Single Emissive Layer Research Articles

Luminance uniformity study of OLED lighting panels depending on OLED device structures.

This paper describes the luminance uniformity of OLED lighting panels depending on OLED device structures of single emission layer (single-EML), 2-tandem, and 3-tandem. The luminance distribution is evaluated through the circuit simulation and the fabricated panel measurement. In the simulation results with yellow-green color panels of 30 × 80 mm2 emission area, a 3-tandem structure shows the lowest non-uniformity (1.34% at 7.5V), compared to single-EML (5.67% at 2.8V) and 2-tandem (2.78% at 5.3 V) structures at 1,000 cd/m2. The luminance non-uniformity is germane to the OLED conductance showing that the high luminance-current efficiency is of the most importance to achieve the uniform voltage and luminance distribution. In measurement, a 3-tandem structure also achieves the most uniform luminance distribution with non-uniformity of 4.1% while single EML and 2-tandem structures accomplish 9.6%, and 6.4%, respectively, at ~1,000 cd/m2. In addition, the simulation results ensure that a 3-tandem structure panel is allowed to be enlarged the panel size up to about 5,000 mm2 for lower luminance non-uniformity than 10% without any auxiliary metal electrodes.

Design, synthesis and properties of triple-color hyperbranched polymers derived from poly(9,9-dioctylfluorene) with phosphorescent core tris(1-phenylisoquinoline)iridium(Ⅲ)

A series of triple-color hyperbranched polymers based on poly(9,9-dioctylfluorene) (PF) were designed and synthesized, in which phosphorescent tris(1-phenylisoquinoline)iridium(Ш) (Ir(piq)3) acts as red core and poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)] (PFBT) is inserted as green-emission segment. The synthesized polymers have good thermostability, film forming ability and high fluorescence quantum yield. By tuning the contents of red core and green-emission segment, the optimized triple-color hyperbranched polymer is acquired, named PF-BT250-Ir(piq)325. Furthermore, the PF-BT250-Ir(piq)325 is utilized as single emission layer in white polymer light emitting diode with the configuration of ITO/PEDOT:PSS (50 nm)/PF-BT250-Ir(piq)325 (80 nm)/TPBI (40 nm)/LiF (1 nm)/Al (100 nm). The diode exhibits a maximum luminous efficiency of 1.60 cd/A, a maximum luminance of 3267 cd/m2 and the Commission Internationale de l’Eclairage coordinate of (0.32, 0.34) very close to the pure white point of (0.33, 0.33).

Efficient single-emitting layer hybrid white organic light-emitting diodes with low efficiency roll-off, stable color and extremely high luminance

An efficient single-emitting layer hybrid white organic light-emitting diode (WOLED) was developed, simultaneously fulfilling low efficiency roll-off, stable color and extreme luminance. At the practical luminance of 1000cd/m2, a total current efficiency of 42.8cd/A and power efficiency of 19.2lm/W are achieved, maintaining as high as 40.5cd/A and 15.5lm/W even at 5000cd/m2. Besides, a slight color variation [(0.025, 0.011)] and extreme luminance (∼106cd/m2) are obtained. The working mechanism is unveiled and it is demonstrated that the triplet-exciton-confining ability of electron transport layers plays a more vital role in device performances, particularly for the lifetime.

Efficiency enhancement in a single emission layer yellow organic light emitting device: Contribution of CIS/ZnS quantum dot

Electroluminescence (EL) efficiency from a single emission layer solution processed yellow emitting polymer, i.e. poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,10,3}-thiadiazole)] end-capped with dimethylphenyl (ADS233YE), is firstly enhanced by the optimization of stock polymer concentrations and the coating rates, and then with the addition of copper indium disulfide/zinc sulfide (CIS/ZnS) core/shell quantum dots (QDs). Using these bare core/shell QDs as the active layer in the studied device gave no EL at all. However, yellow EL with the maximum brightness of 56834cd/m2, maximum current efficiency of 4.7cd/A and maximum power efficiency of 2.3lm/W is obtained from the device structure of indium tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)/ADS233YE:0.4wt.% CIS/ZnS QD/Ca/Al those of which correspond to approximately 4 and 2 folds of enhancements in the brightness and luminous and power efficiency values, respectively, compared to that of the device without CIS/ZnS.

