Phosphorescent White Organic Light-emitting Devices Research Articles

Patterned OLEDs: effect of substrate corrugation pitch and height

An ongoing OLED challenge is cost-effective enhancement of light extraction, i.e., increasing the external quantum efficiency (EQE ∼20% in conventional devices). OLEDs on corrugated substrates often show enhanced EQEs providing insight into light emission processes. In particular, patterned plastic substrates directly imprinted easily at room temperature and amenable to low-cost R2R production are ideal for studying/optimizing various structures, further elucidating the extraction process. We show new semi-quantitative data of the effect of the pitch (a) and height/depth (h) of plastic substrate patterns on the OLEDs’ stack and EQE, focusing on new designs, interestingly, some showing surprisingly enhanced EQEs that were neither reported nor discussed before. These include: (i) shallow (h < 200 nm) convex polycarbonate with a ∼ 750 versus∼400 nm, where the h gradually decreases as the OLED stack is built and (ii) concave PET/CAB with large a (∼2.8 and ∼7.8 μm), where the EQE enhancement of conformal OLEDs may be due largely to scattering. EQEs of green, blue, and white phosphorescent OLEDs were measured. OLEDs on substrates with narrow a ∼ 400 nm and low h < 200 nm showed no enhancement, resembling flat devices. In contrast, OLEDs on substrates with comparable or smaller h, but larger a ∼ 750 nm show significant EQE enhancement despite h reduction across the stack. Green OLEDs with a ∼ 750 nm and h ∼ 160 to ∼180 nm, showed EQEs ∼30%, reaching ∼58% with substrate mode extraction. Surprisingly, fully conformal OLEDs on a PET/CAB substrate with a ∼ 7.8 μm showed blue and white EQEs reaching ∼33%, without substrate mode extraction. The enhancing patterns increase the OLEDs’ EQE by reducing surface plasmon excitation and internal waveguiding.The experimental results for OLEDs on substrates with a < 2 μm are supported by scattering matrix simulations that assume conformal stacks, incorporating diffraction for internal losses reduction. EQE enhancement not predicted by simulations may be due additionally to scattering mostly for substrates with a significantly larger than the emitting wavelength.

Solution-processable cyclometalated gold(III) complexes for high-brightness phosphorescent white organic light-emitting devices

We report two yellow-emitting cyclometalated gold(III) complexes serving as phosphorescent dopants for high-brightness solution-processed white organic light-emitting devices (WOLEDs). Dichromatic WOLEDs consisting of our newly synthesized Au–Y or Au–wY for yellow emission in combination with FIrpic for blue emission have been studied. Notably, white emissions in terms of Commission Internationale de L’Eclairage coordinates of (0.31, 0.39) and (0.40, 0.47) and color rendering indices of 66 and 69 have been accomplished with Au–Y- and Au–wY-based WOLEDs. In addition, current efficiencies of 26.0 and 25.7 cd/A corresponding to external quantum efficiencies of 11.0% and 10.5% are achieved and can be maintained at 19.8 and 25.3 cd/A at a high brightness level of 10,000 cd/m2 for these devices. Such high-performance WOLEDs in terms of small efficiency roll-offs over a wide brightness range are believed to be due to the use of gold(III) complexes with very short-lived emission lifetimes of ~ 1 μs, high photoluminescence quantum yields of up to 86% and broad yellow emission bandwidths of up to 107 nm. This study opens up the possibility of using solution-processable cyclometalated organogold(III) complexes with short-lived emissive excited states for the fabrication of WOLEDs, especially for high-brightness applications.

Energy transfer process in white organic light-emitting devices based on carbazole/thioxanthene-S,S-dioxide host material

Phosphorescent white organic light-emitting devices (Ph-WOLEDs) fabricated utilizing carbazole/thioxanthene-S,S-dioxide (EBCz-ThX) host material were investigated to enhance their efficiency by understanding the energy transfer process from the host to the dopant materials. Time-resolved photoluminescence (PL) spectra at room and low temperatures demonstrated that the energy transfer occurs between the EBCz-ThX host and the bis(2-benzo[b]thiophen-2-ylpyridine)(acetylacetonate)-iridium(III) (Ir(bt)2acac) orange dopant or the (bis(2-(4,6-difluorophenyl)pyridyl-N,C2′)iridium(III) picolinate (FIrpic) blue dopant. The energy transfer process was inferred from the PL decay curves of Ir(bt)2acac doped EBCz-ThX with doping concentrations of 5, 10, and 20%. The quantum efficiency in the Ph-WOLEDs with 5% Ir(bt)2acac doped EBCz-ThX is the highest since its energy transfer process from EBCz-ThX to Ir(bt)2acac involves the charge transfer states and is most efficient. The quantum efficiency of the Ph-WOLEDs with 5% Ir(bt)2acac doped EBCz-ThX emission layers at 1,000 cd/m2 showed the highest value of 10.2%, and the commission internationale de l'Eclairage (CIE) coordinates were (0.45, 0.46), indicative of the warm white emission.

