Solution-processed Organic Light-emitting Diodes Research Articles

Thermally Activated Delayed Fluorescence Emitters with Intramolecular Proton Transfer for High Luminance Solution-Processed Organic Light-Emitting Diodes.

We report an organic emitter containing a β-triketone electron acceptor core and phenoxazine as the electron donors (TPXZBM) for solution-processed organic light-emitting diodes (OLEDs). The resulting molecule is very unusual because it shows both thermally activated delayed fluorescence and intramolecular proton transfer. We compare its performance with the previously reported diketone analogue PXZPDO. Solution-processed OLEDs of PXZPDO and TPXZBM show maximum external quantum efficiencies of 20.1 and 12.7%, respectively. The results obtained for the solution-processed PXZPDO-based device are as good as the previously reported evaporated device. At a very high luminance of 10,000 cd m-2, the efficiencies of the OLEDs were 10.6% for PXZPDO and 4.7% for TPXZBM, demonstrating a relatively low efficiency roll-off for TADF materials. The low efficiency roll-off was rationalized on the basis of the short delayed lifetimes of 1.35 μs for PXZPDO and 1.44 μs for TPXZBM. Our results suggest that intramolecular proton transfer may be useful for the design of OLED materials with a low efficiency roll-off.

Donor engineered Deep-Blue emitters for tuning luminescence mechanism in Solution-Processed OLEDs

Abstract Three novel solution-processable A-π-2D-type deep-blue emitters, namely BCz, BBFCz, and BICz, were developed to investigate their luminescence mechanisms and performances in organic light-emitting diodes (OLEDs). The emitters were uniquely designed by connecting two carbazole analogs as donors and a boron-fused unit as an electron acceptor to the benzene core; they exhibited aggregation-induced emission properties in the film states. Theoretical calculations and time-resolved photoluminescence (TRPL) experimental results indicate that the luminescence mechanism of the three emitters changed from fluorescence to thermally activated delayed fluorescence (TADF) as the donor unit was changed from carbazole to indenocarbazole. BCz was found to act like a fluorescent emitter, but BBFCz and BICz displayed TADF characteristics. Efficient reverse intersystem crossing (RISC) in BICz was confirmed by small ΔEST, Ea, and kISC/kRISC ratio. Consequently, non-doped solution-processed TADF-OLEDs based on BICz as an emitter exhibited the highest external quantum efficiency (EQE) of 10.11%, with deep-blue commission International de ĺEclairage (CIE) color coordinates (0.16, 0.08). In contrast, BCz- and BBFCz-based devices showed relatively lower EQEs of 3.44% and 6.78%, respectively. The results showed that BICz as an emitter displayed exceptional performance in a non-doped solution-processed deep-blue TADF-OLED.

Simple-Structured Blue Thermally Activated Delayed Fluorescence Emitter for Solution-Processed Organic Light-Emitting Diodes with External Quantum Efficiency of over 20.

Solution-processed organic light-emitting diodes (OLEDs) are much preferred for the manufacture of low-temperature, low-cost, large-area, and flexible lighting and displaying devices. However, these devices with high external quantum efficiency are still limited, especially for blue ones. In addition, the molecular configurations of emitters are usually complicated, indicative of high costs. In this study, two simple-structured thermally activated delayed fluorescent emitters M1 and its polymer P1 were synthesized with acridine as a donor and benzophenone as an acceptor. Solution-processed OLEDs were prepared based on M1 and P1 as doped light-emitting layer, and M1-based doped device could achieve maximum external quantum efficiency of up to 20.6% with blue-light emission.

