Inkjet Printing of Mixed-Host Emitting Layer for Electrophosphorescent Organic Light-Emitting Diodes

Controlling Drying Conditions in Vacuum for Uniform Film Formation in Inkjet-Printed OLEDs

The drying process of inkjet-printed organic light-emitting diodes (OLEDs) is influenced by both ink properties and external environmental factors, which ultimately affect the film profile. First, we conducted a detailed investigation of the drying time based on changes in the boiling point (BP) of mixed solvents and analyzed the correlation with the film profile. Under atmospheric drying conditions in a nitrogen (N2) atmosphere, the increased drying time under capillary-driven flow leads to greater particle movement toward the edges, significantly increasing the coffee-ring effect. Additionally, using a high-boiling-point solvent mixture of ethyl 4-methylbenzoate (EMB) and 2-ethylhexyl benzoate (EHB), we produced uniform thin films both between and within pixels (inter and intrapixel uniformity) through a vacuum drying process. In particular, we proposed a drying process model that divides the drying of inkjet pixels into a microfluidic phase and a gelation phase. Through five gelation phase-controlled vacuum drying experiments, the morphology within the pixels was precisely investigated. By sufficiently removing residual solvents after the microfluidic phase and then proceeding with heating, we produced uniform thin films. Furthermore, we fabricated OLED devices using this gelation phase-controlled vacuum drying process, achieving uniform pixel emission and improved device performance.

Interphase-Controlled Inkjet Printing of MicroInlaid OLEDs: Effects of Solvent- and Solute-Polymer Interactions

Inkjet printing, a highly promising technique for the cost-effective fabrication of large-scale organic light-emitting devices (OLEDs), typically necessitates the intricate alignment of precisely patterned insulating layers. Recently, we introduced a unique single-step inkjet printing process that produces well-patterned microinlaid spots of functional compounds through insulating polymer layers. This approach exploits lateral phase separation between the solute of functional compounds and the polymer, allowing the simultaneous spatial etching of the polymer and the infilling of the solute using a single inkjet-printed sessile droplet. Here, we demonstrate that the interaction between the solvent and polymer, as well as the solute and polymer, critically determines the precision and efficiency of printing. This is particularly evident when using either the insulating poly(vinylpyridine) isomer of poly(4-vinylpyridine) (P4VP) or poly(2-vinylpyridine) (P2VP) with chloroform as a solvent, which allows for a detailed examination of these interactions based on certain solubility parameters. Micro-Raman spectroscopy reveals that the self-organizing capability of the microinlaid spots with P4VP is superior to that with P2VP. This is due to the fact that P2VP shows higher affinity to the solvent and causes imperfect phase separation as compared to P4VP. As a result, a performance evaluation demonstrates enhanced device performance for inkjet-printed green micro-OLEDs with P4VP, exhibiting a higher external quantum efficiency of 3.3% compared to that of 2.3% achieved with P2VP. These findings elucidate the important roles of solvent-polymer and solute-polymer interactions in the inkjet printing process, leading to interfacial control of inkjet printing technique for the cost-effective production of high-performance and high-resolution micro-OLEDs.

Single-Step Inkjet-Printed Dielectric Template for Large Area Flexible Signage and Low-Information Displays

In recent years, the proliferation of smart gadgets has increased the demand for information displays; fortunately, organic light-emitting diodes (OLEDs) show great promise for use in display, lighting, and signage contexts. This research demonstrates inkjet printing of dielectric materials to provide maskless emission area patterning and electrical isolation for large-area OLEDs on flexible/rigid indium tin oxide (ITO)-coated substrates, avoiding the need for typical photolithography steps, including etching and lift-off processes. We have studied the impact of impinged droplets' velocity fluctuations, which are measured in relation to their interaction with the substrate, allowing for the determination of the drop diameter and shape. The inkjet parameters, such as pulse waveform, pulse voltage, and pulse width, are controlled to provide consistently repeatable ejection of dielectric ink droplets. The single-step patterning of complex designs with a minimum opening of 18 μm features is successfully printed with high fidelity. The effect of substrate temperature on the printed template/structure size and shape is explored. We have successfully demonstrated an ultralarge-area (120 × 120 mm2) OLED signage application on inkjet-printed dielectric template (IJPDt). Standard small-area OLEDs (4 × 4 mm2) achieved a maximum brightness of 24480 cd m-2 at 10 V and a maximum current efficiency of 17 cd A-1 with a low turn-on voltage of 2.7 V.