1,2,4]Triazolo[1,5-a]pyridine as Building Blocks for Universal Host Materials for High-Performance Red, Green, Blue and White Phosphorescent Organic Light-Emitting Devices.

The electron-accepting [1,2,4]triazolo[1,5-a]pyridine (TP) moiety was introduced to build bipolar host materials for the first time, and two host materials based on this TP acceptor and carbazole donor, namely, 9,9'-(2-([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (o-CzTP) and 9,9'-(5-([1,2,4]triazolo[1,5-a]pyridin-2-yl)-1,3-phenylene)bis(9H-carbazole) (m-CzTP), were designed and synthesized. These two TP-based host materials possess a high triplet energy (>2.9 eV) and appropriate highest occupied molecular orbital/lowest unoccupied molecular orbital levels as well as the bipolar transporting feature, which permits their applicability as universal host materials in multicolor phosphorescent organic light-emitting devices (PhOLEDs). Blue, green, and red PhOLEDs based on o-CzTP and m-CzTP with the same device configuration all show high efficiencies and low efficiency roll-off. The devices hosted by o-CzTP exhibit maximum external quantum efficiencies (ηext) of 27.1, 25.0, and 15.8% for blue, green, and red light emitting, respectively, which are comparable with the best electroluminescene performance reported for FIrpic-based blue, Ir(ppy)3-based green, and Ir(pq)2(acac)-based red PhOLEDs equipped with a single-component host. The white PhOLEDs based on the o-CzTP host and three lumophors containing red, green, and blue emitting layers were fabricated with the same device structure, which exhibit a maximum current efficiency and ηc of 40.4 cd/A and 17.8%, respectively, with the color rendering index value of 75.

Aminoborane-based bipolar host material for blue and white-emitting electrophosphorescence devices

A novel aminoborane-based host material, 9-(dimesitylboryl)-9′-phenyl-9H, 9′H-3,3′-bicarbazole (BCzBMes) was developed for blue and white phosphorescent OLEDs (PHOLEDs). The thermal, photophysical and electrochemical properties were systematically investigated. BCzBMes not only has a high triplet energy but also shows a bipolar behavior. To validate the superior properties of BCzBMes, blue and white PHOLEDs were fabricated using BCzBMes as a bipolar host material. A blue PHOLED containing Bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrPic) as a dopant exhibited excellent performance with a maximum external quantum efficiency (EQE) of 16.7%. In particular, the blue PHOLED exhibited an extraordinary low efficiency roll-off of 10.1% at a brightness of 5000 cd/m2. Meanwhile, an all-phosphor near-white device hosted by BCzBMes was also fabricated, and a high EQE of 18.8% was achieved. This excellent performance suggests that BCzBMes is a potential bipolar host material for the PHOLEDs.

Highly efficient white organic light-emitting devices with optimized electron transporting layers

We fabricated three phosphorescent blue organic light-emitting devices based on blue phosphor of iri-dium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′](Firpic) with different electron transporting layer(ETL) materials. By analyzing efficiency curves and spectral characteristics, the significant effect of ETL on many aspects of device performance was demonstrated. With optimized ETL, the characteristics of devices, such as voltage and efficiency were significantly improved. Combined with a yellow phosphor of iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2′) acetylacetonate(PO-01), phosphorescent white organic light-emitting devices(PhWOLEDs) were obtained. Then, with an aim to promote the performance of the PhWOLEDs, a thin layer of 4,4′,4″-tri(N-carbazolyl)-triphenylamine(TCTA) was introduced between two light emission layers, and the diffusion of excitons was confined. The outperformance device fabricated with 4,7-diphenyl-1,10-phenanthroline(Bphen) as the electron transporting layer exhibited a peak current efficiency of 36.6 cd/A, a peak power efficiency of 19.2 lm/W, and the International Commission on Illumination coordinates(0.37, 0.46).

