The first tandem, all-exciplex-based WOLED

Unraveling non-radiative decay channels of exciplexes to construct efficient red emitters for organic light-emitting diodes

Exciplex emitters naturally have thermally activated delayed fluorescence characteristics due to their spatially separated molecular orbitals. However, the intermolecular charge transfer potentially induces diverse non-radiative decay channels, severely hindering the construction of efficient red exciplexes. Thus, a thorough comprehension of this energy loss is of paramount importance. Herein, different factors, including molecular rigidity, donor-acceptor interactions and donor-donor/acceptor-acceptor interactions, that impact the non-radiative decay were systematically investigated using contrasting exciplex emitters. The exciplex with rigid components and intermolecular hydrogen bonds showed a photoluminescence quantum yield of 84.1% and a singlet non-radiative decay rate of 1.98 × 106 s-1 at an optimized mixing ratio, respectively, achieving a 3.3-fold increase and a 70% decrease compared to the comparison group. In the electroluminescent device, a maximum external quantum efficiency of 23.8% was achieved with an emission peak of 608 nm, which represents the state-of-the-art organic light-emitting diodes using exciplex emitters. Accordingly, a new strategy is finally proposed, exploiting system rigidification to construct efficient red exciplex emitters that suppress non-radiative decay.

Advances in Host-Free White Organic Light-Emitting Diodes Utilizing Thermally Activated Delayed Fluorescence: A Comprehensive Review

The ever-growing prominence and widespread acceptance of organic light-emitting diodes (OLEDs), particularly those employing thermally activated delayed fluorescence (TADF), have firmly established them as formidable contenders in the field of lighting technology. TADF enables achieving a 100% utilization rate and efficient luminescence through reverse intersystem crossing (RISC). However, the effectiveness of TADF-OLEDs is influenced by their high current density and limited device lifetime, which result in a significant reduction in efficiency. This comprehensive review introduces the TADF mechanism and provides a detailed overview of recent advancements in the development of host-free white OLEDs (WOLEDs) utilizing TADF. This review specifically scrutinizes advancements from three distinct perspectives: TADF fluorescence, TADF phosphorescence and all-TADF materials in host-free WOLEDs. By presenting the latest research findings, this review contributes to the understanding of the current state of host-free WOLEDs, employing TADF and underscoring promising avenues for future investigations. It aims to serve as a valuable resource for newcomers seeking an entry point into the field as well as for established members of the WOLEDs community, offering them insightful perspectives on imminent advancements.

Progress in Research on White Organic Light-Emitting Diodes Based on Ultrathin Emitting Layers

White organic light-emitting diodes (WOLEDs) hold vast prospects in the fields of next-generation displays and solid-state lighting. Ultrathin emitting layers (UEMLs) have become a research hotspot because of their unique advantage. On the basis of simplifying the device structure and preparation process, they can achieve electroluminescent performance comparable to that of doped devices. In this review, we first discuss the working principles and advantages of WOLEDs based on UEML architecture, which can achieve low cost and more flexibility by simplifying the device structure and preparation process. Subsequently, the successful applications of doping and non-doping technologies in fluorescent, phosphorescent, and hybrid WOLEDs combined with UEMLs are discussed, and the operation mechanisms of these WOLEDs are emphasized briefly. We firmly believe that this article will bring new hope for the development of UEML-based WOLEDs in the future.

Exciplex-forming cohost systems with 2,7-dicyanofluorene acceptors for high efficiency red and deep-red OLEDs

Two 2,7-dicyaonfluorene-based molecules 27-DCN and 27-tDCN are utilized as acceptors (A) to combine with hexaphenylbenzene-centered donors (D) TATT and DDT-HPB for probing the exciplex formation. The photophysical characteristics reveal that the steric hindered 27-tDCN not only can increase the distance of D and A, resulting in a hypsochromic emission, but also dilute the concentration of triplet excitons to suppress non-radiative process. The 27-tDCN-based exciplex-forming blends exhibit better photoluminescence quantum yield (PLQY) as compared to those of 27-DCN-based pairs. In consequence, among these D:A blends, the device employing DDT-HPB:27-tDCN blend as the emissiom layer (EML) exhibits the best EQE of 3.0% with electroluminescence (EL) λmax of 542 nm. To further utilize the exciton electrically generated in exciplex-forming system, two D-A-D-configurated fluorescence emitter DTPNT and DTPNBT are doped into the DDT-HPB:27-tDCN blend. The nice spectral overlap ensures fast and efficient Förster energy transfer (FRET) process between the exciplex-forming host and the fluorescent quests. The red device adopting DDT-HPB:27-tDCN:10 wt% DTPNT as the EML gives EL λmax of 660 nm and maximum external quantum efficiency (EQEmax) of 5.8%, while EL λmax of 685 nm and EQE of 5.0% for the EML of DDT-HPB:27-tDCN:10 wt% DTPNBT. This work manifests a potential strategy to achieve high efficiency red and deep red OLED devices by incorporating the highly fluorescent emitters to extract the excitons generated by the exciplex-forming blend with bulky acceptor for suppressing non-radiative process.

Unusual Excimer/Dimer Behavior of a Highly Soluble C,N Platinum(II) Complex with a Spiro-Fluorene Motif

In this work, we introduce a spiro-fluorene unit into a phenylpyridine (CN)-type ligand as a simple way to deplanarize the structure and increase the solubility of the final platinum(II)···complex. Using a spiro-fluorene unit, orthogonal to the main coordination plane of the complex, reduces intermolecular interactions, leading to increased solubility but without significantly affecting the ability of the complex to form Pt···Pt dimers and excimers. This approach is highly important in the design of platinum(II) complexes, which often suffer from low solubility due to their mainly planar structure, and offers an alternative to the use of bulky alkyl groups. The nonplanar structure is also beneficial for vacuum-deposition techniques as it lowers the sublimation temperature. Importantly, there are no sp3 hybridized carbon atoms in the cyclometalating ligand that contain hydrogens, the undesired feature that is associated with the low stability of the materials in OLEDs. The complex displays high solubility in toluene, ∼10 mg mL-1, at room temperature, which allows producing solution-processed OLEDs in a wide range of doping concentrations, 5-100%, and EQE up to 5.9%, with a maximum luminance of 7400 cd m-2. Concurrently, we have also produced vacuum-deposited OLEDs, which display luminance up to 32 500 cd m-2 and a maximum EQE of 11.8%.