Blue-green phosphorescent imidazole-based iridium(III) complex with a broad full width at half maximum for solution-processed organic light-emitting diodes

We designed and synthesized a blue-green phosphorescent iridium(III) complex for solution-processed organic light-emitting diodes (OLEDs), (DMDPI)2Ir(tftap), that contains 1,2-dimethyl-4,5-diphenyl-1H-imidazole (DMDPI) as the main ligand and 2-(3-(trifluoromethyl)-1H-1,2,4-triazol-5-yl) pyridine (tftap) as an ancillary ligand. (DMDPI)2Ir(tftap) showed good solubility in common organic solvents such as dichloromethane, chloroform, toluene, and chlorobenzene. The photoluminescence (PL) spectra of (DMDPI)2Ir(tftap) showed a maximum emission peak (λmax) of 532nm in dichloromethane solution. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of (DMDPI)2Ir(tftap) were measured to be −5.36 and −2.76eV, respectively. Solution-processed blue-green OLEDs were fabricated based on (DMDPI)2Ir(tftap) with an ITO/PEDOT:PSS (30nm)/26DCzPPy:(DMDPI)2Ir(tftap) (11%) (40nm)/TPBi (60nm)/CsF (1nm)/Al structure. The electroluminescent (EL) spectra of (DMDPI)2Ir(tftap) exhibited a maximum emission peak at 522nm with a broad full-width at half-maximum (FWHM) of 115nm and Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.54) at 1000cdm−2. The device with 11% doping concentration of (DMDPI)2Ir(tftap) exhibited a maximum luminance of 6304cdm−2, a maximum luminous efficiency of 7.14cdA−1, a power efficiency of 3.63lmW−1, and an external quantum efficiency of 2.59%. Moreover, white-light-emitting devices containing a single emissive layer consisting of PVK as a polymer host, (DMDPI)2Ir(tftap) as a blue-green dopant, and (PIQ)2Ir(acac) as a red dopant were fabricated. These devices exhibited CIE coordinates of (0.42, 0.48) at 1000cdm−2, which are close to the standard warm white-light CIE coordinates of (0.44, 0.40).

Fabrication of a white electroluminescent device based on bilayered yellow and blue quantum dots.

Until now most work on colloidal quantum dot-light-emitting diodes (QLEDs) has been focused on the improvement of the electroluminescent (EL) performance of monochromatic devices, and multi-colored white QLEDs comprising more than one type of QD emitter have been rarely investigated. To demonstrate a white EL as a result of color mixing between blue and yellow, herein a unique combination of two dissimilar QDs of blue- CdZnS/ZnS plus a yellow-emitting Cu-In-S (CIS)/ZnS is used for the formation of the emitting layer (EML) of a multilayered QLED. First, the QLED consisting of a single EML randomly mixed with two QDs is fabricated, however, its EL is dominated by blue emission with the contribution of yellow emission substantially weaker. Thus, another EML configuration is devised in the form of a QD bilayer with two stacking sequences of CdZnS/ZnS//CIS/ZnS QD and vice versa. The QLED with the former stacking sequence shows an overwhelming contribution of blue EL, similar to the mixed QD EML-based device. Upon applying the oppositely stacked QD bilayer of CIS/ZnS//CdZnS/ZnS, however, a bicolored white EL can be successfully achieved by means of the effective extension of the radiative excitonic recombination zone throughout both QD EML regions. Such QD EML configuration-dependent EL results, which are discussed primarily using the proposed device energy level diagram, strongly suggest that the positional design of individual QD emitters is a critical factor for the realization of multicolored, white emissive devices.

Solution processed single-emission layer white polymer light-emitting diodes with high color quality and high performance from a poly(N-vinyl)carbazole host.

