The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N′-dicarbazolyl-1,1′-biphenyl

Organic Light-Emitting Diodes with Electrospun Electrodes for Double-Side Emissions

Transparent conductive electrodes (TCE) obtained by the electrospinning method and gold covered were used as cathodes in the organic light-emitting diodes (OLEDs) to create double side-emission. The electro-active nanofibers of poly(methyl methacrylate) (PMMA) with diameters in the range of several hundreds of nanometers, were prepared through the electrospinning method. The nanofibers were coated with gold by sputtering deposition, maintaining optimal transparency and conductivity to increase the electroluminescence on both electrodes. Optical, structural, and electrical measurements of the as-prepared transparent electrodes have shown good transparency and higher electrical conductivity. In this study, two types of OLEDs consisting of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT-PSS)/ Ir(III) complex (8-hydroxyquinolinat bis(2-phenylpyridyl) iridium-IrQ(ppy)₂ 20 wt% embedded in N, N'-Dicarbazolyl-4,4'-biphenyl (CBP) sandwich structure and either gold-covered PMMA electrospun nanoweb (OLED with electrospun cathode) were fabricated together with a similar structure containing thin film gold cathodes (OLED with thin film cathode). The luminance-current-voltage characteristics, the capacitance-voltage, and the electroluminescence properties of these OLEDs were investigated.

Numerical Study of Triplet Dynamics in Organic Semiconductors Aimed for the Active Utilization of Triplets by TADF under Continuous-Wave Lasing

The limitation of lasing duration less than nanosecond order has been a major problem for realizing organic solid-state continues-wave (CW) lasers and organic semiconductor laser diodes. Triplets accumulation under CW excitation has been well recognized as a critical inhibiting factor. To overcome this issue, the utilization of thermally activated delayed fluorescence (TADF) emitters is a promising mechanism because of efficient reverse intersystem crossing. Herein, we model the triplet accumulation processes under lasing and propose the active utilization of TADF for lasing based on our simulation analysis. We used the rate constants experimentally determined from the optical properties of a boron difluoride curcuminoid fluorophore showing both TADF and lasing. We demonstrate that the intersystem crossing efficiency is gradually increased after the convergence of relaxation oscillation, i.e., terminating laser oscillation. In addition, we found that when the reverse intersystem crossing rate is close to the intersystem crossing rate, CW lasing becomes dominant.

Unraveling exciton processes in Ir(ppy)3:CBP OLED films upon photoexcitation

Emissive layers in phosphorescent organic light-emitting diodes commonly make use of guest-host blends such as Ir(ppy)₃:CBP to achieve high external quantum efficiencies. However, while the Ir(ppy)₃:CBP blend has been studied experimentally, crucial questions remain regarding how exciton diffusion is dependent on the distribution of the guest in the host, which can currently only be addressed at the atomic level via computational modeling. In this work, kinetic Monte Carlo simulations are utilized to gain insight into exciton diffusion in Ir(ppy)₃:CBP blend films. The effects of both guest concentration and exciton density on various system properties are analyzed, including the probability of singlet excitons being converted to triplets, and the probability of those triplets decaying radiatively. Significantly, these simulations suggest that triplet diffusion occurs almost exclusively via guest-guest Dexter transfer and that concentration quenching of triplets induced by guest-guest intermolecular dipole-dipole interactions has a negligible effect at high exciton densities due to the prevalence of triplet-triplet annihilation. Furthermore, results for vacuum deposited morphologies derived from molecular dynamics simulations are compared to the results obtained using a simple cubic lattice approximation with randomly distributed guest molecules. We show that while differences in host-based processes such as singlet diffusion are observed, overall, the results on the fate of the excitons are in good agreement for the two morphology types, particularly for guest-based processes at low guest concentrations where guest clustering is limited.

tert-Butyl-substituted bicarbazole as a bipolar host material for efficient green and yellow PhOLEDs

We designed a novel bipolar host material with a high E_T and thermal stability for multi-colour PhOLEDs.