Recombination of X-ray-Generated Radical Ion Pairs in Alkane Solution Assembles Optically Inaccessible Exciplexes from a Series of Perfluorinated para-Oligophenylenes with N,N-Dimethylaniline

We demonstrate that a series of perfluorinated para-oligophenylenes C₆F₅-(C₆F₄)_n-C₆F₅ (n = 1-3) produce exciplexes with N,N-dimethylaniline (DMA) in degassed X-irradiated n-dodecane solutions. The optical characterization of the compounds shows that their short fluorescence lifetimes (ca. 1.2 ns) and UV-Vis absorption spectra, overlapping with the spectrum of DMA with molar absorption coefficients of 2.7-4.6 × 10⁴ M^-1cm^-1, preclude the standard photochemical exciplex formation pathway via selective optical generation of the local excited state of the donor and its bulk quenching by the acceptor. However, under X-rays, the efficient assembly of such exciplexes proceeds via the recombination of radical ion pairs, which delivers the two partners close to each other and ensures a sufficient energy deposition. The exciplex emission is completely quenched by the equilibration of the solution with air, providing a lower bound of exciplex emission lifetime of ca. 200 ns. The recombination nature of the exciplexes is confirmed by the magnetic field sensitivity of the exciplex emission band inherited from the magnetic field sensitivity from the recombination of spin-correlated radical ion pairs. Exciplex formation in such systems is further supported by DFT calculations. These first exciplexes from fully fluorinated compounds show the largest known red shift of the exciplex emission from the local emission band, suggesting the potential of perfluoro compounds for optimizing optical emitters.

Advances in Blue Exciplex–Based Organic Light-Emitting Materials and Devices

Exciplexes possessing thermally activated delayed fluorescence (TADF) characteristics have received much attention in the fields of organic light-emitting materials and devices over the past decade. In general, an exciplex is a physical mixture between a donor (D) with hole transport properties and an acceptor (A) with electron transport characteristics, and the energy difference between the lowest excited singlet state and the lowest excited triplet state is usually fairly small in terms of the long-range charge-transfer process from D to A. In the processes of photoluminescence and electroluminescence, triplet excitons can be converted to singlet excitons through reverse intersystem crossing and then radiate photons to achieve TADF. As a consequence, triplet excitons can be effectively harvested, and the exciton utilization can be significantly enhanced. Up to now, a large number of exciplexes have been developed and applied to organic light-emitting devices. Notably most of them showed green or red emission, while blue exciplexes are relatively few owing to the spectrum characteristics of the large red-shift and broadened emission. In this study, the latest progress of blue exciplex–based organic light-emitting materials and devices is briefly reviewed, and future research is prospected.

Novel Easy to Synthesize Benzonitrile Compounds with Mixed Carbazole and Phenoxazine Substituents Exhibiting Dual Emission and TADF Properties

The photophysical and electrochemical properties of a new class of fluorinated benzonitrile compounds substituted with mixed phenoxazine and carbazole units have been investigated. When absorbing in a large range of the UV-vis spectrum due to both localized and charge-transfer absorptions, these compounds show dual broad emission in solution and intense emission in PMMA films, with photoluminescence quantum yields changing from a few percent in solution to 18% in a more rigid environment. The compounds also exhibit thermally activated delayed fluorescence demonstrated by the role of oxygen in the quenching of delayed fluorescence and by time-resolved luminescence studies, with an efficiency directly related to the number of phenoxazine substituents. Electrochemistry reveals dramatic changes in the reduction mechanisms according to the number of remaining fluorine atoms on the benzonitrile core. All these results demonstrate how it is possible to tune the photophysical and electrochemical properties of easily synthesizable derivatives by controlling the nature and relative number of the substituents on a simple aromatic platform.

Nanosecond-Pulsed Perovskite Light-Emitting Diodes at High Current Density

While metal-halide perovskite light-emitting diodes (PeLEDs) hold the potential for a new generation of display and lighting technology, their slow operation speed and response time limit their application scope. Here, high-speed PeLEDs driven by nanosecond electrical pulses with a rise time of 1.2 ns are reported with a maximum radiance of approximately 480 kW sr^-1  m^-2 at 8.3 kA cm^-2 , and an external quantum efficiency (EQE) of 1% at approximately 10 kA cm^-2 , through improved device configuration designs and material considerations. Enabled by the fast operation of PeLEDs, the temporal response provides access to transient charge carrier dynamics under electrical excitation, revealing several new electroluminescence quenching pathways. Finally, integrated distributed feedback (DFB) gratings are explored, which facilitate more directional light emission with a maximum radiance of approximately 1200 kW sr^-1 m^-2 at 8.5 kA cm^-2 , a more than two-fold enhancement to forward radiation output.

