Crosslinked Hole Transport Layer Research Articles

Nondestructive Direct Patterning of Both Hole Transport and Emissive Layers for Pixelated Quantum-Dot Light-Emitting Diodes.

Considering the increasing demand for high-resolution light-emitting diodes (LEDs), it is important that direct fine patterning technologies for LEDs be developed, especially for quantum-dot LEDs (QLEDs). Traditionally, the patterning of QLEDs relies on resin-based photolithography techniques, requiring multiple steps and causing performance deterioration. Nondestructive direct patterning may provide an easy and stepwise method to achieve fine-pixelated units in QLEDs. In this study, two isomeric tridentate cross-linkers (X8/X9) are presented and can be blended into the hole transport layer (HTL) and the emissive layer (EML) of QLEDs. Because of their photosensitivity, the in situ cross-linking process can be efficiently triggered by ultraviolet irradiation, affording high solvent resistance and nondestructive direct patterning of the layers. Red QLEDs using the cross-linked HTL demonstrate an impressive external quantum efficiency of up to 22.45%. Through lithographic patterning enabled by X9, line patterns of HTL and EML films exhibit widths as narrow as 2 and 4 μm, respectively. Leveraging the patterned HTL and EML, we show the successful fabrication of pixelated QLED devices with an area size of 3 × 3 mm2, alongside the successful production of dual-color pixelated QLED devices. These findings showcase the promising potential of direct patterning facilitated by engineered cross-linkers for the cost-effective fabrication of pixelated QLED displays.

Low-Temperature Cross-Linked Hole Transport Layer for High-Performance Blue Quantum-Dot Light-Emitting Diodes.

Quantum-dot light-emitting diodes (QLEDs), a kind of promising optoelectronic device, demonstrate potential superiority in next-generation display technology. Thermal cross-linked hole transport materials (HTMs) have been employed in solution-processed QLEDs due to their excellent thermal stability and solvent resistance, whereas the unbalanced charge injection and high cross-linking temperature of cross-linked HTMs can inhibit the efficiency of QLEDs and limit their application. Herein, a low-temperature cross-linked HTM of 4,4'-bis(3-(((4-vinylbenzyl)oxy)methyl)-9H-carbazol-9-yl)-1,1'-biphenyl (DV-CBP) with a flexible styrene side chain is introduced, which reduces the cross-linking temperature to 150 °C and enhances the hole mobility up to 1.01 × 10-3 cm2 V-1 s-1. More importantly, the maximum external quantum efficiency of 21.35% is successfully obtained on the basis of the DV-CBP as a cross-linked hole transport layer (HTL) for blue QLEDs. The low-temperature cross-linked high-mobility HTL using flexible side chains could be an excellent alternative for future HTL development.

Photothermal Synergic Cross-Linking Hole Transport Layer for Highly Efficient RGB QLEDs.

Developing an insoluble cross-linkable hole transport layer (HTL) plays an important role for solution-processed quantum dots light-emitting diodes (QLEDs) to fabricate a multilayer device with separated quantum dots layers and HTLs. In this work, a facile photothermal synergic cross-linking strategy is simultaneous annealing and UV irradiation to form the high-quality cross-linked film as the HTL without any photoinitiator, which efficiently reduces the cross-linking temperature to the low temperature of 130 °C and enhances the hole mobility of the 3-vinyl-9-{4-[4-(3-vinylcarbazol-9-yl)phenyl]phenyl}carbazole (CBP-V) thin films. The obtained high-quality cross-linked CBP-V films exhibited smooth morphology, excellent solvent resistance, and high mobility. Moreover, the high-performance red, green, and blue (RGB) QLEDs are successfully fabricated by using the photothermal synergic cross-linked HTLs, which achieved the maximum external quantum efficiency of 25.69, 24.42, and 16.51%, respectively. This work presents a strategy of using the photothermal synergic cross-linked HTLs for fabrication of high-performance QLEDs and advancing their related device applications.

Building one-dimensional hole transport channels in cross-linked polymers to enable efficient deep blue QLED

Quantum dot light-emitting diodes (QLEDs) emerged as very competitive candidates for next generation display and lighting applications. Cross-linked hole transport materials (HTMs) permit the flourish of solution-processed QLEDs. Whereas, unbalanced charge injection and energy level mismatch happened to cross-linked HTMs severely suppress the efficiency of blue QLEDs, which limits the full-color display application. Here, discoid molecules with self-assembly behavior were firstly proposed to construct one-dimensional transport channels in cross-linked hole transport layer (HTL) to advance the carrier transport property of the target composite HTL (CHTL). Compared with CHTLs prepared from amorphous and disordered crystalline discoid molecules, the carrier mobility of CHTL (T5DP36: V-CBP) with one-dimensional transport channels was signally boosted by two orders of magnitude from 6.6 × 10−5 to 2.16 × 10−3 cm2 V−1 s−1. The relevant blue QLED was qualified with notably enhanced external quantum efficiency of 18.16 %, deep blue emission with Commission International de I’Eclaiiage of (0.14, 0.04) and extended T50 lifetime from 177 to 417 h under 100 cd m−2. This is at a cutting edge of deep blue light and eligible for high-end display applications.