Cross-linkable deep-blue small molecular material for solution-processed organic light-emitting diodes

The development of deep-blue electroluminescent fluorescent materials with solution-processability and small molecule has exerted a great influence on the application in organic light-emitting diodes (OLED) field. However, so far there still lack of small molecule deep-blue materials suitable for solution-processed OLEDs and its wide band gap and low carrier mobility are requisite. Notably, the poor solvent resistance of small molecule usually incurs the inter-layer corrosion while using solution process to deposit multiple layers, which would result in poor device performance. In this work, a cross-linkable deep-blue light-emitting material DPCS-DPS containing diphenylsulfone/carbazole units and styrene groups has been designed and synthesized. The hole and electron transport parts composed of diphenylsulfone/carbazole exhibited bipolar charge-transporting properties, and the styrene group exhibited good solvent-resistance after thermal cross-linking, which is beneficial for the preparation of subsequent functional layers by solution process. By using the DPCS-DPS, the non-doped OLED achieved deep-blue emission wavelength of 415 nm, and the doped device approached a maximum current efficiency of 1.19 cd A−1 and maximum power efficiency of 0.75 lm W−1.

Solution-Processed OLEDs Based on Thermally Activated Delayed Fluorescence Copper(I) Complexes with Intraligand Charge-Transfer Excited State.

A new series of tetrahedral heteroleptic copper(I) complexes exhibiting efficient thermally-activated delayed fluorescence (TADF) in green to orange electromagnetic spectral regions has been developed by using D-A type N^N ligand and P^P ligands. Their structures, electrochemical, photophysical, and electroluminescence properties have been characterized. The complexes exhibit high photoluminescence quantum yields (PLQYs) of up to 0.71 at room temperature in doped film and the lifetimes are in a wide range of 4.3–24.1 μs. Density functional theory (DFT) calculations on the complexes reveal the lowest-lying intraligand charge-transfer excited states that are localized on the N^N ligands. Solution-processed organic light emitting diodes (OLEDs) based on one of the new emitters show a maximum external quantum efficiency (EQE) of 7.96%.

Enhanced Upconversion of Triplet Excitons for Conjugated Polymeric Thermally Activated Delayed Fluorescence Emitters by Employing an Intramolecular Sensitization Strategy.

Endowed by a thermally activated delayed fluorescence (TADF) sensitizer with a high constant rate of reverse intersystem crossing, the singlet excitons could be accumulated and then delivered to emitting states through favorable Förster resonance energy transfer, bypassing the inefficient intersystem transition processes of emitters. However, the conventional intermolecular sensitization strategies suffer from inherent aggregation-induced quenching and inevitable phase segregation of TADF sensitizers and emitters. In this context, we proposed a novel intramolecular sensitization strategy by covalently incorporating the TADF sensitizer into conjugated polymeric emitters. After rationally regulating the proportions of sensitizer and emitter units in polymers, the intramolecular sensitized conjugated TADF polymers with anticipated photophysical properties and stable device performance were obtained. A superior kRISC value over 106 s-1 accompanied by a suppressed nonradiative transition of the triplet exciton could be gained; therefore, the photoluminescence quantum yield (PLQY) could reach nearly 90%. In accord with the superior PLQY values enhanced by our intramolecular sensitization strategy, the solution-processed organic light-emitting diodes (OLEDs) can achieve a maximum external quantum efficiency (EQE) value of 17.8% while still maintaining 16.0% at 1000 cd/m2 with extremely low efficiency roll-off. These results convincingly manifest the significance of an intramolecular sensitization strategy for designing high-efficiency polymeric TADF emitters.

Adjusting the photophysical properties of AIE-active TADF emitters from through-bond to through-space charge transfer for high-performance solution-processed OLEDs

Intramolecular through-space charge transfer (TSCT) excited state has been exploited for developing thermally activated delayed fluorescence (TADF) emitters, while the adjustment of excited state dynamics by conformational engineering is still rare. Herein, we designed a series of AIE-active TADF emitters, bearing triphenylamine as donor and phenyl ketone as acceptor unit, and the evolution of charge transfer from through-bond to through-space is realized by controlling the relative position of the donor and acceptor units. Upon transforming the charge transfer categories from through-bond to through-space, the reverse intersystem crossing (RISC) rate is progressively improved by virtue of the narrower energy split between singlet and triplet states (ΔΕST), meanwhile, the restricted intramolecular motion suppresses the non-radiative decay effectively, resulting in obviously improved luminescence efficiency. Appointing a bipolar blue TADF material as host to sensitize these emitters, solution-processed organic light-emitting diodes (OLEDs) based on TSCT emitter 1TCPM-Cz achieve maximum current efficiency (CE) of 35.5 cd A−1 and external quantum efficiency (EQE) of 13.3%, respectively, which is higher than that of through-bond charge transfer (TBCT) analogues. The work demonstrates a convenient design strategy to achieve high-performance TSCT emitters by regulating the conformation of the donor and acceptor.

