Bottom-emitting Organic Light-emitting Diodes Research Articles

Precise Regulation of Multiple Resonance Distribution Regions of a B,N-Embedded Polycyclic Aromatic Hydrocarbon to Customize Its BT2020 Green Emission.

Recently, boron (B)/nitrogen (N)-embedded polycyclic aromatic hydrocarbons (PAHs), characterized by multiple resonances (MR), have attracted significant attention owing to their remarkable features of efficient narrowband emissions with small full width at half maxima (FWHMs). However, developing ultra-narrowband pure-green emitters that comply with the Broadcast Service Television 2020 (BT2020) standard remains challenging. Precise regulation of the MR distribution regions allows simultaneously achieving the emission maximum, FWHM value, and spectral shape that satisfy the BT2020 standard. The proof-of-concept molecule TPABO-DICz exhibited ultrapure green emission with a dominant peak at 515 nm, an extremely small FWHM of 17 nm, and Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.76). The corresponding bottom-emitting organic light-emitting diode (OLED) exhibited a remarkably high CIEy value (0.74) and maximum external quantum efficiency (25.8 %). Notably, the top-emitting OLED achieved nearly BT2020 green color (CIE: 0.14, 0.79) and exhibited a state-of-the-art maximum current efficiency of 226.4 cd A-1 , thus fully confirming the effectiveness of the above strategy.

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.

Structural characteristics of thermally advanced polymer with intrinsic microporosity for application as flexible OLED substrate

The chemical and structural characteristics of solution-processed polymer with intrinsic microporosity (PIM) based on 9, 9-bis (4-hydroxyphenyl) fluorene (BHPF) film is investigated. A fully flexible and transparent PIM film exhibiting uniform and high transmittance of more than 90% within the visible light range is demonstrated. The stability of the PIM film is further demonstrated with relatively stable coefficient of thermal expansion (CTE) characteristics. Additionally, the film has high heat resistance that can withstand high-temperature processes at 250 °C. The PIM film exhibits a remarkably high glass transition temperature ( of 294.7 °C, indicating the thermal stability of the film at elevated temperatures. The PIM film also exhibits relatively high surface energy, low surface roughness (), and peak-to-valley values of 0.45 and 4.4 nm, respectively. This surface morphology confirms the PIM film’s superior characteristic in preventing short circuits or leakage in current paths in an organic electronic device. Finally, the PIM film is successfully tested as a substrate for a bottom-emitting organic light-emitting diode (OLED). The investigation has proven PIM film to be a good candidate to be adopted as a substrate in fabricating advanced organic electronic devices and next-generation displays.

Toward Narrowband and Efficient Blue Fluophosphors by Locking the Stretching Vibration of Indolocarbazole Skeletons.

Developing efficient and color-saturated deep-blue emitting molecules with small Commission Internationale de L'Eclairage (CIE) y values is challenging and has great potential for wide-color gamut displays. Herein, we introduce an intramolecular locking strategy to restrain molecular stretching vibrations of the emission spectral broadening. By cyclizing rigid fluorenes and connecting electron-donating groups to the indolo[3,2,1-jk]-indolo[1',2',3':1,7]indolo[2,3-b]carbazole (DIDCz) framework, the in-plane swing of peripheral bonds and stretching vibrations of the indolocarbazole skeleton are restricted due to an increased steric hindrance from cyclized groups and diphenylamine auxochromophores. As a result, reorganization energies at the high-frequency region (1300-1800 cm-1) are reduced, realizing pure blue emission with a small full-width-at-half-maximum (FWHM) of 30 nm by suppressing shoulder peaks of polycyclic aromatic hydrocarbon (PAH) frameworks. The fabricated bottom-emitting organic light-emitting diode (OLED) exhibits an efficient external quantum efficiency (EQE) of 7.34% and deep-blue coordinates of (0.140, 0.105) at a high brightness of 1000 cd/m2. The FWHM of the electroluminescent spectrum is only 32 nm, which is one of the narrowest electroluminescent emissions among the reported intramolecular charge transfer fluophosphors. Our current findings provide a new molecular design strategy to conceive efficient and narrowband emitters with small reorganization energies.

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.

