Layer In Organic Light-emitting Diodes Research Articles

Paracyclophane-based bipolar near-ultraviolet emitters showing advanced circularly polarized luminescent properties

<p>As linking unit, [2.2]paracyclophane (PCP) has been inserted in between electron-donating groups (<em>i.e.</em> carbazole and 10<em>H</em>-phenoxazine) and electron-accepting groups (<em>i.e.</em> triphenyl-1,3,5-triazine and diphenylsulfone) to prepare a series of linear bipolar racemic near-ultraviolet (NUV) emitters owing to the planar chirality associated with the PCP. In toluene, the racemic bipolar PCP-based emitters can show emission wavelength in the range of 400-444 nm. Critically, very small Commission Internationale de L��Eclairage (CIE) chromaticity coordinate of y value of 0.03 has been achieved for the photoluminescent spectra of these bipolar racemic NUV emitters in solution. The PCP-based bipolar emitters can show efficient NUV emission with photoluminescent quantum yields (PLQY) of <em>ca.</em> 0.5 in doped mCP film. In addition, the enantiomers of the NUV emitters have been obtained to show obvious circular dichroism (CD) behavior and circularly polarized luminescence (CPL). The absorption asymmetric factor (<em>g</em><sub>abs</sub>) of <em>ca.</em> 4.4��10<sup>-4</sup> and photoluminescent asymmetric factor (<em>g</em><sub>PL</sub>) of <em>ca.</em> 1.5��10<sup>-3</sup> can be achieved by the PCP-based NUV enantiomers. After doped in the emission layer of organic light-emitting diodes (OLEDs), efficient NUV and deep blue-emitting OLEDs have been constructed. The NUV OLEDs based on the racemic emitters can achieve forward-viewing maximum external quantum efficiency (<em>��</em><sub>ext</sub>) of 5.03% and electroluminescence (EL) peak at 404 nm with CIE<sub>y</sub> of 0.05, while the OLEDs with deep blue emission can show <em>��</em><sub>ext</sub> of 4.12% and EL peak at 412 nm with CIE<sub>y</sub> of 0.08. Furthermore, the NUV chiral enantiomers can bring <em>��</em><sub>ext</sub> of 5.25% and EL peak at 404 nm with CIE<sub>y</sub> of 0.05, while the deep blue chiral enantiomers can give <em>��</em><sub>ext</sub> of 4.40% and EL peak at 412 nm with CIE<sub>y</sub> of 0.08. Clearly, these encouraging EL data show the great potential of these linear PCP-based bipolar emitters in the field of OLEDs.</p>

The effect of gas composition on the properties of silicon oxynitride thin film prepared by low-pressure inductively coupled Ar/N2 plasma

Silicon nitride (Si3N4) has been widely used in semiconductor devices and applications, including the passivation layer in organic light-emitting diode (OLED). However, it becomes more important to adjust the refractive index value of silicon nitride layers. For example, the refractive index difference between the passivation layer and emission layer is one of the problems degrading OLED performance. Recently, silicon oxynitride (SiOxNy) thin films possessing different refractive indexes have been fabricated by varying their nitrogen and oxygen content. In this work, we control the refractive index of SiOxNy thin film by changing the N2 and Ar flow ratio in 13.56 MHz radio-frequency inductively coupled plasma (RF-ICP). To study the effects of gas composition in the plasma on the properties of thin films, nitrogen atom (N) and nitrogen molecular ion (N2+) density are investigated using actinometry and simulation via the synthetic spectrum method, respectively. We observed that when the gas ratio defined as [N2/(N2+Ar)] increases from 0.1 to 0.9, the density of N2+ increases from 4.76×1015m − 3 to 1.71×1016m − 3 and the refractive index of the thin film increases from 1.54 to 1.57. The results indicate that N2+ plays the important role in changing the properties of thin film during the plasma process.