Secondary Exciplex by Electromer Mediated Charge Transfer for Multiband Electroluminescence

Charge-transfer states have been observed extensively in heterojunctions of organic semiconductors, which are also referred to as exciplexes in polymer blends. Such mechanisms have been well understood in the conventional material systems. However, electromer states may be produced only in some polymeric molecules with folded chains. We report here the interaction between exciplex and electromer states, which facilitates the formation of a new electrically excited state that we define as a secondary exciplex. This is an indirect process understood as an electromer-mediated heterojunction. We discovered such an optoelectronic mechanism in the blend film of poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-pheny-l,4-phenylene-diamine) (PFB) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)] (F8BT). Four emission bands can be resolved from the electroluminescence spectrum, including those based on the excitons, the electromers, and the primary and secondary exciplexes. The whole electroluminescence spectrum thus extends from the green (500 nm) to the near-infrared (900 nm) with a full bandwidth of 400 nm. These new discoveries with the conventional light-emitting polymers are important not only for polymeric optoelectronics, but also for the development of broadband light-emitting devices.

Thermally Activated Delayed Fluorescence in Commercially Available Materials for Solution-Process Exciplex OLEDs

Organic light-emitting diodes (OLEDs) have developed rapidly in recent years. Thermally activated delayed fluorescent (TADF) molecules open a path to increase exciton collection efficiency from 25% to 100%, and the solution process provides an alternative technology to achieve lower cost OLEDs more easily. To develop commercial materials as exciplex hosts for high-performance and solution-processed OLEDs, we attempted to use 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine (TAPC), poly(9-vinylcarbazole) (PVK), N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), and poly(N,N’-bis-4-butylphenyl-N,N’-bisphenyl)benzidine (Poly-TPD) as the donors and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (POT2T) as the acceptor to obtain the TADF effect. All donors and the acceptor were purchased from chemical suppliers. Our work shows that excellent TADF properties and high-efficiency exciplex OLEDs with low turn-on voltage and high luminance can be achieved with a simple combination of commercial materials.

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.

Modulation for efficiency and spectra of non-doped white organic light emitting diodes by combining an exciplex with an ultrathin phosphorescent emitter

Herein, structured non-doped white organic light-emitting diodes (WOLEDs) were designed by combining the emission of a blue exciplex and orange-red phosphorescent ultrathin layer. The device efficiency and spectra were modulated successfully by adjusting the thickness of the exciplex layer and ultrathin layer, respectively. Meanwhile, high efficiency with external quantum efficiency (EQE) ranging from 15% to 22%, power efficiency from 33 lm W⁻¹ to 47 lm W⁻¹ and warm white emission with correlated color temperature (CCT) from 1600 K to 2600 K were realized. The energy transfer process and emission mechanism is also discussed, and the results reveal that the efficient charge trapping and recombination contribute to the improvement of device efficiency and reduce the roll-off efficiency.

Non-doped WOLED with modulated efficiency and spectra were designed by combining blue exciplex with orange-red phosphorescent ultrathin layer.

Solution-Processable Csp3-Annulated Hosts for High-Efficiency Deep Red Phosphorescent OLEDs

In this report, a solution-processable cohost system incorporating N,N'-di(naphtalene-1-yl)-N,N'-diphenylbenzidine (NPB) and Csp³-annulated phenylquinoline derivatives, including spiro[indeno[1,2-b]quinoline-11,8'-indolo[3,2,1-de]acridine] (IAIQ), 10-phenyl-10H-spiro[acridine-9,11'-indeno[1,2-b]quinoline] (PAIQ) and 3,3'-(11H-indeno[1,2-b]quinoline-11,11-diyl)bis(N-phenyl-N-(m-tolyl)aniline) (m-TPA-DPIQ), is developed for highly efficient saturated red phosphorescent organic light emitting diodes (OLEDs). IAIQ, PAIQ, and m-TPA-DPIQ, designed with the increase of molecular flexibility, are systematically investigated. Solution-processable devices based on the efficient phosphorescent emitter bis[2-(3,5-dimethylphenyl)isoquinolinato](2,8-dimethyl-4,6-nonanedionato)Iridium [Ir(mpiq)₂divm] are successfully fabricated, and give electroluminescent peaks at 634-636 nm with Commission Internationale de L'Eclairage coordinates of (0.70, 0.30). Under optimized conditions, the devices incorporating IAIQ, PAIQ, and m-TPA-DPIQ exhibit high external quantum efficiency with the maximum value at 25.1%, 23.4%, and 23.3%, respectively, and all exceeding 18% at the luminance of 1000 cd/m². In application, the supersaturated red devices with excellent performance could facilitate the development of wet-made displays. The newly developed Csp³-annulated host materials with their excitonic properties also showoff the tactic to construct cohost system for high-quality phosphorescent OLEDs.

Exciplex energy transfer through spacer: White electroluminescence with enhanced stability based on cyan intermolecular and orange intramolecular thermally activated delayed fluorescence

Capability of exciplex energy transfer through a spacer was investigated using three exciplex-forming solid mixtures which contained the well-known electron accepting 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine and appropriately designed bipolar cyanocarbazolyl-based derivatives functionalized by attachment of carbazolyl, acridanyl or phenyl units. These novel cyanocarbazolyl-based derivatives were used as both the spacer and exciplex-forming donor. Efficient organic light-emitting diodes with electroluminescence in cyan-yellow region and maximum external quantum efficiency of up to 7.7% were fabricated owing to efficient thermally activated fluorescence (TADF) of the newly discovered exciplexes. An approach of exciton separation by the spacer between the studied exciplexes and selected orange TADF emitter was proposed for the fabrication of white electroluminescent devices with prolonged lifetime comparing to that of single-color exciplex-based devices. Exciplex-forming systems were tested for exciton separation between inter- and intramolecular TADF. Exciplex energy transfer through a spacer was observed on relatively long distance for one system due to the energy resonance between triplet levels of the exciplex and spacer. First time observed here exciplex energy transfer through a spacer can be useful for both improvement of device stability and obtaining of white electroluminescence.