Revising of the Purcell effect in periodic metal-dielectric structures: the role of absorption

Periodic metal-dielectric structures attract substantial interest since it was previously proposed that the spontaneous emission amplification rates (the Purcell factor) in such structures can reach enormous values up to 10⁵. However, the role of absorption in real metals has not been thoroughly considered. We provide a theoretical analysis showing that absorption leads to diminishing values of Purcell factor. We also suggest that using emitting organic compounds such as CBP (4,4-Bis(N-carbazolyl)-1,1-biphenyl) can lead to a moderate increase of about an order of magnitude in the Purcell factor. Defining the experimentally measured Purcell factor as a ratio between the excited state lifetimes in bare CBP and in periodic structure, this increase in the fabricated periodic structure is demonstrated through a 4–8 times decrease in excited state radiative lifetime compared to a bare organic material in a wide emission spectrum.

Root Causes of the Limited Electroluminescence Stability of Organic Light-Emitting Devices Made by Solution-Coating

Although organic electroluminescent materials have long promised the prospect of making organic light emitting devices (OLEDs) via low-cost solution-coating techniques, the electroluminescence stability of devices made by such techniques continues to be rather limited making them unsuitable for commercialization. The root causes of the lower stability of OLEDs made by solution-coating versus the more conventional vacuum-deposition remain unknown. In this work, we investigate and compare between solution-coated and vacuum-deposited materials under prolonged excitation, using the archetypical host material 4,4'-bis( N-carbazolyl)-1,1'-biphenyl as a model OLED material. Results show that solution-coated films are more susceptible to degradation by excitons in comparison to their vacuum-deposited counterparts, resulting in a faster decrease in their luminescent quantum yield. The degradation rate also depends on the choice of solvent that was used in the solution-coating process. Results also show that the decrease in quantum yield is caused by exciton-induced chemical decomposition in the material as well as some possible molecular reorganization or aggregation, both of which are induced by excitons and proceed more quickly in case of solution-coated films. The faster degradation in the solution-coated films appears to originate primarily from their different morphological makeup and not due to chemical impurities. The findings uncover what appears to be one of the fundamental root causes of the lower stability of solution-coated OLEDs in general.

Increased Electromer Formation and Charge Trapping in Solution-Processed versus Vacuum-Deposited Small Molecule Host Materials of Organic Light-Emitting Devices

We investigate and compare between organic light-emitting devices (OLEDs) fabricated by solution-coating versus vacuum-deposition. Electroluminescence, photoluminescence, and chromatographic measurements on typical OLED host materials reveal significant electromer formation in layers fabricated by solution-processing, pointing to stronger intermolecular interactions in these systems. Delayed electroluminescence measurements reveal that solution-processed layers also have increased charge traps. The findings provide insights on the morphological differences between solution-processed and vacuum-deposited materials and shed light on the root causes behind the lower electroluminescence stability of solution-processed OLEDs.

Solution-state photophysics of N-carbazolyl benzoate esters: dual emission and order of states in twisted push–pull chromophores

The stepwise photoinduced charge transfer in a series ofN-carbazolyl benzoate ester push–pull chromophores has been studied in solution.

Chemical degradation in organic light-emitting devices: mechanisms and implications for the design of new materials

Degradation of the materials in organic light-emitting devices (OLEDs) is the major impediment for the development of economically feasible, highly efficient and durable devices for commercial applications. Even though this chemical degradation is complex and the least understood of the different degradation modes in OLEDs, scientists were successful in providing insight into some of the responsible processes. In this progress report we will review recent advances in the elucidation of chemical degradation mechanisms: First possible reasons for defect formation and the most common and important methods to investigate those processes are covered before discussing the reactions and their products for the different types of materials present in a device. We summarize commonalities in the occurring mechanisms, and identify structural features and moieties that can be detrimental to operational stability. Some of the resulting implications on the development of new materials are presented and backed by concrete examples from literature.