Highly efficient and stable deep-blue OLEDs based on narrowband emitters featuring an orthogonal spiro-configured indolo[3,2,1-de]acridine structure.

High-efficiency and stable deep-blue bottom-emitting organic light-emitting diodes with Commission Internationale de l'Eclairage y coordinates (CIEys) < 0.08 remain exclusive in the literature owing to the high excited-state energy of the emitters. Here, we propose the utilization of narrowband emitters to lower the excited-state energy for stable deep-blue devices by taking advantage of their high color purity. Two proof-of-concept deep-blue emitters with nitrogen-containing spiro-configured polycyclic frameworks are thereafter developed to introduce a multi-resonance effect for narrow emissions and sterically orthogonal configurations for alleviated molecular interactions. Both emitters show bright ultrapure deep-blue emissions with an extremely small full-width-at-half-maxima of only 18–19 nm, which can be maintained even in heavily doped films. Small CIEys of 0.054 and 0.066 are therefore measured from the corresponding electroluminescence devices with peak energies of only 2.77 eV (448 nm) and 2.74 eV (453 nm), accounting for the remarkably long LT80s (lifetime to 80% of the initial luminance) of 18 900 and 43 470 hours at 100 cd m−2, respectively. Furthermore, by adopting a thermally activated delayed fluorescence sensitizer, impressive maximum external quantum efficiencies of 25% and 31% are recorded respectively, representing state-of-the-art performances for deep-blue devices.

Nanoslot metasurface design and characterization for enhanced organic light-emitting diodes

We investigate bottom-emitting organic light-emitting diodes (B-OLEDs) integrated with metasurface (MS) to analyze the effect of the structural parameters on the output performance. The performance of the MS-integrated B-OLED (MIB-OLED) is evaluated by out-coupling efficiency (OCE) and reflection of the ambient light, while attention is paid mainly to dielectric capping and metal structure of MS that may influence excitation of surface plasmon (SP). The results suggest that layer thicknesses affect the performance by as much as 10% for the OCE and up to 32% for reflectance. The OCE is in general weakly affected by the structural parameters of MS. In contrast, the reflectance characteristics are found to be dominated by localized SP that is largely determined by the length and the width of a unit slot of MS. An optimization factor introduced to evaluate the performance based on out-coupling power to the radiation mode and reflectance of MIB-OLEDs confirms that integration with MS improves performance by 16% over conventional planar structure. In particular, MIB-OLED is found to enhance OCE by 51% with Lambertian-like pattern. Enhanced performance is experimentally confirmed. The findings provide insights on how to optimize the MS structure to produce MIB-OLEDs with enhanced out-coupled power and contrast ratio.

Improvement of viewing angle dependence of bottom-emitting green organic light-emitting diodes with a strong microcavity effect.

The current efficiency and color purity of organic light-emitting diodes (OLEDs) can be easily improved by means of a microcavity structure, but this improvement is typically accompanied by a deterioration in the characteristics of viewing angle. To minimize the angular dependence of the color characteristics exhibited by these strong microcavity devices, we investigated the changes in the optical properties of the green OLED with a bottom resonant structure. This investigation was based on varying the hole transport layer and semitransparent anode thicknesses. The results of optical simulations revealed that the current efficiency and viewing angle characteristics can be simultaneously improved by adjusting the thickness of the two layers. Furthermore, optical simulations predicted that the angular color dependence could be limited to 0.019 in the International Commission on Illumination (CIE) 1976 coordinate system. This optimum condition yielded a current efficiency of ∼134 cd/A. To further reduce this color shift, a nanosized island array (NIA) was introduced through the dewetting process of cesium chloride. By employing NIAs, the color coordinate shift value was reduced to 0.016 in the CIE 1976 coordinate system, and a current efficiency of 130.7 cd/A was achieved.

