Color-variable highly efficient organic electrophosphorescent diodes manipulating molecular exciton and excimer emissions (original) (raw)
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Mixing of molecular exciton and excimer phosphorescence to tune color and efficiency of organic LEDs
Organic Electronics, 2010
We report highly efficient, variable-color light-emitting diodes (LEDs) realized via mixing of molecular exciton and excimer phosphorescent emissions from a new organic phosphor platinum N^C 2^N -[1,3-di(4-methoxy-pyrid-2-yl)-4,6-difluorobenzene] chloride (PtL 26 Cl), a member of the Pt(N^C^N) complex series, where N^C^N represents a cyclometallating tridentate ligand based on 1,3-dipyridylbenzene, L. PtL 26 Cl-doped TCTA blends [TCTA = 4 ,4 0 ,4 00 -tris(N-carbazolyl-triphenylamine)] have been used as either the low-concentration bluish-green (molecular) phosphorescence emitter or high-concentration red (excimer) phosphorescence emitter. By adjusting the relative amount of blue and red emissive species, the color of the light emission was tuned from bluish-green through green and white up to red. The concentration-optimized devices can easily reach high brightness up to B = 10,000 cd/m 2 , achieving extremely high external quantum efficiency (EQE) up to 20% at low-current densities (<10 À2 mA/cm 2 ) and up to 17% at B = 500 cd/m 2 . High purity white light emission [CIE coordinates (x = 0.34, y = 0.35)] was realized at this B, QE = 11.5%, power efficiency of 6.8 lm/W and color rendering index (CRI) = 74, though CRI = 81 has been achieved with CIE coordinates (x = 0.42, y = 0.38). The reported performance data are among the best achievements reported for color and white emission organic LEDs.
Organic Electronics, 2009
A new phosphorescent platinum(II)-pyridyltriazolate complex, bis[3,5-bis(2-pyridyl)-1,2,4-triazolato]platinum(II), Pt(ptp) 2 , was synthesized and incorporated into organic light emitting diodes for evaluation as electrophosphorescent dopant. Single crystal X-ray diffraction shows that the complex forms columnar stacks stabilized via strong intermolecular Pt(II)Á Á ÁPt(II) interactions (3.289 Å), making it amenable to form excimers. Three types of phosphorescent emissions are seen for neat and doped thin films of Pt(ptp) 2 into 4,4 0bis(carbazol-9-yl)triphenylamine (CBP): structured monomer emission in the blue-green region (k max $ 480 nm), unstructured excimer emission in the yellow region (k max $ 550 nm), and rather broad unstructured extended excimer emission in the orange-red region (k max $ 600 nm). By varying the doping level in x% Pt(ptp) 2 :CBP thin films, the ratio of monomer to excimer emission could be adjusted leading to tuning both the photoluminescence (PL) and electroluminescence (EL) wavelengths. Doping levels of 5−105-10%v were found to be optimal for both EL efficiency and white color coordinates resulting from simultaneous monomer and excimer emissions. Peak power and luminous efficiencies obtained were 9.8 lm/W and 14 cd/A, respectively, while the peak external quantum efficiency was 6.6% in a standard OLED device structure. The emissions for the 7.5-15%v Pt(ptp) 2 :CBP devices are characterized by CIE coordinates of 5−10(0.3, 0.5), rendering near-white EL with green hue. The CRI increased with dopant concentration, reaching 56 at the 45% doping level. More optimal white EL color could not be attained by fine-tuning the doping level. The devices exhibit good stability at higher current density and brightness levels, which is atypical of phosphorescent organic light emitting diodes. Strategies to attain higher efficiencies and CRI are proposed based on the results of this work.
2014
Blue to red organic light-emitting diodes based on a series of newly synthesized distyrylbenzenes have been demonstrated. Their optical properties have been theoretically and experimentally studied in order to inquire into the substitution effects (such as electron-donating, electron-withdrawing, and steric hindrance) on the emission color. Density functional theory at B3LYP/6-311+G(d) level of calculation was employed to obtain the molecular structures and highest occupied molecular orbital and lowest unoccupied molecular orbital surfaces. Electroluminescence emission range of compounds could be tuned by changing the strength of the acceptor component and using push-pull and nonplanarity effects from 483 (blue) to 600 (red) nm. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)
Journal of Materials Chemistry, 2006
This work describes bipolar 2,5-diaryl-1,3,4-oxadiazole-fluorene hybrids which incorporate triphenylamine or carbazole units within the p-electron system, viz. compounds 7, 8, 14 and 16. A related bipolar bis(oxadiazolyl)pyridine system 20 is reported. The syntheses of these five new materials are discussed, along with their optoelectronic absorption and emission properties, and their solution electrochemical redox properties. Anodic electropolymerisation of 20 was observed. Calculations using DFT (density functional theory) establish that they all possess a significantly higher HOMO energy level (by 0.60-1.02 eV) than 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-7) due to the presence of electron-rich amine moieties and increased conjugation lengths, thereby leading to more balanced charge-transport characteristics. Devices were fabricated by spin-coating techniques using the bipolar compounds as the emitters in the simple device architecture ITO:PEDOT-PSS:X:Ca/Al (X = 7, 8, 14, 16 or 20). The turn-on voltages were 2.9, 5.5, 3.6, 4.5 and 3.4 V for the devices incorporating 7, 8, 14, 16 and 20, respectively. The highest external quantum efficiency (EQE) was observed for compound 7: viz. EQE 0.36%; current efficiency 1.00 cd A 21 ; power efficiency 0.56 lm W 21 at 5.7 V. The EQE of the device fabricated from 8 was considerably lower than for devices using other materials due to low light emission. The EL emission peaked at l max 430, 487, 487 and 521 nm for 8, 14 and 16, and 7, respectively. For the 20 device l max = 521 nm and 564 nm. Thus the HOMO-LUMO gap has been modified, allowing the colour of the emitted light to vary from light blue through to green by the systematic chemical modification of the molecular subunits. The high chemical and thermal durability of these materials combined with the simplicity of the device structure and low turn-on voltages offers considerable potential for OLED applications. { Electronic supplementary information (ESI) available: Synthesis of 11; cyclic voltammetry data for compounds 7, 8, 14, 16 and 20, X-ray crystallographic data for compounds 11, 12, 13 and 20 including diagrams and discussion of the structures; B3LYP/6-31G(d) optimised geometries ; orbital energy level diagrams and frontier orbital localisation for compounds 7a, 8a, 14a, 16a and 20; EL spectra of blended layer devices of MEH-PPV and compound 7. See
