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

Abstract

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 CuIn ″ and CuCu • 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.

Methanol-induced fast CsBr release results in phase-pure CsPbBr3 perovskite nanoplatelets

“…Different from traditional covalent semiconductor nanocrystals (NCs), whose synthesis requires a high temperature to decompose the precursor, [32][33][34][35] perovskite NCs can be synthesized at room temperature via a ligand assisted reprecipitation (LARP) approach, 36,37 which is due to its ionic nature. 38,39 By controlling the surface ligands, reaction temperature and additives, several shape variations of perovskite nanomaterials have been achieved, including NCs, nanorods, nanowires, and nanoplatelets (NPLs).…”

Stoichiometry-controllable optical defects in CuxIn2−xSy quantum dots for energy harvesting

“…18 To address this issue, recently it has been proposed that there are two sets of Cu defects: "Cu 1+ defects" that undergo a self-trapped excitonlike reorganization, and "Cu 2+ defects," which do not require excited-state reorganization. 20 Stoichiometry-dependent optical spectra confirms this interpretation, but could only hypothesize the precise chemical identity for each of these defects. 20 Developing a clear understanding of structure-property relationships in CIS QDs is of significant importance to energy harvesting applications.…”

“…20 Stoichiometry-dependent optical spectra confirms this interpretation, but could only hypothesize the precise chemical identity for each of these defects. 20 Developing a clear understanding of structure-property relationships in CIS QDs is of significant importance to energy harvesting applications. [21][22][23][24][25][26][27][28][29][30][31][32] For example, a key performance limiting factor for LSCs is reabsorption, which occurs when there is spectral overlap between the fluorophore's absorption and emission.…”

“…Transient absorption (TA) spectroscopy has detected the presence of "Cu 1+ " defects (CuIn''), and confirmed our predictions whereby Cu-rich (mostly CuIn'') and near-stoichiometric CIS QDs (similar CuIn'' and CuCu  depending on the exact Cu:In) have broader spectra than Cu-deficient (more CuCu  ) structures due to strong overlap between hυCu,a and hυx,a. 20 The strength of hυCu,a decreases with increasing Cu-deficiency until eventually the spectral shape evolves into a Bi-Gaussian with a strong hυx,a bleach and weak hυCu,a shoulder. 20 As the concentration of VCu' grows, the strength of the VB electron  CuCu  hole IR transition also becomes stronger, but is outside of the detection range of typical TA experiments, and obscured in FTIR measurements by molecular vibrations of similar energy from surface passivating ligands.…”

“…This was first proposed in ref. 20, which experimentally detected the presence of "Cu 1+ " and "Cu 2+ " defects in CIS QDs. Our calculations agree with this interpretation, and attach clear chemical identities to these defects.…”