Simple method to extract extinction coefficients of films with the resolution of 10-5 using just transmission data and application to intermolecular charge-transfer absorption in an exciplex-forming organic film

A simple method is presented to determine the thickness, refractive index and extinction coefficient of a film with the resolutions of ±1 nm, ±2 × 10^-3, and 6 × 10^-5, respectively. The method requires only ultraviolet-visible-near-infrared spectroscopy measurement of the transmittance of films with thicknesses of a few micrometers. A very small extinction coefficient of intermolecular charge transfer (CT) absorption of an exciplex-forming organic film is measured in the sub-bandgap wavelength region. This is to demonstrate that the CT absorption with extinction coefficient in the range of 10^-3 to 10^-5 can be measured indeed using the method, which is in the same resolution as photothermal deflection spectroscopy and Fourier transform photocurrent spectroscopy. The simplicity and feasibility of the proposed approach is expected to promote active study of intermolecular CT absorption in exciplex-forming films.

TADF and exciplex emission in a xanthone-carbazole derivative and tuning of its electroluminescence with applied voltage

Materials showing white light emission have found applications in a variety of solid state devices especially in display technology. For white light emission, doping of red (R), green (G) and blue (B) emitters in a host matrix is commonly practised. However, finding RGB emitters of similar stability with homogenous doping is challenging. Furthermore, such devices suffer from color purity in the long run. Small organic light emitters, capable of colour tuning and having a broad emission spectrum are in high demand as they provide colour stability, reproducibility, a simple device geometry and high efficiency. Recently, it has been shown that the efficiency of OLEDs can be enhanced by employing thermally activated delayed fluorescence (TADF) materials. Here, we designed and synthesised a xanthone-carbazole based D-A-D material (Xan-Cbz) for TADF properties. Blue TADF emission, in neat thin films, at 470 nm was observed and further investigated by studying delayed fluorescence and lifetime measurements. In addition, a blend of Xan-Cbz with NPD shows exciplex emission at 525 nm in thin film. OLEDs based on Xan-Cbz were fabricated using several device configurations. OLEDs having the device configuration ITO/PEDOT:PSS/NPD/Xan-Cbz/Bphen/LiF-Al showed a luminance of 1.96 × 104 Cd m-2 (at a current density of 50 mA cm-2) and V ON at ∼6 V. Electroluminescence showed the features of both neat emission (470 nm) of Xan-Cbz and its exciplex (525 nm) with NPD. Further, colour tuning was observed as a function of applied voltage and the ratio of light intensity (I 525/I 470) of neat and exciplex emission was found to decrease with increasing voltage. Greenish-blue emission (CIE coordinates: 0.202, 0.382) from Xan-Cbz OLEDs was obtained. Xan-Cbz showed its neat emission (at 470 nm) in ITO/PEDOT:PSS/CBP/Xan-Cbz/Bphen/LiF-Al and pure exciplex emission (at 525 nm) in ITO/PEDOT:PSS/NPD:Xan-Cbz/Bphen/LiF-Al device configurations. Thus in this article we showed blue TADF emission, exciplex emission and voltage dependent color tuning in OLEDs based on a small organic emitter.

Superior Efficiency and Low-Efficiency Roll-Off White Organic Light-Emitting Diodes Based on Multiple Exciplexes as Hosts Matched to Phosphor Emitters

Based on exciplexes as hosts, the monochromatic organic light-emitting diodes (OLEDs) have achieved high power and external quantum efficiencies. However, the high-quality white OLEDs (WOLEDs) with high color rendering index (CRI) have the unsatisfactory efficiencies at high luminance, particularly in terms of power efficiency (PE), resulting in high energy consumption. Here, a new design concept using multiple exciplexes as hosts to match different phosphors has been demonstrated to develop high-performance WOLEDs. It can be seen that the resulting WOLEDs work at a low turn-on voltage of 2.3 V and exhibit the large forward-viewing PE and external quantum efficiency (EQE) of 79 lm W^-1 and 22.5%, respectively, without light out-coupling techniques. Significantly, the PE and EQE still remain 48.0 lm W^-1 and 21.4% at 1000 cd m^-2, showing extremely low efficiency roll-off. The CRI is as high as 81. The keys to success are the selection of the different exciplex hosts matched to different phosphors and the reasonable arrangement of emissive layers, which are beneficial to regulate the exciton distribution and reduce the energy losses.

Tricomponent Exciplex Emitter Realizing over 20% External Quantum Efficiency in Organic Light-Emitting Diode with Multiple Reverse Intersystem Crossing Channels

With the naturally separated frontier molecular orbitals, exciplexes are capable of thermally activated delayed fluorescence emitters for organic light-emitting diodes (OLEDs). And, the current key issue for exciplex emitters is improving their exciton utilization. In this work, a strategy of building exciplex emitters with three components is proposed to realize multiple reverse intersystem crossing (RISC) channels, improving their exciton utilization by enhancing upconversion of nonradiative triplet excitons. Accordingly, a tricomponent exciplex DBT-SADF:PO-T2T:CDBP is constructed with three RISC channels respectively on DBT-SADF, DBT-SADF:PO-T2T, and CDBP:PO-T2T. Furthermore, its photoluminescence quantum yield and rate constant of the RISC process are successfully improved. In the OLED, DBT-SADF:PO-T2T:CDBP exhibits a remarkably high maximum external quantum efficiency (EQE) of 20.5%, which is the first report with an EQE over 20% for the OLEDs based on exciplex emitters to the best of our knowledge. This work not only demonstrates that introducing multiple RISC channels can effectively improve the exciton utilization of exciplex emitters, but also proves the superiority of the tricomponent exciplex strategy for further development of exciplex emitters.

