Molecular Interdiffusion between Stacked Layers by Solution and Thermal Annealing Processes in Organic Light Emitting Devices

Advances in Solution-Processed OLEDs and their Prospects for Use in Displays

This review outlines problems and progress in development of solution-processed organic light-emitting diodes (SOLEDs) in industry and academia. Solution processing has several advantages such as low consumption of materials, low-cost processing, and large-area manufacturing. However, use of a solution process entails complications, such as the need for solvent resistivity and solution-processable materials, and yields SOLEDs that have limited luminous efficiency, severe roll-off characteristics, and short lifetime compared to OLEDs fabricated using thermal evaporation. These demerits impede production of practical SOLED displays. This review outlines the industrial demands for commercial SOLEDs and the current status of SOLED development in industries and academia, and presents research guidelines for the development of SOLEDs that have high efficiency, long lifetime, and good processability to achieve commercialization.

Probing impact of interface mixing on the charge carrier dynamics of a solution-processed organic light emitting diode via impedance spectroscopy

Recently, several studies have revealed that the thermal annealing process induces intermixing at the interfaces of multilayered solution-processed organic light emitting diodes (OLEDs) and enhances their device performance. Depth profiling measurements, such as neutron reflectometry, have meticulously shown that significant intermixing occurs when the annealing temperature exceeds the glass transition temperature (T_g) of OLED materials. However, electrical characterization to unveil the physical origins of the correlation between interfacial characteristics and device performance is still lacking. Here, we introduce impedance spectroscopy (IS) analysis to examine the thermally induced modifications of charge carrier dynamics in a solution-processed bilayer OLED, consisting of an emission layer and an electron transporting layer (ETL). The characteristic relaxation frequency and capacitance extracted from the capacitance-frequency spectra of the OLEDs thermally annealed at varying temperatures were utilized to separately assess the conductance of the ETL and interfacial carrier accumulation, respectively. The results show that the improved charge transport of the ETL upon thermal annealing is mainly responsible for the performance enhancement since annealing the OLEDs at a temperature above the T_g of the ETL, at which significant intermixing occurs, promotes non-radiative trap-assisted recombination and thereby deteriorates the current efficiency. The proposed IS analysis exhibits that IS can separately probe the charge transport, interfacial charge accumulation and recombination process which are crucial for accurate analysis of charge carrier dynamics in solution-processed OLEDs and can thus be utilized to identify the key factors limiting the device performance.

Diffusion in Organic Film Stacks Containing Solution-Processed Phosphorescent Poly(dendrimer) Dopants

Efficient organic light-emitting diodes (OLEDs) consist of an emissive layer comprising a blend of a light-emitting and host material in contact with one or more charge transporting layers. The distribution of the active material in the guest-host emissive layer blend and the changes that may occur upon thermal annealing are two important factors in determining the stability and efficiency of OLEDs. We have combined neutron reflectometry and photoluminescence measurements to investigate the structures of films comprising an emissive layer containing a phosphorescent poly(dendrimer) material blended with 4,4'-N,N'-di(carbazolyl)biphenyl. This combination has been shown to give rise to highly efficient OLEDs. Here, we show that the emissive poly(dendrimer) material was not uniformly distributed in the host, but formed a concentration gradient within the emissive layer. Upon heating, the adjacent electron transport layer was found to intermix with the emissive layer, accompanied by changes in the material distribution in the emissive layer. The intermixing of the materials led to a decrease in the photoluminescence from the poly(dendrimer) within the film. The decrease in the photoluminescence was ascribed to an increase in interchromophore interactions that could arise from a conformational change of the poly(dendrimer) or phase separation leading to aggregation. The results indicate that, while uniform mixing of the guest and host is not essential for efficiency, the thermal stabilities of both host and charge transport materials are important for device durability.

Molecular Stacking Effect on Small-Molecular Organic Light-Emitting Diodes Prepared with Solution Process

The light-emitting layer (EML) is generally prepared by mixing the host and dopant to realize an organic light-emitting diode (OLED). However, phase separation is often observed during the fabrication process to prepare OLEDs, depending on the structure of the host materials. In particular, phase separation because of π-π stacking is frequently observed during thermal annealing for the solution process. The annealing process is required for solvent removal and complete relaxation of the molecule. Hence, the materials with a high glass transition temperature (T_g) are ideal because phase separation occurs because of π-π stacking during the annealing process, if T_g is too low. To understand this phenomenon, we compared two host materials with similar molecular weights but different three-dimensional connectivity, which causes different rotational freedom. Then, we investigated the effect on the device properties, depending on the annealing conditions. In both materials, when the annealing temperature rises above 120 °C, the dopant completely escaped from the EML. However, the material that does not disturb the molecular stacking order by annealing because of its limited free rotation through the internal bond shows much better device characteristics even after annealing at a higher temperature than T_g. The results show that interdiffusion at the interface and unstable internal density distribution with annealing temperature are responsible for the device degradation behavior.