Solution-Processed Efficient Blue Phosphorescent Organic Light-Emitting Diodes (PHOLEDs) Enabled by Hole-Transport Material Incorporated Single Emission Layer.

Solution-processed blue phosphorescent organic light-emitting diodes (PHOLEDs) based on a single emission layer with small-molecule hole-transport materials (HTMs) are demonstrated. Various HTMs have been readily incorporated by solution-processing to enhance hole-transport properties of the polymer-based emission layer. Poly(N-vinylcarbazole) (PVK)-based blue emission layer with iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C2′)picolinate (FIrpic) triplet emitter blended with solution-processed 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) gave luminous efficiency of 21.1 cd/A at a brightness of 6220 cd/m2 with an external quantum efficiency (EQE) of 10.6%. Blue PHOLEDs with solution-incorporated HTMs turned out to be 50% more efficient compared to the reference device without HTMs. The high hole mobility, high triplet energy of HTM, and favorable energy transfer between HTM blended PVK host and FIrpic blue dopant were found to be important factors for achieving high device performance. The results are instructive to design and/or select proper hole-transport materials in solution-processed single emission layer.

Synthesis, crystal structure and photophysical properties of deep-blue emitting cationic iridium(III) complexes with 2ʹ,6ʹ-difluoro-2,3ʹ-bipyridine cyclometalated ligand and pyrazole-type ancillary ligands

In this work, we report three deep-blue emitting cationic iridium complexes Ir1–Ir3 with 2ʹ,6ʹ-difluoro-2,3ʹ-bipyridine cyclometalated ligand and pyrazole-type ancillary ligands. Synthesis, crystal structure, photophysical and electrochemical properties of Ir1–Ir3 are studied in detail. Ir1–Ir3 exhibit efficient phosphorescence emissions at 439, 438 and 437 nm with CIE 1931 coordinates of (0.14, 0.12), (0.14, 0.12) and (0.14, 0.13), and luminescence quantum yields of 0.35, 0.27 and 0.50 in CH2Cl2 solutions, respectively, which are among the highest levels of ever reported cationic iridium(III) complexes emitting deep-blue light. High luminescence quantum yields, excellent color purity and short lifetimes demonstrates the great potential of Ir1–Ir3 for light-emitting electrochemical cells (LECs) and solution-processable organic light-emitting diodes (OLEDs) as efficient blue emitters.

Various Structural Design Modifications: para-Substituted Diphenylphosphinopyridine Bridged Cu(I) Complexes in Organic Light-Emitting Diodes.

The well-known system of dinuclear Cu(I) complexes bridged by 2-(diphenylphosphino)pyridine (PyrPhos) derivatives Cu2X2L3 and Cu2X2LP2 (L = bridging ligand, P = ancillary ligand) goes along with endless variation options for tunability. In this work, the influence of substituents and modifications on the phosphine moiety of the NP-bridging ligand was investigated. In previous studies, the location of the lowest unoccupied molecular orbital (LUMO) of the copper complexes of the PyrPhos family was found to be located on the NP-bridging ligand and enabled color tuning in the whole visible spectrum. A multitude of dinuclear Cu(I) complexes based on the triple methylated 2-(bis(4-methylphenyl)phosphino)-4-methylpyridine (Cu-1b-H, Cu-1b-MeO, and Cu-1b-F) up to complexes bearing 2-(bis(4-fluorophenyl)phosphino)pyridine (Cu-6a-H) with electron-withdrawing fluorine atoms over many other variations on the NP-bridging ligands were synthesized. Almost all copper complexes were confirmed via single crystal X-ray diffraction analysis. Besides theoretical TDDFT-studies of the electronic properties and photophysical measurements, the majority of the phosphino-modified Cu(I) complexes was tested in solution-processed organic light-emitting diodes (OLEDs) with different heterostructure variations. The best results of the OLED devices were obtained with copper emitter Cu-1b-H in a stack architecture of ITO/PEDOT-PSS (50 nm)/poly-TPD (15 nm)/20 wt % Cu(I) emitter:CBP:TcTA(7:3) (45 nm)/TPBi (30 nm)/LiF(1 nm)/Al (>100 nm) with a high brightness of 5900 Cd/m2 and a good current efficiency of 3.79 Cd/A.