Multilayer solution‐processed phosphorescent organic light‐emitting diodes based on a crosslinkable and solvent‐resistant hole transport material

Phosphorescent organic light-emitting diodes (PhOLEDs) with multiple functional layers are favorable for high device performance. However, due to the erosion between functional layers, it is still a great challenge to prepare multilayer devices with a low-cost solution process. In this manuscript, 4,4′-bis (4-vinylphenyl) triphenylamine (VBTPA) can be in-situ polymerized as the thermally crosslinkable hole transport layer and successfully applied to solution-processed green light PhOLEDs by spin-coating method. It had the styryl groups as the thermally crosslinking position and can be crosslinked at 175°C with short crosslinking time (20 min). The high thermal stability (446.0°C), good photoelectric property and excellent solvent resistance and so forth, made the crosslinked VBTPA achieve a good device performance in solution-processed PhOLEDs. The turn-on voltage was 6.1 V, the maximum luminance was 8271 cd m−2 and the maximum current efficiency was 12.4 cd A−1. It indicated that the hole transport material with styryl groups as the thermally crosslinking position can be crosslinked under lower temperature (<200°C) for shorter time and applied to the preparation of multilayer solution-processed stacked PhOLEDs.

Low-temperature processed cross-linkable hole transport layer for efficient and stable perovskite solar cells

To the best of our knowledge, poor environment stability is the main challenge for perovskite solar cells (PSCs). In this study, cross-linked hole transport layer (HTL) is built on perovskite crystal to guarantee the operation stability of mesoporous PSCs for the first time. Here, N,N'-di-p-tolyl-N,-N'-bis(4-vinylphenyl)-[1,1′-biphenyl]-4,4′-diamine (V-p-TPD) was applied as a hole-transportable polymerizable monomer with the assistance of ultraviolet photoinitiator 4-octyloxydiphenyliodonium hexafluoroantimonate (OPPI). Cross-linking process is conducted at a distinctly low temperature of only 100 °C by adjusting the V-p-TPD: OPPI ratio to 10:1, which is tolerable for the active black phase of perovskite. The resultant cross-linked V-p-TPD film is qualified with a high cross-linking degree of 99%, a high hole mobility of 3.84 × 10−4 cm2V−1s−1 and a hydrophobic surface with wetting angle of 94.6°. Mesoporous PSCs based on the obtained cross-linked HTL achieved a tremendous Power conversion efficiency (PCE) of 18.49%, which represents the state-of-the-art records of mesoporous PSCs with cross-linkable HTL. PCE of the unpackaged device can remain 88% of the initial value after 30 d in an atmospheric environment with a relative humidity of 60 ± 5%, and maintain 86% of the initial PCE after 200 h in 65 ± 2 °C argon atmosphere. This work presents a low-temperature processing strategy for the preparation of cross-linked HTL layer, which does favor to the stable and efficient energy harvesting application.

P‐104: Improvement of Efficiency of a New Solution Processed Red Organic Light Emitting Diodes by Applying a New Crosslinked Hole Transport Layer

We synthesized a new set of solution processable light emitting materials for phosphorescent red organic light emitting diodes (OLEDs). To improve the device performance, we controlled the electron mobility of electron transport layer (ETL). However, the effect of changing the electron mobility of ETL was very limited. So, we applied a new crosslinkable hole transport layer which was perfectly crosslinked at 135°C and achieved much more improved efficiency by up to 200%.

Interfaces and pattern resolution of inkjet-printed organic light-emitting diodes with a novel hole transport layer

In this paper, the effects of the combination of solvents on the uniformity of a dried emitting layer and on device performance were studied. Green emitter inks with various solvents were prepared, and it was seen that good film uniformity of printed layers could be obtained with a solvent mixture with different boiling points, which reduced the coffee ring effect on a dried surface. During the inkjet process, the drop injection condition aiming was also controlled for better drop-to-drop spacing and reduction of the line-edge roughness. Printed patterns of a small molecular/polymer hybrid emitting layer, composed of the chlorobenzene-dichlorobenzene solvent mixture, showed significantly reduced roughness of the microscopic surface and improved efficiency of the inkjet device, at levels nearly comparable to those of spin-coated patterns. A suitable hole transport layer (HTL) was also further developed as an interfacial material, prior to the solution processing of the emitter. The cross-linkable HTL was composed of triphenylamine as the cross-linking unit and a fluorene-based compound. At the optimized condition of the solvent formulation, printed droplets within 100 µm× 300 µm pixels formed a uniform stripe without remarkable coffee-ring defects and line-edge surface roughness.

