Top-emitting Organic Light-emitting Diodes Research Articles

High-efficiency top-emitting organic light-emitting diodes based on metal/ITO composite electrodes

High-performance top-emitting organic light-emitting diodes (TEOLEDs) integrating the metal/ITO composite anodes were developed. The results revealed that covering the surface of Al anode with ITO layer could effectively improve the charge injection efficiency and balance the hole–electron charge. The effect of the thickness of ITO on the performance of TEOLEDs with Al/ITO anodes was further studied. The TEOLEDs with Al/ITO (5 nm) anode showed optimized performance, with the current efficiency (CE) and external quantum efficiency (EQE) enhanced by 40.0% and 34.5% compared to that of the OELD with pure Al anode, and increased by 93.1% and 33.5% compared to that of bottom-emitting OLEDs. In addition, the full width at half maximum (FWHM) was narrowed to 36 nm due to the micro-cavity effect of the top-emitting structure and the turn-on voltage decreased to as low as 2.3 V owing to the efficient charge injection and well-matched energy level. In addition, TEOLEDs using Ag/ITO as anode exhibited a slow roll-off of CE and EQE and a narrower FWHM of 30 nm, greatly improving the color purity. The strategy is simple and can significantly improve the efficiency of the TEOLEDs, which promotes the applications of OLEDs in the fields of ultra-high-definition displays and micro-displays.

Operando measurements of top-emission organic light-emitting diode using electric-field-induced doubly resonant sum-frequency generation vibrational spectroscopy

We performed sum-frequency generation (SFG) measurements on top-emission (TE) organic light-emitting diodes (OLEDs) during operation. These measurements require the detection of SFG light using probe laser beams transmitted through a 9 nm thick semitransparent Mg:Ag metal electrode. Only the SFG signals from the hole transport material (4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl, NPD) are observed for the device configuration of the OLEDs in this study. The SFG peak (at approximately 1603 cm−1) shows a linear dependence on the applied voltage. This indicates that the NPD-derived SFG is enhanced by an electric-field-induced (EFI) effect owing to injected charges. In addition, the SFG efficiency of the TE OLED is considerably higher than that of a bottom-emission (BE) OLED manufactured in the same batch using the same material and film thickness. The difference in the SFG responses of the BE and TE OLEDs is caused by structural differences rather than the layer composition. The results of this study indicate that EFI-doubly resonant-SFG operando measurements can be applied to analyze the carrier behavior and charge transport of not only BE OLEDs but also TE OLEDs, the latter of which are used more commonly as commercial products.

High-efficiency and reduced efficiency roll-off of top-emitting organic light-emitting diodes

High-efficiency and reduced efficiency roll-off of top-emitting organic light-emitting diodes

High light extraction performance using evanescent waves for top emission OLED applications with thin film encapsulation.

We report high light extraction from the top emission OLED (TEOLED) device structure by improving mainly the waveguide mode loss in the atomic layer deposition processed thin film encapsulation (TFE) layer. A novel structure incorporating the light extraction concept using evanescent waves and the hermetic encapsulation of a TEOLED device is presented here. When the TEOLED device is fabricated using the TFE layer, a substantial amount of generated light is trapped inside the device due to the difference in refractive index (RI) between the capping layer (CPL) and the aluminum oxide (Al2O3) layer. By inserting a low RI layer at the interface between the CPL and Al2O3, the direction of the internal reflected light is changed by the evanescent waves. The high light extraction with the low RI layer is attributed to the presence of evanescent waves and an electric field in the low RI layer. The novel fabricated TFE structure, CPL/ low RI layer/ Al2O3/ polymer/ Al2O3, is reported here. The current efficiency of the fabricated blue TEOLED device using this low RI layer is improved by about 23% and the blue index value is enhanced by about 26%. This new approach for light extraction will be applicable to future encapsulation technology for flexible optoelectronic devices.