Low cost and high performance white polymer light-emitting diodes (PLEDs) are very important as solid-state lighting sources. In this research three commercially available phosphors were carefully chosen, bis[2-(4,6-difluorophenyl)pyridinato-N,C(2)](picolinate)iridium(III) (FIrpic), bis[2-(2-pyridinyl-N)phenyl-C](2,4-pentanedionato-O(2),O(4))iridium(III) [Ir(ppy)2(acac)], and bis(2-phenyl-benzothiazole-C(2),N)(acetylacetonate)iridium(III) [Ir(bt)2(acac)], plus a home-made red phosphor of tris[1-(2,6-dimethylphenoxy)-4-(4-chlorophenyl)phthalazine]iridium(III) [Ir(MPCPPZ)3], and their photophysical and morphological properties were systematically studied as well as their applications in single-emission layer white PLEDs comprising poly(N-vinylcarbazole) as host. Additionally, the electrochemical properties and energy level alignment, possible energy transfer process, and thin-film morphology were also addressed. The binary blue/orange complementary white PLEDs exhibit stable electroluminescence spectra, wide spectrum-covering region range from 380-780 nm, and high color rendering index (CRI) over 70 with Commission Internationale de l'Eclairage coordinates x,y (CIEx,y) of (0.388, 0.440), correlated color temperature (CCT) of around 4400, plus high efficiency of 25.5 cd A(-1). The optimized red-green-blue white PLEDs showed a satisfactory CRI of around 82.4, maximum current efficiency of 20.0 cd A(-1) and external quantum efficiency (EQE) of 10.8%, corresponding to a CCT of 3700-2800, which is a warm-white hue. At last, stable and high color quality, red-green-orange-blue four component white PLEDs, with a CRI of over 82, a high efficiency of 24.0 cd A(-1), EQE of 11.5%, and high brightness of 43,569.9 cd m(-2) have been obtained.

Study of triplet exciton’s energy transfer in white phosphorescent organic light-emitting diodes with multi-doped single emissive layer

The performance of three color single emissive layer white phosphorescent organic light-emitting diodes (PHOLEDs), with different hole transporting materials of N,N′-diphenyl-N,N′-bis(l-naphthyl-phenyl)-(l,l′-biphenyl)-4,4′-diamine (NPB), 4,4_,4_-tris_N-carbazolyl_tri-Phenylamine (TCTA) and 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), was analyzed. It is found that the luminous efficiency of white PHOLEDs is closely related to the triplet exciton energy level in the hole transporting layer (HTL). White PHOLEDs with a TAPC as HTL, having the highest triplet exciton energy level amongst the three different hole transporting materials, yielded external quantum efficiency of 25.5% at 4.5V and a high luminous efficiency of 51cd/A at 4.5V with CIE color coordinates of (0.34, 0.39) at 10V. The results reveal that the effective confinement of triplet excitons in white PHOLEDs with a high triplet exciton energy level HTL (TAPC) allows improving the luminous efficiency, as compared to the devices made with NPB and TCTA. Additionally, we demonstrated possibility for the loss of intensity in EL spectra of the white PHOLED devices with NPB and TCTA as HTL compared to that of the device with TAPC as HTL.

The effect of the hole injection layer on the performance of single layer organic light-emitting diodes

Efficient single-layer organic light-emitting diodes (OLEDs) were reported based on a green fluorescent dye 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7–tetramethyl-1H,5H,11H-(1) benzopyropyrano (6,7-8-I,j)quinolizin-11-one (C545T). Herein, poly(3,4-ethylenedioxy thiophene) poly(styrene sulfonate) were, respectively, applied as the injection layer for comparison. The hole transport properties of the emission layer with different hole injection materials are well investigated via current-voltage measurement. It was clearly found that the hole injection layers (HILs) play an important role in the adjustment of the electron/hole injection to attain transport balance of charge carriers in the single emission layer of OLEDs with electron-transporting host. The layer of tris-(8-hydroxyquinoline) aluminum played a dual role of host and electron-transporting materials within the emission layer. Therefore, appropriate selection of hole injection layer is a key factor to achieve high efficiency OLEDs with single emission layer.

