Color temperature tunable white organic light-emitting diodes

In this work, we demonstrated color-tunable white organic light-emitting diodes by stacking upper orange transparent and lower blue bottom emission organic light-emitting diodes (OLEDs). By independently operating each OLED, it was possible to tune the color temperature in a range of 1500–10,000 K, which covers the full Planckian locus in the 1931 CIE space. In designing stable and efficient OLEDs, in addition to the electrical characteristics, the importance of internal microcavity was emphasized and implemented. In fabricating the upper transparent OLED, special attention was paid to the capping layer for enhancing the emission. Our results presented a general guideline that is practically useful in designing high-performance color-tunable OLEDs with transparent OLEDs.     

High-performance green phosphorescent top-emitting organic light-emitting diodes based on FDTD optical simulation

We have successfully applied finite-difference time-domain (FDTD) method in top-emitting organic light-emitting diodes (TOLEDs) for structure optimization, demonstrating good agreement with experimental data. A mixed host with both hole transport and electron transport materials is employed for the green phosphorescent emitter to avoid charge accumulation and broaden the recombination zone. The resulting TOLEDs exhibit ultra-high efficiencies, low current efficiency roll-off, and a highly saturated color, as well as hardly detectable spectrum shift with viewing angles. In particular, a current efficiency of 127.0 cd/A at a luminance of 1000 cd/m² is obtained, and maintains to 116.3 cd/A at 10,000 cd/m².     

Sputter-patterned ITO-based organic light-emitting diodes with leakage current cut-off layers

We demonstrate the cost-effective fabrication of organic light-emitting diodes (OLEDs) using a sputter-patterned indium–tin-oxide (ITO). This scheme brings in a leakage current on the slope of the sputter-patterned ITO edges due to spike-like surface. To suppress it, we place thermally evaporated organic insulating molecules right on the ITO edges for preventing hole leakage, just below the aluminum (Al) cathode for blocking electron leakage, or both on the ITO edges and below the Al cathode. It is demonstrated that blocking off both hole- and electron-leak pathways (via the spikes) is highly desired to enhance the current efficiency and lifetime of the sputter-patterned ITO-based OLEDs.     

Investigation and optimization of each organic layer: A simple but effective approach towards achieving high-efficiency hybrid white organic light-emitting diodes

A highly efficient hybrid white organic light-emitting diode based on a simple structure has been successfully fabricated and characterized. By systematically investigating the influence of the emissive layer thickness, electron transporting layer thickness, spacer and hole transporting layer, the forward-viewing current efficiency and power efficiency of the resulting device without any out-coupling schemes or n-doping strategies can be as high as 59.4 cd/A and 58.4 lm/W, respectively. Besides, a Commission International de l’Eclairage of (0.412, 0.393) and a color rendering index of 60 are obtained at the current density of 11 mA/cm². Through the optimization and investigation, the origin of this unique device is explored comprehensively. Undoubtedly, such presented results will be beneficial to the design of both material and device architecture for ultra high-performance white organic light-emitting diodes.     

Phenylimidazole-based homoleptic iridium(III) compounds for blue phosphorescent organic light-emitting diodes with high efficiency and long lifetime

In order to obtain triplet emitters with high stability and efficiency, three homoleptic iridium(III) compounds — specifically, Ir(tpim)₃ (1), Ir(mtpim)₃ (2), and Ir(itpim)₃ (3), where tpim = 1-([1,1′:3′,1″-terphenyl]-2′-yl)-2-(4-fluorophenyl)-1H-imidazole, mtpim = 2-(4-fluorophenyl)-1-(5′-methyl-[1,1′:3′,1″-terphenyl]-2′-yl)-1H-imidazole, and itpim = 2-(4-fluorophenyl)-1-(5′-isopropyl-[1,1′:3′,1″-terphenyl]-2′-yl)-1H-imidazole — were prepared by one-pot reaction of the corresponding phenylimidazole ligand with an Ir(I) complex as a starting material. Compounds 1–3 emit bright sky-blue phosphorescence with λ_max = 459–463 nm and phosphorescent quantum efficiencies of 0.38–0.50. Multi-layer phosphorescent organic light-emitting diodes using compounds 1–3 as the triplet emitters and mCBP (3,3-di(9H-carbazol-9-yl)biphenyl) as the host have been fabricated. Compound 3 doped in the emissive layer demonstrate external quantum efficiency as high as 20.1% at 1000 cd/m². In addition, the device based on compound 1 as an emitter shows a stable lifetime greater than 300 h at 1000 cd/m², which is one of the best results concerning the device lifetime.     

