Yuwen Chen

Correction for ‘Ultra-smooth and robust graphene-based hybrid anode for high-performance flexible organic light-emitting diodes’ by Zhikun Zhang et al. , J. Mater. Chem. C , 2021, 9 , 2106–2114, DOI: 10.1039/D0TC05213B.

Large surface roughness, especially caused by the large particles generated during both the transfer and the doping processes of graphene grown by chemical vapor deposition (CVD) is always a critical issue that impedes the utility of graphene in the next-generation flexible thin-film opto-electronic devices such as organic light-emitting diodes (OLEDs), because it yields pinhole defects, leakage current, and electrical shorts, etc., causing non-emitting areas and device performance degradation over time. Here, an extremely smooth and robust PEDOT:PSS/graphene/PET hybrid anode is reported, prepared by direct and polymer-free transfer of CVD-grown graphene onto the smooth PET substrate. Flexible OLEDs with such hybrid anodes have the maximum brightness, current efficiency (CE) and power efficiency (PE), as high as 19 000 cd m⁻², 76 cd A⁻¹ and 61 lm W⁻¹, respectively, which are superior to those of the ITO/glass-based OLEDs. Moreover, a 2.5-inch monolithic flexible OLED has been successfully fabricated. The luminescence is very uniform and no dark spots are observed in the unencapsulated OLEDs, demonstrating the effectiveness of our strategies and the great prospect of graphene-based flexible OLEDs in the future foldable and rollable displays.

Conventional fluorescence-based organic light-emitting diodes (OLEDs) have attracted wide interest due to their long operational lifetime, high color purity, low efficiency roll-off and potential to be manufactured at low cost in the applications of next-generation full-color display and solid state lighting. However, the external quantum efficiency (EQE) of fluorescent OLEDs is limited to 5% due to the inability to use the generated triplet excitons. Here, we report a blue fluorescent OLED that realizes an EQE as high as 14.1% by using a thermally activated delayed fluorescent (TADF) material as an assistant host in the emitting layer, which permits the efficient energy transfer of all generated excitons to the blue fluorescent dopant. Furthermore, color-stable hybrid warm-white OLEDs with improved lifetime also have been obtained. The maximum forward-viewing power efficiency (PE), current efficiency (CE), and EQE are as high as 68.5 lm W⁻¹, 56.7 cd A⁻¹ and 20.1%, respectively, and the T₅₀ lifetime at L₀ = 1000 cd m⁻² reaches 1657 h. The achievement of these results fully exhibits the effectiveness of the TADF material as the assistant host to fabricate high efficiency and long lifetime fluorescent OLEDs.

The simultaneous achievement of high efficiency, low efficiency roll-off, good spectral stability and a high color rendering index (CRI) is still a challenge in hybrid white organic light-emitting diodes (WOLEDs). In this paper, we strategically design a device structure by simply inserting ultrathin red/green phosphorescent emitters in a blue exciplex to efficiently utilize excitons for light emission. The resulting WOLEDs not only exhibit high efficiency and low efficiency roll-off, but also maintain a high CRI and stable spectral emission. The maximum power efficiency (PE), current efficiency (CE) and external quantum efficiency (EQE) reached 66.2 lm W⁻¹, 50.6 cd A⁻¹ and 21.2% and remained at 50.9 lm W⁻¹, 45.2 cd A⁻¹ and 18.3% at 100 cd m⁻² and 34.6 lm W⁻¹, 35.5 cd A⁻¹ and 15.8% at 1000 cd m⁻², respectively. In addition, the electroluminescence (EL) spectra were also extremely stable with a high CRI of 86 and Commission Internationale de L’Eclairage (CIE) coordinates of (0.40 ± 0.01, 0.46 ± 0.01) from 500 to 10 000 cd m⁻². The achievement of good comprehensive performance is attributed to the efficient device structure of ultrathin phosphors in the exciplex that can realize 100% exciton utilization without affecting the electrical properties.

Hybrid white organic light-emitting diodes (WOLEDs) combining blue fluorophores and green/red (or yellow) phosphors are still preferred for OLED production applications due to their advantages of good stability and high efficiency. One major challenge is to have a suitable blue fluorophore material to fully utilize the electrically generated excitons for fluorophore and phosphor emitters. Here, we employed an efficient blue fluorophore (PBI-PPI-TPA) with a high triplet energy of 2.48 eV as the blue emitter as well as the host for phosphors, whereby a series of highly efficient hybrid WOLEDs were successfully achieved. The two-color (BO) WOLEDs exhibited a maximum forward-viewing EQE/PE of up to 23.6%/77.8 lm W⁻¹, and retained 20.0%/49.5 lm W⁻¹ at 1000 cd m⁻², thus exhibiting high-efficiency and low-efficiency roll-off. Furthermore, three-color (BYR) hybrid WOLEDs were also fabricated by adopting blue-yellow-red cascade energy transfer to ensure that all the excitons could be effectively utilized for the white emission. The resulting devices achieved a peak EQE/PE of 21.5%/53.2 lm W⁻¹, retained 17.2%/26.8 lm W⁻¹ at 1000 cd m⁻², and the color rendering index (CRI) was doubled with respect to the two-color WOLEDs, showing a good trade-off between the efficiency and CRI. All these results are among the best reported for hybrid WOLEDs.