Highly efficient blue TADF emitters incorporating bulky acridine moieties and their application in solution-processed OLEDs

Two bulky acridine moieties, 9,9-diphenyl-9,10-dihydroacridine (DPAc) and 10H-spiro [acridine-9,9′-fluorene] (SpiroAc), were selected as electron donors. Compared with commonly used 9,9-dimethyl-9,10-dihydroacridine (DMAc), phenyl modified acridines exhibited low-lying highest occupied molecular orbital (HOMO) levels and relatively weak electron-donating abilities, which make them potential units to create blue TADF emitter. DPAc and SpiroAc were connected with a triarylboron group and constructed centrosymmetric 3D-A architectures, namely 3DPAc-TB and 3SpiroAc-TB, respectively. Density functional theory calculations showed that both molecules exhibited large dihedral angles between the triarylboron unit and acridine planes. The well-separated frontier molecular orbitals contributed to small energy gaps between S1 and T1 states of the designed molecules (0.12 and 0.07 eV, respectively). Both molecules exhibited distinct thermally activated delayed fluorescence (TADF) character. The bulky acridine donors endowed both molecules with rigid structures and realized high photoluminescent quantum yields (71.6% and 84.3%, respectively). Solution-processed organic light-emitting diodes (OLEDs) utilizing 3DPAc-TB and 3SpiroAc-TB as emitters exhibited blue emissions with emission peaks of 472 nm and 490 nm, respectively. The maximum external quantum efficiencies of 3DPAc-TB and 3SpiroAc-TB based devices were 12.8% and 17.3%, respectively.

Highly efficient carbazolylgold(iii) dendrimers based on thermally activated delayed fluorescence and their application in solution-processed organic light-emitting devices.

A new class of C^C^N ligand-containing carbazolylgold(iii) dendrimers has been designed and synthesized. High photoluminescence quantum yields of up to 82% in solid-state thin films and large radiative decay rate constants in the order of 105 s−1 are observed. These gold(iii) dendrimers are found to exhibit thermally activated delayed fluorescence (TADF), as supported by variable-temperature emission spectroscopy, time-resolved photoluminescence decay and computational studies. Solution-processed organic light-emitting diodes (OLEDs) based on these gold(iii) dendrimers have been fabricated, which exhibit a maximum current efficiency of 52.6 cd A−1, maximum external quantum efficiency of 15.8% and high power efficiency of 41.3 lm W−1. The operational stability of these OLEDs has also been recorded, with the devices based on zero- and second-generation dendrimers showing maximum half-lifetimes of 1305 and 322 h at 100 cd m−2, respectively, representing the first demonstration of operationally stable solution-processed OLEDs based on gold(iii) dendrimers.

Intermolecular locking design of red thermally activated delayed fluorescence molecules for high-performance solution-processed organic light-emitting diodes

An intermolecular locking strategy was proposed to improve both the solution processibility and photoluminescence efficiency of red TADF emitters for solution-processed OLEDs with an EQE up to 8.2% and an efficiency roll-off of 9.0% at 1000 cd m−2.