Straight-forward control of the degree of micro-cavity effects in organic light-emitting diodes based on a thin striped metal layer

We investigated the control of micro-cavity (MC) effects in organic light-emitting diodes (OLEDs) with the introduction of a striped thin metal layer between the indium tin oxide (ITO) layer and the hole transporting layer (HTL). With an enhanced MC effect obtained through the inserted metal layer, the forward emission of the OLED became stronger and the angular distribution became more forward-directed, leading to a current efficiency (CE) that was nearly 1.45 times higher than that of the reference device without the inserted metal layer. The net CE of the OLEDs with a striped metal layer was found to be determined by the area-weighted average of the CE’s of full-cavity-enhanced OLEDs and non-cavity OLEDs. It was also observed that the trade-off between resonance enhancement in efficiency and angle-dependent color stability, often found problematic in MC-based OLEDs, could be mitigated in a straight-forward manner by changing the relative portion of the metal-covered area.     

Deep blue phosphorescent organic light-emitting diodes with excellent external quantum efficiency

Highly efficient deep blue phosphorescent organic light-emitting diodes (PHOLEDs) using two heteroleptic iridium compounds, (dfpypy)₂Ir(acac) and (dfpypy)₂Ir(dpm), as a dopant and 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazol-3-yl)diphenylphosphine oxide as a host material have been developed. The electroluminescent device of (dfpypy)₂Ir(dpm) at the doping level of 3 wt% shows the best performance with external quantum efficiency of 18.5–20.4% at the brightness of 100–1000 cd/m² and the color coordinate of (0.14, 0.18) at 1000 cd/m².     

Degradation pattern of contact resistance and characteristic trap energy in blue organic light-emitting diodes

Thin and lightweight organic light-emitting diodes (OLEDs) are promising candidates for next-generation rollable displays; they offer numerous advantages, such as scalable manufacturing, high color contrast ratio, flexibility, and wide viewing angle. Despite the numerous merits of OLEDs, the insufficient lifetime and stability of blue OLEDs remain unresolved, thereby necessitating a feedback strategy for lifetime extension. Herein, we propose a simple yet effective methodology to determine the contact resistance (R_CT) and characteristic trap energy (E_T) of OLEDs simultaneously in the trapped-charge-limited-conduction regime, where electroluminescence occurs primarily. To validate our approach, the extracted R_CT and E_T values are directly compared with each other by connecting a commercial resistor (R_C) to a blue OLED in series. The percent errors discovered in R_C and E_T are less than 7% and 4%, demonstrating the high feasibility and accuracy of our approach. We further employ this method to study the degradation mechanism of a blue OLED by presenting the electrical stress time- and cycle-dependent R_CT, E_T, ideality factor, and turn-on voltage, revealing different degradation patterns of the metal-to-transport layer interface and emission layer, respectively. Our results provide better insights into the electrical parameter extraction method and electrical current degradation mechanism in blue OLEDs.     

Single walled carbon nanotube anodes based high performance organic light-emitting diodes with enhanced contrast ratio

Commercially-available single walled carbon nanotubes (SWCNTs) were used to fabricate SWCNT sheets for anodes of organic light-emitting diodes (OLEDs) by spray-coating process without any use of surfactant or acid treatment. A layer of DMSO doped PEDOT:PSS was spray-coated on the SWCNT sheets to not only lessen the surface roughness to an acceptable level, but also improve the conductivity by more than three orders of magnitude. For our SWCNT-based OLEDs of tris-(8-hydroxquinoline) aluminum (Alq₃) emission layers, a maximum luminance 4224 cd/m² and current efficiency 3.12 cd/A were achieved, which is close to the efficiency of ITO-based OLEDs. We further found out that our OLEDs based on the PEDOT:PSS covered SWCNT anodes tripled the contrast ratio of the conventional indium tin oxide (ITO) based OLEDs.     