Silicone engineered anisotropic lithography for ultrahigh-density OLEDs

Ultrahigh-resolution patterning with high-throughput and high-fidelity is highly in demand for expanding the potential of organic light-emitting diodes (OLEDs) from mobile and TV displays into near-to-eye microdisplays. However, current patterning techniques so far suffer from low resolution, consecutive pattern for RGB pixelation, low pattern fidelity, and throughput issue. Here, we present a silicone engineered anisotropic lithography of the organic light-emitting semiconductor (OLES) that in-situ forms a non-volatile etch-blocking layer during reactive ion etching. This unique feature not only slows the etch rate but also enhances the anisotropy of etch direction, leading to gain delicate control in forming ultrahigh-density multicolor OLES patterns (up to 4500 pixels per inch) through photolithography. This patterning strategy inspired by silicon etching chemistry is expected to provide new insights into ultrahigh-density OLED microdisplays.

Efficient OLEDs Based on Slot-Die-Coated Multicomponent Emissive Layer

The optimization of multicomponent emissive layer (EML) deposition by slot-die coating for organic light-emitting diodes (OLEDs) is presented. In the investigated EMLs, the yellow-green iridium complex (Ir) was doped in two types of host: a commonly used mixture of poly(N-vinylcarbazole) (PVK) with oxadiazole derivative (PBD) or PVK with thermally activated delayed fluorescence-assisted dopant (10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-spiro[acridine-9,9′-fluorene], SpiroAC-TRZ). In this article, OLEDs with EML prepared in air by slot-die coating, facilitating industrial manufacturing, are confronted with those with spin-coated EML in nitrogen. OLEDs based on PVK:PBD + 2 wt.% Ir-dopant exhibit comparable performance: ~13 cd A⁻¹, regardless of the used method. The highest current efficiency (21 cd A⁻¹) is shown by OLEDs based on spin-coated PVK with 25 wt.% SpiroAC-TRZ and 2 wt.% Ir-dopant. It is three times higher than the efficiency of OLEDs with slot-die-coated EML in air. The performance reduction, connected with the adverse oxygen effect on the energy transfer from TADF to emitter molecules, is minimized by the rapid EML annealing in a nitrogen atmosphere. This post-treatment causes more than a doubling of the OLED efficiency, from 7 cd A⁻¹ to over 15 cd A⁻¹. Such an approach may be easily implemented in other printing techniques and result in a yield enhancement.

High-Resolution Patterning of Organic Emitting-Layer by Using Inkjet Printing and Sublimation Transfer Process

We implemented ultra-high resolution patterns of 2822 pixels-per-inch (PPI) via an inkjet printing and vacuum drying process grafted onto a sublimation transfer process. Co-solvented ink with a 1:1 ratio of N,N-dimethylformamide (DMF) to ortho-dichlrorobenzene (oDCB) was used, and the inkjet driving waveform was optimized via analysis of Ohnesorge (Oh)-Reynolds (Re) numbers. Inkjet printing conditions on the donor substrate with 2822 PPI microchannels were investigated in detail according to the drop space and line space. Most sublimation transferred patterns have porous surfaces under drying conditions in an air atmosphere. Unlike the spin-coating process, the drying process of inkjet-printed films on the microchannel has a great effect on the sublimation of transferred thin film. Therefore, to control the morphology, we carefully investigated the drying process of the inkjet-printed inks in the microchannel. Using a vacuum drying process to control the morphology of inkjet-printed films, line patterns of 2822 PPI resolution having a root-mean-square (RMS) roughness of 1.331 nm without voids were successfully fabricated.

Broadband Reflective Liquid Crystal Films Prepared by Rapid Inkjet Printing and Superposition Polymerization

Inkjet printing is a non-contact, material saving and on-demand material manufacturing technology, which is able to be applied to the fabrication of functional materials with high efficiency. A new method for preparing broadband reflective cholesteric films based on inkjet printing and non-stick technology was proposed in this paper. The feasibility of automatic mixing of liquid crystal and doped materials in inkjet printing was studied. The spectral data of samples prepared by manual mixing and automatic mixing by inkjet printing were compared. It was found that the spectral error of the printed film was only less than 0.17 wt%, which reached or even exceeded the effect of manual mixing. The feasibility of preparing liquid crystal films with broadband reflection characteristics by stacking polymerization based on in situ UV polymerization and non-stick technology was verified. By changing the printing amount of chiral doped ink, the bandwidth of PSCLC film can be accurately controlled. This technology is expected to play an important role in scientific research and practical application.