Exciplex Organic Light-Emitting Diodes with Nearly 20% External Quantum Efficiency: Effect of Intermolecular Steric Hindrance between the Donor and Acceptor Pair

Exciplex emitters have emerged as an important class of thermally activated delayed fluorescence (TADF) materials for highly efficient OLEDs. A TADF exciplex emitter requires an intermolecular donor/acceptor pair. We have synthesized a bipolar donor-type material, DPSTPA, which was used to pair with known acceptor materials (2CzPN, 4CzIPN, or CzDBA). The OLEDs based on the exciplex emitters, DPSTPA/X, where X = 2CzPN and CzDBA, give green and orange-red colors with record-high external quantum efficiencies (EQEs) of 19.0 ± 0.6 and 14.6 ± 0.4%, respectively. In contrast, the exciplex pair DPSTPA/4CzIPN gave a very low photoluminescence quantum yield (PLQY) and a very low EQE value of the device. The DFT calculations indicate that the intermolecular distance between the donor and the acceptor plays a key factor for the PLQY and EQE. The observed low PLQY and the poor device performance for the DPSTPA/4CzIPN pair are probably because of the relatively long distance between the DPSTPA and 4CzIPN in the thin film caused by the four congested carbazole (Cz) groups of 4CzIPN, which effectively block the interaction of the nitrile acceptor with the triphenylamino donor of DPSTPA.

Recent Applications of Interfacial Exciplex as Ideal Host of Power-Efficient OLEDs

Currently, exploring the applications of intermolecular donor-acceptor exciplex couple as host of OLEDs with phosphorescence, thermally activated delayed fluorescence (TADF) or fluorescence emitter as dopant is a hot topic. Compared to other host strategies, interfacial exciplex has the advantage in various aspects, such as barrier-free charge injection, unimpeded charge transport, and the energy-saving direct exciton formation process at the “Well”-like heterojunction interface region. Most importantly, due to a very fast and efficient reverse intersystem-crossing (RISC) process, such a host is capable of regulating singlet/triplet exciton populations in itself as well as in the dopant emitters both under photoluminescent (PL) and electroluminescent (EL) driving conditions. In this mini-review, we briefly summarize and comment on recent applications of this ideal host in OLEDs (including both thermal-evaporation OLEDs and solution-processed OLEDs) with diverse emitters, e.g., fluorescence, phosphorescence, delayed fluorescence, or others. Special attention is given to illustrate the peculiar achievement of high overall EL performance with superiorities of low driving voltages, slow roll-off rate, high power efficiencies and satisfied device lifetime using this host strategy, which is then concluded by personal perspectives on the relevant next-step in this field.

Realizing 20% External Quantum Efficiency in Electroluminescence with Efficient Thermally Activated Delayed Fluorescence from an Exciplex

The investigation of nondoped exciplex blends of 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), working as the one-electron acceptor molecule, with different electron donors is reported. The emissions of these exciplexes span from the blue to orange-red regions, showing clear contribution from thermally activated delayed fluorescence (TADF) and delayed fluorescence originated from nongeminate recombination of charge carriers created by the dissociation of optically generated exciplexes. We focus our studies on the properties of TADF in these systems, covering in particular the physical meaning of the different transient components observed in their luminescence decays. Our results unravel the intricate role of reverse intersystem crossing due to spin-orbit coupling and possibly also due to hyperfine interactions and internal conversion, which affect the efficiency of the TADF mechanism. Remarkable performances are obtained in prototype organic light-emitting diodes fabricated with some of these blends. Green exciplex blends, in particular, exhibited the current efficiency of 60 cd A^-1, power efficiency of 71 lm W^-1, and external quantum efficiency of 20%. We believe that our results will contribute significantly to highlight the potential advantages of intermolecular exciplexes in the area of organic light-emitting diodes.

Development of Materials for Blue Organic Light Emitting Devices

The success of organic light emitting diodes (OLED) has been witnessed by the commercialization of this technology for manufacturing the vivid and colorful displays used in our daily life now. The prospective growth of OLED technology on display industry will be optimistic. Over the last three decades, many different approaches on material and device designs have been implemented for improving the efficiency and stability of OLED devices. These efforts install main cornerstones to support the great achievement of OLED technology. However, until now, the performance and stability of blue OLEDs still have some concerns. This troublesome issue should be totally conquered before the large-scale manufactures dominated over other display technologies, particularly liquid crystal-based displays, takes place. Though significant progress has already been made to achieve high performance and long lifetime blue OLEDs, this topic still remains as one of the hot researches in OLEDs. We have been working on this area for about two decades and made some notable contributions. Consequently, in this personal account we have outlined our efforts to obtain better performing blue OLEDs by utilizing a range of emitters based on fluorescence, phosphorescence, delayed fluorescence and exciplex systems. We have also developed some novel host materials for blue OLEDs, which are worth mentioning in this account.

Recent Developments in Tandem White Organic Light-Emitting Diodes

Tandem white organic light-emitting diodes (WOLEDs) are promising for the lighting and displays field since their current efficiency, external quantum efficiency and lifetime can be strikingly enhanced compared with single-unit devices. In this invited review, we have firstly described fundamental concepts of tandem device architectures and their use in WOLEDs. Then, we have summarized the state-of-the-art strategies to achieve high-performance tandem WOLEDs in recent years. Specifically, we have highlighted the developments in the four types of tandem WOLEDs (i.e., tandem fluorescent WOLEDs, tandem phosphorescent WOLEDs, tandem thermally activated delayed fluorescent WOLEDs, and tandem hybrid WOLEDs). Furthermore, we have introduced doping-free tandem WOLEDs. In the end, we have given an outlook for the future development of tandem WOLEDs.

Doping-Free White Organic Light-Emitting Diodes

Doping-free white organic light-emitting diodes (WOLEDs) have great potential to the next-generation solid-state lighting and displays due to the excellent properties, such as high efficiency, bright luminance, low power consumption, simplified structure and low cost. In this account, our recent developments on doping-free WOLEDs have been summarized. Firstly, fundamental concepts of doping-free WOLEDs have been described. Then, the effective strategies to develop doping-free WOLEDs have been presented. Particularly, the manipulation of charges and excitons distribution in different kinds of doping-free WOLEDs have been highlighted, including doping-free fluorescent/phosphorescent hybrid WOLEDs, doping-free thermally activated delayed fluorescent WOLEDs and doping-free phosphorescent WOLEDs. In the end, an outlook for the future development of doping-free WOLEDs have been clarified.

