Exciplex Type Research Articles

P‐206: Key factor of Device Lifetime in Exciplex Host Based Blue Phosphorescent Organic Light Emitting Diodes

In this study, we investigated the key factor of device lifetime in blue phosphorescent organic light emitting diodes (PhOLEDs) by managing hosts and dopants in the emitting layer. We manufactured four phosphorescent devices using the exciplex type mixed host and single bipolar host doped with either Pt type and Ir type phosphors. The mixed host was better than single host in the Pt emitter doped device, while it was similar to the single host in the Ir emitter doped device. Investigation of the light‐emission mechanism of the PhOLEDs revealed that the mixed host is essential in the device showing strong hole trapping behavior. The strong p‐type host in the mixed host relieves the hole trapping by the Pt phosphor, resulting in less polaron formation and polaron accumulation. As a result, the mixed host reduces triplet‐polaron annihilation and extends the device lifetime of the Pt emitter doped PhOLEDs.

Creation of Dual Thermally Activated Delayed-Fluorescence Exciplexes in a Bulk Emitting Layer and Its Interface with an Electron Transport Layer for Promoting the Performance of Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes Fabricated by a Solution Process.

An exciplex, which is composed of electron donor and acceptor molecules and formed by intermolecular charge transfer, is an excited-state species that is able to emit light or transfer its energy to a lower-energy emitter. In reported exciplex-based organic light-emitting diodes (OLEDs), their working mechanism is to generate exciplexes either in the bulk emitting layer (bulk exciplex) or at its interface with an electron transport layer (interface exciplex); both types give promising device performance. Here, we propose a novel strategy of creating both types of exciplexes simultaneously (dual exciplexes) for the generation of more exciplexes for better device performance as indicated in the improved photoluminescence quantum yield (PLQY). Impressively, the dual exciplex-based device with blue thermally activated delayed fluorescence (TADF) emitter 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) exhibits a record-high maximum external quantum efficiency (EQEmax) of 26.7% among the solution-processed TADF blue OLEDs. By further doping with the red-emitting phosphor emitter into the EML, the white device also gives a record-high EQEmax of 24.1% among the solution-processed TADF-phosphor hybrid white OLEDs (T-P WOLEDs) with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.34, 0.42), color rendering index of 70, and correlated color temperature of 5198 K. Furthermore, both blue and white devices show an ultralow efficiency roll-off with external quantum efficiencies at a practical brightness value of 1000 cd m-2 (EQE1000) of 25.1 and 23.9%, respectively. This is the first report of employing a dual exciplex-based OLED with excellent device performance.

The Effect of Acceptor Structure on Emission Color Tuning in Organic Semiconductors with D-π-A-π-D Structures.

A series of novel donor–acceptor D–π–A–π–D compounds were synthesized and characterized in order to determine the influence of different acceptor units on their properties. The introduction of acceptor moieties had a direct impact on the HOMO and LUMO energy levels. Fluorescence spectra of compounds can be changed by the choice of an appropriate acceptor and were shifted from the green to the near-infrared part of spectra. Due to observed concentration induced emission quenching, the green exciplex type host was used to evaluate the potential of synthesized molecules as emitters in organic light emitting diodes (OLEDs).

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.

Investigating the evolution of exciplex states in thermally activated delayed fluorescence organic light-emitting diodes by transient measurement

Exciplex states play a crucial role in governing the ultimate device performance, as both current density-voltage-luminescence characteristics and external quantum efficiency directly depend on it. For electron-hole evolution, exciplex states represent the intermediate but essential step between exciton recombination and dissociation. Therefore, the fundamental understanding of the processes involving the exciplex state is essential for utilizing such mediate state to achieve high emission efficiencies approaching the theoretical limits with simple structured organic light-emitting diodes (OLEDs). Herein, we systemically investigate the evolution of exciplex states using transient photoluminescence measurements compiling with transient electroluminescence measurements on high-performance exciplex type thermally activated delayed fluorescence (TADF) OLEDs. The dynamic properties of the long-lived fluorescent exciplex states upon both photo-excitation and electro-excitation offer in-depth insight to the fundamental mechanisms driving its decay pathways, and by studying the electric-field dependent motion of exciplex, we can explore its transport and emission evolution.

