Xi Yuan

Dual emissive manganese and copper Co-doped Zn-In-S quantum dots as a single color-converter for high color rendering white-light-emitting diodes.

Novel white light emitting diodes (LEDs) with environmentally friendly dual emissive quantum dots (QDs) as single color-converters are one of the most promising high-quality solid-state lighting sources for meeting the growing global demand for resource sustainability. A facile method was developed for the synthesis of the bright green-red-emitting Mn and Cu codoped Zn-In-S QDs with an absorption bangdgap of 2.56 eV (485 nm), a large Stokes shift of 150 nm, and high emission quantum yield up to 75%, which were suitable for warm white LEDs based on blue GaN chips. The wide photoluminescence (PL) spectra composed of Cu-related green and Mn-related red emissions in the codoped QDs could be controlled by varying the doping concentrations of Mn and Cu ions. The energy transfer processes in Mn and Cu codoped QDs were proposed on the basis of the changes in PL intensity and lifetime measured by means of steady-state and time-resolved PL spectra. By integrating these bicolor QDs with commercial GaN-based blue LEDs, the as-fabricated tricolor white LEDs showed bright natural white light with a color rendering index of 95, luminous efficacy of 73.2 lm/W, and color temperature of 5092 K. These results indicated that (Mn,Cu):Zn-In-S/ZnS QDs could be used as a single color-converting material for the next generation of solid-state lighting.

High color purity ZnSe/ZnS core/shell quantum dot based blue light emitting diodes with an inverted device structure

Deep-blue, high color purity electroluminescence (EL) is demonstrated in an inverted light-emitting device using nontoxic ZnSe/ZnS core/shell quantum dots (QDs) as the emitter. The device exhibits moderate turn-on voltage (4.0 V) and color-saturated deep blue emission with a narrow full width at half maximum of ∼15 nm and emission peak at 441 nm. Their maximum luminance and current efficiency reach 1170 cd/m2 and 0.51 cd/A, respectively. The high performances are achieved through a ZnO nanoparticle based electron-transporting layer due to efficient electron injection into the ZnSe/ZnS QDs. Energy transfer processes between the ZnSe/ZnS QDs and hole-transporting materials are studied by time-resolved photoluminescence spectroscopy to understand the EL mechanism of the devices. These results provide a new guide for the fabrication of efficient deep-blue quantum dot light-emitting diodes and the realization of QD-based lighting sources and full-color panel displays.

Efficient white light emitting diodes based on Cu-doped ZnInS/ZnS core/shell quantum dots

We report the fabrication of efficient white light-emitting diodes (WLEDs) based on Cu : ZnInS/ZnS core/shell quantum dots (QDs) with super large Stokes shifts. The composition-controllable Cu : ZnInS/ZnS QDs with a tunable emission from deep red to green were prepared by a one-pot noninjection synthetic approach. The high performance Cu : ZnInS QD-WLEDs with the colour rendering index up to 96, luminous efficacy of 70-78 lm W(-1), and colour temperature of 3800-5760 K were successfully fabricated by integration of red and green Cu-doped QDs. Negligible energy transfer between Cu-doped QDs was clearly found by measuring the photoluminescence lifetimes of the QDs, consistent with the small spectral overlap between QD emission and absorption. The experimental results indicated low toxic Cu : ZnInS/ZnS QDs could be suitable for solid state lighting.

Efficient energy transfer from hole transporting materials to CdSe-core CdS/ZnCdS/ZnS-multishell quantum dots in type II aligned blend films

We studied the energy transfer between CdSe core/shell quantum dots (QDs) and hole transporting materials (HTMs) in type II aligned inorganic/organic blend films by using steady-state and time-resolved photoluminescence (PL) spectroscopy. The lengthening and shortening in PL lifetimes of the QDs in HTMs under resonant excitation condition were explained by energy transfer and charge separation processes. Surprisingly, the maximum energy transfer efficiency from 4,4′,4″-Tris (carbazol-9-yl)-triphenylamine (TcTa) to CdSe/CdS/ZnCdS/ZnS core/multishell QDs was determined to be 86% by calculating the excited state lifetime of the TcTa molecules participating in the energy transfer process.

Efficient inverted quantum-dot light-emitting devices with TiO2/ZnO bilayer as the electron contact layer.

We have demonstrated an efficient inverted CdSe/CdS/ZnS core/shell quantum-dot light-emitting device (QD-LED) using a solution-processed sol-gel TiO2 and ZnO nanoparticle composite layer as an electron-injection layer with controllable morphology and investigated the electroluminescence mechanism. The introduction of the ZnO layer can lead to the formation of spin-coated uniform QD films and fabrication of high-luminance QD-LEDs. The TiO2 layer improves the balance of charge injection due to its lower electron mobility relative to the ZnO layer. These results offer a practicable platform for the realization of a trade-off between the luminance and efficiency in the inverted QD-LEDs with TiO2/ZnO composites as the electron contact layer.

Ultrafast Carrier Dynamics and Hot Electron Extraction in Tetrapod-Shaped CdSe Nanocrystals.

The ultrafast carrier dynamics and hot electron extraction in tetrapod-shaped CdSe nanocrystals was studied by femtosecond transient absorption (TA) spectroscopy. The carriers relaxation process from the higher electronic states (CB2, CB3(2), and CB4) to the lowest electronic state (CB1) was demonstrated to have a time constant of 1.04 ps, resulting from the spatial electron transfer from arms to a core. The lowest electronic state in the central core exhibited a long decay time of 5.07 ns in agreement with the reported theoretical calculation. The state filling mechanism and Coulomb blockade effect in the CdSe tetrapod were clearly observed in the pump-fluence-dependent transient absorption spectra. Hot electrons were transferred from arm states into the electron acceptor molecules before relaxation into core states.