Efficient larger size white quantum dots light emitting diodes using blade coating at ambient conditions
Graphical abstract
Introduction
As an inorganic semiconductor material, colloid quantum dots (QDs) have attracted great attention in the field of flat panel display and solid-state lighting, due to its unique luminescent properties such as outstanding photoluminescent quantum yield (PLQY), narrow full-width at half maximum (FWHM), high color purity and good light stability [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. Compared with traditional display devices, quantum dot light-emitting diodes (QLEDs) have the advantages of high stability, high color saturation and excellent solution processability [[10], [11], [12]]. QLEDs can realize the leap of light source from point source to surface source by self-illumination, and can also be applied to flexible display devices. Since the first electroluminescent device based on QDs was reported by Colvin et al. in 1994 [13], the performances of QLEDs have been undergoing rapid progress with the combined optimization of QDs quality, device structure and luminescence mechanism. The external quantum efficiency (EQE) of monochromatic QLEDs has exceeded 20% [[14], [15], [16], [17]], which is already comparable to organic light-emtting diodes (OLEDs). In 2014, Peng et al. reported the record-high red QLEDs with EQE of 20.5% and the lifetime more than 100,000 h at brightness of 100 cd/m2 by adding an insulating layer PMMA between the electron transport layer and light-emitting layer to optimize the carrier balance of the device [15]. In 2015, Manders et al. reported the green QLED with EQE of 21%, the current efficiency (ηA) of 82 cd/A and the CIE of (0.17,0.73), which is the best performances to date [18]. In 2018, Wang et al. demonstrated the blue QLED with a high EQE exceeding 18%, current efficiency of 3.4 cd/A and low efficiency roll-off by optimizing the QDs core/shell structures [19]. Even though the performance of monochromatic QLED has been progressively enhanced, that of the white QLED still remains inferior, especially for devices integrating various quantum dots in a single light emitting layer. In 2014, Bae et al. reported the white devices consisting of three primary-colored QDs showed the EQE of 1.3% [20], which was still not high enough for practical applications. Lee et al. also demonstrate the white QLED devices using RGB QD-mixed as emitting layer with the record high EL performance such as the peak values of 21.8 cd/A in current efficiency, and 10.9% in EQE [21].
Most of the reported work have been focused on the development of small-size QLEDs fabricated by spin-coating method. In practical application, spin coating is not suitable for large-size equipment manufacturing and mass production. Although inkjet printing and roll-to-roll manufacturing have attracted wide attention in the display industry, many problems still need to be overcomed in order to achieve high-speed and high-precision signal pixel fabrication of large area devices. Other fabrication methods have been tried to control the pixel patterning and QD morphology. For example, Lee Ann et al. demonstrated a solvent-fee contact printing process for deposition of patterned and unpatterned QD thin film as light-emitting layers of hybrid organic QLEDs [22]. Moon et al. reported a wearable red green blue (RGB) QLEDs array using high-resolution intaglio transfer printing and a white device with a EQE of 2.35% [23]. Blade coating is an economical and effective manufacturing process for roll to roll compatible optoelectronic device [[24], [25], [26], [27]]. Compared with ink-jet printing and spin coating, the blade coating has the advantages of high material utilization, good compatibility with different substrates, and large scale continuous production and processing of thin films. It is thus expected that blade coating favorites the all-solution-processed large-area QLEDs. However, up to now, the fabrications of blade-coated QLEDs are still rarely reported. It is expected that efficient large size white QLEDs with uniform luminance could be obtained by using doctor blade coating since the technique could give a more uniform RGB QDs distribution in the light-emitting layer.
