Emitters In Organic Light-emitting Diodes Research Articles

High-performance three-coordinated organoboron emitters for organic light-emitting diodes

Three-coordinated organoboron compounds of diversified structures and multifunctionality are highly attractive materials for high-performance organic light-emitting diodes.

Spiro-configured dibenzosuberene compounds as deep-blue emitters for organic light-emitting diodes with a CIEy of 0.04

Deep blue electroluminescence is highly required for organic light-emitting diode (OLED) technology. We report, in this work, blue emitting fluorophores for OLED with low CIEy.

Enhanced Hole Injection Characteristics of a Top Emission Organic Light-Emitting Diode with Pure Aluminum Anode.

A top emitting organic light-emitting diode (OLED) device with pure aluminum (Al) anode for high-resolution microdisplays was proposed and fabricated. The low work function of the Al anode, even with a native oxide formed on the Al anode surface, increases the energy barrier of the interface between the anode and hole injection layer, and has poor hole-injection properties, which causes the low efficiency of the device. To enhance the hole-injection characteristics of the Al anode, we applied hexaazatriphenylene hexacarbonitrile (HATCN) as the hole-injection layer material. The proposed OLED device with a pure Al anode and native oxide on the anode surface improved efficiency by up to 35 cd/A at 1000 nit, which is 78% of the level of normal OLEDs with indium tin oxide (ITO) anode.

Conformation-dependent mechanochromic delayed fluorescence of AIE-active tetra-coordinated B–N complexes

Mechanochromic luminescence (MCL) materials have attracted great attention due to their promising prospect in mechanical sensors, optoelectronic devices and data storage. In this work, two novel molecules bearing electron deficient tetra-coordinated phenylpyridine boron (PPYB) and electron donating phenothiazine (PTZ) are developed, and their thermal stability, optical and electronic structures, electrochemical property, photophysical property and MCL behaviors are investigated. Both molecules display obvious MCL effect with large emission color change from blue to yellow, which are ascribed to the changes of molecular conformation from quasi-axial conformation to quasi-equatorial one. In quasi-axial conformation, they show short fluorescence lifetimes without obvious delayed components, but in quasi-equatorial conformation, they have longer fluorescence lifetimes with prominent delayed components. Moreover, they are weakly fluorescent in solutions, but the emission intensity and delayed components are significantly enhanced in the solid state, demonstrating typical aggregation-induced delayed fluorescence (AIDF) characteristics. The application of both molecules as emitters in organic light-emitting diodes (OLEDs) is also evaluated.

A solution-processable hybridized local and charge-transfer (HLCT) phenanthroimidazole as a deep-blue emitter for efficient solution-processed non-doped electroluminescence device

Most highly efficient hybridized local and charge-transfer (HLCT)-based organic light‐emitting diodes (OLEDs) are multi-layer devices fabricated by thermal vacuum evaporation techniques, which are inappropriate for the simplified device fabrication process. However, there are only a couple of reported examples of solution-processed HLCT OLEDs, particularly deep-blue OLEDs. Herein, we design and synthesize a solution-processable HLCT emitter, CPPI, in which 1,2-diphenyl-phenanthroimidazole (PI) as an acceptor core is substituted by three 3,6-di-tert-butyl-N-phenyl-carbazole (CP) moieties as a donor and hole-transporting/solubilizing unit. The HLCT and photophysical properties are thoroughly examined by theoretical and experimental methods. CPPI exhibits HLCT characteristics with intense deep-blue color emission, high thermal and electrochemical stability, and decent hole-transporting ability. The molecule is successfully fabricated as a solution-processed non-doped emitter in organic light-emitting diodes (OLED). The solution-processed OLED retains efficient and stable deep-blue color emission (CIE coordinates of (0.157, 0.089)) with a narrow FWHM of 66 nm, an EQEmax of 3.39% and a high exciton utilization efficiency of 42.4%. Crucially, this result signifies an advance in developing a solution-processable HLCT molecule as an emitter for solution-processed non-doped OLEDs.