Characterization of the material and electrical properties depending on the Mg:Ag ratio as a cathode for TEOLED

In this study, Mg:Ag mixed alloy films were comprehensively analyzed as a potential transparent cathode, depending on the deposition ratio of Mg:Ag atoms. It was determined that the work function, refractive index, and film formation characteristics varied depending on the deposition ratio of Mg:Ag, and affected electro-optical characteristics such as transmittance and sheet resistance. In addition, the applicability of the proposed transparent cathode for top emitting organic light-emitting diodes (TEOLEDs) was analyzed by comprehensively considering the TEOLED performance in relation to the Mg:Ag deposition ratio. The TEOLED performance was optimized by adjusting the Mg:Ag transmittance, sheet resistance and appropriate work function, and the emission spectrum could be controlled by localized surface plasmon resonance (LSPR) and cavity mode coupling effect.

Research on the High Light Out-Coupling Efficiency Deep-Blue Top-Emitting Organic Light-Emitting Diode through FDTD Optical Simulation.

We have studied high light out-coupling efficiency top-emitting organic light-emitting diodes (TOLEDs) under the guidance of the finite-difference time-domain (FDTD) simulation. TOLED achieves an extraordinarily high light extraction efficiency at 468 nm, in deep-blue regions, of 49.70%, which is approximately 3.5 times that of the bottom light-emitting diode (BOLED) by changing the thickness of the organic layer and the position of the light-emitting layer in the FDTD simulation. Based on the simulation results, the TOLED with ultrahigh efficiency and narrow full width at half maximum is successfully fabricated, and the maximum external quantum efficiency of TOLED is almost 3.3 times that of the BOLED, which is perfectly consistent with the FDTD simulation results. Meanwhile, the shift of the electroluminescence spectrum of the TOLED is restricted within 10° in the angular-dependence test (0° to 80°). The optimized performance of the OLED indicates a new method to develop a high-performance device under the guidance of simulation.

39.1: Invited Paper: Study on Filler Film Degradation Mechanism in OLED Encapsulation

Dam & Fill is the main encapsulation method for large-sized top-emitting Organic light-emitting diodes (OLEDs) displays, in which light transmits through filler encapsulation film. Transparency of the filler film is a critical factor in obtaining a high efficiency and high reliability OLED display. In this paper, we study the degradation mechanism of filler film in OLED encapsulation. The results reveal that Ultraviolet (UV) light below 325 nm degrades filler film and reduces the film transmittance, CF film and low temperature effectively mitigate UV light damage to filler film, and high temperature (100 ℃) has no effect on filler film transparency.

Cathode Metal Patterning in Top Emission OLED through Insertion of Low Surface Energy Organic Molecules

Cathode Metal Patterning in Top Emission OLED through Insertion of Low Surface Energy Organic Molecules

Performance improvement of blue TADF top-emission OLEDs by tuning hole injection barriers using a nickel-doped silicon dioxide buffer layer

The severe exciton self-quenching and twisted structure of typical thermally activated delayed fluorescence (TADF) emitters result in a low external quantum efficiency (EQE) and broad emission spectrum, especially in blue organic light-emitting diodes (OLEDs). These challenges have been overcome by employing complex device fabrication steps and complex molecular structures requiring fine stoichiometric adjustments. However, only a few attempts have been made to improve the performance of OLEDs through device structure engineering, particularly in top-emission OLEDs (TEOLEDs). Herein, we report blue TADF TEOLEDs fabricated by employing a second-order microcavity structure. These TEOLEDs simultaneously exhibit a high EQE (∼20.2 %) and narrow full width at half maximum (29 nm) owing to the incorporation of a Ni-doped SiO2 buffer layer that balances the injection charges. The wide bandgap of SiO2 prevents rapid hole injection from the anode and modifies the anode–organic layer interface, while tuning of the Ni doping concentration reduces the turn-on voltage of the device through co-sputtering. The charge balance mechanism is elucidated in detail by analyzing tunneling effects across the buffer layer and the bonding states of Ni atoms in the SiO2 film. This study can promote the development of TADF top-emission devices with high efficiency and high color purity.

