Solution-processed White Organic Light-emitting Diodes Research Articles

Red-emitting IrIII(C^N)2(P-donor ligand)Cl-type complexes showing aggregation-induced phosphorescent emission (AIPE) behavior for both red and white OLEDs

Two red-emitting IrIII(C^N)2(P-donor ligand)Cl-type complexes bearing C^N ligands with carbazole functional group have been successfully prepared with different P-donor ligands of triphenylphosphine and triethylphosphine, respectively. The investigation of their phosphorescent behavior in the mixture of THF and water to reveal their aggregation induced phosphorescent emission (AIPE) ability, which is also indicated by their much higher phosphorescent quantum yield (ΦP) in doped film than those in the dilute solution. Mainly, their AIPE are induced by the blocked stretching motion of aromatic segments in C^N ligand and restrained the deformation of their coordinating skeletons. The AIPE complexes can possess AIE factor (αAIE) of ca. 7.4. In addition, the carbazole group can effectively promote hole transporting ability of the concerned AIPE emitters, which can benefit their electroluminescent ability. Hence, the IrIII(C^N)2(P-donor ligand)Cl-type complexes can show decent EL efficiencies in the solution-processed red-emitting organic Light-emitting diodes (OLEDs) with a maximum external quantum efficiency (ηext) of 8.5%, a maximum current efficiency (ηL) of 22.0 cd A−1 and a maximum power efficiency (ηP) of 15.9 lm W−1. Furthermore, as long-wavelength emitter, solution-processed white OLEDs (WOLEDs) have been constructed based on the red-emitting AIPE IrIII(C^N)2(P-donor ligand)Cl-type complexes, which can play critical role in achieving stable white electroluminescent spectra at high luminescence. The concerned WOLEDs can show attractive EL efficiencies of 6.6%, 23.7 cd A−1 and 16.0 lm W−1. All these results can provide valuable information for developing new AIPE materials with high EL ability.

Highly efficient solution-processed white organic light-emitting diodes based on a co-host system by controlling energy transfer among different emitters

Efficient solution-processed white organic light-emitting diodes with FIrpic, Ir(mppy)3 and (MDQ)2Ir(acac) as the blue, green and red emitters, respectively, were realized by employing TcTa and CzSi as hosts.

Solution-processed white OLEDs with power efficiency over 90 lm W-1 by triplet exciton management with a high triplet energy level interfacial exciplex host and a high reverse intersystem crossing rate blue TADF emitter.

Solution-processed white organic light-emitting diodes (WOLEDs) have shown much lower device efficiency than their vacuum-deposited counterparts, due to the lack of triplet exciton management in a single-emissive-layer device structure, which will induce triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA). Here, two kinds of solution-processed WOLEDs, including thermally activated delayed fluorescence (TADF)/phosphorescence hybrid WOLEDs and all-TADF WOLEDs, with high power efficiency are developed by using a high triplet energy level (T1) interfacial exciplex as a host and a high reverse intersystem crossing (RISC) rate TADF emitter as a blue dopant for triplet exciton management. The interfacial exciplex host with high T1 can ensure that triplet excitons transfer from the host to the blue emitter, and the blue TADF emitter with high RISC rate (1.15 × 107 s-1) can rapidly upconvert triplet excitons to singlet ones to avoid TTA and TPA. The solution-processed TADF/phosphorescence hybrid and all-TADF WOLEDs exhibit maximum external quantum efficiencies of 31.1% and 27.3%, together with maximum power efficiencies of 93.5 and 70.4 lm W-1, respectively, which are the record efficiencies for solution-processed WOLEDs, and quite comparable to those of most vacuum-deposited counterparts.

