Red Dopant Research Articles

Synthesis and tunable emission of novel polyfluorene co-polymers with 1,8-naphthalimide pendant groups and application in a single layer–single component white emitting device

New luminescent polymers containing two individual emission species-poly(fluorene- alt-phenylene) as a blue host and variable amounts of 1,8-naphthalimide as red dopant have been designed and synthesized. Optical studies (optical absorption (OA) and steady-state photoluminescence emission (PL)) in diluted solutions and thin solid films reveal that the emission spectrum can be tuned by varying the content of 1,8-naphthalimide moieties. Although no significant interaction can be observed between both moieties in the ground state, after photoexcitation an efficient energy transfer takes place from the PFP backbone to the red chromophore, indeed, by adjusting the polymer/naphthalimide ratio it is possible to obtain single polymers which emit white light to the human eye in solid state. Energy transfer is more effective in the co-polymers than in physical mixtures of the two chromophores. We prepared single-layer electroluminescent simple devices with structure: ITO/poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS)/active layer/Ba/Al. With this single layer–single component device structure, white light with Commission Internationale de l’Eclairage (CIE) color coordinates (0.3, 0.42) is obtained for the electroluminescence (EL) emission with an efficiency of 22.62 Cd/A.

Red polymer light-emitting devices based on dye-dispersed poly (9,9-dioctylfluorene-alt-benzothiadiazole)

A polymer–organic host–guest emitter (POHGE) system for obtaining polymer light-emitting diodes (PLEDs) with pure red emission and high luminance is proposed. The POHGE system was prepared by the fluorescent dye 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljulolidy-9-enyl)-4H-pyran (DCJTB) doped in green-emitting polymer poly (9,9-dioctylfluorene-alt-benzothiadiazole) (BTF8). A photoluminescence (PL) quenching phenomenon was observed owing to the higher doping amount of DCJTB dopant. In this study, the red DCJTB dopant emits with high luminance and without concentration quenching when the host BTF8 emitting layer is doped with a small amount of the red guest DCJTB dopant. An optimal amount of DCCJTB dopant was obtained at 2 mg. Devices with a configuration of indium tin oxide (ITO)/poly (ethylenedioxythiophene) doped with poly (styrenesulfonate) (PEDOT:PSS)/BTF8:DCJTB/Ca/Al were fabricated. This device achieved an electroluminescence (EL) efficiency of 1.33 cd/A at 6 V. The emission area is 1.5 × 0.5 cm 2. The devices show the red light emission with a peak at about 630 nm. The color coordinate in Commission Internationale d’Eclairage (CIE) chromaticity is at x = 0.64 and y = 0.36.

Color Stable White Organic Light-Emitting Diodes Having High Color Rendering Index Utilized by Simple-Hybrid Structures

This article reports the highly color stable white organic light-emitting diodes (WOLEDs) having a high color rendering index (CRI) utilized by simple-hybrid structures involving emitting layers (EMLs) of phosphorescent yellow polymer and a thermally deposited fluorescent blue. To derive the yellow EML, a highly soluble red dopant is developed and used with a highly soluble green co-dopant. Using a simple-hybrid structure, the emission tuned WOLEDs are successfully demonstrated with a high CRI of 93 and a highly color stable CIE 1931 coordinate over the full driving voltages from (0.33 ± 0.00, 0.35 ± 0.01) to (0.42 ± 0.01, 0.44 ± 0.01) depending on the blue EML host.

Solution processed efficient orange phosphorescent organic light-emitting device with small molecule host

We present excellent orange red dopants, carbazole-phenylquinoline-based Ir(III) complexes, with a small molecule host system for highly efficient solution processed phosphorescent orange devices. A hole type material of m-MTDATA (4, 4′, 4″-tri-(N-3-methylphenyl-N-phenylamino)triphenylamine) and a TPBI (2, 2′, 2″-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole)) electron transporting material are employed as a soluble small molecule mixed host for these orange devices. Maximum current- and power-efficiency values of 20.0 cd A−1 and 16.8 lm W−1 and maximum external quantum efficiency of 10.8% are demonstrated. Results reveal a practical way to fabricate the highly efficient orange solution processed phosphorescent organic devices, useful for low cost production of white light-emitting devices.

