition, we show that both Cu¹⁺ and Cu²⁺ centers are emissive but are characterized by distinct activation mechanisms and slightly different emission energies due to different crystal lattice environments. An important overall conclusion of this study is that the relative importance of the Cu²⁺ vs Cu¹⁺ emission/absorption channels can be controlled by tuning the Cu-to-In ratio, suggesting that the control of sample stoichiometry represents a powerful tool for achieving functionalities (e.g., strong intragap emission) that are not accessible with ideal, defect-free materials.

• Research Organization: Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: 89233218C; NA000001
• OSTI ID: 1601424
• Report Number(s): LA-UR-20-20156
• Journal Information: ACS Nano, Vol. 14, Issue 2; ISSN 1936-0851
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
copper indium sulfide
quantum dot
native defects
Cu1+
Cu2+
copper vacancy