Spectral and kinetic properties of silver sulfide quantum dots in an external electric field

The effect of an external electric field on the luminescence characteristics of silver sulfide nanoparticles embedded in a film based on an optically passive dielectric matrix has been studied. The luminescence characteristics were studied using methods of optical and time-resolved spectroscopy involving the time-correlated single-photon counting technique. The morphology of the nanoparticles was studied using transmission electron microscopy. It was shown that in an external electric field, an increase in the intensity of the recombination luminescence band is observed for silver sulfide nanoparticles, together with an increase in the electronic relaxation rate. This effect is explained by the fact that the electric field enhances the transport of free holes to electron traps which are radiative recombination centers. The observed effects indicate that silver sulfide nanoparticles can be effectively used as active layers of organic light-emitting diodes, where an external field of the order of 500 kV/cm will not lead to a deterioration in their operating luminescence characteristics.

Nanocrystalline copper iodide enabling high-efficiency organic LEDs

High-efficiency organic light-emitting diodes (LEDs) can be achieved by employing efficiency-effective materials coupling with suitable device architectures. Among them, there have been numerous hole-injection and -transport materials reported, but having issues of high-cost, solution-process feasibility, and toxic solvent involvement. Here, we demonstrated a low-cost solution-processable nanocrystalline copper iodide (CuI) as a hole-injection and -transport material for the fabrication of high-efficiency organic LEDs. The CuI dispersion can be prepared by using a non-toxic ammonia-water solution with processing being carried out at a relatively low temperature of 110 o C. The annealed CuI film exhibited more superior optical properties, a better hole injection capability, and a better crystalline nature as comparing with the unannealed one or the typical hole-injection counterpart. The annealing resulted nanocrystalline CuI had enabled a green device with a maximum external quantum efficiency (EQE max ) of 17%, a maximum power efficacy (PE max ) of 64 lm W −1 and a maximum current efficacy (CE max ) of 62 cd A −1 , an increment of 54, 33 and 15%, respectively, as comparing with the unannealed counterpart. The respective increments are 35, 41 and 38% as comparing with the typical hole-injection material. Additionally, a blue hazard-free candlelight organic LED has also been fabricated with an EQE max of 15%, PE max of 43 lm W −1 and CE max of 41 cd A −1 . Electrical simulation was also performed to get recombination rate and electric field distribution in active layer. CuI based OLED exhibited higher recombination rate and lower electric filed distribution as comparing with PEDOT:PSS based OLED. The superior performance may be attributed to the enhanced recombination due to efficient injection, transport of holes, and a potential inner light scattering medium aiding light extraction due to its nanocrystalline nature. Our results show that the nanocrystalline CuI can be an inexpensive substitute for the conventionally used hole-injection materials to achieve high-efficiency lighting sources. • Nanocrystalline CuI as an effective Hole-injection/transport layer for organic LEDs. • CuI dispersion is prepared in a non-toxic ammonia-water solution. • Electro-simulation study comparison of CuI and PEDOT:PSS. • Superior device performance in comparison with PEDOT: PSS and amorphous counterpart.

Small D-π-A-π-D organic dyes for near-infrared emitting OLEDs with excellent external quantum efficiency

Organic-light emitting diodes (OLEDs) with near-infrared (NIR) emission have an extremely broad range of attractive applications. Compounds containing donor (D) and acceptor (A) fragments are promising materials for use as active emitting layer in OLEDs due to their fast luminescence and because their luminescence properties can be fine-tuned at the synthesis stage. In this work, we synthesized and investigated two D-π-A-π-D NIR-emitting dyes based on a new pyridazine-thiadiazole electron acceptor with two different π linkers. The introduction of π linkers, together with the geometry modification, provides an additional redshift of the emission maximum from 700 to 850 nm. These dyes demonstrate moderate quantum yields up to 15% in chloroform solutions and to 4% in the solid state. Spectroscopic studies were supported by Time Dependent Density Functional Theory (TD-DFT) and multireference (XMCQDPT/CASSCF) calculations. A series of OLEDs with electroluminescence at 700 and 850 nm was fabricated. One of the devices exhibited maximum optical power 76 μW/cm2 and maximum external quantum efficiency (EQE) 1.2%, which is close to the theoretical limit calculated for fluorescent NIR OLEDs based on low band gap small organic molecules.