Outcoupling efficiency enhancement of a bottom-emitting OLED with a visible parylene film

We have investigated an effective and a single-step chemical vapor deposition (CVD) method to achieve conformal visible poly-dichloro-para-xylylene (parylene C) film for light extraction enhancement in bottom-emitting organic light-emitting diodes (OLEDs) at room temperature. We report that sublimed parylene dimers pyrolyzed between 400 °C and 500 °C resulted in visible parylene films with tunable transmittance and haze, exhibiting light scattering properties due to the formation of uniformly distributed dimer crystals. We achieved a novel conformal visible parylene film with total transmittance and high haze of 79.5% and 93.6%, respectively. It is observed that the outcoupling efficiency of the OLEDs employing the visible parylene film is enhanced up to 45.8%. Additionally, the OLED with the visible parylene light extraction film shows limited angle-dependency of emission spectrum over viewing angles. The single-step room temperature fabrication process of this conformal outcoupling film paves the way to achieving commercial high-performance OLEDs.

Achieving Pure Green Electroluminescence with CIEy of 0.69 and EQE of 28.2% from an Aza-Fused Multi-Resonance Emitter.

Pure green emitters are essential for realizing an ultrawide color gamut in next-generation displays. Herein, by fusing the difficult-to-access aza-aromatics onto B (boron)-N (nitrogen) skeleton, a hybridized multi-resonance and charge transfer (HMCT) molecule AZA-BN was successfully synthesized through an effective one-shot multiple cyclization method. AZA-BN shows pure green fluorescence with photoluminance quantum yield of 99.7 %. The corresponding green device exhibits a maximum external quantum efficiency and power efficiency of 28.2 % and 121.7 lm W-1 , respectively, with a full width half maximum (FWHM) of merely 30 nm and Commission Internationale de l'Eclairage (CIE) coordinate y of 0.69, representing the purest green bottom-emitting organic light-emitting diode.

2-Methyl-9,10-bis(naphthalen-2-yl)anthracene Doped Lithium Carbonate as an Effective Electron Injecting Layer for Both Inverted and Conventional Organic Light-Emitting Diode Structures

High driving voltage, low power efficiency, and insufficient device stability are the most critical complications for organic light-emitting diodes (OLEDs) on their way to practical applications. Particularly in the case of active-matrix organic light emitting device (AMOLED) displays, inferior electron injection from commonly-used ITO electrodes is a critical issue. In this work, 2-Methyl-9,10-bis(naphthalen-2-yl)anthracene doped rubidium carbonate (MADN:Li2CO3) is used as an effective electron injecting layer for both inverted and normal bottom-emission organic light-emitting diodes. When the concentration of Li2CO3-doped MADN is optimized, the device exhibits improved characteristics, including improvements in turn-on voltage, luminance, and efficiency. Ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) analyses reveal an energy level shift in MADN:Li2CO3, which indicates the Fermi level of MADN is moving close to its lowest unoccupied molecular orbital (LUMO) and therefore facilitating electron injection from ITO. In addition, the AFM measurement showed the morphology of the Li2CO3-doped MADN films, revealing good thermal stability in the material related to enhanced lifetime. The results unveiled in this work indicate that Li2CO3:MADN is a promising electron injecting layer for OLEDs with different device structures and provide a vision of the mechanisms behind this phenomenon.

Enhanced Light Extraction from Bottom Emission OLEDs by High Refractive Index Nanoparticle Scattering Layer.

High refractive index nanoparticle material was applied as a scattering layer on the inner side of a glass substrate of a bottom emission organic light emitting diode (OLED) device to enhance light extraction and to improve angular color shift. TiO2 and YSZ (Yttria Stabilized Zirconia; Y2O3-ZrO2) were examined as the high refractive index nanoparticles. The nanoparticle material was formed as a scattering layer on a glass substrate by a coating method, which is generally used in the commercial display manufacturing process. Additionally, a planarization layer was coated on the scattering layer with the same method. The implemented nanoparticle material and planarization material endured, without deformation, the subsequent thermal annealing process, which was carried out at temperature ranged to 580 °C. We demonstrated a practical and highly efficient OLED device using the conventional display manufacturing process by implementing the YSZ nanoparticle. We obtained a 38% enhanced luminance of the OLED device and a decreased angular color change compared to a conventional OLED device.