Development of an exciplex type mixed host using a pyrrolocarbazole type material for extended lifetime in green phosphorescent organic light-emitting diodes

Abstract Lifetime improvement of green phosphorescent organic light-emitting didoes (PHOLEDs) by an exciplex type host was studied by mixing a hole transport type host and an electron transport type host. A pyrrolocarbazole type material was developed as the hole transport type host and a triazine type material was the electron transport type host. The exciplex type mixed host showed much longer lifetime and improved efficiency compared with each host material constituting the exciplex type mixed host. Hole and electron stability of the exciplex host was proposed as the key factor for the long lifetime of the green phosphorescent device.

White Light Emission in Butadiyne Bridged Pyrene-Phenyl Hybrid Fluorophore: Understanding the Photophysical Importance of Diyne Spacer and Utilizing the Excited-State Photophysics for Vapor Detection.

Generation of white light emission (WLE) from a single organic fluorophore is challenging because of the need to get fluorescence covering the visible region (400-700 nm) upon excitation of the dye at near-ultraviolet wavelength. Herein, we report WLE from a butadiyne bridged pyrene-phenyl hybrid fluorophore in mixed-aqueous solvents as well as in polymer film matrices. The ability of the butadiynyl dye to emit from multiple excited states such as locally excited (LE; 400-500 nm), aggregate (excimer type; 475-600 nm), and charge transfer (CT; 500-750 nm) states spanning the emission almost throughout the visible range has made the generation of the white light to be possible. In highly polar solvent such as acetonitrile, the butadiynyl dye emits from the LE and CT states, and the WLE is achieved through a control of the dye concentration such that intermolecular CT (exciplex type) contributes along with the intramolecular CT and LE emissions. In mixed-aqueous systems such as water-acetonitrile and water-N,N-dimethylformamide, the CT emission is red-shifted (because of the high dielctric constant of water), and the contribution of the aggregate emission (originated because of the poor solvent water) is important in maintaining the relative distribution of the fluorescence intensities (LE, excimer, and CT) in the entire visible region. The significance of the diyne spacer in achieving the WLE is delineated through a control study with a single acetylenic analogue. The LE, aggregate, and CT emissions are involved in generating bluish-white light in a poly(vinyl alcohol) film matrix of the butadiynyl dye. Blue emission is noted in a poly(methyl methacrylate) (PMMA) film matrix of the dye with a major contribution from the LE and a minor contribution from the aggregate state. Exposure of the PMMA film of the dye to polar aprotic vapors assists in gaining the CT state emission such that the LE, aggregate, CT emissions cover the entire visible region to produce the WLE. This opens a new strategy for selective vapor sensing.

Photophysics of Diphenylbutadiynes in Water, Acetonitrile-Water, and Acetonitrile Solvent Systems: Application to Single Component White Light Emission.

Diacetylenes have been the subject of current research because of their interesting optoelectronic properties. Herein, we report that substituted diphenylbutadiynes exhibit locally excited (LE) and excimer emissions in water and multiple emissions from the LE, excimer, and intramolecular charge transfer (ICT) states in acetonitrile-water solvent systems. The LE, excimer, and ICT emissions are clearly distinguishable for a diphenylbutadiynyl derivative with push (-NMe2)-pull (-CN) substituents and those are closely overlapped for non-push-pull analogues. In neat acetonitrile, the excimer emission disappears and the LE and ICT emissions predominate. In the case of the push (-NMe2)-pull (-CN) diphenylbutadiyne, the intensity of the ICT emission increases with increasing the fluorophore concentration. This suggests that the ICT emission accompanies with intermolecular CT emission which is of exciplex type. As the LE and exciplex emissions of the push-pull diphenylbutadiyne together cover the visible region (400-700 nm) in acetonitrile, a control of the fluorophore concentration makes the relative intensities of the LE and exciplex emissions such that pure white light emission is achieved. The white light emission is not observed in those diphenylbutadiynyl analogues in which the peripheral substituents of the phenyl rings do not possess strong push-pull character.