Optical and Electrical Characterization of Visible Parylene Films.

Poly-dichloro-para-xylylene (parylene-C) film is formed through a chemical vapor deposition process, where monomeric gases are polymerized on the target surface at room temperature and are used as transparent insulating coating films. The thin parylene-C films exhibit uniform conformal layers even when deposited on substrates or surfaces with fine cracks, structures, and bumps. However, the film is highly transparent in the visible range (transmittance > 90%); thus, it is difficult to visually identify, inspect the coating process and check for any defects when used as an insulation film. Some reports have demonstrated the deposition of visible (hazy) parylene films through the control of the vaporization or pyrolysis of the parylene-C powder and sublimed dimers, respectively. Even though these films have been applied as device substrates and light extraction layers in organic light-emitting diodes (OLEDs), their optical and electrical characteristics have not been extensively explored, especially for their applications as insulation coatings. In this study, the characteristics of visible parylene films produced by tuning the ratio of dimer to monomer gases via the adjustments of the pyrolysis temperature are analyzed with electrical and optical methods. Parylene-C films deposited within the pyrolysis temperature of 400–700 °C exhibited a haze range of 10–90%. A relative reflectance of 18.8% at 550 nm of the visible light region was achieved in the visible parylene film deposited with a pyrolysis temperature of 400 °C. Resistivity in the order of 1010 Ω cm was achieved for the visible parylene films measured with the transmission line measurement (TLM) method. The films can be applied in advanced insulation coatings for various optical systems and electronic devices.

Facile Preparation of Four-Layer MoS2 Nanosheets and Their Application to Organic Light-Emitting Diode

High-quality four-layer molybdenum disulfide (MoS2) nanosheets with lateral dimension of about 11 µm were prepared by ultrasonic treatment of MoS2 powder with assistance of 1-methyl-2-pyrrolidone (NMP) solvent. The optimal preparation conditions for the preparation of MoS2 nanosheets were investigated from the aspects of ultrasonic processing time, ultrasonic power and amount ratio of MoS2 powder and NMP solvent. At the same time, the MoS2 nanosheets were employed as anode buffer layer in organic light-emitting diode (OLED) with copper nanowire (CuNW) film being anode. MoS2 nanosheets can reduce roughness of CuNW film, protect CuNW film from oxidation and improve work function of CuNW film. Experiments show that MoS2 nanosheets can significantly improve the current density and brightness of the OLED with CuNW film being anode. The maximum brightness of the OLED with MoS2 anode buffer layer is 2.15 times that of the OLED without MoS2 anode buffer layer. The current density of the OLED with MoS2 anode buffer layer is also obviously increased compared with the OLED without MoS2 anode buffer layer.

Europium(II) Complexes with Substituted Tris(2‐aminoethyl)amine/Triethanolamine Ligand and their Application in Blue Spin‐Coated Organic Light‐Emitting Diodes

AbstractLuminescent Eu(II) complexes with characteristic d–f transitions have potential applications in many fields, especially organic light‐emitting diodes (OLEDs), due to their satisfactory advantages including short excited‐state lifetimes, 100% theoretical exciton utilization efficiency and tunable emission colors. Up to now, most light‐emitting layers in OLEDs are typically fabricated through high‐vacuum thermal evaporation process, which is uneconomical for large‐area and multi‐dopant devices. In this work, eight Eu(II) complexes with substituted tris(2‐aminoethyl)amine or triethanolamine ligands are designed and synthesized, and their structural and photophysical properties are studied. These complexes exhibit tunable maximum emission wavelengths in the range of 437–553 nm and high photoluminescence quantum yields up to 75%. Furthermore, this work demonstrates the application of Eu(II) complex in solution‐processed OLEDs with high efficiency and luminance for the first time. The optimized device show a maximum external quantum efficiency of 9.2% and a maximum luminance of 1780 cd m−2 with an emission peak at 485 nm.