Semi-transparent perovskite solar cells with a cross-linked hole transport layer

Semi-transparent perovskite solar cells (ST-PeSCs) have received great attention because of their excellent performance and promising application in areas such as tandem devices and building integrated photovoltaics (BIPVs). Critical across all these applications is achieving both high efficiency and stable photovoltaic performance of such devices. Realizing both of these properties simultaneously has not been possible using device architectures featuring the archetypal doped Spiro-OMeTAD as a hole transport layer (HTL). As such, in this work we explore the use of a solution-processed cross-linked HTL formed from N4,N4′-di(naphthalen-1-yl)-N4,N4′-bis(4-vinylphenyl)biphenyl-4,4′-diamine (VNPB) molecules as an alternative to the conventional Spiro-OMeTAD within an FTO/SnO2/C60-SAM/Perovskite/HTL/MoOx/ultra-thin gold/MoOx ST-PeSC device architecture. Through an optimized multi-step thermal treatment process that maximizes charge extraction and reduces recombination from these devices, we can achieve ST-PeSCs that exhibit record power conversion efficiencies for Spiro-OMeTAD-free devices with average visible transmittance values between 10 and 30%. These devices exhibit comparable efficiencies to their Spiro-OMeTAD counterparts, with the additional benefit that the use of the poly-VNPB as the HTL material provides significant improvements in long-term device stability under both continuous illumination and high humidity conditions.

High triplet energy crosslinkable hole transport material for blue phosphorescent organic light-emitting diodes

A high triplet energy crosslinkable hole transport material was developed for solution processed blue phosphorescent organic light-emitting diodes. A high triplet energy core structure, 3,3-di(9H-carbazol-9-yl)biphenyl, was functionalized with a vinylbenzene crosslinking unit to form the crosslinked hole transport layer. Crosslinking of the hole transport layer at 220°C produced an insoluble film which is stable under exposure to solvents. A blue emitting layer of iridium(III) bis(2-(4,6-difluorophenyl)-pyridinato-N,C2)picolinate doped diphenyldi(4-(9-carbazolyl)phenyl)silane was spin-coated on the crosslinked hole transport layer without damaging the underlying hole transport layer, which afforded solution processed blue phosphorescent organic light-emitting diodes with an external quantum efficiency of 19.1%.

Comparative Study of Printed Multilayer OLED Fabrication through Slot Die Coating, Gravure and Inkjet Printing, and Their Combination

In this study, multilayer organic light-emitting diodes (OLEDs) consisting of three solution-processed layers are fabricated using slot die coating, gravure printing, and inkjet printing, techniques that are commonly used in the industry. Different technique combinations are investigated to successively deposit a hole injection layer (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)), a cross-linkable hole transport layer (N,N′-bis(4-(6-((3-ethyloxetan-3-yl)methoxy)-hexyloxy)phenyl)-N,N′-bis(4-methoxyphenyl)biphenyl-4,4′-diamin (QUPD)), and a green emissive layer (TSG-M) on top of each other. In order to compare the application techniques, the ink formulations have to be adapted to the respective process requirements. First, the influence of the application technique on the layer homogeneity of the different materials is investigated. Large area thickness measurements of the layers based on imaging color reflectometry (ICR) are used to compare the application techniques regarding the layer homogeneity and reproducible film thickness. The total stack thickness of all solution-processed layers of 32 OLEDs could be reproduced homogeneously in a process window of 30 nm for the technique combination of slot die coating and inkjet printing. The best efficiency of 13.3 cd A−1 is reached for a process combination of slot die coating and gravure printing. In order to enable a statistically significant evaluation, in total, 96 OLEDs were analyzed and the corresponding 288 layers were measured successively to determine the influence of layer homogeneity on device performance.