Highly efficient exciplex organic light-emitting diodes using thermally activated delayed fluorescent emitters as donor and acceptor materials

Highly efficient exciplex type organic light-emitting diodes were developed using thermally activated delayed fluorescent emitters as donors and acceptors of an exciplex. Blue emitting bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) was a donor and 9,9′-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)-1,3-phenylene)bis(9H-carbazole) (DDCzTrz) and 9,9′,9″-(5-(4,6-diphenyl-1,3,5-triazin-2-yl)benzene-1,2,3-triyl)tris(9H-carbazole) (TCzTrz) were acceptor materials. The exciplexes of DMAC-DPS:TCzTrz and DMAC-DPS:DDCzTrz resulted in high photoluminescence quantum yield and high quantum efficiency in the green exciplex organic light-emitting diodes. High quantum efficiencies of 13.4% and 15.3% were obtained in the DMAC-DPS:DDCzTrz and DMAC-DPS:TCzTrz exciplex devices.

Bimodal Exciplex Formation in Bimolecular Photoinduced Electron Transfer Revealed by Ultrafast Time-Resolved Infrared Absorption.

The dynamics of a moderately exergonic photoinduced charge separation has been investigated by ultrafast time-resolved infrared absorption with the dimethylanthracene/phthalonitrile donor/acceptor pair in solvents covering a broad range of polarity. A distinct spectral signature of an exciplex could be identified in the -C≡N stretching region. On the basis of quantum chemistry calculations, the 4-5 times larger width of this band compared to those of the ions and of the locally excited donor bands is explained by a dynamic distribution of exciplex geometry with different mutual orientations and distances of the constituents and, thus, with varying charge-transfer character. Although spectrally similar, two types of exciplexes could be distinguished by their dynamics: short-lived, "tight", exciplexes generated upon static quenching and longer-lived, "loose", exciplexes formed upon dynamic quenching in parallel with ion pairs. Tight exciplexes were observed in all solvents, except in the least polar diethyl ether where quenching is slower than diffusion. The product distribution of the dynamic quenching depends strongly on the solvent polarity: whereas no significant loose exciplex population could be detected in acetonitrile, both exciplex and ion pair are generated in less polar solvents, with the relative population of exciplex increasing with decreasing solvent polarity. These results are compared with those reported previously with donor/acceptor pairs in different driving force regimes to obtain a comprehensive picture of the role of the exciplexes in bimolecular photoinduced charge separation.

Essential electro-optical differences of exciplex type OLEDs based on a starburst carbazole derivative prepared by layer-by-layer and codeposition processes

The star-shaped compound tri(9-hexylcarbazol-3-yl) amine (THCA) and 4,7-diphenyl-1,10-phenanthroline (Bphen) were used for the preparation of bilayer and bulk exciplex based organic light emitting diodes (OLEDs) in which THCA and Bphen were selected as donor and acceptor components, respectively. The dependencies of the electroluminescence and impedance characteristics of the exciplex type OLEDs depending on the technologies of the formation of the active layers were studied. The correlations between the appearance of negative differential resistance in current–voltage characteristics and negative capacitance observed at low frequencies in impedance spectra of OLEDs with the resonant tunneling of electrons into THCA through Bphen/THCA barrier and generation–recombination processes in THCA were established.

Light emission mechanism of mixed host organic light-emitting diodes

Light emission mechanism of organic light-emitting diodes with a mixed host emitting layer was studied using an exciplex type mixed host and an exciplex free mixed host. Monitoring of the current density and luminance of the two type mixed host devices revealed that the light emission process of the exciplex type mixed host was dominated by energy transfer, while the light emission of the exciplex free mixed host was controlled by charge trapping. Mixed host composition was also critical to the light emission mechanism, and the contribution of the energy transfer process was maximized at 50:50 mixed host composition. Therefore, it was possible to manage the light emission process of the mixed host devices by managing the mixed host composition.

