QD Products

Heavy Metal-Free Colloidal Quantum Dot Light Emitting Diode

 

Colloids of semiconductor nanocrystals or quantum dots (Qdots) are promising materials to serve as light emitters or color converters in light emitting diodes (LEDs). Attempts to apply these technologies to flat panel display (FPD) and solid state lighting have produced promising results. Especially for flat panel display, Qdots provide a new approach for higher performance color display with a much wider color gamut (or a greater quantity of colors displayable) that has not been achieved yet in current liquid crystal displays (LCDs), organic LEDs (OLEDs), or polymer OLEDs (POLEDs) systems. Cadnium chalcogenide based Qdots (CdS, CdSe, and CdTe) have been intensively studied with Qdot-LEDs emitting color-saturated blue, green, and red emissions have been developed.


While above mentioned progress with Qdot-LED promises great results in FPD applications, efforts are still in need to achieve Qdot-LEDs with desired features of eco-friendly composition, high efficiency, high color purity, and long lifetime, especially for blue-emissive Qdot-LED. Contributing towards this goal, Mesolight has manufactured high quality Qdot and applied them successfully into blue-emitting Qdot-LED with record high efficiency, brightness, color purity, and lifetime.

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Figure 1. Photoluminescence spectra of core/shell ZnSe/ZnS Qdot FWHM of (a) 10.6 nm, and (b) 10.8 nm, and (c) evolution in QY during a batch of synthesis under room temperature. (ex: 350 nm, QY reference: Exciton Coumarine 460 fresh in methanol) (inserts: optical images of photoluminescence)


The high quality cadmium-free Qdots are core/shell structure colloidal ZnSe/ZnS Qdots. These Qdots are manufactured from all air stable precursors with our proprietary technique (USPTO Provisional Patent Application Number 61506700). They present superior features of exceptionally narrow photoluminescence emissions as shown in Figure 1, with exemplary FWHM values as small as 10.6 nm peaked at 430 nm and 10.8 nm peaked at 440 nm, respectively. These values are indicative of the ultra narrow size distribution of these monodisperse Qdots. It is noteworthy that the emission is all from the band gap emission of Qdots without any noticeable side emissions in the visible range. Under optimized conditions, ZnSe/ZnS core/shell Qdot emit bright violet-blue emissions in 410-442 nm, with quantum yield (QY) up to 60-85% for the core/shell products (highest values reported to date for blue Qdots). Single molecular fluorescence spectroscopy study on single Qdots revealed that these Qdots showed “blinking” phenomenon as that observed for CdSe Qdot by emitting lights in ~85% of the time within the tested time of 62 seconds, confirming these core/shell ZnSe/ZnS Qdots have high quantum efficiency. Shelf life of Qdot in powder was tested to be longer than 12 months, while Qdot solution at 2 mg/mL showed a drop of QY to ~50% from initial value within 3 months.

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Figure 2. Cadmium-free blue-emitting Qdot-LED device. (a) Electroluminescence spectrum of the Qdot-LED measured at the bias of 7V, with the peak at 442 nm and a spectral FWHM value of 14.6 nm. Insert shows the EL spectra from 5.5 to 7 V. (b) Image of a Qdot-LED with saturated blue emission at the luminance of 300 cd/m2.


As presented in Figure 2, LED fabricated from the high quality Qdots display exciting electroluminescence properties: (a) heavy-metal free composition; (b) violet to blue emission with peaks in the range of 411-445 nm; (c) high efficiency (peak EQE up to 3.4%, an order of magnitude higher than prior cadmium based blue Qdot-LEDs; peak power efficiency up to 23.22 lm/W); (d) ultra narrow emission (FWHM as small as 14.6 nm) ; (e) free-of side emissions (>99% of emission are from band gap emissions of ZnSe/ZnS Qdot); (f) peak luminance up to 620 cd/m2 at 440 nm; (g) turn on voltage as low as 2.8 V; (h) first reported half lifetime of 118 h at 30 cd/m2 luminance in ambient condition.

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Figure 3. Chromaticity coordinates of three ZnSe/ZnS Qdot-LEDs on CIE 1976 u’v’ color space: (0.242, 0.051), (0.235, 0.075), and (0.214, 0.097), respectively.
The exceptional narrow emission (FWHM=14.6 nm) and high color purity (side emission <1%) of the cadmium-free Qdot-LEDs (FWHM=14.6 nm) are of great importance for future FPD applications. The emission bandwidth of blue Qdot-LED demonstrated here is sharper than that of single crystal semiconductor LEDs (such as GaN or InGaN LED at 440 nm with emission FWHM of ~20-25 nm) but getting close to that of blue lasers. In display, such high color purity means a deep color and consequently a wider possible color gamut. Correspondingly, Qdot-LEDs have color coordinates (Figure 3) lying outside of the National Television System Committee (NTSC) color gamut, and comes close to the left edge on the CIE 1976 u’v’ color space (which presents better color uniformity than CIE 1932 or 1960 color spaces). Coupled with the short-blue emission near 440 nm, Qdot-LED herein could be an ideal blue emitter to produce a much wider trichromatic color gamut than that in current LCDs, OLEDs, or PhOLEDs.


With our demonstrated expertise in the manufacturing of high quality cadmium-free Qdot and Qdot-LED, Mesolight is now developing the cadmium-free Qdot-LED based next generation display technology with deep color, high energy efficiency, and more vivid colors that have not been achieved in current LCDs and OLEDs.

 

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