Red, green and blue (RGB) emission doped Y 3Al 5O 12 (YAG) phosphors prepared by non-hydrolytic sol–gel route

An evolutionary optimization process involving combination chemistry was employed in an attempt to develop Y 3Al 5O 12 (YAG). The combination chemistry process utilized here consisted the doping of the YAG host with appropriate amounts of red (R), green (G), and blue (B) dopants in a single layer, for use in tricolor white light. The doped YAG was acieved by means of the non-hydrolytic sol–gel route. Four samples were prepared, three of which were mono-doped samples containing 1.0% of a certain lanthanide (Eu 3+, Tb 3+, or Tm 3+) ion, while the fourth contained the three ions. The samples were characterized by X-ray diffractometry and photoluminescence. The diffraction pattern of the mono-doped samples synthered at 800 °C for 16 h displayed peaks corresponding to the Y 3Al 5O 12 (YAG) phase, while the sample doped with the three ions revealed the presence of a mixture of Y 3Al 5O 12 (YAG) and Y 4Al 2O 9 (YAM) phases. The emission spectra of the three mono-doped YAG samples displayed the typical bands of the blue, green, and red emission of the corresponding lanthanide ions. As for the sample doped with the three lanthanide ions; it simultaneously emitted R, G and B lights. The green emission (546 nm) was more intense and narrow in relation to the red and blue emissions, which may be due to differences in the size of the three incorporated ions.

Electrical Current Aging of Mixed-Host Organic Light-Emitting Devices with Thin Doped Layer

Time-evolved electrical and optical characteristics of an organic light-emitting device (OLED), which exhibited a bipolar emitting layer (EML) with thin doped layer insertion at different positions, were investigated. When the OLED was “fresh,” the dopant near the recombination zone peak of the bipolar EML resulted in a voltage reduction due to the enhancement of the recombination current, and a strong red dopant emission was observed from the electroluminescence (EL) spectra. After constant electrical current aging, spectral changes were observed by analyzing the time-evolved EL spectra, which exhibits two discernible peaks from the green host and the red dopant. By optically exciting OLED at different wavelengths, it was observed that the loss of green emission from the host varied with different dopant positions, which resulted in a nonuniform degradation in the EML. For the dopant near the recombination zone peak, a reduction in green emission was most serious. It suggested that the enhancement of the recombination current induced by the dopant resulted in an increase in local temperature, which helped to deteriorate the organic material nearby, resulting in such nonuniform degradation behaviors.

Fabrication of White Organic Light-Emitting Diodes Using Two Complementary Color Methods

White organic light-emitting diodes (WOLEDs) were fabricated using two complementary color methods with two emissive layer structure such as NPB (500 Å)/DPVBi (65 Å)/MADN:DCM2-0.5% (170 Å)/Bphen (300 Å)/Liq (20 Å)/Al (1000 Å). A deep blue and orange emissions were obtained from DPVBi layer and MADN host doped with a red fluorescent DCM2 dopant each. White emission was achieved through controlling hole-electron recombination by optimization of emissive layers thicknesses and DCM2 concentrations. Optimized WOLED device shows emission efficiency of 5.04 cd/A, current density of 2.64 mA/cm2 and luminance of 938 cd/m2 at 6 V with CIEx,y color coordinate of (0.338, 0.330). These results indicate that DPVBi layer effects on hole blocking and the exciton generation due to difference of energy level in highest occupied molecular orbital. The CIEx,y coordinates of device was slightly changed change from (0.338, 0.330) at 6 V to (0.341, 0.333) at 12 V, which is almost independent on driving voltages.

White organic electroluminescence from fluorescent bis (2-(2-hydroxyphenyl) benzoxazolate)zinc doped with phosphorescent material

Efficient white electroluminescence has been obtained by using an electroluminescent layer comprising of a blue fluorescent bis (2-(2-hydroxyphenyl) benzoxazolate)zinc [Zn(hpb) 2] doped with red phosphorescent bis (2-(2′-benzothienyl) pyridinato-N,C 3′)iridium(acetylacetonate) [Ir(btp) 2acac] molecules. The color coordinates of the white emission spectrum was controlled by optimizing the concentration of red dopant in the blue fluorescent emissive layer. Organic light-emitting diodes were fabricated in the configuration ITO/α-NPD/Zn(hpb) 2:0.01 wt%Ir(btp) 2acac/BCP/Alq 3/LiF/Al. The J– V– L characteristic of the device shows a turn on voltage of 5 V. The electroluminescence (EL) spectra of the device cover a wide range of visible region of the electromagnetic spectrum with three peaks around 450, 485 and 610 nm. A maximum white luminance of 3500 cd/m 2 with CIE coordinates of ( x, y=0.34, 0.27) at 15 V has been achieved. The maximum current efficiency and power efficiency of the device was 5.2 cd/A and 1.43 lm/W respectively at 11.5 V.