Low-voltage inverted organic light-emitting diodes with double-layer election injection layer based on silicon dioxide

We demonstrated low operating voltage and highly efficient inverted bottom-emission organic light-emitting diodes using SiO2/n-doped 4,7-diphenyl-1,10-phenanthroline (Bphen) as an electron injection layer (EIL) at the interface between the indium tin oxide (ITO) cathode and the organic electron transport layer. The current and power efficiencies exceed 79.2 cd A−1 and 67.8 lm W−1 at 3.6 V. The ultraviolet photoelectron spectroscopy measurement showed that the insertion of the SiO2 layer reduced the electron injection barrier between ITO and Li doped Bphen. Based on the energy levels and the current-voltage characteristics of electron-dominated devices, we demonstrate that the interface between ITO and the n-doped electron transporting layer (ETL) is important for efficient electron injection into ETL.

Generalized Poynting vector model to calculate the spatial and spectral profiles of the electric field intensity, optical power flow, and optical absorption for all optical modes of organic light-emitting diodes.

We propose a generalized Poynting vector model (GPVM) that can simultaneously calculate the spatial and spectral distributions of the electric field intensity, optical power flow, and optical absorption as well as the power dissipation spectrum for all optical modes of organic light-emitting diodes (OLEDs). The theoretical formulation of the GPVM with respect to the dipole orientation and light polarization is derived by combining the dipole source term and transfer matrix method as a function of the normalized in-plane wave vector u. In the GPVM, the theoretical expression of the spectral power density, derived from the time-averaged Poynting vector at the emission layer, proves to be identical to that presented by the currently-used point dipole model. In a bottom-emitting OLED, the electric field profiles of the waveguide (WG) and surface plasmon polariton (SPP) modes obtained by the GPVM are nearly same as those calculated by the boundary eigenvalue solver except the slight difference at the position of the dipole emitter, which only occurs in the case that the excitation efficiency of a WG or SPP mode is relatively small. Finally, two-dimensional plots of the internal optical power flow and optical absorption, providing physical and intuitive information on the internal emission process as well as the absorption loss of all the optical modes, are calculated as a function of the longitudinal position and normalized in-plane wave vector. Compared with the currently-used electromagnetic methods of the Green's function, dipole radiation, and point dipole models, the proposed GPVM has the advantage that it can provide all the spatial and spectral calculation results of the electric field intensity, optical power flow, and optical absorption with respect to the dipole orientation and light polarization, which are essential in the optical modeling and analysis of OLEDs.

Theoretical comparison of the excitation efficiency of waveguide and surface plasmon modes between quantum-mechanical and electromagnetic optical models of organic light-emitting diodes.

We theoretically compare the excitation efficiency of waveguide and surface plasmon modes between quantum-mechanical and classical electromagnetic optical models of organic light-emitting diodes (OLEDs). A sophisticated optical model combining the two approaches is required to obtain an accurate calculation result and a comprehensive understanding of the micro-cavity effect in OLEDs. In the quantum-mechanical approach based on the Fermi's golden rule, the mode expansion method is used to calculate the excitation efficiency. In the classical electromagnetic approach, the spectral power density calculated by the point dipole model is fitted by the summation of the Lorentzian line shape functions, which provide the excitation probability of each waveguide and surface plasmon modes. The mode coupling efficiencies on the basis of the two approaches are calculated in a bottom-emitting OLED when the position of a dipole emitter is varied. By comparing the calculation results, we confirm the equivalence of two approaches and obtain the better optical interpretation to the calculated excitation efficiency of waveguide and surface plasmon modes. The ratio of mode excitation efficiencies calculated by two approaches agrees well with each other except the contribution of the near-field absorption component.

Tailoring Mg:Ag functionalities for organic light-emitting diodes

Abstract Magnesium (Mg) and silver (Ag) alloys are staple electrodes for organic light-emitting diodes (OLEDs). In this study, the Mg:Ag binary phase diagram is used to establish an electrode materials selection guide for both bottom- and top-emitting OLEDs. To achieve low turn-on voltage and high efficiency in bottom-emitting OLEDs, it is found that Mg:Ag cathodes must be selected from AgMg3 Mg phase region which provides the best figure of merits in low work function and high optical reflectance. For top-emitting OLEDs, however, Mg:Ag cathodes in the AgMg3 Mg region show large imaginary refractive index, and thus a stacked Mg:Ag/Ag is found essential to achieve optimal device performance. Furthermore, insertion of a thin LiF layer is found to create an effective charge injection cathode. In order to theoretically compute the devices’ optical characteristics, a database of complex optical constants of various materials including the metals is obtained by using a spectroscopic ellipsometer. This new database is found essential for accurate device optical design and engineering.