UV-ozone treated Ti3C2Tx-MXene nanosheets as hole injection layer for organic light-emitting diodes

In this work, we successfully synthesized Ti3C2Tx-MXene nanosheets with a lateral dimension of 1 ∼ 2 μm by using wet-chemical etchant and sonication-assisted liquid-phase exfoliation method. After UV-ozone treatment, the work function for Ti3C2Tx-MXene nanosheets was adjusted to 5.14 eV well matching to indium tin oxide (ITO) anode (4.80 eV). Using Ti3C2Tx-MXene nanosheets as hole injection layer (HIL), Ir(ppy)3-based green organic light-emitting diodes (OLEDs) were fabricated, and the resulting devices exhibit a low turn-on voltage of 3.0 V and the high current efficiency, power efficiency and external quantum efficiency of 72.79 cd/A, 54.44 lm/W and 20.34 %, which are obviously better than these of 67.38 cd/A, 50.38 lm/W and 18.65 % for PEDOT:PSS-based reference device. Further, FIrpic- and Ir(pq)2acac-based blue and red OLEDs with Ti3C2Tx-MXene nanosheets HIL were demonstrated, which also show the significantly improved device efficiency of 44.56 cd/A, 32.44 lm/W and 22.07 % for blue device and 39.17 cd/A, 38.54 lm/W and 21.90 % for red device than these for corresponding PEDOT:PSS-based devices, proving the universal applicability of Ti3C2Tx-MXene nanosheets as HIL in OLEDs. The higher performance for Ti3C2Tx-MXene nanosheets-based devices can be attributed to the appropriate work function, high transmittance, high carrier mobility and good film forming property for Ti3C2Tx-MXene nanosheets. This is the first report about the MXene using as HIL in OLEDs, which opens up a new way to develop high-performance holes injection materials for solution-processed OLEDs.

When Poor Light‐Emitting Spiro Compounds in Solution Turn into Emissive Pure Layers in Organic Light‐Emitting Diodes: The Key Role of Phosphine Substituents

Spiro compounds are widely used as host matrices in organic light‐emitting diodes (OLEDs). Here, inspired by the recent developments in thermally activated delayed fluorescence (TADF) materials, the potential as a light emitter of two spiro‐configured organic semi‐conductors is investigated, constructed by the association of quinolinophenothiazine (QPTZ) or indoloacridine (IA) as the electron‐rich fragment and diphenylphosphine oxide fluorene as the electron‐poor fragment. By comparison with structural analogues lacking phosphine oxides, the crucial role played by these electron‐accepting substituents not only on the photo‐physical properties but also on the device performances is evidenced. Despite a very low quantum yield in solution, these two compounds unexpectedly display good emission properties when incorporated as a pure layer in OLEDs, thus highlighting the role of intermolecular effects.

Large effects of tiny structural changes on the AIE-TADF type xanthone derivatives in mechano-responsive luminescence and electroluminescence

In this work, two compounds with aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF) properties (XT-T and XT-OT) which have the xanthone moiety as an electron withdrawing group were synthesized and studied. Both molecules exhibited the mechano-responsive luminescence (MRL) effect, in which a crystal of XT-OT could exhibit a bathochromic shift of 101 nm to achieve a wide-range emission transition from blue to yellow after grinding, compared to 34 nm for crystal XT-T. Such a high color-difference in the maximum emission peak was rarely reported in MRL materials. Analysis of crystal structures showed that small differences in structure could give rise to variable conformations and packing patterns in different aggregated states. The application of two compounds as light-emitting layers in organic light-emitting diodes (OLEDs) revealed the impact on device performance by their structural difference. The results presented here could provide a complementary strategy to design new multi-function materials.