Further Studies on Relaxed and Un-Relaxed Exciplexes in Pyrene-N,NDimethylaniline System in Benzene-Acetonitrile Binary Solvents

Magnetic field effect (MFE) on pyrene-N,N-dimethylaniline (Py-DMA) exciplex system in benzene-acetonitrile (Bz-ACN) mixtures of varying bulk permittivity (e) has been studied. The MFE has been wavelength resolved over the emission band of the exciplex along with the measurement of lifetime. The data establishes the concept of formation of two types of exciplexes (Relaxed and Un-relaxed) at higher bulk value of permittivity. At lower value of permittivity when solvent reorganization energy is not so prominent effectively only a single type of exciplex (Relaxed) is formed. The data also indicates that the benzene π cloud favors formation of relaxed exciplex.

Study of magnetic field effect on Py-DMA exciplex luminescence in THF–DMF binary solvents: Evidence of multiple exciplex formation at higher bulk dielectric constant

Magnetic field effects (MFE) on pyrene-DMA (N,N-dimethylaniline) exciplex system in THF–DMF binary solvents of varying bulk dielectric constant ( ε) has been studied. The MFE has been wavelength resolved over the emission band of the exciplex. The data indicates formation of two types of exciplexes at higher bulk value of ε. At lower value of ε ( ε < 9) essentially there is a single type of exciplex which is supported by the wavelength resolved MFE and life-time ( τ) measurements. However, at higher value of ε ( ε > 15) when the solvent reorganization energy is more prominent; the wavelength resolved MFE and life-time studies show evidence of two types of exciplexes.

Temperature-Dependent Magnetic Field Effect Study on Exciplex Luminescence: Probing the Triton X-100 Reverse Micelle in Cyclohexane

The microenvironment within the reverse micelle of the nonionic surfactant Triton X-100 (TX-100) in cyclohexane has been investigated by studying the magnetic field effect (MFE) on pyrene-dimethylaniline exciplex luminescence. The nature of exciplex fluorescence and its behavior in the presence of a magnetic field have been found to vary significantly with the water content of the medium. Results are discussed in light of multiple exciplex formation within the micelle which is further supported by the fluorescence lifetime measurements. Those exciplexes emitting at longer wavelength are found to be magnetic field sensitive while those emitting toward the blue region of the spectrum are insensitive toward magnetic field. Since the exciplex's emission characteristics and magnetic field sensitivity depend on its immediate surrounding, it has been concluded that the environment within the micelle is nonuniform. With an increase in hydration level, different zones of varying polarity are created within the reverse micelle. It has been pointed out that the magnetic field sensitive components reside inside the polar core of the micelle while those located near the hydrocarbon tail are field insensitive. However it has been presumed that an interconversion between the different types of exciplexes is possible. The environment within the reverse micelle is found to be largely affected by the change in temperature, and this is reflected in the exciplex emission property and the extent of magnetic field effect. Interestingly, the variation of MFE with temperature follows different trends in the dry and the wet reverse micelle. A comparison has been drawn with the reverse micelle of the ionic surfactant to get an insight into the difference between the various types of micellar environment.

Magnetic field effect on pyrene–DMA exciplex luminescence in non-aqueous AOT reverse micelle

Magnetic field effect (MFE) on pyrene–DMA exciplex luminescence and exciplex lifetime have been studied in reverse micelle of Aerosol-OT in n-heptane in presence of N, N-dimethylformamide and methanol as the polar solvent at different W s ( W s = [polar solvent]/[surfactant]) values. It is found that two types of exciplexes are residing at different locations of the non-aqueous reverse micelle. Out of these only the exciplex emitting at longer wavelength is found to be field sensitive. The higher MFE and shorter lifetime of the exciplex in non-aqueous reverse micelle compared to that of aqueous micelle have been explained on the basis of presence of hydrogen bonding and dipolar interaction between the solvent and the polar head group of the micelle.