Effect of various red phosphorescent dopants in single emissive white phosphorescent organic light-emitting devices

In order to realize single emissive white phosphorescent organic light-emitting devices (PHOLEDs) with three color phosphorescent dopants (red, green, and blue), the energy transfer between the host material and the three dopants, as well as the among the three dopants themselves, should be considered and optimized. To explore the effect of red phosphorescent dopant on the color rendering index (CRI), the authors investigate the wavelength position of the maximum emission peak from three phosphorescent dopants. The CRI and luminous efficiency of white PHOLED in which Ir(pq)2(acac) acts as the red phosphorescent dopant are found to be greater than those of devices prepared using Ir(piq)3 and Ir(btp)2(acac) as the emission spectrum has a relatively high intensity near the human perception of blue, red, and green wavelengths. Furthermore, we demonstrate that the performance of the three dopants is related to the absorption characteristics of the red phosphorescent dopant. With a maximum emission peak at 600 nm, Ir(pq)2(acac) has a higher intensity in the concave section between 550 and 600 nm seen for red and blue dopants. In addition, the long metal-to-ligand charge transfer (MLCT) absorption tail of Ir(pq)2(acac) overlaps with the emission spectra of the green dopant, enhancing emission. Such energy transfer mechanisms are confirmed to optimize white emission in the single emissive white PHOLEDs.

Quenching in single emissive white phosphorescent organic light-emitting devices

To explore energy loss by diffusive triplet excitons in single emissive white phosphorescent organic light-emitting devices, the authors investigated collisional quenching between the electron transport materials 4,7-diphenyl-1,10-phenanthroline (Bphen), 2′,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), or 1,3,5-tri(3-pyrid-3-yl-phenyl)benzene (TmPyPB) and the blue phosphorescent material, 3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl) Iridium III (FIrpic) spectroscopically in solution. The luminous efficiency and the external quantum efficiency (EQE) of an emissive white phosphorescent organic light-emitting device, in which TmPyPB acted as the electron transport material, was found to be greater than those of devices prepared using Bphen or TPBi due to the lack of collisional quenching. In addition, it was found that to prevent triplet exciton loss, an ETL material should have a low bimolecular quenching rate constant kq of less than 1.458 × 107 s−1 M−1, which is the kq of TmPyPB.

De Novo Design of Boron-Based Host Materials for Highly Efficient Blue and White Phosphorescent OLEDs with Low Efficiency Roll-Off.

Borane is an excellent electron-accepting species, and its derivatives have been widely used in a variety of fields. However, the use of borane derivatives as host materials in OLEDs has rarely reported because the device performance is generally not satisfactory. In this work, two novel spiro-bipolar hosts with incorporated borane were designed and synthesized. The strategies used in preparing these materials were to increase the spatial separation of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) in the molecules, tune the connecting positions of functional groups, and incorporate specific functional groups with desirable thermal stability. Based on these designs, phosphorescent OLEDs with borane derivatives as hosts and with outstanding device performances were obtained. In particular, devices based on SAF-3-DMB/FIrpic exhibited an external quantum efficiency (EQE) of >25%. More encouragingly, the device was found to have quite a low efficiency roll-off, giving an efficiency of >20% even at a high brightness of 10000 cd/m(2). Furthermore, the EQE of the three-color-based (R + G + B) white OLED employing SAF-3-DMB as a host was also as high as 22.9% with CIE coordinates of (x, y) = (0.40, 0.48). At a brightness of 5000 cd/m(2), there was only a 3% decrease in EQE from its maximum value, implying a very low efficiency roll-off.

Efficient white organic light-emitting devices based on a novel phosphine oxide host with low driving voltage and excellent color stability

Efficient phosphorescent white organic light-emitting devices (WOLEDs) based on a novel phosphine oxide host, 4,6-bis(diphenylphosphoryl)dibenzofuran (DBFDPO), with particularly low turn-on voltage and high color stability are reported. Without any electrical doping (p-doing or n-doping) strategies, the simplified optimal WOLEDs utilizing DBFDPO as the same host for blue/yellow emitters show low operating voltages, 1cd/m2at 2.5V (turn-on voltage), 46cd/m2 at 3V and 2787cd/m2at 5V, and 12840cd/m2 at 7V, respectively. A forward-viewing current efficiency (CE) of 41.3cd/A (power efficiency (PE) of 34.0lm/W) is achieved at the luminance of 673.4cd/m2 without any out-coupling enhancement structures. Furthermore, improved color stability is achieved through adjusting the yellow emitting layer. The Commission Internationale de I’Eclairage coordinates (CIE) of the optimal WOLED merely shift from (0.330, 0.405) to (0.327, 0.400) over a wide range of luminance (10–10,000cd/m2). We have revealed that the competition of the two mechanisms of energy transfer and carrier-trapping/-assist transport severely impact on the spectra stability.