Preserving the work function of Ultra-Violet-ozone treated indium tin oxide by triarylamine-based small molecule modification for solution-processed organic light-emitting diodes with increased external quantum efficiency

UV-ozone treatment is one of the most common ways to increase the work function of indium tin oxide (ITO), which is used as the transparent conducting anode in organic light-emitting diodes (OLEDs). However, the work function increase is time sensitive when the samples are left or processed in air, often returning to a similar value to that measured before UV-ozone treatment. We found that for OLEDs formed by solution processing and containing a poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole injection layer that there was no advantage in terms of device efficiency of using UV-ozone treated ITO. However, if a triarylamine-based small molecule [N1,N3,N5-tris(4-n-butylphenyl)-N1,N3,N5-triphenylbenzene-1,3,5-triamine, 4-n-BTDAB] was introduced onto the UV-ozone treated ITO then the work function was stabilised. When 4-n-BTDAB was introduced between UV-ozone treated ITO and PEDOT:PSS, the resultant solution processed OLEDs were found to have a maximum external quantum efficiency ≈20% higher (corresponding to an absolute increase of ≈2% to around 12%) compared to devices with the same structure but without the triarylamine layer.

Green Ir(III) complexes with multifunctional ancillary ligands for highly efficient solution-processed phosphorescence organic light-emitting diodes with high current efficiency

Abstract Two new highly efficient green emitting heteroleptic Ir(III) complexes, namely, bis[5-(2-ethylhexyl)-8-(trifluoromethyl)benzo[c][1,5]naphthyridin-6(5H)-one]iridium-4-((3,5-di(9H-carbazol-9-yl)benzyl)oxy)picolinate (Ir-HT) and bis[5-(2-ethylhexyl)-8-(trifluoromethyl) benzo[c][1,5]naphthayridin-6(5H)-one]iridium-4-((4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzyl) oxy)picolinate (Ir-ET) were designed and synthesized for solution-processed phosphorescence organic light-emitting diodes (PHOLEDs). These new Ir(III) complexes are based on amide-bridged trifluoromethyl (-CF3) substituted phenylpyridine unit as main ligand and 1,3-bis(N-carbazolyl)benzene (mCP) unit and 1,3,4-oxadiazole (OXD) unit functionalized picolinate (pic) as an ancillary ligand. These multifunctional groups were attached into the 4-position of pic ancillary ligands via ether linkage. Interestingly, the solution-processed PHOLED device using Ir-HT as a dopant exhibited a maximum external quantum efficiency (EQEmax) of 20.92% and a maximum current efficiency (CEmax) of 64.04 cd A−1. Whereas PHOLED device using Ir-ET displayed a EQEmax of 20.68% and a CEmax of 65.02 cd A−1. This is one of best CE with high EQE for green Ir(III) complexes via solution-processed PHOLEDs using multifunctional ancillary ligands so far.

Photo and electroluminescence of a phenylene vinylene conjugated polymer containing bipirydine units and chelated europium complex

A conjugated polymer, poly[5,5'-(2,2′-bipyridylene)vinylene-alt-2,5-(1,4-bis(octyloxy)vinylene)] (PBPyVOctV), was synthesized by the polymerization of 5,5′-divinyl-2,2′-bipyridine with 1,4-dibromo-2,5-bis(octyloxy) benzene through a Heck cross-coupling reaction. Then, a metallopolymer, PBPyVOctV-Eu(tta)3, was obtained by the bipyridine chelating coordination of PBPyVOctV with europium (Eu(III)) complex, Eu(tta)3.xH2O (where tta is 2-thenoyltrifluoroacetonate). The incorporation of an europium center into the polymer backbone changed the optical and electronic properties of the conjugated polymer. The photoluminescence of the materials and electroluminescence properties of solution-processed organic light-emitting diodes (OLEDs) consisting of composites of PVK as a host with PBPyVOctV, Eu(tta)3.xH2O or PBPyVOctV-Eu(tta)3 as guests were studied. The diode containing PVK:PBPyVOctV-Eu(tta)3 as emissive layer delivered the highest luminance and highest current efficiency, with an orange color emission, as a result of combined emissions from the europium ions and polymer structure.