High-performance organic red-light-emitting device based on DCJTB and a new host material

An efficient red-light-emitting device using a new host material (DPF) and a red dopant (DCJTB) with a configuration of ITO/NPB (50 nm)/DCJTB:DPF (2%, 10 nm)/TPBI (30 nm)/LiF (0.5 nm)/Mg:Ag has been fabricated and investigated. The red OLED yields a brightness of 9270 cd/m 2 at 10 V, a maximum current efficiency of 4.2 cd/A and a maximum power efficiency of 3.9 lm/W. Using DPF as host material, the performance is much better than that of a prototypical Alq 3-based device, which has a maximum efficiency of 1.9 cd/A and 0.6 lm/W. The performance is even comparable with red OLEDs using an assist dopant or a cohost emitter system. Results of this work indicate that DPF is a promising host material for red OLEDs with high efficiency and simple device structure.

Spectral Analysis of WOLEDs with Combination of Fluorescent Red and Phosphorescent Blue Emitting Layers

The electroluminescence (EL) spectra of white organic light emitting diodes (WOLEDs) with combination of red fluorescent and blue phosphorescent emitting layers were analyzed spectroscopically as a function of applied voltage. The red dopant was (2Z,2′Z)-3,3′-[4,4″-bis(dimethylamino)-1,1′:4′,1″-terphenyl-2′,5′-diyl]bis(2-phenylacrylonitrile) (ABCV-P) and blue one was iridium (III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′]-picolinate (FIrpic). Two devices with and without hole blocking layer (HBL) were fabricated. EL emission of device with HBL was shifted to a greenish white color compared to that of device without HBL. However, EL emission from both devices with and without HBL showed a rather low color temperature with Commission Internationale de L'Eclairage (CIExy) coordinates from (0.416, 0.428) to (0.407, 0.431). Gaussian fitting of the EL spectra for fabricated WOLEDs supported that red emission was significantly affected by device structures and emission region became broader under high applying voltage.

Efficient single layer RGB phosphorescent organic light-emitting diodes

We report efficient single layer red, green, and blue (RGB) phosphorescent organic light-emitting diodes (OLEDs) using a “direct hole injection into and transport on triplet dopant” strategy. In particular, red dopant tris(1-phenylisoquinoline)iridium [Ir(piq) 3], green dopant tris(2-phenylpyridine)iridium [Ir(ppy) 3], and blue dopant bis(3,5-difluoro-2-(2-pyridyl)phenyl-(2-carboxypyridyl)iridium [FIrpic] were doped into an electron transporting 1,3,5-tris( N-phenylbenzimidazole-2-yl)benzene (TPBi) host, respectively, to fabricate RGB single layer devices with indium tin oxide (ITO) anode and LiF/Al cathode. It is found that the maximum current efficiencies of the devices are 3.7, 34.5, and 6.8 cd/A, respectively. Moreover, by inserting a pure dopant buffer layer between the ITO anode and the emission layer, the efficiencies are improved to 4.9, 43.3, and 9.8 cd/A, respectively. It is worth noting that the current efficiency of the green simplified device was as high as 34.6 cd/A, even when the luminance was increased to 1000 cd/m 2 at an extremely low applied voltage of only 4.3 V. A simple accelerated aging test on the green device also shows the lifetime decay of the simplified device is better than that of a traditional multilayered one.