Effects of surface treatment of ITO anode layer patterned with shadow mask technology on characteristics of organic light-emitting diodes

We investigated the effects of various surface treatments of indium tin oxide (ITO) on the electrical and optical characteristics of organic light-emitting diodes (OLEDs). A 150-nm-thick ITO anode layer was patterned directly with a shadow mask during the sputtering process without the use of a conventional photolithography patterning method. The sputtered ITO layer was subjected to thermal and oxygen plasma treatments to reduce the sheet resistance and improve surface roughness. The thermal treatment was performed for 1 h at temperatures of 250 and 380 °C, which were chosen so that the glass substrates would not deform from thermal damage. The measured sheet resistance decreased from 30.86 Ω/sq for the as-sputtered samples to 8.76 Ω/sq for the samples thermally treated at 380 °C for 1 h followed by oxygen plasma treatment. The root-mean-square surface roughness measured by atomic force microscopy considerably decreased to 3.88 nm with oxygen plasma treatment. The thermal treatment considerably decreased the sheet resistance of the ITO anode layer patterned with the shadow mask. The spike-like structures that are often formed and observed in shadow mask-patterned ITO anode layers were almost all removed by the oxygen plasma treatment. Therefore, a smooth surface for shadow mask-patterned ITO layers with low sheet resistance can be obtained by combining thermal and oxygen plasma treatments. A smooth surface and low sheet resistance improves the electrical and optical characteristics of OLEDs. The surface-treated ITO layer was used to fabricate and characterize green phosphorescent OLED devices. The typical characteristics of OLED devices based on surface-treated shadow mask-patterned ITO layers were compared with those fabricated on untreated and photolithography-patterned ITO layers to investigate the surface treatment effects. The OLED devices fabricated by thermal treatment at 380 °C for 1 h followed by oxygen plasma treatment for 180 s showed the highest luminance and current density. Furthermore, the leakage current that might be induced by the rough ITO surface was dramatically reduced to 0.112 mA/cm². Our study showed that the shadow mask-patterned ITO anode layer treated by heat and plasma and having a low sheet resistance and surface roughness yielded excellent electrical and optical properties for OLEDs compared to those based on an untreated ITO layer. The fabricated OLED devices using the surface-treated shadow mask-patterned ITO layer exhibited comparable characteristics to those obtained from a conventional photolithography-patterned ITO anode.     

Improved efficiency in green polymer light-emitting diodes with air-stable electrodes

By dispersing an electron transporting molecular dopant into the active semiconducting luminescent polymer, we have achieved improved efficiencies for green light-emitting diodes (LEDs). These green emitting LEDs were fabricated by adding an electron transporting molecular dopant, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-l,3,4-oxadiazole (PBD), into the semiconducting luminescent polymer as the emitting layer in the polymer LEDs. The devices used poly(2-cholestanoxy-5-thexyldimethylsilyl-l,4-phenylene vinylene) (CS-PPV), a new soluble green light emitter, as the semiconducting luminescent polymer and either aluminum or indium as the electron injection electrodes. Quantum efficiencies of LEDs with the electron transporting molecular additive in the luminescent polymer and an Al electrode are about 0.3% photons per electron, better by a factor of 18 than similar devices made without the addition of the electron transport molecular dopant; quantum efficiencies of similar LEDs fabricated with an In electrode are 0.23% photons per electron, better by a factor of 16 than devices without the electron transport molecular additive.     

Corrugated structure through a spin-coating process for enhanced light extraction from organic light-emitting diodes

We have demonstrated a simple fabrication method for an out-coupling structure to enhance light extraction from organic light-emitting diodes (OLEDs). Spin-coating of SiO₂ and TiO_x sol mixture solution develops corrugated film. The structural evolution of the corrugation was explained by the localization of surface tension during the solvent evaporation. The structural parameters of the corrugated structure were characterized by varying the spin-coating speed and the mixing ratio of the solution. Compared to conventional devices, OLEDs with a corrugated structure at the backside of the glass substrate showed increased external quantum efficiency without change in the electroluminescence spectrum. The light extraction enhancement is attributed to the decreased incidence angle at the interface of glass substrate and air.     