Efficient and Good Color Quality Single-Emitting-Layer Fluorescent White Organic Light-Emitting Diode Employing a Novel 8-Hydroxyquinoline Derivative as Yellow Emissive Component

Efficient, good color quality, two-component single-emitting layer fluorescent white organic light emitting diode (WOLED) has been developed using a novel 2-substitutd-8-hydroxyquinoline derivative, (E)-2-(2-(9-p-toly1-9H-carbazo1-3-y1)viny1) quinolin-Zinc(MCzHqZn), as yellow dopant. The WOLED with the optimizing MCzHqZn-doped concentration of 2 wt% exhibits maximum current efficiency of 2.3 cd/A with the Commission Internationale del'Eclairage (CIE) coordinates of (0.320, 0.347). From the brightness at 1000 to 5000 cd/m(2), the current efficiency roll-off of the device is 9.0%. White light emission is achieved by combining blue and yellow emissions emitted from NPB and MCzHqZn, respectively. The MCzHqZn emission is mainly attributable to the direct carrier trapping (DCT) at the dopant sites. Moreover, the WOLED exhibits excellent color stability, high color rendering index (CRI) and appropriate correlated color temperature (CCT) in a wide range of voltage. The CIE coordinate region is (0.310-0.324, 0.333-0.335) at various current density (20-858 mA/cm(2)), corresponding CRI of (78-81). The CIE coordinates correspond to CCT is 5914 K at 8532 cd/m(2).

Study of sequential dexter energy transfer in high efficient phosphorescent white organic light-emitting diodes with single emissive layer.

In this study, we report our effort to realize high performance single emissive layer three color white phosphorescent organic light emitting diodes (PHOLEDs) through sequential Dexter energy transfer of blue, green and red dopants. The PHOLEDs had a structure of; ITO(1500 Å)/NPB(700 Å)/mCP:Firpic-x%:Ir(ppy)3-0.5%:Ir(piq)3-y%(300 Å)/TPBi(300 Å)/Liq(20 Å)/Al(1200 Å). The dopant concentrations of FIrpic, Ir(ppy)3 and Ir(piq)3 were adjusted and optimized to facilitate the preferred energy transfer processes attaining both the best luminous efficiency and CIE color coordinates. The presence of a deep trapping center for charge carriers in the emissive layer was confirmed by the observed red shift in electroluminescent spectra. White PHOLEDs, with phosphorescent dopant concentrations of FIrpic-8.0%:Ir(ppy)3-0.5%:Ir(piq)3-0.5% in the mCP host of the single emissive layer, had a maximum luminescence of 37,810 cd/m2 at 11 V and a luminous efficiency of 48.10 cd/A at 5 V with CIE color coordinates of (0.35, 0.41).

Synthesis and optoelectric properties of fluorene-alt-benzene organic phosphorescent polymers containing pendent cyclometalated iridium complex

A series of novel phosphorescent polymers consisting of fluorene-alt-benzene as building block in the main chain and cyclometalated iridium complex as the pendent in the branch were synthesized by coupling polymerization. All of these resulting polymers presented a good thermal stability, an intense absorption in the region of 300–400 nm and a bright blue emission in solution. On contrary, the polymers exhibited dual emission spectra originated from the backbones and iridium complex unit in the neat film. Polymer organic light-emitting diodes employing these polymers as the single emissive layer with the configuration of ITO/PEDOT : PSS (50 nm)/phosphorescent polymer (45 nm)/LiF (0.5 nm)/Al (150 nm) were fabricated. The devices revealed a maximum brightness 43 cd/m2 with a low molar concentration of iridium complex (1%).

Synthesis, characterization, and application of a novel orange–red iridium(III) phosphor for solution-processed single emissive layer white organic light-emitting diodes