60.3: Two‐Stacked White Organic Light‐Emitting Diodes Consisting of Fluorescent and Phosphorescent Hybrid Structure with High Efficiency and Good Color Characteristics

AbstractWe report an improved bottom‐emission two‐stacked white OLED structure consisting of one unit emitting fluorescent blue light and the other unit emitting phosphorescent yellow in one‐host and two‐dopant (R+G) system. By searching EL condition optimizing both phosphorescent green and red dopants within just one light‐emitting layer, we can get WOLEDs suitable for a display application with very high current and power efficiencies, and excellent color characteristics. Our device exhibits high external quantum efficiency of 28.8%, current efficiency of 56.5 cd/A, and power efficiency of 30 lm/W at 5.9 V and 1000 nit where color point is (0.368, 0.385) in x‐y color space (CIE 1931). Half‐lifetime (LT50) of the device at 1000 nit is estimated more than 31,000 hrs and color variation by aging is suppressed within less than 0.02 in u'‐v'color space (CIE 1976). Color filter developed for our WOLED enables color gamut to become wider than the 100% NTSC in uniform color space (u', v').

23.4: Invited Paper: Ideal Host‐Dopant System for Highly Efficient Phosphorescent OLEDs

AbstractUsing a narrow band‐gap fluorescent host material with a phosphorescent red dopant, highly efficient red phosphorescent OLEDs (PHOLEDs) exhibiting an ideal external quantum efficiency with a doping concentration of 1% were developed. The origin of such a low doping is revealed by the ideal Dexter doping condition study for efficient energy transfer in PHOLEDs. Besides this cause, the strong fluorescent host materials function very well in phosphorescent OLEDs. Singlet excitons generated by fluorescent host materials in phosphorescent OLEDs can produce dopant triplet excitons via the efficient Förster energy transfer mechanism.

OVPD®technology

The precise control of organic thin film processing by organic vapor phase deposition (OVPD®) is presented and analyzed on device level. OVPD® offers accurate and individual control of deposition layer properties like mixing of several materials (co-deposition) and the control of various morphologies by a wide process parameter space given by, e.g. substrate temperature, deposition rate and pressure. The benefit of precise co-deposition is demonstrated by an OLED with a sensitive twofold-doped emissive layer and revealed a doping level of 0.26% for the red dopant with a std. dev. of 0.38%. The effect of the various morphologies is investigated by optimizing the efficiency of molecular organic solar cells consisting of copper phthalocyanine (CuPc) and C60. With defined process parameters efficiencies of up to 3.0% were demonstrated.

The facile synthesis and high efficiency of the red electroluminescent dopant DCINB: A promising alternative to DCJTB

The cost of manufacturing 4-(dicyanomethylene)-2- tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4 H-pyran (DCJTB) is prohibitively high because the synthesis of one of the key intermediates, 1,1,7,7-tetramethyljulolidine, is costly, produces by-products and is of low yield. As an alternative low-cost process, a novel red dopant 4-(dicyanomethylene)-2- tert-butyl-6-(8-(4-methyl)phenyl-2,3-dihydro-1 H-cyclopenta [3 a,8 a]indolin-5-enyl)-4 H-pyran (DCINB) bearing an indoline unit instead of 1,1,7,7-tetramethyljulolidine as electron donor has been conveniently synthesized in high yield. The dopant exhibited very similar absorption and emission behaviour to DCJTB, but displayed superior fluorescence quantum yield and electroluminescence efficiency, which was attributed to high recombination ratio of excitons imparted by the introduced indoline unit. The synthesis and purification stages are simple and can be scaled-up, thereby offering a possible alternative to the commercial DCJTB.

Highly efficient single-layer phosphorescent white organic light-emitting diodes using a spirofluorene-based host material

Highly efficient phosphorescent white organic light-emitting diodes (PHWOLEDs) were developed by the doping of phosphorescent blue and red dopants in a spirofluorene-based phosphine oxide host material. A high quantum efficiency of 18.3% and a current efficiency of 34.2 cd/A at 100 cd/m(2) were obtained from the PHWOLED with the spirofluorene-based phosphine oxide host material. In addition, a high power efficiency of 28.3 lm/W was achieved in the PHWOLED. The wide triplet bandgap of host and charge balance in the light-emititng layer were responsible for the high efficiency.