Solution-processed deep-blue (y∼0.06) fluorophores based on triphenylamine-imidazole (donor-acceptor) for OLEDs: computational and experimental exploration

Developing solution-processable deep-blue emitters for organic light-emitting diodes (OLEDs) is still a challenging task. In this context, two new solution-processable deep-blue emitters, N, N-diphenyl-4’-(1-(3-(trifluoromethyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-[1,1’-biphenyl]-4-amine(4-PIMCFTPA) and 4’-(4,5-diphenyl-1-(3-(trifluoromethyl)phenyl)-1H-imidazol-2-yl)-N,N-diphenyl-[1,1’-biphenyl]-4-amine (4-BICFTPA), were successfully designed and synthesized by incorporating phenanthroimidazole (PI)/diphenylimidazole (BI)-triphenylamine (TPA), which is functional at the N1 position of the imidazole, with Ph-mCF3. The thermal, photophysical, and electrochemical properties of both fluorophores were systematically explored. These fluorophores showed a deep-blue emission in the solution as well as in the solid state. The highest occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) energy level of the fluorophores was calculated using electrochemical studies and compared with the theoretical calculation [the density functional theory (DFT)]. The asymmetrically twisted conformation of 4-PIMCFTPA between PI-TPA efficiently showed a high photoluminescence quantum yield. OLED (undoped and doped) devices were fabricated with the newly synthesized emitters, and 4-PIMCFTPA demonstrated better electroluminescence (EL) performance than the BI-based emitter. Thus, the OLED based on 4-PIMCFTPA (1 wt% in the CBP host) had the best EL performance, with a maximum external quantum efficiency 1.7% and CIE coordinates of (0.17, 0.06).

Theoretical Study of Triphenylphosphine Oxide Derivatives for Blue Thermally Activated Delayed Fluorescence.

We designed novel thermally activated delayed fluorescence (TADF) materials by combining the electron donor 10,10-diphenyl-5,10-dihydrodibenzo[b,e][1,4]azasiline (DDA) with the electron acceptor triphenylphosphine oxide (PO) unit (mDDA-PO and o-mDDA-PO) and compared their characteristics with those of a reference material using 1,3-Bis(N-carbazolyl)benzene (mCP) as an electron donor (mCP-PO) for blue organic light-emitting diodes (OLEDs). Using density functional theory (DFT) and time-dependent DFT calculations, we obtained the electron distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as the energies of the lowest singlet (S1) and lowest triplet (T1) excited states. The calculated energy difference (ΔEST) between the S1 and T1 states of mDDA-PO (0.16 eV) and o-mDDA-PO (0.07 eV) were smaller than that of mCP-PO (0.48 eV). The results showed that o-mDDA-PO is a suitable blue OLED emitter because it has sufficiently small ΔEST values, which is favorable in a reverse-intersystem process crossing from the T1 state to S1 states, as well as an emission wavelength of 446.7 nm.

Bis(Triphenylamine)Benzodifuran Chromophores: Synthesis, Electronic Properties and Application in Organic Light-Emitting Diodes.

A series of bis(triphenylamine)benzodifuran chromophores have been synthesized and fully characterised. Starting from suitably functionalized benzodifuran (BDF) precursors, two triphenylamine (TPA) moieties are symmetrically coupled to a central BDF unit either at 4,8-positions through double bonds (1) and single bonds (2) respectively, or at 2,6-positions through double bonds (3). Their electronic absorption and photoluminescence properties as well as redox behaviour have been investigated in detail, indicating that the π-extended conjugation via vinyl linkers in 1 and 3 leads to comparatively strong electronic interactions between the relevant redox moieties TPA and BDF. Due to intriguing electronic properties and structural planarity, 3a has been applied as a dopant emitter in organic light-emitting diodes. A yellowish-green OLED exhibits a high external quantum efficiency (EQE) of 6.2%, thus exceeding the theoretical upper limit most likely due to energy transfer from an interface exciplex to an emissive layer and/or favorable horizontal orientation.

Modeling Molecular Emitters in Organic Light-Emitting Diodes with the Quantum Mechanical Bespoke Force Field.

Combined molecular dynamics (MD) and quantum mechanics (QM) simulation procedures have gained popularity in modeling the spectral properties of functional organic molecules. However, the potential energy surfaces used to propagate long-time scale dynamics in these simulations are typically described using general, transferable force fields designed for organic molecules in their electronic ground states. These force fields do not typically include spectroscopic data in their training, and importantly, there is no general protocol for including changes in geometry or intermolecular interactions with the environment that may occur upon electronic excitation. In this work, we show that parameters tailored for thermally activated delayed fluorescence (TADF) emitters used in organic light-emitting diodes (OLEDs), in both their ground and electronically excited states, can be readily derived from a small number of QM calculations using the QUBEKit (QUantum mechanical BEspoke toolKit) software and improve the overall accuracy of these simulations.