Improvement of Colour Gamut in Bottom-Emission Organic Light-Emitting Diodes Using Micro-Cavity Structure Embedded Cathodes

We demonstrate an approach for improving the colour gamut of bottom-emission organic light-emitting diodes (OLEDs) through micro-cavity structure embedded cathodes. The devices with micro-cavity structure embedded cathodes showed an improved colour gamut of 91.5% (National Television System Committee (NTSC)), 95.8% (Adobe RGB), and 129.2% (sRGB), compared to those of the devices without micro-cavity structure embedded cathodes—59.2% (NTSC), 62.0% (Adobe RGB), 83.6% (sRGB). In addition, the performance of red, green, and blue devices are also investigated depending on the optical length of the micro-cavity structure.

2-Methyl-9,10-bis(naphthalen-2-yl)anthracene doped rubidium carbonate as an effective electron injecting interlayer on indium-tin oxide cathode in inverted bottom-emission organic light-emitting diodes

2-Methyl-9,10-bis(naphthalen-2-yl)anthracene doped rubidium carbonate (MADN:Rb2CO3) is used as an effective electron injecting interlayer on an indium-tin oxide (ITO) cathode for inverted bottom-emission organic light-emitting diodes (IBOLEDs). At a Rb2CO3 doping concentration of 20% in MADN, the device exhibits enhanced characteristics, some of which are turn-on voltage, luminance at a given current density, and current efficiency. The attained performance is better than that of the device using lithium fluoride (LiF) as an n-type dopant. Space-charge-limited current acknowledges improved electrical properties of Rb2CO3 doped MADN. Ultraviolet and X-ray photoelectron spectroscopy investigation unveils an interfacial dipole layer induced by charge transfer between Rb2CO3 and ITO, leading to a lowered ITO work function and an electron injection barrier. The improved electron injection and transport capabilities contribute to better charge balance in IBOLED, thus resulting in advanced luminance efficiency. In addition, the morphology stability of organic films is also amended, which benefits long-term reliability under operationally induced thermal stress. Moreover, the effectiveness of using Rb2CO3:MADN as an electron injecting layer for IBOLEDs is superior to many of its alkali-based counterparts demonstrated in the literature, with high compatibility with different types of sophisticated ITO-based IBOLEDs.

Enhancement of electron injection in inverted bottom-emitting organic light-emitting diodes using Al/LiF compound thin film

The inverted bottom-emitting organic light-emitting devices (IBOLEDs) were prepared, with the structure of ITO/Al (x nm)/LiF (1 nm)/Bphen (40 nm)/CBP: GIr1 (14%):R-4b (2%) (10 nm)/BCP (3 nm)/CBP:GIr1 (14%):R-4b (2%) (20 nm)/TCTA (10 nm)/NPB (40 nm)/MoO3 (40 nm)/Al (100 nm), where the thickness of electron injection layer Al (x) are 0 nm, 2 nm, 3 nm, 4 nm and 5 nm, respectively. In this paper, the electron injection condition and luminance properties of inverted devices were investigated by changing the thickness of Al layer in Al/LiF compound thin film. It turns out that the introduction of Al layer can improve electron injection of the devices dramatically. Furthermore, the device exerts lower driving voltage and higher current efficiency when the thickness of electron injection Al layer is 3 nm. For example, the current efficiency of the device with 3-nm-thick Al layer reaches 19.75 cd·A-1 when driving voltage is 7 V, which is 1.24, 1.17 and 17.03 times larger than those of the devices with 2 nm, 4 nm and 5 nm Al layer, respectively. The device property reaches up to the level of corresponding conventional device. In addition, all inverted devices with electron injection Al layer show superior stability of color coordinate due to the adoption of co-evaporation emitting layer and BCP spacer-layer, and the color coordinate of the inverted device with 3-nm-thick Al layer only changes from (0.580 6, 0.405 6) to (0.532 8, 0.436 3) when driving voltage increases from 6 V to 10 V.