Jen-La Plante, Ilan | Search NASA

DOE OSTI2023

Stable Cadmium-Free Quantum Dot Optical Down-Converters for Solid State Lighting

Quantum dots (QDs) have been used in commercial solid-state lighting (SSL) applications to improve the energy efficiency of light generation at warm color temperatures. By increasing the amount of QDs used in each product, further energy savings are possible; however, traditional QDs contain cadmium, which has regulatory limits on its concentration in consumer products. As described within this final technical report, we have established that heavy-metal free QDs made from indium phosphide (InP) exhibit highly efficient emission at the temperatures and fluxes relevant to SSL. However, the maintenance of this emission is compromised during long term operation due to QD oxidation. We have identified multiple methods to slow the oxidation rate, which has improved the operational stability of these materials more than 200 times longer than at the project start. Beyond these improvements, heavy-metal free QDs require a further hundred-fold increase in stability to enable use in mid-power SSL and a ten-fold increase in stability to enable use in diffuse SSL applications. The outcomes of this project demonstrate feasibility for the use of heavy-metal free QDs in commercial SSL applications with potential use in diffuse SSL applications in the near term (1-2 years) pending market need.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

DOE OSTI2023

Spectral widths and Stokes shifts in InP-based quantum dots

InP-based quantum dots (QDs) have Stokes shifts and photoluminescence (PL) line widths that are larger than in II–VI semiconductor QDs with comparable exciton energies. The mechanisms responsible for these spectral characteristics are investigated in this paper. Upon comparing different semiconductors, we find the Stokes shift decreases in the following order: InP > CdTe > CdSe. We also find that the Stokes shift decreases with core size and decreases upon deposition of a ZnSe shell. We suggest that the Stokes shift is largely due to different absorption and luminescent states in the angular momentum fine structure. The energy difference between the fine structure levels, and hence the Stokes shifts, are controlled by the electron–hole exchange interaction. Luminescence polarization results are reported and are consistent with this assignment. Spectral widths are controlled by the extent of homogeneous and inhomogeneous broadening. Here, we report PL and PL excitation (PLE) spectra that facilitate assessing the roles of homogeneous and different inhomogeneous broadening mechanisms in the spectra of zinc-treated InP and InP/ZnSe/ZnS particles. There are two distinct types of inhomogeneous broadening: size inhomogeneity and core–shell interface inhomogeneity. The latter results in a distribution of core–shell band offsets and is caused by interfacial dipoles associated with In–Se or P–Zn bonding. Quantitative modeling of the spectra shows that the offset inhomogeneity is comparable to but somewhat smaller than the size inhomogeneity. The combination of these two types of inhomogeneity also explains several aspects of reversible hole trapping dynamics involving localized In 3+ /V Zn 2– impurity states in the ZnSe shells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