Role of the Trifluoropropynyl Ligand in Blue-Shifting Charge-Transfer States in Emissive Pt Diimine Complexes and an Investigation into the PMMA-Imposed Rigidoluminescence and Rigidochromism.

Square-planar PtII complexes are of interest as dopants for the emissive layer of organic light-emitting diodes. Herein, the photophysics of three Pt bipyridyl complexes with the strongly e- withdrawing, high-field, 3,3,3-trifluoropropynyl ligand has been investigated. One complex, (phbpy)PtC2CF3 (phbpy = 6-phenyl-2,2'-dipyridyl), has also been characterized by single-crystal X-ray diffraction. All complexes reported are emissive in both RT CH2Cl2 solution (ΦPL = 0.007 to 0.027) and PMMA film (ΦPL = 0.25 to 0.42). The trifluoropropynyl ligand elevates the energy of the MLCT and LL'CT states above that of the IL π-π* state, resulting in IL emission in all cases. The emission energies of the trifluoropropynyl compounds are also blue-shifted relative to the analogous pentafluorophenylethynyl compounds, suggesting that the trifluoropropynyl ligand is one of the most electron-withdrawing alkynyl ligands. Rate constants for radiative and nonradiative deactivation were determined from experimentally determined values of ΦPL and excited-state lifetimes in both solution and PMMA films. The increase in ΦPL upon incorporation into PMMA film (rigidoluminescence) results from a decrease in the rate constant for non-radiative relaxation. Experimental activation energies for excited-state decay in combination with TDDFT are consistent with the rigidoluminescence resulting from an increase in the energy of the non-emissive triplet metal-centered state. Two of the complexes investigated, (Ph2bpy)Pt(C2CF3)2 and (t-Bu2bpy)Pt(C2CF3)2, where t-Bu2bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl and Ph2bpy = 4,4'-diphenyl-2,2'-dipyridyl, exhibit concentration-dependent excimer emission (orange) along with monomer emission (blue), enabling fine-tuning of the emission color. However, excimer emission was absent in cured PMMA films up to the solubility limit for solution processing of (Ph2bpy)Pt(C2CF3)2 in CH2Cl2, demonstrating the diffusional nature of excimer formation.

Enhanced performance in solution-processed blue emission layer of organic light-emitting diodes with an alcohol soluble amphiphilic polymer as the hole modify layer

Solution-processed emission layer of organic light-emitting diodes (OLEDs), especially blue OLEDs, are generally inefficient. In this work, we use an alcohol-soluble amphiphilic polymer, Poly[9, 9- bis[3- (ethyldimethylammoni o) propyl] 9′, 9′- dioctyl[2, 2′- bi- 9 H- fluorene] - 7, 7′- diyl bromide (1:2)] (PFN-Br), to modify the hole transport layer, which is deposited on the surface of Poly(9-vinyl carbazole) (PVK) layer for interface modification to achieve high efficient blue OLEDs based on the thermally activated delayed fluorescent (TADF) material 10-[4-(4,6-Diphenyl-1,3,5-triazin-2-yl)− 2-methylphenyl]-spiro[acridine-9(10 H),9’-[9 H]fluorene] (TTSA) as the emitter. It is shown that adding PFN-Br layer can reduce the defect density at the interface between PVK and the emission layer, then the exciton quenching of TTSA is reduced. After optimizing the thickness of PFN-Br layer, the maximum external quantum efficiency and current efficiency of the optimal OLED achieve 22% and 42 cd/A, respectively. To the best of our knowledge, these values are superior to the highest values reported for this material in the literature.

