Enhancing the efficiency and the luminance of quantum dot light-emitting diodes by inserting a leaked electron harvesting layer with thermal-activated delayed fluorescence material
Graphical abstract
Introduction
Colloidal quantum dots (QDs), a kind of inorganic nanocrystal luminescent material with quantum-confined characteristics, have attracted great interests due to their excellent properties, such as tunable bandgap, narrow linewidth for saturating emitting, pretty high photoluminescence quantum yield (∼90%) and better chemical stability than organic materials [[1], [2], [3]], what makes quantum dot light-emitting diodes (QLEDs) a promising candidate for next generation display technology after OLED [[3], [4], [5]]. It is well known that the architecture of the QLEDs can be either forward or inverted [[6], [7], [8]], in the forward architecture, ITO is used as the anode and the QLEDs can be fabricated by all-solution processed method [1,2,9]. However, the selection of hole transporting materials (HTM) is limited due to the requirement for high performance QLEDs, such as the good film-forming property from the solution procedure. In the inverted architecture situation, there are few limits for HTM benefiting from the thermal evaporation deposition technology. In addition, the inverted QLEDs can be directly connected to the drain electrode of n-type thin film transistor (TFT), and thus is beneficial to tangible products and avoids the QD layer damaged problems induced by the polymer and solvent, mentioned in previous reports [[10], [11], [12]]. Therefore, increased attentions are paid on inverted QLEDs.
In both forward and inverted QLEDs, ZnO or Metal-doped ZnO is typically used as the electron transport layer (ETL), due to their high electron mobility and suitable energy level of conducting band for electron injection to QD EML [1,2,4,6,9,10,[12], [13], [14], [15]]. However, the hole injection to QD EML is much more difficult than that of the electron, due to the relatively larger energy level difference between hole transport layer (HTL) and the QD EML. Meanwhile, the hole mobility of the HTMs are very low (10−5∼10−4 cm [2]/V·s) [3,[16], [17], [18], [19]]. Thus, the carrier distribution in the QLED is imbalanced, leading to the electron leakage from EML and the charged QDs by the excessed electrons which cause the excitons are consumed without giving the photons and the efficiency of QLED will be reduced [20]. To solve these problems and enhance efficiency of QLEDs, several approaches have been proposed, including the utilization of luminescent materials as energy transfer medium [[21], [22], [23]], inserting a carrier blocking layer in QLEDs [2,18,24], using two HTLs to improve hole injection [4,16,17,25,26], doping metal nanoparticles in function layer to enable localized surface plasma resonance (LSPR) [[27], [28], [29]]. etc. Among these, the utilization of luminescent materials, which harvest the leaked electrons and then transfer the excitons energy to the QD EML, is demonstrated to be an efficient strategy. For example, Evren Mutlugun et al. reported a red QLED with green phosphorescent (Irppy3) material doped in host material as the energy transfer medium, the maximum external quantum efficiency (EQE) has been raised by 6-fold–8.62% [21]. However, the color purity of EL emission is contaminated by the green phosphorescence. This scheme of utilizing phosphorescent luminescent materials has also been reported in inverted QLEDs. Hany Aziz et al. utilized blue phosphorescent material FIrpic doped in CBP host as an inserting layer between green QD EML and HTL, the maximum luminance of 65000 cd/m2 is achieved, compared with 20000 cd/m2 of standard device [22]. Chen et al. applied FIrpic in red QLEDs, the maximum current efficiency and the luminance are increased to 6.6 cd/A and 47410 cd/m2 in the optimized device, respectively [23]. Though these approaches utilizing phosphorescent luminescent material in QLEDs as the leaked electron harvesting layer are working for improving the device efficiency, all these reported phosphorescent materials require the corporation of the corresponding hosts and are fabricated by co-evaporating method, leading to the increased complexity in fabrication. In comparison, TADF materials, which also possess high quantum yield and efficiency due to the utilization of triplet and singlet excitons simultaneously, can be used individually without host material [30,31], whereas their utilization in QLEDs have rarely been reported [32].
Here, we proposed the first example of using blue TADF material, DMAC-DPS, as the leaked electron harvesting layer by inserting it between HTL and QD EML. This blue TADF material presents the emission peak around 487 nm in which region the green CdSe/ZnS core-shell QDs show strong absorption, enabling the efficient energy transfer from TADF material to QDs. In addition, it also has a matched energy level with that of the QDs and the HTL material (TAPC) used here [26,33,34], which benefit for the hole injection from TAPC HTL to QD EML. Optimal device with 5 nm DMAC-DPS layer has a drastic increment in luminance and current efficiency compared to standard device, the maximum values of these two indexes are 63458 cd/m2 and 10.44 cd/A, respectively, which are comparable to these of the reported highly efficient inverted QLEDs.
Section snippets
Experimental section
Materials: The ZnO precursor solution used in devices was synthesized by dissolving zinc acetate (99.99%, Sigma-Aldrich) in 2-methoxyethanol (99.8%, Sigma-Aldrich) and ethanolamine (99.5%, Aldrich), then the mixed solution was stirred for 12 h at 60 °C for hydrolysis reaction [35]. Green CdSe/ZnS Core-shell QD dispersion was purchased from Mesolight Inc. The emission peak wavelength, full width at half maximum (FWHM) and photoluminescence quantum yield (PLQY) of green QDs are 530 nm, 28 nm and
Results and discusstion
The schematic device structures of the standard QLEDs and the QLEDs with TADF material (abbreviated as TADF-QLEDs hereafter) are shown in Fig. 1a. A layer of the TADF material, DMAC-DPS, is inserted between QD EML and TAPC HTL for TADF-QLEDs. Generally, the electrons are expected to be excess in QD EML because of the larger hole injection barrier from HTL to QD EML and the relatively lower mobility of holes in the organic HTMs as compared to that of electrons in ZnO electron transport layer
Conclusion
Blue TADF DMAC-DPS layer is successfully used as the exciton harvesting layer to improve inverted G-QLED's performance. The device performance of the QLEDs, such as the turn-on voltage, the luminance, the efficiency and the efficiency roll-off characteristic, are significantly improved by introducing TADF layer with proper thickness. A high luminance of 63458 cd/m2 and a current efficiency of 10.44 cd/A are obtained in TADF-QLEDs. The TADF layer are considered to play three functions including
Notes
The authors declare no competing financial interest.
Acknowledgments
This research was supported by the National Key Research and Development Program of China under Grant No. 2016YFB0401302, the National Natural Science Foundation of China under Grant No. 61775013 and No. 11474018, and the Fundamental Research Funds for the Central Universities under the Grant No. 2017RC015 and No. 2017JBC027.
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