(765b) On the Molecular Origin of Intra-Gap Emission from CuInSe2-XSx Quantum Dots
CuInSe2-xSx (CISeS) quantum dots (QDs) have recently gained attention as heavy-metal free alternatives to Cd- and Pb-based QDs for applications in next generation solar and solid-state lighting technologies. CISeS QDs exhibit several unusual photophysical properties that are distinct from those of structurally similar II-VI QDs, and have broad implications for their use in photonic devices. The large Stokes shifts (Îsâ¥250 meV), for example, inhibits reabsorption losses in luminescent solar concentrators by reducing the spectral overlap between the emission and absorption spectra. On the other hand, the broad linewidths limit their use in high-color-definition displays. A possible explanation for these spectral features is the involvement of intra-gap states containing a localized hole capable of coupling with a conduction band electron for a radiative transition. However, the origin of these intra-gap states, and their role in both emissive and non-emissive decay channels still remain poorly understood. Here, we address these questions with quantum chemical calculations supported by spectral electrochemistry (SEC) and ultra-fast spectroscopy experiments. Density-functional theory (DFT) calculations predict the formation of intra-gap states, which arise from either copper anti-site swapping (CuIn2- + InCu2+), or from excess positive charge accumulated on a lattice copper in order to charge-balance a copper vacancy (VCu- + CuCu+, which is a Cu2+ atom on a Cu+ site). The density of states (DOS) for both defect-free QDs and QDs with copper defects are then calculated and used to predict the âtheoretical Stokes shiftâ, which is determined by the energy offset between the DOS localized at the Cu-based defect and the valence band. The theoretical Stokes shifts for the combined set of copper defects vary from ~0.3 eV to ~0.7 eV based on their proximity with their corresponding charge-balancing paired defect (InCu2+ in the case of CuIn2- and VCu- in the case CuCu+). These results are in excellent agreement with experimental studies, which show that individual CIS QDs have narrow emission linewidths with Stokes shifts that vary from QD to QD. Finally, we compare DFT calculations to SEC and ultra-fast spectroscopy measurements to determine the relationship between the copper defectâs oxidation state (Cu+ in the case of CuIn2- and Cu2+ in the case of CuCu+) on carrier trapping and nonradiative decay. The DOS for the VCu- and CuCu+ clearly show magnetization and intra-gap states below the CuCu+ state, which can trap photoexcited holes prior to emission. This suggests that Cu2+ emission occurs via a pre-existing hole due to its unfilled d shell, which leads to paramagnetism and requires capture of a valence band hole at a separate intra-gap defect state prior to emission. Consequently, quantum yields for CISeS QD ensembles with a high relative concentration of CuCu+ defects are not significantly affected by hole trapping. This differs from QD ensembles with a high relative concentration of CuIn2- defects (filled d shell) where a valence band hole must be captured at the Cu-defect site before emission. Further considering that the emission transition arises from a conduction band electron to a Cu defect regardless of its origin or initial oxidation state, nonradiative excited-state deactivation for either CuIn2- or CuCu+ defects primarily occurs through electron traps. Each of these predictions are confirmed by SEC and ultra-fast spectroscopy experiments comparing stoichiometric QDs (predominantly CuIn2- defects) and Cu-deficient QDs (predominantly CuCu+). Lastly, we show that electron traps form as a result of inadequate passivation of the QD surface and can be removed by overcoating with a ZnS shell to achieve emission quantum yields nearing 80%.