A novel iridium(III) complex bis(2-(naphthalen-1-yl)-6-(trifluoromethyl)benzothiazole)iridium(acetylacetonate) (CF3BT-N)2Ir(acac) with an orange–red emission was synthesized. The incorporation of CF3 and naphthalene groups into benzimidazole to act as a new ligand causes a significant change of both HOMO and LUMO energy levels of the iridium complex compared with the yellow emissive parent compound comprising the 2-phenylbenzimidazole ligand. As a result, a bathochromic shift by as much as 50nm of the phosphorescence emission peak can be achieved. Electronic properties of (CF3BT-N)2Ir(acac) were examined by time-dependent density functional theory (TD-DFT) calculations. The influence of such substituent groups on the photophysical, electrochemical and electroluminescent properties of iridium complex was studied. Furthermore, two- and three-element solution-processed white organic light-emitting diodes (WOLEDs) with high performance can be realized by using this kind of orange–red phosphorescent emitter in conjunction with other phosphors in a single emissive layer. A maximum luminous efficiency of 10.9cdA−1 with CIE coordinates of (0.27, 0.37) was obtained for the two-element WOLEDs. For the three-element WOLED, a maximum luminance of 22716cdm−1 and a maximum luminous efficiency of 14.6cdA−1 with CIE coordinates of (0.33, 0.42) can be achieved.

Highly efficient inverted polymer light-emitting diodes using surface modifications of ZnO layer.

Organic light-emitting diodes have been recently focused for flexible display and solid-state lighting applications and so much effort has been devoted to achieve highly efficient organic light-emitting diodes. Here, we improve the efficiency of inverted polymer light-emitting diodes by introducing a spontaneously formed ripple-shaped nanostructure of ZnO and applying an amine-based polar solvent treatment to the nanostructure of ZnO. The nanostructure of the ZnO layer improves the extraction of the waveguide modes inside the device structure, and a 2-ME+EA interlayer enhances the electron injection and hole blocking in addition to reducing exciton quenching between the polar-solvent-treated ZnO and the emissive layer. Therefore, our optimized inverted polymer light-emitting diodes have a luminous efficiency of 61.6 cd A(-1) and an external quantum efficiency of 17.8%, which are the highest efficiency values among polymer-based fluorescent light-emitting diodes that contain a single emissive layer.

Synthesis and optoelectronic properties of novel fluorene-bridged dinuclear cyclometalated iridium (III) complex with A–D–A framework in the single-emissive-layer WOLEDs

To explore the influence of push–pull chromophores on properties of emitter in organic light-emitting devices (OLEDs), an acceptor–donor–acceptor (A–D–A)-based dinuclear iridium (III) complex of (dfppy)4Ir2(dipic-FL) was synthesized via Suzuki coupling reaction, in which dfppy is 2-(2,4-difluorophenyl)pyridine and dipic-FL is 2,7-di(5-pyridyl-2-carboxyl)-9,9-dioctyl-9H-fluorene. An intense emission peak at about 480nm resulting from the (dfppy)2Ir(pic) chromophore and a weak long-wavelength emission band at 580–660nm attributed to intramolecular charge transfer transition were exhibited for (dfppy)4Ir2(dipic-FL) in dichloromethane solution. But a remarkably hypsochromic photoluminescence profile with an intense characteristical emission peak at 422nm was observed, which is attributed to the intraligand (IL) π–π∗ excited states in its thin film. White emission with a maximum luminance of 1040cd/m2 and current efficiency of 1.2cd/A was obtained in its single-emissive-layer (SEL) OLEDs with a configuration of ITO/PEDOT:PSS/(dfppy)4Ir2(dipic-FL) (10 wt%):TCTA/TPBi/LiF/Al. To our knowledge, this is one of the best examples in term of dinuclear iridium complex as single dopant in the high-performance white-emitting SEL-OLEDs.

Efficient red electroluminescent devices with sterically hindered phosphorescent platinum(II) Schiff base complexes and iridium complex codopant.

Sterically hindered platinum(II) Schiff base complexes were prepared. Complex 4, which displays red emission with a quantum yield of 0.29 in a thin film and a self-quenching rate constant of 1×10(-7) dm(3) mol(-1) s(-1), was used to fabricate organic light-emitting diodes with single or double emissive layers (EMLs). An iridium(III) complex with a wide band gap was codoped into the electron-dominant EML to act as a deep electron trapper, and red-light-emitting devices with the highest current, power, and external quantum efficiencies of 20.43 cd A(-1) 18.33 Lm W(-1), and 11.7%, respectively, were fabricated. A high current efficiency and EQE of up to 14.69 cd A(-1) and 8.3%, respectively, were achieved at a high brightness of 1000 cd m(-2). The significant delay of efficiency roll-off is attributed to the bulky 3D structure of the norbornene moiety at the periphery of the Schiff base ligand of 4 and to the new device design strategy. The fabricated device had a projected lifetime (LT50) of 18,000 h.