Design of Blue Thermally Activated Delayed Fluorescent Emitter with Efficient Exciton Gathering Property for High-Performance Fully Solution-Processed Hybrid White OLEDs

The blue thermally activated delay fluorescence (TADF) emitters are highly attractive in the fields of constructing hybrid white organic light-emitting diodes (WOLEDs) due to its high efficiency and color stability. However, few blue TADF emitters can withstand sequential orthogonal solvents, making it impossible to fabricate the fully solution-processed hybrid WOLEDs. Here, two TADF materials, PCz-4CzCN and TPA-4CzCN, were designed and synthesized by equipping the emissive core with nonconjugated bulky units, which can effectively enhance the solvent resistance ability without disturbing the TADF feature. The photophysical investigation indicates that phenylcarbazole unit can efficiently block the electromer formation to enhance the energy transfer and exciton utilization of the emitter. Accordingly, the blue OLEDs of PCz-4CzCN shows higher external quantum efficiency (EQE) of 22.6%, which is the best performance recorded among the fully solution-processed blue OLEDs. Upon further doping, the yellow phosphor PO-01, the fully solution-processed TADF-phosphor (T-P) hybrid WOLEDs was successfully obtained with high performance for the first time. Thanks to the efficient exciplex formation, the turn-on voltage of the white device is only 2.8 V, and the maximum brightness and power efficiency are as high as 53 300 cd m^-2 and 38.5 lm W^-1, respectively, which are even higher than the previous reported T-P hybrid WOLEDs with a vacuum-deposited electron transfer layer.

A molecular dynamics study on the interface morphology of vapor-deposited amorphous organic thin films

The interfaces between amorphous organic layers play an important role in the efficiency and lifetime of organic light emitting diodes (OLEDs). However, an atomistic understanding of the interface morphology is still poor. In this study, we theoretically investigate the interfacial structure of amorphous organic films using molecular dynamics simulations that mimic vapor-deposition processes. We find that molecularly sharp interfaces are formed by the vapor-deposition process as the interface thickness spans only a mono- or double-layer in terms of lie-down geometry. Interestingly, the interface is more diffusive into the upper layer due to asymmetric interdiffusion during the vapor-deposition process, which is well described by a simple random-walk model. Additionally, we investigate the change in the molecular orientation of interdiffused molecules, which is crucial for device performance.

Highly Efficient Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with Fully Solution-Processed Organic Multilayered Architecture: Impact of Terminal Substitution on Carbazole-Benzophenone Dendrimer and Interfacial Engineering

A series of second-generation carbazole-benzophenone dendrimer substituted by several functional groups at terminal positions (subG2B) was investigated toward a thermally activated delayed fluorescence (TADF) emitter for nondoped emissive layer (EML) application in a solution-processed organic light-emitting diode (OLED). Substitution was found to dramatically alter the photophysical properties of the dendritic TADF emitters. The introduction of tert-butyl and phenyl group endows the subG2Bs with aggregation-induced emission enhancement character by suppression of internal conversion in singlet excited states. In the meantime, the introduction of a methoxy group resulted in aggregation-caused quenching character. The device performance of the OLED, where subG2B neat films were incorporated as nondoped EMLs, was found to be highly enhanced by adopting fully solution-processed organic multilayer architecture in comparison to the devices with a vacuum-deposited electron transporting layer (ETL), achieving a maximum external quantum efficiency of 17.0%. Such improvement was attributable to the improved carrier balance via intermixing at solution-processed EML/ETL interfaces. It was also found that the post-thermal annealing of the OLED at appropriate temperatures could be beneficial to enhance OLED performance by promoting the intermixing EML/ETL interface to some extent. Our findings emphasize the potential utility of dendritic TADF emitters in the solution-processed TADF-OLED and increase the importance to manipulate dendrimer/small molecule interfaces.

