Top-emitting Organic Light-emitting Diodes Research Articles

Patternable Semi‐Transparent Cathode using Thermal Evaporation for OLED Display Applications

AbstractA new patternable semi‐transparent metal cathode, Zn:Mg (4:1 and 10:1) using the thermal evaporation process for organic light‐emitting diode (OLED) display applications is reported. A new Zn:Mg alloy film shows good surface morphology and almost no temperature effect on the shadow mask. The Zn:Mg layer with ratio of 4:1 and 10:1 shows sheet resistance of 33.2 and 39.2 Ω ϒ−1, respectively, at the thickness of 12 nm. The fabricated green top‐emitting OLED (TEOLED) with Zn:Mg cathode shows current efficiency of 79.3 cd A−1 and a color coordinates of (0.166, 0.755). These results show the possibility of high‐quality cathode patterning for various OLED display and sensor applications.

A Deep Blue Strong Microcavity Organic Light‐Emitting Diode Optimized by a Low Absorption Semitransparent Cathode and a Narrow Bandwidth Emitter

Herein, deep blue strong microcavity top‐emitting organic light‐emitting diodes (TEOLEDs) optimized by a low absorbed silver cathode and a narrow bandwidth emitter are reported for realizing high‐efficiency and narrow spectral characteristics. Usually, the Mg:Ag cathode is widely used as a semitransparent cathode in TEOLED devices due to its favorable reflectivity and proper electron injection ability. However, the high extinction coefficient of Mg causes a decrease in the microcavity effect, causing a low optical efficiency of devices. A Ag cathode is applied as a semitransparent cathode to modify the high absorption property for a strong microcavity blue OLED. At optimal conditions, the optically calculated device efficiency of TEOLEDs using a BPPyA emitter is 70.2 and 91.1 cd (A * y)−1 for Mg:Ag and Ag TEOLEDs, respectively. To further improve the device efficiency, an alternative emitter (DABNA‐NP‐TB) is applied which has a narrow bandwidth of 25 nm. The optically calculated efficiency enhancement in the TEOLED is about 23% compared with a control emitter, which is well correlated with the result of fabricated TEOLEDs.

Realizing 8 cd A−1 Current Efficiency for Solution-Processed Inverted Top-Emitting Polymer Light-Emitting Diodes

We report indium tin oxide-free top-emitting organic light-emitting diodes (OLEDs) with solution-processed organic layers. The two electrodes are made of evaporated silver, creating a Fabry-Perot resonator. The main issue of solution-processed OLEDs is the thickness reproducibility, which greatly influences the top-emitting OLED’s performance and color because of the optical resonator. In this work, we combine simulation with experimental results to show the good thickness homogeneity of the spin-coated layers, with a 7% thickness accuracy. Similar efficiencies are obtained for top- and bottom-emitting OLEDs based on the same organic materials, 8.1 ± 0.3 cd A−1 and 8.1 ± 0.2 cd A−1, respectively, with similar Commission Internationale de l’Eclairage (CIE) coordinates. This work demonstrates the feasibility to fabricate microcavity top-emitting OLEDs by solution process and the possibility to increase the performance by using a capping layer.

Technical status of top-emission organic light-emitting diodes

Top-emission organic light-emitting diodes (TEOLEDs) that contain semi-transparent metal top cathodes and highly reflective anodes have been actively researched for display applications due to their many advantages such as their high aperture ratio, good color purity, and high efficiency. In this research, the technical design of TEOLEDs used in organic light-emitting diode (OLED) displays is reviewed, covering the optical theory with the microcavity effect, optical losses, and the importance of the Purcell effect in the optical calculation. The key methodologies for addressing the high efficiency and low driving voltage of TEOLEDs are also discussed.

Optical outcoupling in monochromatic top-emitting organic light-emitting diodes with an Au/Ag semi-transparent electrode

Top-emitting organic light-emitting diodes can achieve high efficiencies due to the strong cavity effect resulting from the relatively thick semi-transparent metallic top electrode. The strong cavity resonance, however, simultaneously brings along negative side effects such as pronounced angular-dependent emission and spectral narrowing. In this work, through numerical simulations, we demonstrate that top-emitting organic light-emitting diodes using a thin Au(2 nm)/Ag(7 nm) top electrode can achieve light-outcoupling efficiency comparable to a thick silver electrode, while reducing spectral narrowing. This can be realized by tuning the organic capping layer thickness without affecting the electrical properties, which can be applied to diodes based on either intrinsic or efficiently doped charge transport layers.

Second-order Microcavity Length Based on Top-emitting Organic Light-emitting Diodes

Second-order Microcavity Length Based on Top-emitting Organic Light-emitting Diodes

A new host material achieving above 75 cd A−1 current efficiency with top-emitting deep-red phosphorescent organic light-emitting diodes

Highly efficient deep-red phosphorescent OLEDs (PhOLEDs) are very important in the practical application of full-color display and automobile lighting.

