Quantum-Dot Light-Emitting Diodes Exhibiting Narrow-Spectrum Green Electroluminescence by Using Ag–In–Ga–S/GaS_x Quantum Dots

Abstract

Quantum dots (QDs), which have high color purity, are expected to be applied as emitting materials to wide-color-gamut displays. To enable their use as an alternative to Cd-based QDs, it is necessary to improve the properties of QDs composed of low-toxicity materials. Although multielement QDs such as Ag–In–Ga–S are prone to spectrally broad emission from defect sites, a core/shell structure covered with a GaS x shell is expected to enable sharp emission from band-edge transitions. Here, QD light-emitting diodes (QD-LEDs) embedded with Ag–In–Ga–S/GaS x core/shell QDs (AIGS QDs) were fabricated, and their electroluminescence (EL) was observed. The EL spectra from the AIGS QD-LEDs were found to contain a large defect-related emission component not observed in the photoluminescence (PL) spectra of the AIGS QD films. This defect-related emission was caused by electrons injected into defect sites in the QDs. Therefore, the AIGS QDs and the electron injection layer (EIL) of ZnMgO were treated with Ga compounds such as gallium chloride (GaCl3) and gallium tris(N,N′-diethyl­dithiocarbamate) (Ga(DDTC)3) to improve the luminescence properties of the QD-LEDs. The added Ga compounds effectively compensated for defect sites on the surface of the QDs and suppressed direct electron injection from the EIL into defect sites. As a result, the defect-related emission components in the EL were successfully suppressed, and the EL exhibited a color purity comparable to the PL of the AIGS QD films. The QD-LEDs exhibited EL spectra with a full width at half-maximum of 33 nm, which is extremely sharp for a low-toxicity QD, and the chromaticity coordinates (0.260, 0.695) for green EL were achieved.

I–III–VI Quantum Dots and Derivatives: Design, Synthesis, and Properties for Light-Emitting Diodes

“…Tsuzuki et al reported a GaS x -passivated AIGS QD, whose defect sites are compensated, and the defect-related direct electron injection are suppressed. As shown in Figure e, the fabricated QLED demonstrated a narrowest green electronic emission (33 nm) . For red LEDs, a common strategy is fabricating with CIS/ZnS QDs as luminous layer, whose strong DAP emission and high PLQY are beneficial to LEDs with low turn-on voltage (2.8 V) and high EQE (3.36%) (Figure f) .…”

“…Because of the high formation energy of I–III–VI QDs and derivatives, the mainstream strategies of synthesizing QDs are still based on high temperature and organic phase medium, which produce QDs with good dispersion, excellent optical properties, high crystallinity, and high PLQY (∼90%). The methods reported currently can be divided into hot-injection (HI), one-pot heating-up (HU), and single-source precursor thermal pyrolysis. − The HI method is the most adopted method for synthesizing quaternary/quinary QDs and especially shows great advantages in precisely constructing core/shell structure. The HU method may show superiority in the uniform nucleation and growth of QDs, and the reaction results can be controlled just by adjusting reaction temperature and precursor ratio.…”

“…Copyright 2017 Royal Society of Chemistry. (d)–(f) EL spectra of blue, green, and red QDs, and the photographs demonstrate the LEDs under voltages. ,, Adapted with permission from ref (copyright 2022 American Chemical Society), ref (copyright 2018 The Optical Society of American, under CC BY 4.0), and ref (copyright 2023 American Chemical Society).…”

“…As shown in Figure 4e, the fabricated QLED demonstrated a narrowest green electronic emission (33 nm). 45 For red LEDs, a common strategy is fabricating with CIS/ZnS QDs as luminous layer, whose strong DAP emission and high PLQY are beneficial to LEDs with low turn-on voltage (2.8 V) and high EQE (3.36%) (Figure 4f). 34 However, the researches in narrowing the FWHM of R-QLEDs are still backward (usually larger than 100 nm), and efficiency improvements of RGB-QLEDs also need to be further developed.…”

Eco-Friendly Zn–Ag–In–Ga–S Quantum Dots: Amorphous Indium Sulfide Passivated Silver/Sulfur Vacancies Achieving Efficient Red Light-Emitting Diodes

“…Recently, eco-friendly I–III–VI QDs and derivatives (I: Ag + , Cu + ; III: Ga 3+ , In 3+ , Al 3+ ; VI: S 2– , Se 2– , and Te 2– ) have emerged as an excellent alternative for Cd- and Pb-based QDs. The characteristics of direct band-gap semiconductors, band structure tunability, and excited-state energy convergence endow I–III–VI QDs with the potential to realize full visible coverage, pure color, and high PLQY. − However, the multitype vacancies, highlighted as V Ag and V S in typical Ag–In/Ga–S QDs, could be nonradiative centers to quench active excitons (Figure a). The resulting inhomogeneous carrier distribution severely limits the improvement of color purity and efficiency in materials and devices.…”