Principles of phosphorescent organic light emitting devices
Physical Chemistry Chemical Physics, 2014
Organic light-emitting device (OLED) technology has found numerous applications in the development of solid state lighting, flat panel displays and flexible screens. These applications are already commercialized in mobile phones and TV sets. White OLEDs are of especial importance for lighting; they now use multilayer combinations of organic and elementoorganic dyes which emit various colors in the red, green and blue parts of the visible spectrum. At the same time the stability of phosphorescent blue emitters is still a major challenge for OLED applications. In this review we highlight the basic principles and the main mechanisms behind phosphorescent light emission of various classes of photofunctional OLED materials, like organic polymers and oligomers, electron and hole transport molecules, elementoorganic complexes with heavy metal central ions, and clarify connections between the main features of electronic structure and the photo-physical properties of the phosphorescent OLED materials. Cherkassy. He has delivered lectures as a guest professor at the Royal Institute of Technology. Prof. Minaev is a specialist in the field of spin chemistry and phosphorescence theory.
Advanced Materials, 2019
During the last two decades, white organic light-emitting diodes (WOLEDs) have been attracting increasing attention due to their potential applications in advanced lighting and display technologies. [1-4] White light emission usually needs simultaneous emission of red, green, and blue (RGB) or of at least blue and orange, two complementary colors. A variety of device architectures have been adopted by researchers to develop efficient whitelight emitting systems, including stacked system with blue fluorescent and red/ green phosphorescent materials, down conversion system with blue LED chips spin coated with orange emitting chromophores, monomer-excimer emitting system which requires high concentration of cyclometallation ligands as dopants. [5-7] However, these approaches do encounter many problems hindering them from commercial applications, such as limited choice of materials, sophisticated device structure, color shift during current stressing process and high fabrication cost. [8-10] One of the best strategies to avoid the above problems is to employ single organic molecule as emissive layer due to improved color stability, simplified fabrication process and reduced material cost. Pioneers' work by attaching different chromophores, including fluorescent and phosphorescent dyes, onto a polymer host (copolymer) has achieved polymer-based white light-emitting diodes (WPLEDs) with highest current efficiency of nearly 10 cd A −1 and the maximum luminance over 10 4 cd m −2. [11-13] Such approach is synthetically driven which require multiple-step synthesis and WPLEDs are for solution based manufacture technology and not for the vacuum thermal evaporation (VTE) method which remains the mainstream fabrication technology of OLED industry nowadays. Thus, for white light LED, it is still imperative to develop single small molecule systems that are compatible for VTE fabrication. Few examples of white light emission based on single organic small molecule have been reported, mainly because fluorophores tend to attain the lowest vibrational states that result in monochromatic emission according to Kasha's rule. [14,15] Pervious research found that simple modification of blue fluorescent molecules could cause an intramolecular Förster White organic light-emitting diode (WOLED) technology has attracted considerable attention because of its potential use as a next-generation solid-state lighting source. However, most of the reported WOLEDs that employ the combination of multi-emissive materials to generate white emission may suffer from color instability, high material cost, and a complex fabrication procedure which can be diminished by the single-emitter-based WOLED. Herein, a colortunable material, tris(4-(phenylethynyl)phenyl)amine (TPEPA), is reported, whose photoluminescence (PL) spectrum is altered by adjusting the thermal annealing temperature nearly encompassing the entire visible spectra. Density functional theory calculations and transmission electron microscopy results offer mechanistic understanding of the PL redshift resulting from thermally activated rotation of benzene rings and rotation of 4-(phenylethynyl) phenyl) amine connected to the central nitrogen atom that lead to formation of ordered molecular packing which improves the π-π stacking degree and increases electronic coupling. Further, by precisely controlling the annealing time and temperature, a white-light OLED is fabricated with the maximum external quantum efficiency of 3.4% with TPEPA as the only emissive molecule. As far as it is known, thus far, this is the best performance achieved for single small organic molecule based WOLED devices.
Advanced Materials, 2014
Organic Light-Emitting Diodes (OLEDs) using the thermally activated delayed fluorescence (TADF) emitter (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) are demonstrated using a novel ambipolar host 3,5-di(carbazol-9-yl)-1-phenylsulfonylbenzene (mCPSOB). When doped in a 5 wt.% concentration, OLEDs with EL efficiency values of more than 81 cd/A for current efficacy and 26.5% for external quantum efficiency are reported. These devices exhibit a low turn-on voltage of 3.2 V at 10 cd/m 2 , as well as reduced efficiency roll-off at high current densities. To the best of our knowledge, these are among the highest ever reported efficiencies for TADF OLEDs, and are even comparable to the highest reported efficiencies for phosphorescent OLEDs.