Effect of a P-doped hole transport and charge generation layer on single and two-tandem blue top-emitting organic light-emitting diodes

The transmittance of the p-doped hole transporting layer (p-HTL) and the charge generation layer (p-CGL) corresponding to the photoluminescence (PL) of blue dopants in an emitting layer decreases as the ratio of the p-dopant increases due to the absorption of the p-dopant. However, there was little difference in the luminous efficiency of blue top-emitting organic light-emitting diodes using p-HTL or p-CGL at a maximum doping ratio of 20%. p-HTL for a single structure required a 5% doping ratio to ensure sufficient electrical characteristics, but p-CGL for the two-stack tandem structure required more than a 10% doping ratio. The optical simulation showed that the device was affected by the specific absorbance of the p-dopant depending on the doping ratio and thickness. Although there was no significant difference in efficiency depending on the doping ratio at a thickness of 10 nm, the reduction rate of the external quantum efficiency increased from over 20 nm due to the doping ratio.

Flexible distributed Bragg reflectors as optical outcouplers for OLEDs based on a polymeric anode

Top-emitting OLEDs (TOLEDs) represent a promising technology for the development of next-generation flexible and rollable displays, thanks to their improved light outcoupling and their compatibility with opaque substrates. Metal thin films are the most used electrodes for the manufacturing of TOLEDs, but they show poor resistance to mechanical deformation, which compromises the long-term durability of flexible devices. This paper reports the exploitation of a dielectric mirror (DBR) based on seven pairs of TiO2 and SiO2 combined with a polymeric electrode as an alternative to the bottom metal electrode in flexible TOLEDs. The DBR showed a maximum reflectivity of 99.9% at about 550 nm, and a stop-band width of about 200 nm. The reflectivity remained unchanged after bending and treatment with water and solvents. Green TOLED devices were fabricated on top of DBRs, and demonstrated good stability in terms of electro-optical and colorimetric characteristics, according to varying viewing angles. These results demonstrate that the combination of the flexible DBR with the polymeric anode is an interesting strategy for improving the durability of flexible TOLEDs for display applications, implemented on different kinds of free-standing ultra-thin substrates.

Luminance compensation in top-emitting organic light-emitting diodes with high color purity as quantum dots using horizontal dipole orientation

Abstract Organic light-emitting diodes (OLEDs) and quantum dots (QDs) are mainstream applications in large-sized displays. Owing to their narrow photoluminescence (PL), QDs offer a wide color gamut. Although OLEDs have a wider PL spectrum bandwidth than that of QDs, a high color purity can be obtained through an optical design of the top-emitting device. However, the luminance improvement from the strong cavity effect in a top-emitting geometry is not significant because of the wide PL spectrum. This insufficient luminance improvement can be compensated by applying organic light-emitting materials with horizontally oriented dipoles. Luminance compensation is color dependent. In a blue light-emitting structure, the luminance improvement by a narrow PL spectrum is more advantageous than by a horizontal dipole orientation. However, in green and red light-emitting structures, even if the full width at half maximum of the PL is about 20-nm-wide, luminance can be compensated using a horizontal dipole orientation while maintaining high color purity.

Multi-mode Organic Light-Emitting Diode to Suppress the Viewing Angle Dependence.

A typical top-emitting organic light-emitting diode (OLED) has a strong microcavity effect because of the two reflective electrodes. The cavity effect causes a serious color shift with the viewing angles and restricts the organic layer thickness. To overcome these drawbacks, we design a multi-mode OLED structure with dual-dielectric spacer layers, which extend the cavity length by more than 10 times. This design completely eliminates the intrinsic cavity effect caused by the top and bottom boundaries and provides freedom for the organic layer thickness. We demonstrate these effects in a white multi-mode OLED using a white emitter, which shows a negligible angular chromaticity shift of Δuv = 0.006 from 0 to 70° and a Lambertian emission profile. The simple design and the perfect angular color profiles make the multi-mode OLED structure promising in large-area displays and solid-state lighting applications.

Impact of pixel surface topography onto thin-film encapsulated top-emitting organic light-emitting diodes performances