A systematic approach to reducing angular color shift in cavity-based organic light-emitting diodes

The white angular dependence (WAD) is considered a serious problem in top emitting OLEDs (TOLEDs), both in monochrome and multi-color subpixel configurations. This work aims at providing a systematic strategy for obtaining WAD-free performance with little compromise in efficiency in TOLED-based multi-color displays. Starting with a slightly blue-detuned microcavity structure leading to low WAD per individual primary colors, we try to look for a condition where angular intensity drop rates are well balanced among the subpixels involved and yet the efficiency can be maintained within a certain threshold value. With the proposed protocol, red, green and blue TOLEDs are demonstrated that show a very low angular color shift of 2.4 “just noticeable color difference” (JNCD), 1.7 JNCD, and 1.4 JNCD for red, green, and blue devices themselves and 1.4 JNCD for a 1:1:1 mixture of red, green and blue. These devices are shown to exhibit not only low WAD but also high efficiency that is still within 70% of the maximum achievable values.     

Transparent and color-tunable organic light-emitting diodes with highly balanced emission to both sides

Future lighting applications will strongly benefit from transparent luminescent devices. Here, we demonstrate transparent organic light-emitting diodes (OLEDs), which provide real-time adjustment of the emission color. Making use of the AC/DC concept, two stacked subunits can be addressed independently via an AC signal. Combining blue and yellow emission leads to the possibility to tune the emitted color between deep blue over cold white and warm white to yellow on both emission sides. For such highly complex device architectures, the thickness of each layer needs to be adjusted carefully in order to achieve balanced and efficient emission in both directions. Therefore, optical simulations are carried out to optimize the OLED. Based on these simulations, we present transparent, indium-free OLEDs that achieve a luminous efficacy of 8.7 lm/W in bottom direction and 9.7 lm/W in top direction at a brightness level of 1000 cd/m² for warm white emission and a peak transmission of 56%. Using an emitter combination providing red, green, and blue emission, we were able to achieve a high color-rendering index (CRI) of 84, which further expands the range of possible applications for this promising device concept.     

Efficient solution-processed blue phosphorescent organic light-emitting diodes with halogen-free solvent to optimize the emissive layer morphology

Efficient solution-processed blue phosphorescent organic light-emitting diodes (OLEDs) featuring with halogen-free solvent processing are fabricated in this study. The organic molecule 3,6-bis(diphenylphosphoryl)-9-(4′-(diphenylphosphoryl) phenyl)-carbazole (TPCz) that possesses good solubility in halogen-free polar solvents is selected to serve as the host of blue phosphorescent iridium(III) [bis(4,6-difluorophenyl)-pyridinato-N,C²]-picolinate (FIrpic) dopant. The morphology of the TPCz:FIrpic emissive layer prepared with different polar solvents including chlorobenzene (CB), n-butanol (ButA) and isopropanol (IPA) and the effect on their electroluminescent performance have been investigated in detail. It is found that the more polar halogen-free solvent IPA restrains the FIrpic aggregation and renders a more densely packed emissive layer as compared to the CB-processed counterpart, which results in the enhanced electroluminescent performance. The luminous efficiency and power efficiency of the blue phosphorescent OLEDs prepared with CB are merely 5.7 cd/A and 3.3 lm/W, respectively. When using more polar halogen-free solvent IPA, the efficiencies are enhanced to 22.3 cd/A and 15.6 lm/W, about 2.9 and 3.7-time increment, respectively. This work provides an approach to fabricate efficient solution-processed phosphorescent OLEDs with environmental-friendly solvents, which is highly required in large-scale solution-processed manufacturing.     

Multiaxial wavy top-emission organic light-emitting diodes on thermally prestrained elastomeric substrates

In this work, we report the fabrication process of wavy top-emission organic light-emitting diodes (WOLEDs), which can sustain multiaxial tensile and compressive strains. The devices are fabricated using standard procedures, comprised of the conventional stacks of OLED materials and transfer printing process. Transferring these devices onto thermally prestrained elastomeric substrates and then releasing this strain configure the devices into random, two-dimensional (2D) wavy layouts. The performance of the WOLEDs is analyzed at ±1.5% (strain ratio = 1.16) and ±3% (strain ratio = 2.33) strain with respect to the prestrain value. The fabricated WOLEDs demonstrate good performance in the green light region within ±1.5% and show comparable results even at ±3% tensile and compression strains, which indicates that the fabricated devices can accommodate high strain ratios without inducing significant stresses in the devices. Finite element simulation demonstrates strong coherence with the experimental results and provides a valuable insight into the strain effects on each layer utilized for the device fabrication. Along with that, the neutral plane is generated around the upper region of emission and cathode layers in the devices. A slight blue shift observed by the electroluminescence analysis reveals that luminescence of various colors can be obtained by changing the dimensions of the wavy buckles. This research work can remarkably contribute to the fabrication of multicolored flexible, wearable indicators or curvilinear displays that require the ability not only to bend and stretch, but also to compress in multiple directions with a high strain ratio.     