Interfacial TADF Exciplex as a Tool to Localize Excitons, Improve Efficiency, and Increase OLED Lifetime

In this work, we employ a thermally activated delayed fluorescence (TADF) exciplex formed between the emissive layer (EML) host, 26DCzPPy, and the electron transport layer (ETL) 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine at the interface between the EML and the ETL to improve the stability and efficiency of a phosphorescence organic light-emitting diode based on Ir(dmpq)₂acac. We show that the presence of the TADF exciplex at the EML-ETL interface induces an efficient localization of the recombination zone, which is confined within the 5 nm thick EML. Furthermore, the TADF exciplex allows harvesting of the holes and electrons that piled up at the EML-ETL interface and transfers the resultant excited state energy to the phosphorescent emitter through Förster and/or Dexter energy transfer. This approach effectively improves the LT90 of devices from <1 min to 6 h by limiting recombination processes outside of the 5 nm EML.

Strategy for the Realization of Highly Efficient Solution-Processed All-Fluorescence White OLEDs-Encapsulated Thermally Activated Delayed Fluorescent Yellow Emitters

Fabrication of highly efficient all thermally activated delayed fluorescence (TADF) white organic light-emitting diodes (WOLEDs) through solution-process still remains a big challenge. Here, two encapsulated TADF molecules with a small singlet-triplet energy gap (Δ E_ST) and high photoluminescence quantum yield (PLQY) were designed and synthesized as yellow emitters for solution-processed WOLEDs. The high current, power, and external quantum efficiencies of 41.6 cd A^-1, 30.4 lm W^-1, and 17.3% were achieved for the solution-processed all-fluorescence WOLEDs with a single-emission layer. In contrast, even with the same Δ E_ST and PLQY, the corresponding unencapsulated parent emitters will account for nearly 50% loss of the potential device efficiency. This is for the first time that the small molecular TADF blue host and TADF yellow guest are used to construct solution-processed all-fluorescence WOLEDs, which exhibit high efficiency comparable with most of the vacuum-deposited all-fluorescence white devices. These results not only demonstrate the great potential of TADF emitters in achieving highly efficient solution-processed WOLEDs, but also testify the key role of molecular encapsulation in reducing polar-exciton quenching and enhancing electroluminescence performance.

Exciplex Cohosts Employing Nonconjugated Linked Dicarbazole Donors for Highly Efficient Thermally Activated Delayed Fluorescence-Based Organic Light-Emitting Diodes

Two new nonconjugated linked dicarbazole materials, dCzPSi and dCzPSO₂, with high triplet energy were synthesized and characterized. dCzPSi and dCzPSO₂ were adopted as unipolar host materials for the green thermally activated delayed fluorescence (TADF) emitter (4CzIPN) to achieve high-efficiency organic light-emitting diodes (OLEDs). The electron-transporting acceptor, PO-T2T, was introduced to mix with dCzPSi and dCzPSO₂ to give two new exciplex-forming systems that can improve the exciton formation propensity in the emitting layer. The relevant properties of these new exciplexes were characterized, and they were suggested as promising cohosts for the green TADF emitter, 4CzIPN. The characteristics of the devices employing single hosts (dCzPSi and dCzPSO₂) and exciplex-forming cohosts (dCzPSi:PO-T2T and dCzPSO₂:PO-T2T) were explored. The obtained results indicate that the Si-bridged dicarbazole compound dCzPSi outperforms its counterpart dCzPSO₂ in which two carbazole groups are linked by an SO₂ group. The device employed with the dCzPSi:PO-T2T cohost with 10 wt % 4CzIPN achieved a low V_on of 2.2 V and maximum efficiencies of 21.1% (external quantum efficiency), 56.4 cd A^-1 (current efficiency), 59.1 lm W^-1 (power efficiency), as compared to those (18.7%, 56.6 cd A^-1, and 68.5 lm W^-1) of the dCzPSO₂:PO-T2T-hosted device. This work verifies the advantages of using a cohost that can form an exciplex for boosting the device efficiency with reduced efficiency roll-off of TADF-based OLEDs.

High-Efficiency White Organic Light-Emitting Diodes Integrating Gradient Exciplex Allocation System and Novel D-Spiro-A Materials

How to maintain high power efficiency (PE) and color stability under operating brightness is critical for the white organic light-emitting diodes (WOLEDs). To this end, two novel spiro-type materials STPy3 and STPy4 were designed. These materials could act as a single host and achieve a remarkable external quantum efficiency of 27.5% at 1000 cd m^-2; to further optimize the PEs of OLEDs, STPy3/4 and PO-T2T were used as co-host-induced exciplexes, which enhanced the PE of green OLED to over 148.0 lm W^-1. Unfortunately, the lower triplet energy level of exciplexes than blue emitters implied it is commonly unsuitable to fabricate WOLEDs. Herein, a new allocation of gradient exciplex (AGE) strategy was developed in which the formed excitons could be rationally allocated in a consequently doped nonuniform profile. The AGE incorporated the advantages of the exciplex with an ultralow turn-on voltage of 2.3 V and efficiency stability of spiro materials. The PE at 1000 cd m^-2 was enhanced to 72.7 lm W^-1, representing the first exciplex WOLED with the performance exceeding that of conventional fluorescent tubes.