Two different designs of top-emitting green OLEDs (Organic Light-Emitting Diodes) have been studied. The first one presents a planar OLED architecture. The second one presents an OLED having a topographic surface, so as to simulate a pixel partitioning of a display using an electrically insulating, 200 nm-thick, resist. It has been observed that the topography has a large impact on OLED performances. Studying devices using an ALD (Atomic Layer Deposition)-deposited Al2O3 barrier film shows that topographic OLEDs have a lower stability under storage in 65 °C/85% RH conditions compared to planar ones, with a difference in ageing models between the two devices. As the ALD deposition technology has a high conformity, which implies that ALD-deposited Al2O3 barrier films should be as good on topographic devices as on planar ones, we inferred that the topographic OLED Achilles’ heel lies rather in the OLED structure rather than in the Al2O3 encapsulation itself. Thus, topographic and planar unencapsulated OLEDs (without Al2O3) were studied: interestingly, it has been observed that planar OLEDs can live several weeks, while topographic OLEDs show a very short shelf lifetime (in laboratory atmosphere, at 21 °C/50% RH), of only a couple of hours. It will be shown that the topographic OLED surface tends to reduce the thickness of the PVD (Physical vapour Deposition)-deposited layers in the OLED, as this is expected for a non-conformal deposition PVD technique, on tapered angle regions of the resist (pixel edges). While this thickness variation would not be critical for thick electrodes, as for instance for bottom-emitting devices made on glass substrates, this thickness reduction turns out to be a critical point for the ultrathin, 15 nm, silver cathode, used as semi-transparent electrode in this top-emitting architecture and will therefore be discussed in the framework of using OLED top-emitting architectures in (micro)display technology.

Top-emission organic light emitting diode with indium tin oxide top-electrode films deposited by a low-damage facing-target type sputtering method

To confirm the effectiveness of the low-damage facing-target sputtering (FTS) method for the deposition of a transparent oxide electrode film on an organic layer, an indium tin oxide (ITO) top electrode was deposited via FTS, and a top emission organic light-emitting diode (OLED) was fabricated. Top and bottom electrode films were deposited using the FTS system; in addition, a top emission a bottom emission OLED were fabricated with an inverse stacking structure. Consequently, both OLEDs exhibited similar operating characteristics, and the top emission OLED had a better luminance efficiency than the bottom emission OLED did. Compared with the OLED fabricated using the evaporation method, the OLEDs fabricated using FTS exhibited similar luminance efficiencies, although they had considerably higher operating voltages. In addition, annealing treatment at 110 °C was observed to lead to a significant reduction in the operating voltages; moreover, the obtained top emission OLEDs exhibited operating characteristics similar to those of the OLED fabricated using the evaporation method. These results indicate that the FTS method is useful for the deposition of the top oxide electrode films of OLEDs.

All solution-processed ITO free flexible organic light-emitting diodes

A top-emitting organic light-emitting diode is fabricated by a solution-process exclusively, using a PEDOT:PSS formulation with a low contact angle.

A Study of Encapsulation Structure for TFT Reliability in Top Emission OLED Display

A Study of Encapsulation Structure for TFT Reliability in Top Emission OLED Display

A Study of Encapsulation Structure for TFT Reliability in Top Emission OLED Display

A Study of Encapsulation Structure for TFT Reliability in Top Emission OLED Display

Light Extraction and Viewing Angle Characteristics of Nano-structure embedded Top-emitting OLEDs fabricated by Vacuum Deposition Processes

Light Extraction and Viewing Angle Characteristics of Nano-structure embedded Top-emitting OLEDs fabricated by Vacuum Deposition Processes

Electro-optically Efficient and Thermally Stable Multilayer Semitransparent Pristine Ag Cathode Structure for Top-Emission Organic Light-Emitting Diodes

To attain high efficiency and stability in a top-emission organic light-emitting diode (TEOLED), the semitransparent metal cathode must have high transmittance, low absorbance, and uniform surface ...

P‐13.5: Top‐Emitting Organic Light‐Emitting Devices with Short Reduction Layer

There is an increasing attention paid to improving transparent conductive electrodes for applications in highly transparent and large size displays. We optimized top‐emitting organic light‐emitting diodes (TE‐OLEDs) using high resistance IZO as a short reduction layer to reduce leakage current through a shorting defect. Leakage current devices have decreased by 5% and brightness decreased by 11% less than the devices with the identical organic layers and standard IZO cathode.

Spontaneously formed organic wrinkle structure for top-emitting organic light emitting diodes

Due to the imperfections of top metal electrodes, top-emitting organic light emitting diodes (TEOLEDs) suffer low efficiencies and non-uniform angular emission spectral characteristics. We demonstrate a novel and simple method of overcoming these drawbacks by directly forming wrinkles on the thin film encapsulation (TFE) surface of TEOLEDs. The developed method relies on the use of a liquid prepolymer that spontaneously forms randomly distributed micron-sized wrinkles upon exposure to ultra-violet light. The wrinkles contribute to the extraction of light confined in the air mode by widening the angle of the escape cone, leading to a 1.4-fold increase in the current efficiency. Due to the spatial randomness of the organic wrinkle structure, almost negligible spectral angular dependency was achieved. As a merit of the simple process at room temperature without complicated processes such as patterning and vacuum processes, the developed organic wrinkle structure can be directly used to enhance the characteristics of TEOLEDs, and is expandable to flexible OLEDs and various optoelectronic devices.