3D-printed flexible organic light-emitting diode displays

The ability to fully 3D-print active electronic and optoelectronic devices will enable unique device form factors via strategies untethered from conventional microfabrication facilities. Currently, the performance of 3D-printed optoelectronics can suffer from nonuniformities in the solution-deposited active layers and unstable polymer-metal junctions. Here, we demonstrate a multimodal printing methodology that results in fully 3D-printed flexible organic light-emitting diode displays. The electrodes, interconnects, insulation, and encapsulation are all extrusion-printed, while the active layers are spray-printed. Spray printing leads to improved layer uniformity via suppression of directional mass transport in the printed droplets. By exploiting the viscoelastic oxide surface of the printed cathode droplets, a mechanical reconfiguration process is achieved to increase the contact area of the polymer-metal junctions. The uniform cathode array is intimately interfaced with the top interconnects. This hybrid approach creates a fully 3D-printed flexible 8 × 8 display with all pixels turning on successfully.

Inkjet Printing of a Benzocyclobutene-Based Polymer as a Low-k Material for Electronic Applications

Polymeric materials with a low dielectric constant are widely used in the electronic industry due to their properties. In particular, polymer adhesives can be used in many applications such as wafer bonding and three-dimensional integration. Benzocyclobutene (BCB) is a very interesting material thanks to its excellent bonding behavior and dielectric properties. Usually, BCB is applied by spin-coating, although this technology does not allow the fabrication of complex patterns. To obtain complex patterns, it is necessary to use a printing technology, such as inkjet printing. However, inkjet printing of BCB-based inks has not yet been investigated. Here, we show the feasibility of printing complex patterns with a BCB-based ink, reaching a resolution of 130 μm. We demonstrate that with a proper dilution, BCB-based inks enter the printability window and drop ejection is achieved without the formation of satellite drops. In addition, we present the conditions in which there is an appearance of the coffee ring effect. Inks that feature a too high interaction with the substrate are more likely to show the coffee ring effect, deteriorating the printing quality. We also observe that it is possible to achieve a better film uniformity by increasing the number of printed layers, due to redissolution of the BCB-based polymer that helps to level possible inhomogeneities. Our work represents the starting point for an in-depth study of BCB-based polymer fabrication using jet printing technologies, as a comparison of the bonding quality obtained with different materials and different technologies could give more information and broaden the perspective regarding this field.

Two-Color Pixel Patterning for High-Resolution Organic Light-Emitting Displays Using Photolithography

Nowadays, the display industry is endeavoring to develop technology to provide large-area organic light-emitting diode (OLED) display panels with 8K or higher resolution. Although the selective deposition of organic molecules through shadow masks has proven to be the method of choice for mobile panels, it may not be so when independently defined high-resolution pixels are to be manufactured on a large substrate. This technical challenge motivated us to adopt the well-established photolithographic protocol to the OLED pixel patterning. In this study, we demonstrate the two-color OLED pixels integrated on a single substrate using a negative-tone highly fluorinated photoresist (PR) and fluorous solvents. Preliminary experiments were performed to examine the probable damaging effects of the developing and stripping processes upon a hole-transporting layer (HTL). No significant deterioration in the efficiency of the develop-processed device was observed. Efficiency of the device after lift-off was up to 72% relative to that of the reference device with no significant change in operating voltage. The procedure was repeated to successfully obtain two-color pixel arrays. Furthermore, the patterning of 15 μm green pixels was accomplished. It is expected that photolithography can provide a useful tool for the production of high-resolution large OLED displays in the near future.

Organic Electropolymerized Multilayers for Light-Emitting Diodes and Displays

In electrochemistry, the carbazole is generally coupled to dimer but not to polymer. This work has reported that organic electropolymerization (OEP) of 4,4',4″-tri(N-carbazolyl)triphenylamine (TCTA) would form a high cross-linked carbazole polymer by its high activity/reversibility and a synchronous viscosity control. It has significantly improved the OEP film quality of both hole-transporting and electroluminescent layers in organic light-emitting diodes. As a result, the conductivity and power efficiency of the organic light-emitting diodes with TCTA are eight and four times of that without TCTA. A prototype display device with a 1.7 in. monochrome passive matrix of 58 ppi under the driving chip is successfully fabricated with accurate pixel size and uniform electroluminescence, which shows a great potential of OEP in the electroluminescent application.