Quantum Dot-Based White Organic Light-Emitting Diodes Excited by a Blue OLED

In this study, white organic light-emitting diodes (OLEDs) consisting of red quantum dots (RQD) and green quantum dots (GQD) were investigated. These are the most exciting new lighting technologies that have grown rapidly in recent years. The white OLED development processes used consisted of the following methods: (a) fabrication of a blue single-emitting layer OLED, (b) nanoimprinting into QD photoresists, and (c) green and red QD photoresists as color conversion layers (CCL) excited by blue OLEDs. To fabricate the blue OLED, the HATCN/TAPC pair was selected for the hole injection/transport layer on ITO and TPBi for the electron transport layer. For blue-emitting material, we used a novel polycyclic framework of thermally activated delayed fluorescence (TADF) material, ν-DABNA, which does not utilize any heavy metals and has a sharp and narrow (FWHM 28 nm) electroluminescence spectrum. The device structure was ITO/HATCN (20 nm)/TAPC (30 nm)/MADN: ν-DABNA (40 nm)/TPBi (30 nm)/LiF (0.8 nm)/Al (150 nm) with an emitting area of 1 cm × 1 cm. The current density, luminance, and efficiency of blue OLEDs at 8 V are 87.68 mA/cm2, 963.9 cd/m2, and 1.10 cd/A, respectively. Next, the bottom emission side of the blue OLED was attached to nanoimprinted RQD and GQD photoresists, which were excited by the blue OLED in order to generate an orange and a green color, respectively, and combined with blue light to achieve a nearly white light. In this study, two different excitation architectures were tested: BOLED→GQD→RQD and BOLED→RQD→GQD. The EL spectra showed that the BOLED→GQD→RQD architecture had stronger green emissions than BOLED→RQD→GQD because the blue OLED excited the GQD PR first then RQD PR. Due to the energy gap architectures in BOLED-GQD-RQD, the green QD absorbed part of the blue light emitted from the BOLED, and the remaining blue light penetrated the GQD to reach the RQD. These excited spectra were very close to the white light, which resulted in three peaks emitting at 460, 530, and 620 nm. The original blue CIE coordinates were (0.15, 0.07). After the excitation combination, the CIE coordinates were (0.42, 0.33), which was close to the white light position.

Effects of electron transport layer thickness on light extraction in corrugated OLEDs.

This study reported the effects of electron transport layer (ETL) thickness on light extraction in corrugated organic light-emitting diodes (OLEDs) and each layer in OLEDs exhibited a periodical corrugated structure, which was determined by depositing thin films on a glass substrate with a nanoimprinted blazed grating structure. The insight is that light extraction in corrugated OLEDs significantly depends on the ETL thickness. Varying the ETL thickness changed the distribution of carrier recombination and led to exciton formation and optical interference, thereby resulting in different attribution of optical loss modes in OLEDs, which increased or even decreased light extraction and device efficiency. Trapped light extraction from the surface plasmon polariton (SPP) and waveguide (WG) modes was identified by splitting the light into transverse electric and transverse magnetic emissions. Thus, the contributions from the individual SPP and WG modes to the external quantum efficiency (EQE) were distinctly clarified by comparing the experimental results with the theoretical calculations. At the ETL thickness of 115 nm, the corrugated OLED exhibited a significantly enhanced (1.83-fold) EQE compared to the planar one due to the effective extraction of trapped light from the SPP and WG modes. The EQE was enhanced by 0.5%, wherein 0.39% came from the WG mode and 0.11% came from the SPP mode.

Isopropanol solvent-treated MoS2 nanosheets from liquid phase exfoliation and their applications to solution-processed anode buffer layer of organic light-emitting diode

Isopropanol solvent-treated MoS2 nanosheets from liquid phase exfoliation and their applications to solution-processed anode buffer layer of organic light-emitting diode

New approach of mechanochromic, thermally activated delayed fluorescence’ dyes consisting of “thioxanthenone derivative as an acceptor unit and two carbazole derivatives as the donor units” used as emitting layer in organic light-emitting diodes