The efficiency roll-off at high luminance and operational lifetime remain a major challenge for the widespread application of white organic light-emitting diode (WOLED) technology. Here we present a novel strategy to improve the efficiency roll-off at high luminance and operational lifetime by employing an assistant layer with triplet–triplet annihilation (TTA) up-conversion characteristics in emitters. It can be seen that at high luminance, the partial triplet energies in emitters will transfer to the TTA assistant layer and finally lead to the TTA emission, which reduces the exciton quenching at high luminance. Therefore, not only the efficiency roll-off, but also the operational lifetime are greatly improved. The resulting hybrid WOLEDs exhibited the maximum forward-viewing external quantum efficiency and power efficiency of 23.6% and 68.8 lm W⁻¹, which only dropped to 18.3% and 38.1 lm W⁻¹ at 1000 cd m⁻² and 17.1% and 25.9 lm W⁻¹ at 5000 cd m⁻², which are significantly higher than 10.5% and 17.9 lm W⁻¹ at 5000 cd m⁻² of WOLEDs without a TTA assistant layer. Furthermore, the operational half-lifetime of the resulting hybrid WOLEDs also reached 600 hours at a luminance of 1000 cd m⁻², which is nearly two-fold longer than that of the reference WOLEDs.

In this work, four phenanthroimidazole (PI) based isomers TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA and NPBI-PPI-TPA for high-efficiency deep-blue organic light-emitting diodes (OLEDs) have been designed and synthesized. The structure–property relationship is systematically studied. Devices based on TPA-PPI-PBI, TPA-PPI-NPBI, PBI-PPI-TPA and NPBI-PPI-TPA achieved deep-blue emissions with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.07), (0.15, 0.07), (0.15, 0.09) and (0.15, 0.05) and high external quantum efficiencies (EQE_max) of 4.12%, 4.66%, 6.88% and 5.59%, respectively. The PBI-PPI-TPA based device exhibited negligible efficiency roll-off with an EQE of 6.48% at practical 1000 cd m⁻². Moreover, the EQE is still above 5% even at a high brightness of 10 000 cd m⁻². Comparing the four isomers, we found that the substituent at the C2 position of the PI core has a significant influence on the emission wavelength and CIE coordinates. This work provides a rational design strategy where modifying an electron acceptor (A) at the C2 position and an electron donor (D) at the N1 position of the PI core will be an effective way to fabricate high-performance PI-based bipolar emitters.

All fluorescent white organic light-emitting diodes (WOLEDs) have aroused significant interest owing to the advantages of excellent operational stability, low efficiency roll-off and low cost. However, it is challenging to further improve the efficiency through the design of the device structure. Here, we demonstrated all fluorescent WOLEDs with high efficiency and low efficiency roll-off using a phosphor sensitization strategy. It can be seen that the energies of triplet excitons can be efficiently transferred to singlet excitons owing to charge carrier trapping and energy transfer processes between the host, phosphor sensitizer and fluorescent emitter. The resulting phosphor-sensitized fluorescent WOLEDs showed maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) values of 42.4 cd A⁻¹, 36.4 lm W⁻¹ and 14.3%, and these values were retained at 34.0 cd A⁻¹, 19.8 lm W⁻¹ and 11.6% at 1000 cd m⁻² luminance, respectively. Furthermore, the resulting WOLEDs also exhibited stable electroluminescence (EL) spectra. The Commission Internationale de l’Eclairage (CIE) values showed very little variation from (0.41, 0.47) to (0.39, 0.46) over a large luminance range from 500 to 5000 cd m⁻². This demonstrates that using a phosphor to sensitize the fluorescent emitter is a potential method for developing high efficiency, low efficiency roll-off fluorescent WOLEDs with stable EL spectra.

Synthesis of the multidentate coordinated chelate N3C–H₂, composed of a linked functional pyridyl pyrazole fragment plus a peripheral phenyl and pyridyl unit, was obtained using a multistep protocol. Preparation of Ir(III) metal complexes bearing a N3C chelate in the tridentate (κ³), tetradentate (κ⁴), and pentadentate (κ⁵) modes was executed en route from two nonemissive dimer intermediates [Ir(κ³-N3CH)Cl₂]₂ (1) and [Ir(κ⁴-N3C)Cl]₂ (2). Next, a series of mononuclear Ir(III) complexes with the formulas [Ir(κ⁴-N3C)Cl(py)] (3), [Ir(κ⁴-N3C)Cl(dmap)] (4), [Ir(κ⁴-N3C)Cl(mpzH)] (5), and [Ir(κ⁴-N3C)Cl(dmpzH)] (6), as well as diiridium complexes [Ir₂(κ⁵-N3C)(mpz)₂(CO)(H)₂] (7) and [Ir₂(κ⁵-N3C)(dmpz)₂(CO)(H)₂] (8), were obtained upon treatment of dimer 2 with pyridine (py), 4-dimethylaminopyridine (dmap), 4-methylpyrazole (mpzH), and 3,5-dimethylpyrazole (dmpzH), respectively. These Ir(III) metal complexes were identified using spectroscopic methods and by X-ray crystallographic analysis of representative derivatives 3, 5, and 7. Their photophysical and electrochemical properties were investigated and confirmed by the theoretical simulations. Notably, green-emitting organic light-emitting diode (OLED) on the basis of Ir(III) complex 7 gives a maximum external quantum efficiency up to 25.1%. This result sheds light on the enormous potential of this tetradentate coordinated chelate in the development of highly efficient iridium complexes for OLED applications.