Light-emitting dendrimer:exciplex host-based solution-processed white organic light-emitting diodes

Solution-processed monochrome and white organic light-emitting diodes (OLEDs) that have a light-emitting layer composed of a 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) and (5-terphenyl-1,3-phenylene)bis(diphenylphosphine oxide) (POPH) exciplex host and soluble blue, green and/or red phosphorescent dendrimers have been fabricated and characterised. The OLED performance characteristics were found to be scan dependent, with the first scans having large external quantum efficiencies (EQEs) at low luminance, with subsequent scans showing stable performance. The monochrome (blue, green and red) films were found to have high photoluminescence quantum yields, relatively balanced hole and electron mobilities, and the OLEDs had stable EQEs of between 9 and 12% at 100 cd m −2 over multiple scans. The blue, green and red emissive materials were blended with the exciplex host, with their ratio tuned to achieve white emission. The optimal blend ratio provided white OLEDs that had EQEs of 11.7% and 10.6% at 100 cd m −2 and 1000 cd m −2 , respectively. The best balance of 1931 Commission Internationale d'Eclairage (CIE) coordinates (0.40,0.40), Colour Rendering Index (70), and Delta uv (0.003) was achieved for an OLED with the light-emitting layer containing the blue, green and red dendrimers in a ratio of 20.0:0.4:0.8 wt%. A feature of the white OLEDs was the stability of the CIE coordinates, with a change of only (0.013,0.005) between a luminance of 100 cd m −2 and 4000 cd m −2 . • A common exciplex host composed for solution processed blue, green, red and white emissive OLEDs. • Monochrome light-emitting dendrimer:exciplex host blend films have relatively balanced hole and electron mobilities. • Monochrome OLEDs with external quantum efficiencies of up to 22.5%. • White emitting OLEDs external quantum efficiencies of 10.6% at 1000 cd m −2 • White OLEDs with stable CIE coordinates across a broad range of luminance.

Highly efficient white-emitting thermally activated delayed fluorescence polymers: Synthesis, non-doped white OLEDs and electroluminescent mechanism

Single white-emitting polymers are greatly desired to develop solution-processed white organic light-emitting diodes (WOLEDs) with simple fabrication processes and low-energy-consumption. Thermally activated delayed fluorescence (TADF) polymers harvesting both triplet and singlet excitons without heavy metals are promising candidates for solution-processable white-emitting devices. However, all-TADF polymers with efficient white emissions have not been well-developed. Herein, a new strategy is proposed to develop a series of all-TADF single white-emitting polymers for highly efficient non-doped solution-processed WOLEDs. A WOLED based on one of these polymers as the emitting layer achieves an EQEmax of 14.2%, a CEmax of 38.8 cd A−1, and a PEmax of 20.3 lm W−1 with the CIE coordinates of (0.33, 0.42) and (0.31, 0.39) at 100 cd m−2 and 2000 cd m−2, respectively. The double-channel trap-assisted and Langevin recombinations account for broad recombination zones, reduced formation of high-energy excitons on hosts and enhanced efficiency of PDTPT-1, 2&3 based OLEDs. As far as it is known, PDTPT-1 is the highest efficient white-emitting TADF polymer so far.

Solution-processed white organic light-emitting diodes with blue fluorescent and orange-red thermally activated delayed fluorescent dendritic luminogens

In this work, dendritic luminogens, namely, 3ICz-Tr as a blue emitter and Tri-tNID as an orange-red emitter, were synthesized and used for the application of white organic light-emitting diodes (WOLEDs). These molecules have appropriately high molecular weights, which are favorable for the fabrication of thin films with fine surface morphologies. Interestingly, the blue emitting 3ICz-Tr displays fluorescent character and the orange-red emitting Tri-tNID exhibited thermally activated delayed fluorescence, which was confirmed by steady state and transient photoluminescence (TRPL) spectroscopy. Eventually, WOLED devices could be realized by mixing the two molecules and optimizing their mixture composition. Among the devices we fabricated, the device with Tri-tNID and 3ICz-Tr (0.5:99.5 wt ratio) without additional host material demonstrated nearly pure white emission, with the maximum EQE of 8.32% and CIE coordinate of (0.32, 0.30). Furthermore, by changing the composition of the two dendritic molecules, the emission color can be adjusted from a warm white to a cool white in the corresponding WOLED.