Improved Surface Morphology of Crosslinked Hole Transport Films by a Mixture of Polymer for OLEDs

Solution process for multilayer organic light-emitting diodes (OLEDs) brings in the dissolution problems between two stacked layers. It can be tackled using crosslinkable organic materials, especially for a hole transport layer (HTL). Similar to noncrosslinked HTL films based on small molecules, however, there appears the aggregation phenomenon in crosslinked HTL films fabricated by spin coating. We have demonstrated that aggregation can be suppressed through a mixture of noncrosslinkable polymer and crosslinkable small molecule HTL materials. Although such a mixed HTL film results in a slight degradation in the solvent resistance and device performance, it ensures the film uniformity and thus OLED stability. When cured under ambient air, however, the mixed HTL films exhibit a number of microaggregates that increase in proportion to the concentration of the polymer chains. It is attributed that the moisture adsorption near the hybrid interfaces causes lateral phase separation. We have found that they disappear by carrying out the thermal curing under inert gas atmosphere or increasing the polymer concentration in ambient air.

Impact of Interface Mixing on the Performance of Solution Processed Organic Light Emitting Diodes-Impedance and Ultraviolet Photoelectron Spectroscopy Study.

We investigated interfacial mixing of solution-processed organic light-emitting devices (OLEDs) using impedance spectroscopy (IS) and ultraviolet photoelectron spectroscopy (UPS) and its impact on device performance. We focused on interfacial mixing between a solution-processed cross-linkable hole transport layer (XM) and an emitting layer (EML), formed either by solution processing or vacuum evaporation. The results of IS and UPS clearly indicated that extensive interfacial mixing was unavoidable, even after the XM was cross-linked to make it insoluble and rinsed to remove residual soluble species, if the subsequent EML was solution processed. In addition, we also demonstrated that interfacial mixing indeed increased hole current density in corresponding hole only device (HOD). In fact, the hole injection efficiency could be an order of magnitude better when the EML was solution processed rather than vacuum evaporated. We investigated such behavior to find the desirable process condition of solution-processed OLEDs.

Cross-Linkable Hole-Transport Materials Improve the Device Performance of Perovskite Light-Emitting Diodes

Hybrid organic/inorganic perovskites are promising candidate materials for use in photovoltaic applications. More recently, they have also become highly attractive as active materials for other optoelectronic devices, including lasers, light-emitting diodes, and photodetectors. Nevertheless, difficulties in forming continuous and uniform films and the existence of a charge-injection barrier between the perovskite layer and the electrodes have hindered the development of high-performance perovskite light-emitting diodes (PeLEDs). In this study, a cross-linked hole-transport layer (HTL) is introduced to improve the hole-injection efficiency of PeLEDs. Furthermore, this layer simultaneously facilitates the formation of smooth perovskite layers, presumably because of the different surface energies. More interestingly, the HTL also exhibits strong solvent effects on the device performance. When the processing solvent for fabricating the HTLs is changed from chlorobenzene to N,N-dimethylformamide (DMF), the perovskite layer becomes more uniform and continuous, leading to better surface coverage and higher device efficiency, presumably because DMF has strong affinity toward the perovskite precursors. The approach presented herein could become a general method for decreasing the hole-injection barrier of PeLEDs and, eventually, lead to higher device performance.

38-4: The Interfacial Effect between HTL and EML on the Efficiency of Solution Processed Green Phosphorescent OLEDs

We have fabricated highly efficient green phosphorescent organic light-emitting diodes by solution process. Very interestingly, we found that the device efficiency is affected by interface between hole transport layer (HTL) and emission layer (EML). It was proved through spin cleaning process of cross-linkable HTL after curing which may affect the interface mixing. In addition, we tried to add complement layer to manipulate such effects. As a result, we significantly improved device efficiency (up to 72.7 cd/A) after spin cleaning process of the cross-linkable HTL material.

Improving the lifetime of white polymeric organic light-emitting diodes

We report on efficient polymeric white organic light-emitting diodes with unprecedented stability. The investigated devices are based on an electroluminescent copolymer of electron and hole-transporting units and red-, green-, and blue-emitting chromophores. We find that the glass transition of the polymer (Tg=182.5 °C) is the process determining the relation between thermal annealing during fabrication and device lifetime. For devices annealed below Tg, the device lifetime significantly increases with increasing annealing temperature. For annealing temperatures above Tg, however, the current density in the devices rapidly increases while their lifetime slightly decreases. Insight into the underlying processes is provided by atomic force microscopy phase imaging and by UV/visible and fluorescence spectroscopy. We also investigated the influence of the operating temperature of the device: besides the commonly known fact that elevated operating temperatures reduce the lifetime, we discovered that the acceleration coefficient, which determines the scaling of the device lifetime with applied current density, was reduced. At the glass transition, the device lifetime no longer depended on the current density. The device lifetime was improved even further by introducing an additional cross-linkable hole-transport layer. Optimized devices achieve a half-luminance lifetime of 1860 h when operated at room temperature and at an initial luminance of 500 cd m−2. As a result of the relatively balanced stability of the three chromophores, the emission spectrum remains virtually unchanged over the entire device lifetime. Finally, to reduce the time required for the lifetime measurements, we propose to analyze the voltage increase over the first 10–50 h of the lifetime test and find that this allows precisely estimating the lifetime of our devices.