Imidazole-based solid-state fluorophores with combined ESIPT and AIE features as self-absorption-free non-doped emitters for electroluminescent devices

Excited-state intramolecular proton transfer (ESIPT) fluorophores with an eye-catching aspect of a large Stokes shift in optical properties have been exceptionally considered as prime candidates for numerous applications. Nevertheless, as non-doped emitters for electroluminescent devices, the device performance is still far behind the traditional fluorescence emitters. Herein, two imidazole-based ESIPT-aggregate induced emission (AIE) fluorophores (HITPE and HPITPE) are designed and synthesized by covalently linked the ESIPT cores of 2-(2-hydroxyphenyl)-1,4,5-triphenylimidazole (HI) and 2-(2-hydroxylphenyl)-1-phenylphenanthroimidazole (HPI) with AIE-active luminogen of tetraphenylethene (TPE) at 5-position of the 2-hydroxyphenyl unit. The ESIPT, AIE, and photophysical properties are theoretically and experimentally studied. Both molecules display ESIPT and AIE characters with intense sky-blue/green-blue color emissions from a pure keto form in the solid-state and photoluminescence quantum yields of 57–64%. They possess high thermal and electrochemical stabilities. They are successfully fabricated as non-doped emitters in organic light-emitting diodes (OLED), and all devices exhibit strong keto-form emissions with low turn-on voltages (3.0–3.2 V). Especially, HPITPE-based OLED achieves a high luminance of 10680 cd m−2, LE value of 3.67 cd A−1, and external quantum efficiency (EQE) of 3.26% with a slight efficiency roll-off. Importantly, this represents an advance in the development of ESIPT molecules as a non-doped emitter for fluorescent OLEDs.

The effect of the electron-donor ability on the OLED efficiency of twisted donor-acceptor type emitters

Two twisted donor-acceptor (D-A) chemical structures, CCDMB and PCDMB, were developed as a new class of thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes (OLEDs). Two emitters consist of 3-substituted carbazole as a first donor and trivalent boron as an electron acceptor in common, and carbazole and phenoxazine as second donors with different electron donor ability. While PCDMB with a strong phenoxazine donor decreased the lowest singlet excited state (S 1 ) level and thus showed a small singlet-triplet energy difference (ΔE ST ) value of 0.13 eV, resulting in effective reverse intersystem crossing (RISC), however, CCDMB with a weak donor showed a large ΔE ST value of 0.21 eV. Efficient triplet harvesting of PCDMB was confirmed by a delayed component in transient PL decay curves of 25 wt% PCDMB-doped bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) films. OLED devices with a CCDMB emitter showed deep-blue emission with Commission Internationale de l’Éclairage (CIE) of (0.16, 0.12) but a low maximum EQE of 5.5%, indicative of insufficient triplet harvesting. PCDMB-based devices showed green emission with CIE of (0.21, 0.45) and a high maximum EQE of 22.3%. Our study revealed the effect of the electron donor ability of structurally similar emitters on ΔE ST values, triplet harvesting, and device efficiency. • We synthesized two thermally activated delayed fluorescence (TADF) emitters with difference only in electron-donor ability. • PCDMB has a smaller ΔE ST value than CCDMB owing to the stronger electron donating ability of phenoxazine than carbazole. • PCDMB -based devices showed a higher EQE compared to CCDMB -based devices. • This study revealed the effect of the electron donor ability of emitters on ΔE ST , triplet harvesting, and device efficiency.

Difluorinated cyclopentadithiophene derivatives for green emitters in organic light-emitting diodes: A theoretical investigation

Two types of difluorinated conjugated systems incorporating bridged cyclopentadithiophene, alternating fumaronitrile and 1,3,4-oxadiazole were designed and theoretically studied by density functional theory and its time-dependent extension with the B3LYP hybrid functional and the 6–311g(d,p) and 6–311++g(d,p) basis sets in tetrahydrofuran. The designed materials are denoted FCFO-1 (>CO) and FCFO-2 (>CC–(CN)2. The electronic band gap ranges from 2.46 eV (FCFO-1) to 2.30 eV (FCFO-2). The difference in optical behaviors may originate from the effect of the bridging group on cyclopentadithiophene derivatives. The materials were examined as green emitters (519–564 nm). Organic light-emitting diodes based on these new fluorescent materials [indium tin oxide anode (100 nm)/N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (hole-injection layer) (150 nm)/FCFO-1 or FCFO-2 (50 nm)/Al cathode (100 nm)], were theoretically designed. The current-voltage characteristics were simulated, and the threshold voltages were estimated to be 3.25 V for FCFO-1 and 2.17 V for FCFO-2. Electric dipole moment calculations show the molecules possess large electric dipole moments (approximately 9.88 D for FCFO-1 and approximately 11 D for FCFO-2) and they thus exhibit excellent nonlinear optical properties.