Thermally Activated Delayed Fluorescence in Polymer-Small-Molecule Exciplex Blends for Solution-Processed Organic Light-Emitting Diodes

The photophysics of an exciplex state formed between a small molecule and a polymer is investigated in this work. The results obtained with this blend show the strong potential of polymer-small-molecule blends for triplet harvesting in organic light-emitting diodes (OLEDs) via thermally activated delayed fluorescence. The exciplex formed between poly( N-vinylcarbazole) (PVK) and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) shows yellow-green emission and is applied in solution-processed OLEDs. The excellent film-forming properties in this blend allow easy spin coating and potential use in other solution-processing techniques, such as slot die coating. In this work, we critically address the reverse intersystem crossing mechanism in the presented exciplex system, including the role of local triplet states. Moreover, we bring a clear physical meaning to the decay components of the exciplex emission, including the decay occurring in a power-law fashion that is often ignored in the literature.

Recent Advances of Exciplex-Based White Organic Light-Emitting Diodes

Recently, exciplexes have been actively investigated in white organic light-emitting diodes (WOLEDs), since they can be effectively functioned as (i) fluorescent or thermally activated delayed fluorescent (TADF) emitters; (ii) the hosts of fluorescent, phosphorescent and TADF dopants. By virtue of the unique advantages of exciplexes, high-performance exciplex-based WOLEDs can be achieved. In this invited review, we have firstly described fundamental concepts of exciplexes and their use in organic light-emitting diodes (OLEDs). Then, we have concluded the primary strategies to develop exciplex-based WOLEDs. Specifically, we have emphasized the representative WOLEDs using exciplex emitters or hosts. In the end, we have given an outlook for the future development of exciplex-based WOLEDs.

Probe exciplex structure of highly efficient thermally activated delayed fluorescence organic light emitting diodes

The lack of structural information impeded the access of efficient luminescence for the exciplex type thermally activated delayed fluorescence (TADF). We report here the pump-probe Step-Scan Fourier transform infrared spectra of exciplex composed of a carbazole-based electron donor (CN-Cz2) and 1,3,5-triazine-based electron acceptor (PO-T2T) codeposited as the solid film that gives intermolecular charge transfer (CT), TADF, and record-high exciplex type cyan organic light emitting diodes (external quantum efficiency: 16%). The transient infrared spectral assignment to the CT state is unambiguous due to its distinction from the local excited state of either the donor or the acceptor chromophore. Importantly, a broad absorption band centered at ~2060 cm^-1 was observed and assigned to a polaron-pair absorption. Time-resolved kinetics lead us to conclude that CT excited states relax to a ground-state intermediate with a time constant of ~3 µs, followed by a structural relaxation to the original CN-Cz2:PO-T2T configuration within ~14 µs.

Exciplex: An Intermolecular Charge-Transfer Approach for TADF

Organic materials that display thermally activated delayed fluorescence (TADF) are a striking class of functional materials that have witnessed a booming progress in recent years. In addition to pure TADF emitters achieved by the subtle manipulations of intramolecular charge transfer processes with sophisticated molecular structures, a new class of efficient TADF-based OLEDs with emitting layer formed by blending electron donor and acceptor molecules that involve intermolecular charge transfer have also been fabricated. In contrast to pure TADF materials, the exciplex-based systems can realize small Δ E_ST (0-0.05 eV) much more easily since the electron and hole are positioned on two different molecules, thereby giving small exchange energy. Consequently, exciplex-based OLEDs have the prospective to maximize the TADF contribution and achieve theoretical 100% internal quantum efficiency. Therefore, the challenging issue of achieving small Δ E_ST in organic systems could be solved. In this article, we summarize and discuss the latest and most significant developments regarding these rapidly evolving functional materials, wherein the majority of the reported exciplex forming systems are categorized into two subgroups, viz. (a) exciplex as TADF emitters and (b) those as hosts for fluorescent, phosphorescent and TADF dopants according to their structural features and applications. The working mechanisms of the direct electroluminescence from the donor/acceptor interface and the exciplex-forming systems as cohost for the realization of high efficiency OLEDs are reviewed and discussed. This article delivers a summary of the current progresses and achievements of exciplex-based researches and points out the future challenges to trigger more research endeavors to this growing field.

Cyanopyrimidine-Carbazole Hybrid Host Materials for High-Efficiency and Low-Efficiency Roll-Off TADF OLEDs

Two isomeric host materials (Sy and Asy) comprising carbazole (donor) and CN-substituted pyrimidine (acceptor) were synthesized, characterized, and utilized as host materials for green and blue thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). Both molecules have high triplet energy and small energy difference between singlet and triplet states, leading to feasible TADF. The different linking topologies of carbazole and CN groups on the pyrimidine core provide distinct photophysical properties and molecular packing manners, which further influence the efficiency as they served as hosts in TADF OLEDs. As compared to Asy-based cases, the Sy-hosted TADF OLED device gave higher maximum external quantum efficiencies (EQE) of 24.0% (vs 22.5%) for green (4CzIPN as a dopant) and 20.4% (vs 15.0%) for blue (2CzTPN as a dopant) and low efficiency roll-off. The high horizontal dipole ratio (Θ ≈ 88%) for both emitters dispersed in Sy and Asy hosts accounts for the high device efficiency. A clear molecular structure-physical property-device performance relationship has been established to highlight the importance of symmetrical structure in TADF host material design.