Merits of narrow spectral blue fluorescence emitters in strong micro-cavity top emission OLEDs

The top emission organic light emitting diode (TEOLED) structure can realize the high efficiency and high color purity through the micro-cavity effect. The characteristics of TEOLED are enhanced by improving the electrodes structures or designing cavities through organic layers. Apart from these factors, the optical characteristic of the emitting material is another very important aspect to improve the TEOLED characteristics. In the present study, we investigated the influence of the narrow full width at half maximum (FWHM) of emitter on the blue light emitting device characteristics through the simulations and experiments. We analyzed the radiance, current efficiency, and blue index characteristics of TEOLED using several emitters having different FWHM values. As the FWHM became narrower, the radiant out-coupling spectrum could overlap sufficiently with original one; so the radiance was improved, but the current efficiency was affected due to the decrease in luminosity factor. Additionally, the blue index value is an important in TEOLED structure, as display applications uses it. TEOLED having the same emission peak value, the narrower FWHM emitter is advantageous in terms of the blue index because of the improved color purity. The narrower the FWHM, the just noticeable color difference is low. The color stability according to the viewing angle is also improved. From the optical simulation results, the optimum condition of the emitter FWHM varies from 14 to 16 nm and the emission peak shifts from 460 to 462 nm. With these emitter conditions, the TEOLED device has excellent color purities and the highest blue efficiency characteristics. • Analyzes the efficiency of TEOLED device according to the FWHM of the blue emitter. • Efficiency is analyzed through optical simulation. • The viewing angle characteristics in terms of the FWHM of the blue emitter were also observed. • In order to obtain a TEOLED device with outstanding characteristics, we suggest the optimal conditions of wavelength and FWHM of blue emitter.

Enhancing hole-injection efficiency of top-emitting organic light-emitting diodes based on Al/MoNx anodes

High reflectivity bottom anodes are essential for high luminance efficiency top-emitting organic light-emitting diodes (TOLEDs). Although the element Al has high optical reflectivity, its poor hole-injection property severely hinders the realization of high efficient TOLED due to its low work function. In this letter, a high work function MoNx layer with different growth time was deposited on Al layers for enhancing the hole-injection efficiency of TOLEDs. Compared with the bottom-emitting OLED (BOLED) using indium-tin oxide (ITO) anode, the TOLED with Al/MoNx (8 s) anode shows a lower driving voltage of 3.1 eV than that (3.4 eV) of the BOLED at 20 mA/cm2, which may attribute to higher work function of MoNx leading to a stronger hole-injection. Moreover, the TOLED with Al/MoNx (8 s) anode exhibits higher brightness (11800 cd/m2) at 20 mA/cm2, higher maximum current efficiency (64.6 cd/A) and narrower electroluminescence spectra. These results indicate that the Al/MoNx layers could be effective hole-injection anodes for achieving high-performance TOLEDs.

Application of optical property-enhancement film to improve efficiency and suppress angle dependence of top-emitting organic light-emitting diodes

In this paper, we propose the application of an optical property-enhancement film that complements the angular dependence and total reflection of top-emitting organic light-emitting diodes (TEOLEDs). The optical property-enhancement film is composed of a porous pyramid arrangement applied on a thin-film encapsulation layer of TEOLEDs and is applied to distribute the transmitted light evenly. The results confirm that the change in the electroluminescence spectrum for each angle was effectively reduced because the TEOLEDs demonstrated uniform light distribution. In addition, reducing the total internal reflection in the film structure made it possible to improve the external quantum efficiency by approximately 35% and current efficiency by 38%.

Robust Laminated Anode with an Ultrathin Titanium Nitride Layer for High-Efficiency Top-Emitting Organic Light-Emitting Diodes.