Fabrication of color tunable organic light-emitting diodes by an alignment free mask patterning method

An alignment free mask patterning method has been proposed for fabricating the side-by-side color tunable organic light-emitting diodes (OLEDs). The demonstrated color tunable OLEDs consists of blue sub-OLEDs and inverted orange sub-OLEDs; both color sub-OLEDs share the same electrodes. With time sequential pulse driving, the blue sub-OLEDs and the inverted orange sub-OLEDs are alternately turned on. Tunable color, resulting from the mixing of the blue and the orange emission, has been realized by simply varying the amplitude ratio of the positive and negative pulses.     

Enhanced light extraction efficiency using self-textured aluminum-doped zinc oxide in organic light-emitting diodes

We report enhanced light extraction efficiency in organic light-emitting diodes (OLEDs) fabricated on a self-textured aluminum-doped zinc oxide (AZO) anode layer. The self-textured AZO (ST-AZO) layer was fabricated by radio-frequency magnetron sputtering with a short period of thermal treatment without employing any additional etching processes. The green-emitting OLEDs exhibited a maximum power efficiency of 56.1 lm/W with 33.7% external quantum efficiency (EQE). We achieved a 3.24-fold enhancement in power efficiency and 2.55-fold increase in EQE for the OLED fabricated on the ST-AZO anode compared to that fabricated on the ITO anode. Furthermore, a low driving voltage and high current efficiency were obtained simultaneously for the OLED fabricated on the ST-AZO layer compared to that fabricated on the flat ITO anode layer. The ST-AZO layer acted as a random scattering layer that enabled the efficient extraction of generated light and served as the anode layer instead of the commonly used ITO. Our study showed that the ST-AZO layer fabricated by a simple sputtering process effectively improved the optical and electrical properties of the OLED.     

Plasma-tolerant structure for organic light-emitting diodes with aluminum cathodes fabricated by DC magnetron sputtering: Using a Li-doped electron transport layer

We fabricate aluminum cathodes that are almost free from plasma damage by DC magnetron sputtering for organic light-emitting diodes (OLEDs). While sputtering is widely known to have numerous advantages over conventional evaporation for mass production of devices, it can cause serious damage to organic layers. In this report, we fabricate devices that are free from plasma damage by introducing a 1%-Li-doped electron transport layer (ETL). The difference of external electroluminescence quantum efficiency between OLEDs with the structure ITO/α-NPD/ETL/Al (where ITO is indium tin oxide and α-NPD is N,N′-di(1-naphthyl)-N,N′-diphenylbenzidine) with Al cathodes deposited by conventional evaporation or sputtering is 0.1%, and their driving voltage is identical. We find that the Li-doped ETL should be thicker than 40 nm. Analysis of the depth profile of the ETL by time-of-flight secondary ion mass spectrometry indicates that considerable damage from sputtering extended to a depth of approximately 30 nm, suggesting that high-energy particles penetrated about 30 nm into the ETL.     

Prism coupling method to evaluate the light extraction efficiency of outcoupling substrates

One of the key challenges in organic light-emitting diodes (OLEDs) for lighting applications is efficient light extraction from the planar, multi-layered OLED stack. Several different light extraction approaches are being explored currently by researchers, however characterizing light extraction films after fabricating OLEDs is not a viable approach when the outcoupling films have large surface roughness and is time consuming as well. Here we apply prism coupling method (PCM), a simple and elegant tool, to characterize outcoupling films. We show the effectiveness of PCM in estimating light extraction efficiency of outcoupling films. PCM can expedite selection and optimization of various light extraction approaches without the need to build OLEDs. The experimental results are corroborated by the optical simulations done using ray tracing method taking into account Mie scattering from wavelength sized spherical inclusions in an outcoupling film.