Triazine-based OLEDs with simplified structure and high efficiency by solution-processed procedure

Solution-processed Organic light-emitting diodes (OLEDs) have aroused serious attention because of its lower cost and easier fabrication. Herein, a novel triazine-based molecular emitter is reported for solution-processed OLEDs for the first time. The emitter, 4,4′,4′'-(1,3,5-triazine-2,4,6-triyl)tris(N,N-bis(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)aniline) denoted as Tri-2tCzPA-TRZ, shows excellent solubility in common organic solvents, facilitating solution-processed OLEDs. The resultant OLEDs doped with 1,3-di(9H-carbazol-9-yl)benzene (mCP) achieved a maximum current efficiency of 18.8 cd A−1 and maximum luminance of 12,580 cd m−2, which is higher than the maximum current efficiency of the most solution-processed OLEDs in simple structures.

Highly Efficient Thermally Activated Delayed Fluorescence via an Unconjugated Donor-Acceptor System Realizing EQE of Over 30.

In this work, two novel thermally activated delayed fluorescence (TADF) emitters, 2tDMG and 3tDMG, are synthesized for high-efficiency organic light-emitting diodes (OLEDs), The two emitters have a tilted face-to-face alignment of donor (D)/acceptor (A) units presenting intramolecular noncovalent interactions. The two TADF materials are deposited either by an evaporation-process or by a solution-process, both of them leading to high OLED performance. 2tDMG used as the emitter in evaporation-processed OLEDs achieves a high external quantum efficiency (EQE) of 30.8% with a very flat efficiency roll-off of 7% at 1000cd m-2 . The solution-processed OLEDs also display an interesting EQE of 16.2%. 3tDMG shows improved solubility and solution processability as compared to 2tDMG, and thus a high EQE of 20.2% in solution-processed OLEDs is recorded. The corresponding evaporation-processed OLEDs also reach a reasonably high EQE of 26.3%. Encouragingly, this work provides a novel strategy to address the imperious demands for OLEDs with high EQE and low roll-off.

Co-doping method used to improve the charge transport balance in solution processed OLEDs

In this paper, co-doping method is used to improve the current efficiency of solution-processed organic light-emitting diodes (OLEDs). By changing the ratio of two thermally activated delayed fluorescent (TADF) emitters, we studied the performance of device and its mechanism. A solution processed OLED with a structure of indium tin oxide (ITO, 150 nm)/PEDOT:PSS (30 nm)/CBP:4CzIPN-x%:4CzPN-y% (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) was fabricated. The current efficiencies of 26.6 cd/A and 26.4 cd/A were achieved by the devices with dopant ratio of 6% 4CzIPN:2% 4CzPN and 2% 4CzIPN:6% 4CzPN in emitting material layer (EML), respectively. By investigating the tendency of current density change in devices with different doping ratio, we suggested that the enhancement of the current efficiency should be due to the charge transport balance improvement induced by assist dopant in EML.

Solution-processed multi-resonance organic light-emitting diodes with high efficiency and narrowband emission

With excellent color purity (full-width half maximum (FWHM) < 40 nm) and high quantum yield, multi-resonance (MR) molecules can harvest both singlet and triplet excitons for highly efficient narrowband organic light-emitting diodes (OLEDs) owing to their thermally activated delayed fluorescence (TADF) nature. However, the highly rigid molecular skeleton with the oppositely positioned boron and nitrogen in generating MR effects results in the intrinsic difficulties in the solution-processing of MR-OLEDs. Here, we demonstrate a facile strategy to increase the solubility, enhance the efficiencies and modulate emission color of MR-TADF molecules by extending aromatic rings and introducing tert-butyls into the MR backbone. Two MR-TADF emitters with smaller singlet-triplet splitting energies (ΔEST) and larger oscillator strengths were prepared conveniently, and the solution-processed MR-OLEDs were fabricated for the first time, exhibiting efficient bluish-green electroluminescence with narrow FWHM of 32 nm and external quantum efficiency of 16.3%, which are even comparable to the state-of-the-art performances of the vacuum-evaporated devices. These results prove the feasibility of designing efficient solution-processible MR molecules, offering important clues in developing high-performance solution-processed MR-OLEDs with high efficiency and color purity.