Carrier conduction mechanism for phosphorescent material doped organic semiconductor

The mobility of charge carriers has been investigated in the pristine and phosphorescent material doped 4,4′,4″-tris(N-carbazolyl) triphenylamine (TCTA) using time-of-flight photoconductivity technique. Doping phosphorescent material fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] increases the electron mobility whereas the hole mobility decreases to the order of 10−4–10−6 cm2/V s measured at room temperature with different bias voltages. The analysis of field and temperature dependences of the mobility agrees well with the Gaussian disorder model. The calculated positional disorders (Σ) for TCTA, Ir(ppy)3-doped TCTA, and tris(1-phenylisoquinoline) iridium [Ir(piq)3]-doped TCTA are 0.12, 2.05, and 1.62 for hole, respectively; 3.89 for electron in only Ir(ppy)3-doped TCTA. The ambipolar transport for holes and electrons is possible by doping TCTA with Ir(ppy)3 (green dopant) whereas only hole transport with reduced mobility is achieved for Ir(piq)3 (red dopant).

Ideal host and guest system in phosphorescent OLEDs

Ideal host-guest system for emission in phosphorescent OLEDs with only 1% guest doping condition for efficient energy transfer have been demonstrated in the present investigation. Using a narrow band-gap fluorescent host material, bis(10-hydroxybenzo[h] quinolinato)beryllium complex (Bebq 2), and red dopant bis(2-phenylquinoline)(acetylacetonate)iridium (Ir(phq) 2acac), highly efficient red phosphorescent OLEDs (PHOLEDs) exhibiting excellent energy transfer characteristics with a doping concentration of 1% were developed. Fabricated PHOLEDs show a driving voltage of 3.7 V, maximum current and power efficiencies of 26.53 cd/A and 29.58 lm/W, and a maximum external quantum efficiency of 21%. Minimized electron or hole trapping at the phosphorescent guest molecules and efficient Förster and Dexter energy transfers from the Bebq 2 host singlet and triplet states to the emitting triplet of Ir(phq) 2acac guest appear to be the key mechanism for ideal phosphorescence emission.

High-Brightness White Organic Light-Emitting Diodes Featuring a Single Emission Layer

We have developed high-brightness, color-stable, broad-spectrum white organic light-emitting diodes featuring a single emission layer comprising a green light-emitting host material [1,3,5-tris(1-pyrenyl)benzene (TPB3)], which has a large bandgap, doped with red [4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)--pyran (DCJTB)], and blue [di(4-fluoro phenyl)amino-di(styryl)biphenyl (DSB)] dyes. A device having the simple configuration indium tin oxide/-bis(1- naphthyl)--diphenyl--biphenyl--diamine (NPB) /TPB3:0.6% DCJTB:6% DSB /tris(8-hydroxy- quinoline)aluminum exhibited a broad-spectrum white emission with a maximum luminance at of , a maximum current efficiency of at , a maximum power efficiency of at , and a maximum external quantum efficiency of 3.08% at . The Commission Internationale de l'Eclairage coordinates of (0.34, 0.39) changed only slightly over a large range of potentials . We attribute the high brightness and color-stability of this system to the strong electroluminescence of the host and the dopants, the relatively high energy-transfer efficiency from the host to the red dopant, effective carrier-direct-recombination on the blue dopant, and the confinement of the charge recombination zone within a single layer.

High-Color-Purity Organic Light-Emitting Diodes Incorporating a Cyanocoumarin-Derived Red Dopant Material

This paper describes a red dopant material, the cyanocoumarin derivative 9-cyano-10-(2- benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro--benzo pyrano[6,7,8- ]quinolizin-11-one (RC545T), and its use in organic light-emitting diodes (OLEDs). Because RC545T exhibits (i) redshifted fluorescence and (ii) a narrower full width at half-maximum (fwhm; 48 nm) relative to the corresponding signal for 4-(dicyanomethylene)- 2-tert-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)--pyran (DCJTB; ; ), which is currently one of the most efficient red dopant materials, we could fabricate red OLEDs exhibiting improved color purity. A device incorporating RC545T as a dopant and rubrene as an assistant dopant in the configuration indium–tin oxide (ITO)/-bis(1-naphthyl)- -diphenyl--biphenyl--diamine /tris(8-hydroxyquinoline)aluminum rubrene: RC545T exhibited a maximum luminance of at , with near-saturated CIE coordinates of (0.64, 0.35), significantly better red color purity than that of the device featuring DCJTB as the dopant material [CIE coordinates: (0.63, 0.37)], thereby rendering RC545T a good red-emitting dopant for OLED applications.