Magnetic field effect on exciplex luminescence: A study of multiple exciplex formation dynamics in biomimicking environment

The interfacial environment of AOT based reverse micelle (RM), which has similarity with bio-membranes, has been probed by measuring: (1) magnetic field effect (MFE) on Pyrene–DMA exciplex luminescence; (2) wavelength-dependence of the exciplex lifetime. At least two different types of exciplexes have been identified at different locations within the RM interface. Among these, only the red edge component of the emission centered at ∼500 nm (Ex 500) is field-sensitive. It makes sense to presume that the Ex 500 is localized at the mobile zone of the interface while the other field-insensitive one, centered at ∼430 nm (Ex 430) resides at the immobile zone of the interface. Time resolutions of luminescence at different wavelengths indicate that the field-sensitive exciplex is formed at the expense of Ex 430.

Optical Properties of Gold Atoms in γ-Irradiated Organic Solid Solutions at 77 K

Au0 atoms were produced via the electron capture of Au+ ions in γ-irradiated solid solutions of MTHF at 77 K. The optical absorption bands observed in the regions of UV and NUV are ascribed to Au0. The steady-state emission and excitation spectra were measured by photoexcitation at 73 K before and after γ-irradiation of the solid solutions containing [Au+] ions. The two-dimensional time-resolved emission spectra were also investigated by photoexcitation at λexc = 337, 380, and 420 nm by using an N2 laser and dye lasers. A set of five emission bands of 385, 430, 480, 520, and 580 nm and another set of emission bands at 456, 482, 484, and 520 nm were observed. The latter set of emission bands showed such characteristic structures as the mirror image of the absorption bands of Au0 in the rare gas matrixes. The emission band at 430 nm observed before γ-irradiation was confirmed to be phosphorescence due to Au+ with a lifetime of about 625 ms. A part of the emission band of 385 nm observed after γ-irradiation was also due to phosphorescence consisting of two components with lifetimes of 146 and 461 ms. Three types of exciplexes (Au+·Ln)*, (Au0·Ln...Au+)*, (Au0·Ln)*, and the excited gold atom (Au0)* are proposed for all the emission bands including phosphorescence. The excited gold atoms in 2P1/2 and 2P3/2 states participate mainly in the formation of these emissive exciplexes. The formation mechanism for these species is also presented.

ESR and Optical Properties of Copper Atoms in γ-Irradiated Organic Solutions at 77 K

EtOH and MTHF solutions containing Cu+ ions are γ-irradiated at 77 K. ESR spectra are attributed, for the first time, to Cu0 that is produced radiation-chemically by reducing Cu+ ions in the solutions at 77 K. A large delocalization of the unpaired spin density of Cu0 onto ligands is deduced. No evidence for formation of the dimer cation of Cu0 has been obtained. This is contrary to the case of Ag0 produced in the solutions containing Ag+ ions. The NUV absorption spectrum of Cu0 is decomposed into five Gaussian bands that are attributed to two types of exciplexes formed between excited Cu0 and ligands: one associates with Cu+ ions as well as solvent molecules, the other consists mainly of solvent molecules. The exciplexes are supposed to be transferred to relaxed states followed by two emission bands, one of which is common to both solvents and the other is characteristic of the solvents. The absorption band at 255 nm is ascribed to a photoionization spectrum of Cu0.

Spectroscopy and Dynamics of Benzonitrile‐Enzene Complexes in A Supersonic Free‐Jet

Excitation and fluorescence spectra of benzonitrile‐benzene and toluene complexes seeded in helium and expanded in a supersonic free jet reveals two types of transition attributed to the coexistence of two isomeric forms. One consists of narrow bands in both excitation and emission and is characteristic of wealdy perturbed van tier Waals complexes. The other involves broadened absorption and gives rise to an exciplex type emission.