Cis-Quinacridone-Based Delayed Fluorescence Emitters: Seemingly Old but Renewed Functional Luminogens.

Herein, we report a material design of linear cis-quinacridone (cis-QA) derivatives as delayed fluorescence luminogens. In contrast to the widely studied traditional trans-isomers, the functionality of cis-QA and its derivatives remains unexplored and unclarified. Through combined computational and experimental investigations, we revealed that cis-QA derivatives can function as fascinating narrowband deep-blue delayed fluorescence emitters for organic light-emitting diodes (OLEDs). The best-performing deep-blue OLEDs incorporating these cis-QA luminogens achieved high external electroluminescence quantum efficiencies of up to 19.0 % and high color purity with chromaticity coordinates of (0.13, 0.14).

A novel molecular design featuring the conversion of inefficient TADF emitters into efficient TADF emitters for deep-blue organic light emitting diodes

Abstract In this report, we unveiled a new molecular design featuring the transformation of inefficient thermally activated delayed fluorescence (TADF) emitters into the efficient TADF emitters by combing them with non-TADF type chromophore units to develop deep-blue TADF emitters for organic light-emitting diodes. The inefficient TADF emitter and non-TADF chromophore were connected directly through a phenyl linker in a sterically hindered configuration by selectively interlocking two donors in the backbone structure. The new TADF emitter demonstrated excellent device performances with maximum external quantum efficiency up to 22% and y color coordinate of 0.15. Based on these results, we believe that the proposed new design strategy is effective with regard to the conversion of inefficient deep-blue TADF emitters to efficient deep-blue TADF emitters and can be diversely applied for the development of efficient TADF emitters.

Multifunctional derivatives of pyrimidine-5-carbonitrile and differently substituted carbazoles for doping-free sky-blue OLEDs and luminescent sensors of oxygen

IntroductionEvolution of organic light-emitting diodes (OLEDs) reached the point, which allows to obtain maximum internal quantum efficiency of 100% partly using heavy-metal-free emitters exhibiting thermally activated delayed fluorescence (TADF). Such emitters are also predictively perfect candidates for new generation of optical sensors since triplet harvesting can be sensitive to different analytes (at least to oxygen). Although many organic TADF emitters have been reported so far as OLED emitters, the investigation of materials suitable for both OLEDs and optical sensors remains extremely rare. ObjectivesAiming to achieve high photoluminescence quantum yields in solid-state and triplet harvesting abilities of organic semiconductors with efficient bipolar charge transport required for application in both blue OLEDs and optical sensors, symmetrical donor–acceptor-donor organic emitters containing pyrimidine-5-carbonitrile electron-withdrawing scaffold and carbazole, tert-butylcarbazole and methoxy carbazole donor moieties were designed, synthesized and investigated as the main objectives of this study. MethodsNew compounds were tested by many experimental methods including optical and photoelectron spectroscopy, time of flight technique, electrochemistry and thermal analyses. ResultsDemonstrating advantages of the molecular design, the synthesized emitters exhibited sky-blue efficient TADF with reverse intersystem crossing rates exceeding 106 s−1, aggregation-induced emission enhancement with photoluminescence quantum yields in solid state exceeding 50%, hole and electron transporting properties with charge mobilities exceeding 10-4 cm2/V·s, glass-forming properties with glass transition temperatures reaching 177 °C. Sky-blue OLEDs with non-doped light-emitting layers of the synthesized emitter showed maximum external efficiency of 12.8% while the doped device with the same emitter exhibited maximum external efficiency of 14%. The synthesized emitters were also used as oxygen probes for optical sensors with oxygen sensitivity estimated by the Stern-Volmer constant of 3.24·10-5 ppm−1. ConclusionThe developed bipolar TADF emitters with pyrimidine-5-carbonitrile and carbazole moieties showed effective applicability in both blue OLEDs and optical sensors.

Donor/Acceptor Pairs Created by Electrostatic Interaction: Design, Synthesis, and Investigation on the Exciplex Formed Within the Pair.

Exciplexes formed between donors and acceptors have been widely explored but isolating them from each other and tuning the interaction between the donor and acceptor have remained challenges. Here, we report donor/acceptor (D/A) pairs created by electrostatic interaction between a carbazole-based anionic donor and a 1,3,5-triazine-based cationic acceptor and the exciplex formed within the pair. In a diluted film, the D/A pair affords an isolated exciplex which shows thermally activated delayed fluorescence (TADF). By changing the anchoring position of the imidazolium cation in the cationic acceptor, interactions between the donor and acceptor can be changed. Compared to the conventional exciplex formed in a neat film, the isolated exciplex exhibits a substantially higher luminescence efficiency. The D/A pairs show intriguing mechanochromic luminescence and mechanical grinding-induced/reinforced TADF in the solid state and promising performances as emitters in organic light-emitting diodes.