Solution-processed white organic light-emitting diodes with bi-component emitting layer based on symmetry blue spiro-sulfone derivative

We report a white organic light-emitting diode (WOLED) of simple construction with a bi-component emitting layer, in which a novel, dual-role deep blue TADF emitter as both the non-doped blue emitter and efficient host of an orange TADF emitter of TXO-TPA. The deep blue molecule (SDPS-4PhCz, CIEy ∼ 0.04) was designed using spiro-diphenylsulfone (SDPS) as electron-accepting group and four 9-phenyl-9H-carbazole (N-PhCz) as the electron-donating groups. SDPS-4PhCz showed delayed fluorescence with short delayed lifetime of 1.4 μs via fast reverse intersystem crossing (RISC) from “hot exciton” triplet excited state. The solution-processed, non-doped deep blue inverted OLED device exhibited a maximum EQE of 4.92% with low efficiency roll-off. The two-color WOLED with extremely simple device architecture showed an EQE above 17%, CIE (0.39, 0.44) and high CRI of 64 by spin-coating method.

High-performance solution-processed white organic light-emitting diodes based on silica-coated silver nanocubes.

Solution-processed white organic light-emitting diodes (WOLEDs) with silica-coated silver nanocubes (Ag@SiO2 NCs) incorporated at the interface of a hole transporting layer and emission layer are studied. The concentration of Ag@SiO2 NCs is varied to investigate the effect of Ag@SiO2 NCs on the performances of WOLEDs. Owing to the sharp edges and corners, Ag NCs greatly improve the radiative rate and emission intensity of nearby blue excitons. The blue emission at different Ag@SiO2 NC concentrations determines the performance of the WOLEDs. The emission of the orange excitons is strengthened by the high concentration of Ag@SiO2 NCs, which slightly influences the device performance. On the other hand, the SiO2 shell and some SiO2 nanospheres coexisting with Ag NCs reduce the hole transporting, improving the carrier balance in the WOLEDs. The experimental and simulated results also show that excessive Ag@SiO2 NCs may cause a rough film surface, unbalanced carrier injection, and fluorescence quenching, which decreases the device performance. The optimized WOLED with a proper concentration of Ag@SiO2 NCs has a peak current efficiency of 53.9cd/A, acquiring a significant enhancement factor of 77.3% compared to the control device without Ag@SiO2 NCs.

Hyperbranched polymers with aggregation-induced emission property for solution-processed white organic light-emitting diodes

In this work, a new series of hyperbranched polymers of PFTPE-Ir(piq)3-X(X = 1, 5, 10) were designed and synthesized, in which tris(1-phenylisoquinoline)iridium(Ш) (Ir(piq)3) acts as red emission core and PFTPE acts as branches. The photophysical study reveals that these hyperbranched polymers exhibit aggregation-induced emission (AIE) characteristic, inducing in much higher photoluminescent quantum yield (ΦY) in neat film than that in dilute tetrahydrofuran (THF) solution. The white-light OLEDs using PFTPE-Ir(piq)3-X as emission layer show rather weaker efficiency roll-off. Especially, the white-light OLED based on PFTPE-Ir(piq)3-5 as emission layer shows a maximum luminance of 4686 cd/m2, a maximum luminous efficiency of 2.43 cd/A, a maximum external quantum efficiency of 1.08% and the Commission Internationale de l’Eclairage coordinate of (0.26, 0.36).

High-Energy-Level Blue Phosphor for Solution-Processed White Organic Light-Emitting Diodes with Efficiency Comparable to Fluorescent Tubes

SummaryA high-energy-level blue phosphor FIr-p-OC8 has been developed for solution-processed white organic light-emitting diodes (WOLEDs) with comparable fluorescent tube efficiency. Benefiting from the electron-donating nature of the introduced alkoxy, FIr-p-OC8 shows not only efficient blue light but also elevated highest occupied molecular orbital/lowest unoccupied molecular orbital levels to well match the dendritic host H2. Consequently, the hole scattering between FIr-p-OC8 and H2 can be prevented to favor the direct exciton formation on the blue phosphor, leading to reduced driving voltage and thus improved power efficiency. By exploiting this approach, a maximum power efficiency of 68.5 lm W−1 is achieved for FIr-p-OC8-based white devices, slightly declining to 47.0 lm W−1 at a practical luminance of 1,000 cd m−2. This efficiency can be further raised to 96.3 lm W−1 @ 1,000 cd m−2 when a half-sphere is applied to increase light out-coupling. We believe that our results can compete with commercial fluorescent tubes, representing an important progress in solution-processed WOLEDs.