The effective reflective anode remains a highly desirable component for the fabrication of reliable top-emitting organic light-emitting diodes (TE-OLEDs) which have the potential to be integrated with complementary metal-oxide-semiconductor (CMOS) circuits for microdisplays. This work demonstrates a novel laminated anode consisting of a Cr/Al/Cr multilayer stack. Furthermore, we implement an ultra-thin titanium nitride (TiN) layer as a protective layer on the top of the Cr/Al/Cr composite anode, which creates a considerably reflective surface in the visible range, and meanwhile improves the chemical stability of the electrode against the atmosphere or alkali environment. Based on [2-(2-pyridinyl-N)phenyl-C](acetylacetonate)iridium(III) as green emitter and Mg/Ag as transparent cathode, our TE-OLED using the TiN-coated anode achieves the maximum current efficiency of 71.2 cd/A and the maximum power efficiency of 66.7 lm/W, which are 81% and 90% higher than those of the reference device without TiN, respectively. The good device performance shows that the Cr/Al/Cr/TiN could function as a promising reflective anode for the high-resolution microdisplays on CMOS circuits.

Highly conductive and transparent thin metal layer for reducing microcavity effect in top-emitting white organic light-emitting diode

The optical and electrical properties of the top electrode are crucial for the structural design of top-emitting organic light-emitting diodes (OLEDs). A uniform metal layer, if obtainable, can be electrically excellent and optically well-tuned. In this study, Ag deposited with Mg at a concentration of 9:1 to form a metal thin film exhibits higher conductivity and higher transmittance than those of a neat Ag thin film within a thickness of 10 nm or less. In addition, the thin Mg:LiF (1:1) layer renders the Mg:Ag metal film more uniform and enhances its electron injection property. If a Mg:LiF/Mg:Ag thin film is applied to top-emitting white OLEDs, the driving voltage is lowered and the efficiency is improved. In addition, color shifts for viewing angles are reduced, and the emission characteristics are close to Lambertian. • 10-nm-thick Mg:Ag (1:9) metal forms a more uniform film compared with neat Ag thin metal of the same thickness. • Thin Mg:LiF (1:1) layer improves the optical and electrical properties of the Mg:Ag thin film. • Top-emitting white OLED device with thin Mg:Ag shows higher efficiency and superior angular emission characteristics.

The Selection of Anode and Cathode Materials for Top Emission Organic Light-Emitting Diodes

In this study top emission organic light-emitting diodes (TE-OLED) were successfully fabricated on flexible PET/multilayer disordered silver nanonetwork (MDSN) substrate with conventional LiF/Al as a semitransparent cathode and Ag as a reflective anode. The effects of the hole injection layer, anode buffer layer, and an electron injection layer on the luminescence characteristics of TE-OLEDs were investigated. At first, the thickness of cathode Al is adjusted based upon the overall transmittance of the top-emission cathode and its conductivity. There is a high energy barrier for the hole between the work function of the anode Ag (4.2 eV) and the NPB highest occupied molecular orbital (HOMO) energy level (5.5 eV), which is not favorable for hole injection. This study tested four types of hole injection layer (HIL) materials. Finally, MoO3 was selected as an optimal HIL material, and the optimum thickness was adjusted, enabling the hole to be injected smoothly from the anode to the NPB and then to an emitting layer. The TE-OLED luminance reached 268 cd/m2. There is a high energy barrier between the work function of Ag and MoO3 HOMO (5.3 eV)—about 1.1 eV—which is still not conducive to the hole injection, so a thin layer of high work function metal Au (work function 5.1 eV) was added to the top of the anode silver, which more matches with the MoO3 energy level. It can make the hole easier to inject from the anode to MoO3 (HIL) and protect the silver from oxidation. At 8 V, the luminance is increased to 413.7 cd/m2, and the current efficiency is 0.81 cd/A. The luminance is significantly improved. An electron transport/hole blocking layer TPBi (10 nm) was added to enhance the electron transport capability and effectively block the holes with higher electron mobility and a higher HOMO energy level of TPBi. So that more holes can remain in the emitting layer and increase the chance of the electron-hole recombination to improve the luminance and current efficiency of TE-OLED. At 8 V, the luminance and current efficiency can reach 611.4 cd/m2 and 0.95 cd/A, respectively.