Hydroxyaromatic Fluorophores.

Many fluorophores that are widely used in analytical biochemistry and in biological microscopy contain a hydroxyaromatic component. One could also find fascinating chemistries of hydroxyaromatic dyes, especially those capable of excited state proton transfer (ESPT) to produce dual emission, in the literature of materials and physical chemistry. The ESPT-capable compounds have attracted interest based on their fundamental intellectual values in molecular photophysics and their potential utilities as light emitters in organic light-emitting diodes (LEDs) or fluorescent sensors. The hydroxyaromatic dyes could undergo either intra- or intermolecular proton transfer in either electronic ground or excited states. Although having long been applied for various purposes, some of their absorption and emission properties have not always been clearly described because of the insufficient attention given to proton transfer equilibria in either the ground or excited state and the challenges in computationally modeling the true emitters of these dyes under any given conditions. In this article, an attempt is made to summarize the spectroscopic properties of a few common hydroxyaromatic dyes that have been studied for both fundamental and practical purposes, with the help from quantum chemical calculations of the absorption and emission energies of these dyes in neutral and anion forms. The goal of this article is to provide readers some clarity in the optical properties of these compounds and the tools to understand and to predict the photon-initiated behaviors of hydroxyaromatic fluorophores.

Synthesis of π-Conjugated Polymers Containing Benzotriazole Units via Palladium-Catalyzed Direct C-H Cross-Coupling Polycondensation for OLEDs Applications.

Four D-π-A conjugated polymers, namely P1–P4, which contain benzotriazole building blocks in their backbone as acceptor, are synthesized via palladium-catalyzed direct C-H cross-coupling polycondensation of 5,6-difluorobenzotriazole with different thiophene derivatives, including 3-octylthiophene, 2,2’-bithiophene, thieno[3,4-b][1,4]dioxine, and 4,4-dioctyl-4H-silolo-[3,2-b:4,5-b’]dithiophene as donor units, respectively. Taking the polymer P1 as an example, the chemical structure of the polymer is demonstrated by 1H and 19F NMR spectra. The optical, electrochemical, and thermal properties of these polymers are assessed by UV–vis absorption and fluorescence spectroscopy, cyclic voltammetry (CV), and thermal gravimetric analysis (TGA), respectively. DFT simulations of all polymers are also performed to understand their physicochemical properties. Furthermore, P1 and P2, which have relatively higher molecular weights and better fluorescent quantum efficiency than those of P3 and P4, are utilized as lighting emitters for organic light-emitting diodes (OLEDs), affording promising green and red luminescence with 0.07% and 0.14% of maximum external quantum efficiency, respectively, based on a device with an architecture of ITO/PEDOT:PSS/PTAA/the polymer emitting layer/TPBi/LiF/Al.

Heavy-Atom-Free Room-Temperature Phosphorescent Organic Light-Emitting Diodes Enabled by Excited States Engineering.

Room temperature phosphorescence materials offer great opportunities for applications in optoelectronics, due to their unique photophysical characteristics. However, heavy-atom-free organic emitters that can realize distinct electrophosphorescence are rarely exploited. Herein a new approach for designing heavy-atom-free organic room temperature phosphorescence emitters for organic light-emitting diodes is presented. The subtle tuning of the singlet and triplet excited states energies by appropriate choice of host matrix allows tailored emission properties and switching of emission channels between thermally activated delayed fluorescence and room temperature phosphorescence. Moreover, an efficient and heavy-atom-free room temperature phosphorescence organic light-emitting diode using the developed emitter is realized.

Thermally activated delayed fluorescence exciplex emitters for high-performance organic light-emitting diodes.

Owing to their natural thermally activated delayed fluorescence (TADF) characteristics, the development of exciplex emitters for organic light-emitting diodes (OLEDs) has witnessed booming progress in recent years. Formed between electron-donating and electron-accepting molecules, exciplexes with intermolecular charge transfer processes have unique advantages compared with unimolecular TADF materials, offering a new way to develop high-performance TADF emitters. In this review, a comprehensive overview of TADF exciplex emitters is presented with a focus on the relationship between the constituents of exciplexes and their electroluminescence performance. We summarize and discuss the latest and most significant developments of TADF exciplex emitters. Notably, the design principles of efficient TADF exciplex emitters are systematically categorized into three systems within this review. These progressive achievements of TADF exciplex emitters point out future challenges to trigger more research endeavors in this growing field.