White organic light-emitting diodes based on a single-emissive layer using electrophosphorescent dopants in a fluorescent host

A single-emissive fluorescent layer co-doped with red and blue phosphorescent dyes in a host [4, 4, bis-9 carbozyl-biphenyl] (CBP) was used to fabricate white organic light-emitting devices (WOLEDs). The electroluminescent spectrum of the generated white light was observed to have two spectral peaks at ∼495 and ∼590 nm and a dip at ∼ 536 nm, with different thicknesses of the emissive layer (EML). By optimizing the ratio of the dopants in the EML, the ratio of the maximum intensity of the two spectral peaks was brought to unity. The Commission Internationale de l'Eclairage (CIE) coordinates and correlated color temperature of the fabricated device were calculated, and the results are reported herein. No significant change in the spectral shape of the emission spectra and the CIE coordinates were obtained when the applied voltages were varied. It is easier to fabricate WOLEDs with a single EML as compared with their multilayer counterparts.

Bipolar Hosts Based on a Rigid 9,10‐Dihydroanthracene Scaffold for Full‐Color Electrophosphorescent Devices

AbstractTwo bipolar hosts, CzDPO and CzDOxa, comprising a hole‐transporting carbazole and an electron‐transporting diphenyl phosphine oxide or oxadiazole with a saturated linker (9,10‐dihydroanthracene) have been synthesized and characterized. The saturated linker limited the effective extension of π conjugation, leading to high triplet energies (ET=2.97–2.98 eV). The diastereomers with a rigid configuration provided an amorphous thin film with high thermal (Td=415–444 °C) and morphological stability (Tg=188–224 °C). The high triplet energies and bipolar carrier transport characteristics (ambipolariy) of CzDPO and CzDOxa can facilitate exothermic energy transfer to the dopants and balance carrier injection/transport in the emission layers. As a result, they were utilized as universal hosts for various phosphorescent OLEDs (from blue to red), showing average external quantum efficiencies (ηext=8.2–13.1 %) and low efficiency roll‐off. In addition, we also fabricated dual‐emitter white organic light‐emitting diodes with a co‐doped single emissive layer. WOLED hosted by CzDOxa exhibited satisfactory device efficiencies (14.4 %, 25.7 cd A−1, 27 lm W−1) with highly stable chromaticity (CIEx=0.26–0.28 and CIEy=0.38) at brightnesses from 560 to 15200 cd m−2.

P‐159: Triplet Exciton Confinement in White Phosphorescent Organic Light‐Emitting Diodes with Multi‐Doped Single Emissive Layer

AbstractWe fabricated white phosphorescent organic light‐emitting diodes (PHOLEDs) with the primary color(Red, Green and Blue) in single emissive layer using NPB, TCTA and TAPC as hole transporting layer (HTL) having different triplet energy level to observe triplet exciton's behavior. White PHOLED with TAPC having higher triplet energy level shows 51cd/A of luminous efficiency, and CIEXY color coordinate of (0.35, 0.40) at 10V.

Effect of PEDOT:PSS vs. MoO3 as the hole injection layer on performance of C545T-based green electroluminescent light-emitting diodes

In order to understand the effect of hole injection on the performance of organic light-emitting diodes (OLEDs) with electron- and hole-type hosts used in the emissive layer, we fabricated OLEDs based on a green fluorescent 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)benzopyropyrano(6,7-8-I,j)quinolizin-11-one (C545T) with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and MoO3 as the hole injection layer, respectively. Here, N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) and tris-(8-hydroxyquinoline) aluminum (Alq3) are, respectively, used as the host in the emissive layer for comparison. It is clearly found that different hole injection layers play different roles in the adjustment of the electron/hole injection and the transport balance, thus the different hosts are needed in the emissive layer for high electroluminescence efficiency OLEDs. This means that the selection of appropriate hole injection layers for OLEDs according to the different hosts in the emissive layer is especially important in the fabrication of high efficiency single emissive layer OLEDs.