M‐Indolocarbazole Derivative as a Universal Host Material for RGB and White Phosphorescent OLEDs

The host materials designed for highly efficient white phosphorescent organic light‐emitting diodes (PhOLEDs) with power efficiency (PE) &gt;50 lm W‐1 and low efficiency roll‐off are very rare. In this work, three new indolocarbazole‐based materials (ICDP, 4ICPPy, and 4ICDPy) are presented composed of 6,7‐dimethylindolo[3,2‐a]carbazole and phenyl or 4‐pyridyl group for hosting blue, green, and red phosphors. Among this three host materials, 4ICDPy‐based devices reveal the best electroluminescent performance with maximum external quantum efficiencies (EQEs) of 22.1%, 27.0%, and 25.3% for blue (FIrpic), green (fac‐Ir(ppy)3), and red ((piq)2Ir(acac)) PhOLEDs. A two‐color and single‐emitting‐layer white organic light‐emitting diode hosted by 4ICDPy with FIrpic and Ir(pq)3 as dopants achieves high EQE of 20.3% and PE of 50.9 lm W−1 with good color stability; this performance is among the best for a single‐emitting‐layer white PhOLEDs. All 4ICDPy‐based devices show low efficiency roll‐off probably due to the excellent balanced carrier transport arisen from the bipolar character of 4ICDPy.

Highly efficient orange and white phosphorescent organic light-emitting devices with simplified structure

We successfully developed efficient orange and white phosphorescent organic light-emitting devices based on simplified structure. The maximum power efficiency and current efficiency of the orange device with only 5nm emission layer are 34.1lm/W and 38.0cd/A. Performances of the simplified orange devices can be greatly improved to 62.6lm/W (79.7cd/A) by introducing one blue phosphorescent dye iridium(III)[bis(4,6-difluorophenyl)-pyridina-to-N,C2′]picolinate into the orange emission layer. The emission mechanism of the above orange devices was also discussed. White light emission with Commission International de L’Eclairage coordinates of (0.36, 0.45) and efficiency of 40.6lm/W (38.7cd/A) was obtained by exactly adjusting the doping concentration of orange phosphorescent dye.

Design and Synthesis of Pyrimidine-Based Iridium(III) Complexes with Horizontal Orientation for Orange and White Phosphorescent OLEDs.

Two phosphorescent Ir(III) complexes Ir(ppm)2(acac) and Ir(dmppm)2(acac) were synthesized and characterized with emission ranged at 584/600 nm and high photoluminescence quantum yields (PLQYs) of 0.90/0.92, respectively. The angle-dependent PL spectra analysis reveals that the two orange iridium(III) complexes embodied horizontal orientation property. The high photoluminescence quantum yield and high horizontal dipoles ratio determine their excellent device performance. The devices based on Ir(ppm)2(acac) and Ir(dmppm)2(acac) achieved efficiencies of 26.8% and 28.2%, respectively, which can be comparable to the best orange phosphorescent devices reported in the literature. Furthermore, with the introduction of FIrpic as sky-blue emitter, phosphorescent two-element white organic light-emitting devices (OLEDs) have been realized with external quantum efficiencies (EQEs) as high as 25%, which are the highest values among the reported two-element white OLEDs.

Efficient simplified orange and white phosphorescent organic light-emitting devices with reduced efficiency roll-off

Efficient orange phosphorescent organic light-emitting devices based on simplified structure with maximum efficiencies of 46.5lm/W and 51.5cd/A were reported. One device had extremely low efficiency roll-off with efficiencies of 50.6cd/A, 45.0cd/A and 39.2cd/A at 1000cd/m2, 5000cd/m2 and 10,000cd/m2 respectively. The reduced efficiency roll-off was attributed to more balanced carrier injection and broader recombination zone. The designed simplified white device showed much lower efficiency roll-off than the control one based on multiple emitting layers. The efficiency of simplified white device was 40.8cd/A at 1000cd/m2 with Commission Internationale de I’Eclairage coordinates of (0.39, 0.46).

Arylfluorene based universal hosts for solution-processed RGB and white phosphorescent organic light-emitting devices

A series of easily synthesized arylfluorenes (AFs)/carbazole universal host materials have been demonstrated for efficient solution-processable phosphorescent OLEDs.

