Hybrid remote quantum dot/powder phosphor designs for display backlights

Abstract

Quantum dots are ideally suited for color conversion in light emitting diodes owing to their spectral tunability, high conversion efficiency and narrow emission bands. These properties are particularly important for display backlights; the highly saturated colors generated by quantum dots justify their higher production cost. Here, we demonstrate the benefits of a hybrid remote phosphor approach that combines a green-emitting europium-doped phosphor with red-emitting CdSe/CdS core/shell quantum dots. Different stacking geometries, including mixed and separate layers of both materials, are studied at the macroscopic and microscopic levels to identify the configuration that achieves maximum device efficiency while minimizing material usage. The influence of reabsorption, optical outcoupling and refractive index-matching between the layers is evaluated in detail with respect to device efficiency and cost. From the findings of this study, general guidelines are derived to optimize both the cost and efficiency of CdSe/CdS and other (potentially cadmium-free) quantum dot systems. When reabsorption of the green and/or red emission is significant compared to the absorption strength for the blue emission of the pumping light emitting diode, the hybrid remote phosphor approach becomes beneficial.

Indium Phosphide‐Based Quantum Dots with Shell‐Enhanced Absorption for Luminescent Down‐Conversion

“…For a 91% absorptance, a PLQY of 43% is measured, while λ max has shifted to the red by only 8 nm as compared to the emission of dilute QD suspensions. [15] Hence, while not entirely Cd-free, a 20-fold reduction of the Cd content in remote phosphor disks can be attained by the approach presented here, while preserving the exquisite self-absorption suppressing characteristics of CdSe/CdS-based QDs. The consequence of this is best appreciated by focusing on the example of ≈77% absorptance layers.…”

“…Displays, LEDs, or solar concentrators all make use of the efficient and spectrally narrow QD photoluminescence (PL) to convert short wavelength incident light to nearly monochromatic emitted light at a longer wavelength of choice, as determined by the dimensions and shape of the QDs. [13][14][15] A major drawback of this approach, however, is its reliance on compounds rich in Cd, which is a toxic heavy metal that is restricted in several countries. Having a bandgap of 2.42 eV (512 nm), a CdS shell can enhance both the photoluminescence quantum yield (PLQY), by passivating the CdSe outer surface, and the absorption cross section of the QDs at blue, violet, and UV wavelengths.…”

“…In spite of this lattice-matched configuration, their approach resulted in rather polydisperse samples with a broad and moderately efficient photoluminescence. [13][14][15] A major drawback of this approach, however, is its reliance on compounds rich in Cd, which is a toxic heavy metal that is restricted in several countries. We show that such core/shell QDs feature an efficient photoluminescence in combination with a considerably enhanced absorption at 450 nm, similar to CdSe/CdS QDs.…”

Counting the Photons: Determining the Absolute Storage Capacity of Persistent Phosphors

“…Details concerning the synthesis, characterization and structural properties of the CaS:Eu nanophosphor were described by Rodríguez Burbano et al [16]. The phosphors were incorporated into a polymer layer by mixing an appropriate amount of the powders in a 20 wt% solution of kraton FG1901X in 3:1 toluene-to-methyl ethyl ketone, after which the mixture was cast on a circular glass plate with a diameter of 18 mm [42]. Care was taken to avoid the inclusion of air bubbles and to obtain a homogeneous phosphor loading over the glass plate area.…”

Stabilizing colour and intensity

“…W hite light-emitting diodes (wLEDs) are rapidly replacing incandescent and fluorescent light sources, both in general lighting and display backlights, due to their long lifetime, small footprint, spectral tunability and, most importantly, their high efficiency in converting electrical to optical power 1,2 . As the emission of LEDs is essentially monochromatic, wLEDs are typically composed of a blue pumping LED and one or more luminescent materials, known as phosphors, which convert part of the blue light to longer wavelengths, the mixture yielding white light (Fig.…”