Efficient solution-processed blue and white OLEDs based on a high-triplet bipolar host and a blue TADF emitter

Abstract In this study, solution-processed blue organic light-emitting diodes (OLEDs) were constructed by employing POBPCz as a bipolar host material with a high triplet of 3.01 eV and 2CzPN as the blue TADF emitter. A maximum EQE of 12.9% was realized in the blue TADF OLEDs, along with a satisfactory CIE coordinate of (0.19, 0.37). These results were among the best ones for the 2CzPN-based solution processed blue OLEDs. Moreover, solution-processed white OLEDs (WOLEDs) with a single-emitting layer were fabricated by Ir(MDQ)2 (acac) as the orange-red iridium complex. The optimized WOLEDs could achieve a maximum EQE of 12.4%, as well as the stable electroluminescence spectra. Notably, the white-light emission was achieved by the incomplete energy transfer from the blue TADF emitter of 2CzPN to the orange-red emitter of Ir (MDQ)2 (acac) via Forster energy transfer (FRET) process.

Ultrahigh Color‐Stable, Solution‐Processed, White OLEDs Using a Dendritic Binary Host and Long‐Wavelength Dopants with Different Charge Trapping Depths

Solution-processed white organic light-emitting diodes (WOLEDs) simultaneously possessing ultrahigh color stability (ΔCIE (0.008, 0.008) from 50 to 17 200 cd m−2) and high power efficiency (41.8 lm W−1) are realized, using a dendritic binary host H2/OXD-7 and selecting long-wavelength phosphors with suitable trap depths and doping concentration, to restrain charge trapping.

Synthesis, characterization, and application of a novel orange–red iridium(III) phosphor for solution-processed single emissive layer white organic light-emitting diodes

A novel iridium(III) complex bis(2-(naphthalen-1-yl)-6-(trifluoromethyl)benzothiazole)iridium(acetylacetonate) (CF3BT-N)2Ir(acac) with an orange–red emission was synthesized. The incorporation of CF3 and naphthalene groups into benzimidazole to act as a new ligand causes a significant change of both HOMO and LUMO energy levels of the iridium complex compared with the yellow emissive parent compound comprising the 2-phenylbenzimidazole ligand. As a result, a bathochromic shift by as much as 50nm of the phosphorescence emission peak can be achieved. Electronic properties of (CF3BT-N)2Ir(acac) were examined by time-dependent density functional theory (TD-DFT) calculations. The influence of such substituent groups on the photophysical, electrochemical and electroluminescent properties of iridium complex was studied. Furthermore, two- and three-element solution-processed white organic light-emitting diodes (WOLEDs) with high performance can be realized by using this kind of orange–red phosphorescent emitter in conjunction with other phosphors in a single emissive layer. A maximum luminous efficiency of 10.9cdA−1 with CIE coordinates of (0.27, 0.37) was obtained for the two-element WOLEDs. For the three-element WOLED, a maximum luminance of 22716cdm−1 and a maximum luminous efficiency of 14.6cdA−1 with CIE coordinates of (0.33, 0.42) can be achieved.

Solution-processed white organic light-emitting diodes with enhanced efficiency by using quaternary ammonium salt doped conjugated polyelectrolyte.

Solution-processed white organic light emitting diodes (WOLEDs) with quaternary ammonium salt doped water/alcohol soluble conjugated polyelectrolyte, poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluoren)] dibromide (PFNBr), as electron transport material has been fabricated. Compared with the undoped devices, the performances of such devices with a doped electron transport layer have been dramatically improved to be nearly twice high in luminous efficiency and nearly one-third in response time when the weight ratio of PFNBr to tetraethylammonium bromide (TEAB) was 10:3. Four kinds of quaternary ammonium salts have been investigated to be dopants in the conjugated polyelectrolyte electron transport layer. It has been shown that both the anions and the cations of quaternary ammonium salts can influence the device performance. The dopant who has both a smaller anion and a smaller cation size can exhibit a better device performance. In addition, ultraviolet photoelectron spectroscopy measurement and single-carrier device testing have been employed to investigate the reason why such quaternary ammonium salt dopants can make an obvious improvement in the device performance of WOLEDs. These findings will be beneficial to the progress in design and fabrication of solution-processed WOLEDs suitable for lighting.