Herein, we report all-solution-processed and high-efficiency white QLEDs based on blade coating technique. The effect of QDs concentration on the morphology of QD films fabricated by blade coating was studied. When the QDs concentration is 15 mg/ml, the red QLED exhibits a maximum luminance of 14,456 cd/m2, a peak current efficiency of 14.1 cd/A and an EQE of 13.3%. For mixed RGB QDs, the trichromatic white QLED was obtained with CIE coordinates within the white region of CIE 1931 over a wide voltage range (5–8 V). In addition, 3 × 8 cm2 uniform QLEDs were demonstrated, which indicate that the efficient large-size white LECs can fabricate by blade coating.
Section snippets
Materials
The luminescent red, green and blue CdSe/ZnS quantum dots were from TCL Co. Ltd. (China) with the fluorescence quantum yield of ~85%. Their emission peaks are 632 nm, 538 nm and 450 nm, with a full-width at half-maximum (FWHM) of 30 nm, 30 nm and 24 nm, respectively. The MoOx was prepared according to previous work [28,29]. Ammonium heptamolybdate (NH4)6Mo7O244H2O was dissolved in deionized water with 0.8 wt% concentration and magnetic stirred for half an hour at 85 °C, then the MoOx precursor
Results and discussion
White QLEDs has been demonstrated with mixed RGB QDs light-emitting layer in multilayer architecture of ITO/molybdenum oxide (MoOx)/poly(N-vinylcarbazole) (PVK)/mixed QD/ZnO/Al (Fig. 1a). MoOx was chosen to instead of conventional poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT:PSS) as a hole injection layer (HIL), due to its stability and deep lying electronic states with the work function of 5.3–5.4 eV [31,32] which facilitated hole injection into PVK. The energy level diagram
Conclusions
In summary, we realized the efficient full-color white QLEDs with mixed RGB QDs by using blade coating in ambient atmosphere. When the QDs concentration is 15 mg/ml, the QDs films prepared by blade coating technique exhibit excellent morphology and well-ordered quantum dot structure. Corresponding red QLEDs device demonstrated the best optoelectronic performance with a maximum luminance of 14,456 cd/m2, a current efficiency of 14.1 cd/A and a EQE of 13.3%. The trichromatic white QLEDs based on
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (61705040), the Natural Science Foundation of Fujian Province (2020J01470).
References (35)
- et al.
Enhanced infrared absorption of spatially ordered quantum dot arrays
Infrared Phys. Technol.
(2007) Electron–electron and electron-hole interactions in small semiconductor crystallites: the size dependence of the lowest excited electronic state
J. Chem. Phys.
(1984)- et al.
Synthesis and characterization of monodisperse nanocrystals and close-Packed nanocrystal assemblies
Annu. Rev. Mater. Res.
(2000) An essay on synthetic chemistry of colloidal nanocrystals
Nano Res
(2009)- et al.
QLEDs for displays and solid-state lighting
MRS Bull.
(2013) - et al.
Bright, multicoloured light-emitting diodes based on quantum dots
Nat. Photon.
(2007) - et al.
Device performance and light characteristics stability of quantum-dot-based white-light-emitting diodes
Nano Res
(2018) Hot exciton relaxation dynamics in semiconductor quantum dots:radiationless transitions on the nanoscale
J. Phys. Chem. C
(2011)- et al.
Quantum-dot light-emitting diodes for large-areadisplays: towards the dawn of commercialization
Adv. Mater.
(2017) - et al.
Investigation on thermally induced efficiency roll-off: toward efficient and ultrabright quantum-dot light-emitting diodes
ACS Nano
(2019)
Highly stable QLEDs with improved hole injection via quantum dot structure tailoring
Nat. Commun.
Quantum-dot light-emitting diodes for outdoor displays with high stability at high brightness
Adv. Opt. Mater.
Electrochemically-stable ligands bridge the photoluminescence- electroluminescence gap of quantum dots
Nat. Commun.
Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer
Nature
High-efficiency quantum dot light-emitting devices with enhanced charge injection
Nat. Photon.
Solution-processed, high performance light-emitting diodes based on quantum dots
Nature
Visible quantum dot light-emitting diodes with simultaneous high brightness and efficiency
Nat. Photon.
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