Highly Efficient Electrophosphorescent Devices with Saturated Red Emission from a Neutral Osmium Complex

Highly efficient electrophosphorescent polymer light-emitting diodes (LEDs) with saturated red emission (Commission Internationale de L'Eclairage chromaticity coordinates exactly at x = 0.67, y = 0.33) are achieved by blending a novel neutral Osmium complex into a single-component bipolar polymer host that possesses balanced hole- and electron-transporting ability. By using a tetraphenylenebiphenyldiamine (TPD)-based cross-linkable hole-transport layer as well as a layer of 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) as an electron-transport layer, a device structure with both effective hole/electron injection and efficient carriers/excitons blocking or confinement at both electrode sides is constituted. In this way, external quantum efficiency 12.8% is reached the first time with an organometallic complex with heavy metal core other than iridium.

Amplified spontaneous emission in an organic semiconductor multilayer waveguide structure including a highly conductive transparent electrode

We demonstrate that the amplified spontaneous emission (ASE) threshold in multilayer waveguide structures suitable for the use in future organic injection lasers can be drastically reduced by inserting a crosslinked hole transport layer (HTL) between a highly conductive indium tin oxide (ITO) electrode and the polymer emission layer. While no ASE is observed when the active layer material is directly spincoated onto the ITO electrode, it can be completely restored upon insertion of a 300-nm-thick HTL. This observation is attributed to reduced attenuation of the waveguided mode enabling the ASE process and is theoretically confirmed by calculations of the mode intensity fraction propagating in the absorptive ITO electrode.

Photo-Patternable Hole-Transport Polymers for Organic Light-Emitting Diodes

A series of soluble arylamine-based hole transporting polymers with various ionization potentials have been synthesized. The synthetic methodology allows for substitution of aryl groups on the amine with electron-withdrawing and electron-donating moieties which permits tuning of the redox potential of the polymer. The arylamine-based monomers have been copolymerized with cinnamate-based moieties to obtain photo-cross-linkable polymers. These materials can be cross-linked and patterned using a standard mask aligner designed for photolithographic applications. UV cross-linking conditions were optimized to obtain an insoluble hole-transport layer with stable electroluminescent properties. Using a photolithographic mask, these materials have been patterned into 10−50-μm-size features. These polymers have been used as hole-transport layers (HTLs) in multilayer light-emitting diodes ITO/HTL/AlQ3/Mg:Ag [ITO = indium tin oxide, AlQ3 = tris(8-hydroxyquinolinato)aluminum]. The electroluminescent properties have been evaluated and compared between devices made from polymers with different ionization potentials. Electroluminescent devices have been fabricated using a multilayered structure of spin-coated hole-transport materials to enhance the overall electroluminescent properties. Fully spin-coated devices have also been fabricated by spinning a blend of polystyrene and AlQ3 on top of a cross-linked hole-transport layer.

New Triarylamine-Containing Polymers as Hole Transport Materials in Organic Light-Emitting Diodes: Effect of Polymer Structure and Cross-Linking on Device Characteristics

A series of poly(norbornenes) with pendant triarylamine (TPA) groups has been synthesized by ring-opening metathesis polymerization and investigated as hole transport materials in organic two-layer light-emitting diodes (LEDs). Efficient device fabrication through spin casting of the hole transport layer (HTL) was possible, since the polymers exhibited excellent film formation properties. LEDs of the form ITO/poly(norbornene)-TPA/Alq3/Mg (ITO = indium tin oxide, Alq3 = tris(8-quinolinato)aluminum) showed bright green emission with external quantum efficiencies of up to 0.77% (1.30 lm/W) for 20 nm thick HTL films. The length and polarity of the linker between the triarylamine functionality and the polymer backbone were varied systematically. The device performance was found to depend strongly on these structural differences. Substitution of ester groups by less polar ether functionalities greatly enhances external quantum efficiencies, lowers the operating voltage, and improves the stability of the device. Further improvement of the device characteristics is achieved by reducing the length of the alkyl linker. The HTL can be conveniently cross-linked by UV irradiation. Cross-linking was found to decrease device performance. A maximum external quantum efficiency of 0.37% was achieved for an Alq3-LED with cross-linked HTL.