Morphology of OLED Film Stacks Containing Solution-Processed Phosphorescent Dendrimers

Organic light-emitting devices containing solution-processed emissive dendrimers can be highly efficient. The most efficient devices contain a blend of the light-emitting dendrimer in a host and one or more charge-transporting layers. Using neutron reflectometry measurements with in situ photoluminescence, we have investigated the structure of the as-formed film as well as the changes in film structure and dendrimer emission under thermal stress. It was found that the as-formed film stacks comprising poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (where the host was deuterated 4,4'-N,N'-di(carbazolyl)biphenyl or tris(4-carbazol-9-ylphenyl)amine, the host:dendrimer layer was solution-processed, and the 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene evaporated) had well-defined interfaces, indicating good wetting of each of the layers by the subsequently deposited layer. Upon thermal annealing, there was no change in the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate/host:dendrimer interface, but once the temperature reached above the T_g of the host:dendrimer layer, it became a supercooled liquid into which 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene dissolved. When the film stacks were held at a temperature just above the onset of the diffusion process, they underwent an initial relatively fast diffusion process before reaching a quasi-stable state at that temperature.

Dependence of Organic Interlayer Diffusion on Glass-Transition Temperature in OLEDs

Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (T_g) of the material with the lowest T_g. Diffusion occurred when the material with the lowest T_g formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative T_g's. If the supercooled liquid formed at a temperature significantly lower than the T_g of the higher-T_g material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two T_g's were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the "burn in" often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes.

Influence of solution- and thermal-annealing processes on the sub-nanometer-ordered organic-organic interface structure of organic light-emitting devices

Solution- and thermal-annealing processed organic-organic interface structures were investigated by neutron reflectometry. We revealed the true picture of interfaces, a polymer hole-transporting layer - a small molecule light-emitting layer - a small molecule electron-transporting layer, and discussed influences of those interface structures on organic light-emitting devices.

All-Solution-Processed Organic Light-Emitting Diodes Based on Photostable Photo-cross-linkable Fluorescent Small Molecules

We demonstrate herein the fabrication of small molecule-based OLEDs where four organic layers from the hole- to the electron-transporting layers have successively been deposited by using an all-solution process. The key feature of the device relies on a novel photopolymerizable red-emitting material, made of small fluorophores substituted with two acrylate units, and displaying high-quality film-forming properties as well as high emission quantum yield as nondoped thin films. Insoluble emissive layers were obtained upon UV irradiation using low illumination doses, with no further need of postcuring. Very low photodegradation was noticed, giving rise to bright layers with a remarkable surface quality, characterized by a mean RMS roughness as low as 0.7 nm after development. Comparative experiments between solution-processed OLEDs and vacuum-processed OLEDs made of fluorophores with close architectures show external quantum efficiencies in the same range while displaying distinct behaviors in terms of current and power efficiencies. They validate the proof of concept of nondoped solution-processable emissive layers exclusively made of photopolymerized fluorophores, thereby reducing the amount of components and opening the way toward cost-effective fabrication of solution-processed OLED multilayer architectures.

Benzophenones as Generic Host Materials for Phosphorescent Organic Light-Emitting Diodes

Despite the fact that benzophenone has traditionally served as a prototype molecular system for establishing triplet state chemistry, materials based on molecular systems containing the benzophenone moiety as an integral part have not been exploited as generic host materials in phosphorescent organic light-emitting diodes (PhOLEDs). We have designed and synthesized three novel host materials, i.e., BP2-BP4, which contain benzophenone as the active triplet sensitizing molecular component. It is shown that their high band gap (3.91-3.93 eV) as well as triplet energies (2.95-2.97 eV) permit their applicability as universal host materials for blue, green, yellow, and red phosphors. While they serve reasonably well for all types of dopants, excellent performance characteristics observed for yellow and green devices are indeed the hallmark of benzophenone-based host materials. For example, maximum external quantum efficiencies of the order of 19.2% and 17.0% were obtained from the devices fabricated with yellow and green phosphors using BP2 as the host material. White light emission, albeit with rather poor efficiencies, has been demonstrated as a proof-of-concept by fabrication of co-doped and stacked devices with blue and yellow phosphors using BP2 as the host material.

Benzophenone-imbedded benzoyltriptycene with high triplet energy for application as a universal host material in phosphorescent organic light-emitting diodes (PhOLEDs)

Benzoyltriptycene functions as a very good host material for blue, green, yellow and red dopants in PhOLED devices.