Spectral variation of solution-processed blue bis[(4,6-difluorophenyl)-pyridinato-N,C2′] (picolinate) iridium(III) phosphor

Spectral properties of a blue phosphorescent bis[(4,6-difluorophenyl)-pyridinato-N,C2′] (picolinate) iridium(III) (FIrpic) complex were systematically investigated. The electroluminescent spectra of FIrpic in solution-processed organic light-emitting diodes (OLEDs) were focused on, and the effects of doping ratio and solvent solution on spectral characterization were explored in solution-processed blue OLEDs. The results showed that there was an unusual spectral variation between shoulder and peak intensities in the electroluminescent spectra of FIrpic, and the intensity ratio of the shoulder to the peak (the Huang-Rhys factor) was obviously increased when FIrpic was at high doping ratios and dissolved in polar solvents. The theoretical mechanism for this phenomenon was analyzed, indicating that the spectral variation was due to solvation effect induced by strong intermolecular interaction between the solvent and FIrpic molecules. Moreover, the spectral behavior of FIrpic in solution-processed white OLEDs with complementary colors was studied, providing further evidence for the spectral variation.

Solution-processed white organic light-emitting diodes with mixed-host structures

Efficient white light-emitting diodes (WOLEDs) were fabricated with a solution-processed single emission layer composed of a molecular and polymeric material mixed-host (MH). The main host used was a blue-emitting molecular material of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) and the assisting host used was a hole-transport-type polymer of poly(9-vinylcarbazole) (PVK). By co-doping 4,4′-bis[2-(4-( N,N-diphenylamino)phenyl)vinyl]biphenyl and 5,6,11,12-tetraphenylnaphacene into the MH, the performances of the fabricated devices made with different mixing ratio of host materials were investigated, and were to depend on the mixing ratios. Under the optimal PVK:DPVBi ratio (3:7), we achieved a maximum luminance of 14 110 cd/m 2 and a maximum current efficiency of 9.5 cd/A. These improvements were attributed to the MH structure, which effectively improved the thermal stability of spin-coated film and enhanced the hole-injection/transporting properties of WOLEDs.

Highly efficient solution-processed white organic light-emitting diodes based on novel copolymer single layer

We fabricate by solution processed methods organic light emitting diodes with single-layer structure (ITO/(PEDOT:PSS)/co-polymer/Ba/Al) and study the transport properties of the final devices. The co-polymer is novel poly(fluorene-alt-phenylene) (PFP) derivatives containing co-monomers, involving red-emitting 1,8-naphthalimide units as pendant groups (0.0005, 0.005, 0.02 and 0.08wt%) covalently attached. All the devices exhibited emission at very low driving currents in the μA range (47–73μA). White emission with luminous efficiency of 9.42Cd/A at 50μA is obtained for the co-polymer with the smallest amount of chromophore. Commission Internationale de L’Eclairage (CIE) coordinates evolve from almost pure white color (0.26, 0.30) for low currents to stable cool white (0.21, 0.23). Increasing naphthalimide contents leads to stable green and orange emission with 3.07, 19.5 and 6.7Cd/A efficiencies. The current–voltage response of the devices is analyzed by means of a numerical model that includes an injection mechanism based in the microscopic hopping theory and a field-dependent carrier mobility for the bulk transport regime. The fitting results allow to estimate the dependence of carrier mobility on polymer composition in the diodes.

Cyclometallated platinum(ii) complexes of 1,3-di(2-pyridyl)benzenes for solution-processable WOLEDs exploiting monomer and excimer phosphorescence

Two cyclometallated platinum(II) complexes, N^C^N-5-fluoro-1,3-di(2-pyridyl)benzene platinum(II) chloride, FPtCl, and N^C^N-5-methyl-1,3-di(2-pyridyl)benzene platinum(II) isothiocyanate, MePtNCS, have been synthesized and characterized. Both complexes are highly efficient phosphorescent green emitters which can also display excimer emission in the red region. They have been studied as triplet emitters in solution-processed, multilayer organic light-emitting diodes (OLEDs), together with the known complex of 5-methyl-1,3-di(2-pyridyl)benzene, MePtCl, for comparison. The trend in efficiencies of the OLEDs prepared correlates with the charge-trapping properties of the complexes. The most efficiently emitting complex, FPtCl, was used as the dopant in a solution-processed white OLED, employing monomer and excimer emission.