Recombination zone shift in phosphorescent white organic light-emitting devices with single host structure of multi-emission layers

We have investigated the recombination zone in the phosphorescent white organic light-emitting devices with single host structure of multi-emission layers. Blue, green, and orange-red phosphorescent emitters were doped into the separate layers of single host material for fabricating the white devices with multi-emission layers. The electroluminescence spectrum was substantially modified by the shift of the recombination zone that was dependent upon the thickness of the electron transport layer. We investigated the recombination zone shift in terms of electric field distribution and carrier injection. A maximum external quantum efficiency of 15.9% and a maximum power efficiency of 28.9 lm/W were achieved by optimizing the recombination zone.

Improved colour stability of phosphorescent white organic light-emitting devices by manipulating electron transport

Phosphorescent white organic light-emitting devices (WOLEDs) with a multilayer structure were fabricated, and colour stability was studied by manipulating electron transport property of the hole-blocking layer 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBi) and the electron transporting layer 4,7-diphenyl-1,10-phenanthroline (Bphen). With 20 nm thick TPBi and 20 nm Bphen tailoring electron transport, the WOLED yielded a slight Commission Internationale de l’Eclairage coordinates variation of (−0.003, 0.007) in a wide range of bias voltages. Additionally, a peak current efficiency of 32.4 cd/A and a peak external quantum efficiency of 12.7% were also realised. It was found that the enhancement of colour stability was due to optimisation of the electron transport, by adjusting the thicknesses of TPBi and Bphen to improve the charge carrier balance.

Flexible white phosphorescent organic light emitting diodes based on multilayered graphene/PEDOT:PSS transparent conducting film

A double-layered graphene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conductive film was prepared, in which the PEDOT:PSS layer was obtained by using spray-coating technique. A flexible white phosphorescent organic light-emitting devices based on the graphene/PEDOT:PSS conductive film was fabricated. Phosphorescent material tris(2-phenylpyridine) iridium (Ir(ppy)3) and the fluorescent dye 5,6,11,12-tetraphenylnapthacene (Rubrene) were co-doped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host. N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) and 4,7-diphenyl-1,10-phenanthroline (Bphen) were used as hole-transporting and electron-transporting layer, respectively, and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) was used as blue light-emitting layer. The device presented pure white light emission with a Commission Internationale De I’Eclairage coordinates of (0.31, 0.33) and exhibited an excellent light-emitting stability during the bending cycle test with a radius of curvature of 10mm.

Highly efficient and color-stable white organic light-emitting diode based on a novel blue phosphorescent host

Highly efficient phosphorescent white organic light-emitting devices (WOLEDs) have been fabricated by using two complementary blue and yellow emitting layers, in which the blue guest iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2′] picolinate (Firpic) was doped in a novel blue phosphorescent host. The efficiencies and spectra of WOLEDs can be easily tuned by inserting an ultrathin interlayer between the two emitters and adjusting the thickness of the yellow emitting layer (Y-EML). The device with 1nm thick interlayer and 3nm thick (Y-EML) obtains very high efficiencies of 42.4cd/A and 47.6lm/W and a luminance of 1144cd/m2 was realized at a low voltage of 3.6V. In addition, another device with 2nm thick interlayer and 5nm thick Y-EML exhibited nearly voltage-independent electroluminescent (EL) spectra. Commission International de L’Eclairage (CIE) coordinates of this device only changes from (0.333, 0.436) at a luminance of 100cd/m2 to (0.330, 0.434) at that of 10,000cd/m2, nearly independent of the driving voltage.

Effect of two yellow delta-emitting layers on device performance of phosphorescent white organic light-emitting devices

Phosphorescent white organic light-emitting devices (WOLEDs) with a structure of ITO/TAPC/δ-EML1/mCP:FIrpic/δ-EML2/Bphen/Mg:Ag were fabricated, wherein two ultrathin and host-free emitting layers (EMLs) were formed by using yellow bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium (acetylacetonate) [(tbt)2Ir(acac)] and referred to as delta-EMLs (δ-EML1 and δ-EML2). By adjusting the thicknesses of δ-EMLs, a maximum current efficiency of 27.6cd/A, an external quantum efficiency (EQE) of 10%, together with low efficiency roll-off at high luminance were achieved. The results showed that δ-EML1 played a dominant role on charge carrier trapping, while δ-EML2 had major impact on yellow light emission, which were highly sensitive to the location of δ-EMLs. Furthermore, by introducing 5-nm Au as anode modifying layer, high device efficiency was maintained along with excellent color stability of warm white emission, displaying color coordinates of (0.38, 0.42) and color temperature of 4348K at a luminance of 7000cd/m2. Importantly, explanation and analysis for the influence of both ultrathin δ-EMLs and anode modifying layer on device performance were proposed.