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

A potential major drawback with organic light-emitting devices, (OLEDs) is the limit of 25% singlet exciton production through spin-dependent charge recombination. Recent device results, however, show that this limit does not hold and far higher efficiencies can be achieved in purely fluorescent-based systems (Wohlgenannt et al. (2001), Dhoot et al. (2002), Lin et al. (2003), Wilson et al. (2001), Cao et al. (1999), Baldo et al. (1999), and Kim et al. (2000)). Thus, the question arises; is recombination spin dependent (Tandon et al. (2003)) or are singlet excitons generated in secondary processes? Direct measurement of the singlet generation rate in working devices of 44% has been shown (Rothe et al. (2006)), which have been verified as being part due to direct singlets formed on recombination and part from triplet fusion, singlets produced during triplet annihilation (Kondakov et al. (2009), King et al. (2011), and Zhang and Forrest (2012)). Here, the various routes by which triplet excitons can generate singlet states are discussed and their relative contributions to the overall electroluminescence yield are given. The materials requirements to obtain maximum singlet production from triplet states are discussed. These triplet contributions can give very high device yields for fluorescent emitters, which in the case of blue devices can be highly advantageous. Further, new devices architectures open up which are simple and have intrinsically low turn on voltages, ideal for large-area OLED lighting applications.

Bipolar molecules with high triplet energies: synthesis, photophysical, and structural properties

This article sheds new light on the interplay of electronic and conformational effects in luminescent bipolar molecules. A series of carbazole/1,3,4-oxadiazole hybrid molecules is described in which the optoelectronic properties are systematically varied by substituent effects which tune the intramolecular torsion angles. The synthesis, photophysical properties, cyclic voltammetric data, X-ray crystal structures, and DFT calculations are presented. Excited state intramolecular charge transfer (ICT) is observed from the donor carbazole/2,7-dimethoxycarbazole to the acceptor phenyl/diphenyloxadiazole moieties. Introducing more bulky substituents onto the diphenyloxadiazole fragment systematically increases the singlet and triplet energy levels (E(S) and E(T)) and blue shifts the absorption and emission bands. The triplet excited state is located mostly on the oxadiazole unit. The introduction of 2,7-dimethoxy substituents onto the carbazole moiety lowers the value of E(S), although E(T) is unaffected, which means that the singlet-triplet gap is reduced (for 7bE(S) - E(T) = 0.61 eV). A strategy has been established for achieving unusually high triplet levels for bipolar molecules (E(T) = 2.64-2.78 eV at 14 K) while at the same time limiting the increase in the singlet energy.

Dynamics of triplet migration in films of N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4''-diamine

We study triplet migration properties in NPB (N, N'-diphenyl-N, N'-bis(1-naphthyl)-1, 1'-biphenyl-4, 4''-diamine) films using time resolved gated spectroscopy and dispersive migration theory as our main tools of analysis. We show that in NPB, a well-known hole transporter in organic light emitting diodes, at high excitation densities triplet migration follows two regimes--a dispersive non-equilibrium regime (distinguished by exciton energetical relaxation within the distribution of hopping sites and as a consequence the hopping frequency being time dependent) that evolves into a second, non-dispersive equilibrium regime. Further, we observe a third region, which we term acceleration. From the turning over time between dispersive and non-dispersive dynamics, we deduce the width of the triplet density of states (DOS). We observe how the DOS variance changes when one decreases the thickness of the NPB film and note how surface effects are becoming important. Furthermore, the DOS variance of NPB changes when another organic layer is evaporated on top, namely Ir(piq)3 (tris(1-phenylisoquinoline)iridium(III)). We believe that these changes are due to the different polarizable media in contact with the NPB film, either vacuum or Ir(piq)3. We also show in this paper that the triplet level when time approaches zero is much higher in energy than the relaxed triplet levels, as quoted in most published papers; these values are thus incorrect for NPB. Lastly, it is possible that even at room temperature, the dispersive regime might be important for triplet migration at high initial triplet concentrations and might affect the diffusion length of triplets to a certain extent. However, more experimentation needs to be performed in order to address this question. Overall, we have characterized the triplet migration dynamics of NPB fully and shown that it agrees with previously published observations for other organic semiconductors and theoretical considerations.

Hybrid light-emitting diodes based on flexible sheets of mass-produced ZnO nanowires

We report the production of free-standing thin sheets made up of mass-produced ZnO nanowires and the application of these nanowire sheets for the fabrication of ZnO/organic hybrid light-emitting diodes in the manner of assembly. Different p-type organic semiconductors are used to form heterojunctions with the ZnO nanowire film. Electroluminescence measurements of the devices show UV and visible emissions. Identical strong red emission is observed independent of the organic semiconductor materials used in this work. The visible emissions corresponding to the electron transition between defect levels within the energy bandgap of ZnO are discussed.