Electronic Level Alignment at an Indium Tin Oxide/PbI2 Interface and Its Applications for Organic Electronic Devices

The electronic level alignment at the indium tin oxide (ITO)/PbI₂ interface is investigated by an ultraviolet photoelectron spectroscopy. An n-type conductivity property is found for PbI₂ as well as a downward shift energy level at the ITO/PbI₂ interface. These indicate that PbI₂ can be used as an anode buffer layer for organic electronic devices. The power conversion efficiency of the organic solar cell based on tetraphenyldibenzoperiflanthene/C₇₀ planar heterojunction is dramatically increased from 1.05 to 3.82%. Meanwhile, the thermally activated delayed fluorescence organic light-emitting diode based on 4,4',4″-tri( N-carbazolyl)triphenylamine-((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) shows a significantly reduced turn-on voltage and enhanced power efficiency from 6.26 to 18.60 lm/W. The improved performance is attributed to the high hole injection/extraction efficiency at the ITO/PbI₂ interface. Besides, the near-infrared (NIR) absorption of lead phthalocyanine (PbPc)-based NIR organic photodetector (NIR-OPD) is dramatically increased, indicating that the PbI₂ layer can also be used as a template layer for the growth of the triclinic phase of PbPc. As a result, the optimized device shows an external quantum efficiency of 26.7% and a detectivity of 9.96 × 10¹¹ jones at 900 nm, which are among the highest ones reported for organic NIR-OPDs.

Nitrogen-Free Bifunctional Bianthryl Leads to Stable White-Light Emission in Bilayer and Multilayer OLED Devices

White organic light-emitting diodes (WOLEDs) are at the center stage of OLED research today because of their advantages in replacing the high energy-consuming lighting technologies in vogue for a long time. New materials that emit white light in simple devices are much sought after. We have developed two novel electroluminescent materials, referred to as BABZF and BATOMe, based on a twisted bianthryl core, which are brilliantly fluorescent, thermally highly stable with high T _d and T _g, and exhibit reversible redox property. Although inherently blue emissive, BABZF leads to white-light emission (CIE ≈ 0.28, 0.33) with a moderate power efficiency of 2.24 lm/W and a very high luminance of 15 600 cd/m² in the fabricated multilayer nondoped OLED device. This device exhibited excellent color stability over a range of applied potential. Remarkably, similar white-light emission was captured even from a double-layer device, attesting to the innate hole-transporting ability of BABZF despite it being non-nitrogenous, that is, lacking any traditional hole-transporting di-/triarylamino group(s). Similar studies with BATOMe led to inferior device performance results, thereby underscoring the importance of dibenzofuryl groups in BABZF. Experimental as well as theoretical studies suggest the possibility of emission from multiple species involving BABZF and its exciplex and electroplex in the devices. The serendipitously observed white-light emission from a double-layer device fabricated with an unconventional hole-transporting material (HTM) opens up new avenues to create new non-nitrogenous HTMs that may lead to more efficient white-light emission in simple double-layer devices.

X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems

Customizable and technology-friendly functional materials are one of the mainstays of emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiation-generated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) monosubstituted derivatives readily form recombination exciplexes with N,N-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas-phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together, the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes.

Low efficiency roll-off and high color stability pure fluorescent white organic light-emitting diode based exciplex host

Pure fluorescent WOLEDs with low efficiency roll-off and high color stability were realized by employing an exciplex host. Due to incomplete energy transfer from the blue exciplex host of mCP : PO-T2T to the orange fluorescent dopant of rubrene, the WOLEDs showed a maximum current efficiency, power efficiency and EQE of 14.2 cd A-1, 12.8 lm W-1 and 4.90%, respectively. To our surprise, a rather small roll-off ratio of 8.2% from the maximum EQE to the EQE at 1000 cd m-2 and stable white light-emitting spectra with CIE coordinates of (0.384 ± 0.001, 0.439 ± 0.002) from 4 V to 8 V were obtained simultaneously. The bipolarity and triplet exciton up-conversion of the exciplex host played key roles in delivering excellent performance. More detailed discussions are also provided.

Strategies to Achieve High-Performance White Organic Light-Emitting Diodes

As one of the most promising technologies for next-generation lighting and displays, white organic light-emitting diodes (WOLEDs) have received enormous worldwide interest due to their outstanding properties, including high efficiency, bright luminance, wide viewing angle, fast switching, lower power consumption, ultralight and ultrathin characteristics, and flexibility. In this invited review, the main parameters which are used to characterize the performance of WOLEDs are introduced. Subsequently, the state-of-the-art strategies to achieve high-performance WOLEDs in recent years are summarized. Specifically, the manipulation of charges and excitons distribution in the four types of WOLEDs (fluorescent WOLEDs, phosphorescent WOLEDs, thermally activated delayed fluorescent WOLEDs, and fluorescent/phosphorescent hybrid WOLEDs) are comprehensively highlighted. Moreover, doping-free WOLEDs are described. Finally, issues and ways to further enhance the performance of WOLEDs are briefly clarified.

An optical and electrical study of full thermally activated delayed fluorescent white organic light-emitting diodes

We report on the engineering of full thermally activated delayed fluorescence – based white organic light emitting diodes (W-OLEDs) composed of three emitters (2,7-bis(9,9-dimethyl-acridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DDMA-TXO2), 2,7-bis(phenoxazin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DPO-TXO2) and 3,11-di(10H-phenoxazin-10-yl)dibenzo[a,j]phenazine (POZ-DBPHZ) in two different hosts. By controlling the device design through the study of the emission of DDMA-TXO2 and DPO-TXO2, the behaviour of POZ-DBPHZ in a device with more than one emitter, and the combination of the three materials, respectively, we show that external quantum efficiencies as high as 16% can be obtained for a structure with a correlated colour temperature close to warm white, together with colour rendering index close to 80. However it is in their performance stability that provides the true breakthrough: at 1000 cd/m² the efficiencies were still above 10%, which is one of the best for this type of devices.

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low-cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a focus on linking their optoelectronic behavior with the performance of the organic light-emitting diode (OLED) and related EL devices. TADF emitters are cross-compared within specific color ranges, with a focus on blue, green-yellow, orange-red, and white OLEDs. Organic small-molecule, dendrimer, polymer, and exciplex emitters are all discussed within this review, as is their use as host materials. Correlations are provided between the structure of the TADF materials and their optoelectronic properties. The success of TADF materials has ushered in the next generation of OLEDs.