Plasmon loss improved top emission organic light-emitting diode with multi capping layer

Here, we report surface plasmon (SP) loss improved top-emitting organic light emitting diodes (TEOLEDs) by applying our unique cathode unit structure. Generally, the TEOLED has high optical loss portion of SP mode compared to the bottom emitting OLED and it causes low out-coupling efficiency in the TEOLED. Commonly used TEOLED structure uses Ag:Mg (10:1, 22 nm) cathode with high refractive index single capping layer, which has high optical loss portion of SP mode. To reduce SP mode loss in the TEOLED, we applied thin Ag:Mg (10:1, 12 nm) cathode with multi-capping layer. Our cathode unit reduces 55% of SP loss and enhances about 20% efficiency. Our approach to enhance out-coupling efficiency will be useful to make high efficiency TEOLEDs.

CaF2/ZnS Multilayered Films on Top-Emission Organic Light-Emitting Diode for Improving Color Purity and Moderation of Dark-Spot Formation.

Organic light-emitting diodes (OLEDs) have been widely used, particularly in display applications. OLEDs are easily degraded without stringent encapsulation owing to their susceptibility to water vapor and oxygen. Therefore, establishing an effective protection method for these devices is essential. In this study, we demonstrate the device protection performance and improvement in color purity by introducing CaF2/ZnS multilayered films on a top-emitting inverted-type OLED (iOLED), which was originally intended to act as a distributed Bragg reflector (DBR). To test the protection performance of each dielectric layer, conventional bottom-emitting OLEDs (cOLEDs) with and without single layers of CaF2 and ZnS were investigated for comparison. All OLEDs were stored in an atmosphere without stringent encapsulation, such as a cover glass. The luminescence area of cOLEDs without the dielectric film decreased by more than 90% after 3 days of fabrication. In contrast, the dark-spot formation was moderated after the same period for the dielectric single-layer deposited cOLEDs. Notably, the iOLED with DBR completely preserved the emitting area even after 2 months of fabrication. This suggests that DBR acted as a protective film for the organic layer, whereas the inverted structure also contributed to reducing the degradation of air- and moisture-sensitive materials.

Fusion of Multi-Resonance Fragment with Conventional Polycyclic Aromatic Hydrocarbon for Nearly BT.2020 Green Emission.

Herein, we report a general strategy for achieving ultra-pure green emissions by suppressing the shoulder peaks in the emission spectra of conventional polycyclic aromatic hydrocarbons (PAHs). Through precise synthetic fusion of multi-resonance (MR) fragments with conventional PAH, extended π-conjugation lengths, increased molecular rigidity, and reduced vibrational frequency could be simultaneously realized. The proof-of-concept emitters exhibited ultra-pure green emissions with dominant peaks at ca. 521 nm, photoluminescence quantum yields that are greater than 99 %, a small full-width-at-half-maximum of 23 nm, and CIE coordinates of (0.16, 0.77). The bottom-emitting organic light-emitting diode (OLED) exhibited a record-high CIEy value of 0.74 and a high maximum external quantum efficiency of 30.5 %. The top-emitting OLED not only achieved a BT.2020 green color (CIE: 0.17, 0.78) for the first time but also showed superior performance among all green OLED devices, with a current efficiency of 220 cd A- .

Ytterbium oxide electron injection interface in organic light-emitting diode

The ytterbium oxide (Yb3+) is found to have an extremely low work function of 2.42 eV, which is even lower than that of its metallic form Yb0 (2.64 eV). The stability of oxides makes Yb3+ an ideal electron injection material for both top-emitting and bottom-emitting organic light-emitting diodes (TOLED and BOLED). The device test data indeed show that at 1000 nit luminance, the TOLED has a 94 cd/A current efficiency and 70 lm/W power efficiency, and BOLED has a 76 cd/A and 60 lm/W efficiency, respectively. X-ray and ultraviolet photoemission spectroscopical studies indicate that the Fermi level of the metal oxide is pinned to the lowest unoccupied molecular orbital of the electron transport layer, leading to the formation of a cathode interface with an ultra-low electron injection barrier.

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