Two dyes containing of thioxanthenone derivative as an acceptor unit, were designed and synthesized for thermally activated delayed fluorescence (TADF) organic light emitting diodes (OLEDs). The photoluminescence (PL) spectra of hammered dyes were shifted and the dyes were mechanochromic. Ionization potential (IP) values of spin-coated dyes on the fluorine-tin oxide film ranged from −5.42 to −5.49 eV and the hole mobility values reached to 2.64 × 10−4 and 3.56 × 10−4 cm2 V−1s−1. Considering the higher values of the photoluminescence quantum yields (PLQYs) of spin-coated dyes on the solid films (16, 21%) that dispersed the dyes in a solution such as toluene (7, 3%), the aggregation induced emission enhancement (AIEE) phenomena were observed. In addition, the doped devices were fabricated based on 50 wt% poly (9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenylamine) (TFB) as a host material through the solution process and physically evaporation deposition (PVD). In this study, the effect of utilizing 4, 4′, 4″-Tris [(3-methylphenyl) phenylamino]triphenylamine (m-MTDATA) as a hole transport layer was impressive and the exciplex forming was confirmed. Finally, the electroluminescence with CIE(x,y) of (0.2076,0.3634) was resulted for the device consisted of low turn on voltage of 4.43 V, maximum brightness of 21362.55 cd m−2, maximum current and power efficiency of 27.03 cd A−1 and 19.65 lm.W−1 and maximum external quantum efficiency of 9.71%.

Uv-Ozone Treated Ti3c2tx-Mxene Nanosheets as Hole Injection Layer for Organic Light-Emitting Diodes

Uv-Ozone Treated Ti3c2tx-Mxene Nanosheets as Hole Injection Layer for Organic Light-Emitting Diodes

Enhancing the optical performance of organic light-emitting diodes using nanoscale random rubbed structure

In this study, we evaluated a nanoscale random rubbed structure (nRRS) used as a scattering layer in organic light-emitting diodes (OLEDs) through an innovative manufacturing method. The rubbing technique, which is conventionally utilized only for liquid crystal alignment, is a manufacturing process with excellent merit in that it can form nanoscale random corrugation in a large area without vacuum equipment even at room temperature, and it is simple and inexpensive. The optimized nRRS, fabricated via rubbing, exhibited a high transmittance of 97.8% and haze of 17.8%, making it suitable as a scattering layer for OLEDs. Owing to its random nature, the scattering effect occurred effectively by rearranging the waveguided light inside the glass substrate. The OLED combined with the optimized nRRS showed a 25.4% improvement in the external quantum efficiency. Additionally, the spectral distortion according to the viewing angle was alleviated, which was confirmed by the negligible difference in the International Commission on Illumination 1931 color space coordinates (∆(x, y) = (0.01, 0.013)). The optical performance of the nRRS–OLED was predicted through a finite-difference time-domain simulation and verified by showing results consistent with those of the fabricated device. This research is expected to be widely applied in many optical devices because it is possible to form a random corrugation on the outside of the device without the difficulty of simply fabricating a beneficial optical structure.

Random rubbed structure for enhanced light extraction in organic light-emitting diodes

In this study, we demonstrated a nanoscale random rubbed structure (nRRS) as an internal scattering layer for organic light-emitting diodes (OLEDs) through an innovative fabrication method. Random corrugation above the SU-8 polymer surface can be simply formed using the rubbing technique commonly employed for liquid crystal alignment. A random structure that is optimized to a nanoscale that does not deteriorate electrical stability can be engineered by controlling the rubbing process. The nRRS exhibited high transmittance (>97%) in the visible wavelength range of 380–700 nm, making it suitable for use as an internal functional layer for OLEDs. Compared to that of the reference device, the external quantum efficiency of the optimized nRRS device increased by 35.47%, and the current and power efficiencies at 3000 cd/m2 were 1.27 times higher. As the viewing angle increased from 0° to 70°, the nRRS-inserted OLED showed a negligible International Commission on Illumination 1931 coordinate shift (Δ (x, y) = (0.016, 0.022)), successfully expressing the alleviation of the viewing-angle dependency. By utilizing finite-difference time-domain simulation tool, the optical results of the device are predicted and verified. This manufacturing approach has the advantages of being applicable to large-scale production and processability at room temperature without the use of a vacuum equipment.