Single-layer Organic Light-emitting Diodes Research Articles

Inverted device architecture for high efficiency single-layer organic light-emitting diodes with imbalanced charge transport

Many wide-gap organic semiconductors exhibit imbalanced electron and hole transport, therefore efficient organic light-emitting diodes require a multilayer architecture of electron- and hole-transport materials to confine charge recombination to the emissive layer. Here, we show that even for emitters with imbalanced charge transport, it is possible to obtain highly efficient single-layer organic light emitting diodes (OLEDs), without the need for additional charge-transport and blocking layers. For hole-dominated emitters, an inverted single-layer device architecture with ohmic bottom-electron and top-hole contacts moves the emission zone away from the metal top electrode, thereby more than doubling the optical outcoupling efficiency. Finally, a blue-emitting inverted single-layer OLED based on thermally activated delayed fluorescence is achieved, exhibiting a high external quantum efficiency of 19% with little roll-off at high brightness, demonstrating that balanced charge transport is not a prerequisite for highly efficient single-layer OLEDs.

Single-Layer Organic Light-Emitting Diode with Trap-Free Host Beats Power Efficiency and Lifetime of Multilayer Devices.

Organic light-emitting diodes (OLEDs) employing a single active layer potentially offer a number of benefits compared to multilayer devices; reduced number of materials and deposition steps, potential for solution processing, and reduced operating voltage due to the absence of heterojunctions. However, for single-layer OLEDs to achieve efficiencies approaching those of multilayer devices, balanced charge transport is a prerequisite. This requirement excludes many efficient emitters based on thermally activated delayed fluorescence (TADF) that exhibit electron trapping, such as the green-emitting bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)methanone (DMAC-BP). By employing a recently developed trap-free large band gap material as a host for DMAC-BP, nearly balanced charge transport is achieved. The single-layer OLED reaches an external quantum efficiency (EQE) of 19.6%, which is comparable to the reported EQEs of 18.9-21% for multilayer devices, but achieves a record power efficiency for DMAC-BP OLEDs of 82lmW-1, clearly surpassing the reported multilayer power efficiencies of 52.9-59lmW-1. In addition, the operational stability is greatly improved compared to multilayer devices and the use of conventional host materials in combination with DMAC-BP as an emitter. Next to the obvious reduction in production costs, single-layer OLEDs therefore also offer the advantage of reduced energy consumption and enhanced stability.

Enhanced Operational Stability by Cavity Control of Single-Layer Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence.

Highly efficient organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have been realized in recent years, but the device lifetime needs further improvement for practical display or lighting applications. In this work, we present a device design principle by tuning the optical cavity of single-layer undoped devices, to realize efficient and long-lived TADF OLEDs. Extending the cavity length to the second-order interference maximum by increasing the emissive layer thickness broadens the recombination zone, while the optical outcoupling efficiency remains close to that of the thinner first-order devices. Such a device design leads to efficient and stable single-layer undoped OLEDs with a maximum EQE of 16%, a LT90 of 452h and LT50 of 3693h at an initial luminance of 1000cd m-2 , which is doubled compared to the first-order counterparts. We further demonstrate that the widely-used empirical relation between OLED lifetime and light-intensity originates from triplet-polaron annihilation, resulting in an extrapolated LT50 at 100cd m-2 of close to 90,000h, approaching the demands for practical backlight applications. This article is protected by copyright. All rights reserved.

All-Solution Processed Single-Layer WOLEDs Using [Pt(salicylidenes)] as Guests in a PFO Matrix.

Herein, we report the synthesis and characterization of two Pt(II) coordination compounds, the new platinum(II)[N,N′-bis(salicylidene)-3,4-diaminobenzophenone)] ([Pt(sal-3,4-ben)]) and the already well-known platinum(II)[N,N′-bis(salicylidene)-o-phenylenediamine] ([Pt(salophen)]), along with their application as guests in a poly [9,9-dioctylfluorenyl-2,7-diyl] (PFO) conjugated polymer in all-solution processed single-layer white organic light-emitting diodes. Completely different performances were achieved: 2.2% and 15.3% of external quantum efficiencies; 2.8 cd A−1 and 12.1 cd A−1 of current efficiencies; and 3103 cd m−2 and 6224 cd m−2 of luminance for the [Pt(salophen)] and [Pt(sal-3,4-ben)] complexes, respectively. The Commission Internationale de l’Eclairage (CIE 1931) chromaticity color coordinates are (0.33, 0.33) for both 0.1% mol/mol Pt(II):PFO composites at between approximately 3.2 and 8 V. The optoelectronic properties of doped and neat PFO films have been investigated, using steady-state and time-resolved photoluminescence. Theoretical calculations at the level of relativistic density functional theory explained these results, based on the presence of the Pt(II) central ion’s phosphorescence emission, considering spin-orbit coupling relationships. The overall results are explained, taking into account the active layer morphological properties, along with the device’s electric balance and the emitter’s efficiencies, according to deep-trap space-charge models. Considering the very simple structure of the device and the ease of synthesis of such compounds, the developed framework can offer a good trade-off for solution-deposited white organic light-emitting diodes (WOLEDs), with further applications in the field of lighting and signage.

A New Benchmark of Charges Storage in Single-Layer Organic Light-Emitting Diodes Based on Electrical and Optical Characteristics.

The storage of charges in organic light-emitting diodes (OLEDs) has drawn much attention for its damage to device performance as well as the loss to carriers. Thus, it is essential to address the issue and do further investigation. The traditional approach to storage analysis is mainly based on transient measurement since it is sensitive to transient instead of steady signal. In this paper, we proposed a new benchmark to investigate the single-layer OLEDs capable of stored charges with poly (methyl methacrylate) (PMMA), which is just based on electrical and optical characteristics. Since the stored charges contribute both to luminance and current of the devices with PMMA, the area between them can be taken as a benchmark and evaluated the storage of charges. In our experiment, the areas of 4 nm, 6 nm, 8 nm, and 10 nm PMMA devices are 0.348, 0.554, 0.808, and 0.894, respectively, indicating a higher capability of storage in thicker PMMA. It is exactly in line with the results taken from transient electroluminescence (EL) measurement. Thus, this new benchmark is practical and provides a more accessible approach to investigate the storage of charges in OLEDs.

Quinolinophenothiazine as an electron rich fragment for high efficiency RGB single-layer phosphorescent organic light-emitting diodes

We report a high efficiency host material constructed on a quinolinophenothiazine core for RGB single-layer phosphorescent organic light emitting diodes.

Highly Efficient Single-Layer Phosphorescent Organic Light-Emitting Diodes Based on Co-Host Structure.

High-efficiency single-layer organic light-emitting diodes (OLEDs) based on a simple structure doped with iridium(III) bis(4-phenylthieno[3,2-c]pyridinato-N,C2′) acetylacetonate (PO-01) as emission dyes are realized, achieving maximum current efficiency (CE) and power efficiency (PE) of 37.1 cd A−1 and 33.3 lm W−1 as well as low turn-on voltage of 3.31 V. Such superior performance is mainly attributed to the employment of a uniform co-host structure and assisted charge transport property of phosphors dyes, which were in favor of the balance of charge carrier injection and transport in the single emitting layer (EML). Moreover, systematic researches on the position of exciton recombination region and the dopant effect on charge carriers were subsequently performed to better understand the operational mechanism. It could be experimentally found that the orange emitting dopants promoted the acceleration of the charge carriers transport and raised the exciton recombination efficiency, eventually leading to an excellent performance of single-layer OLEDs.

Universal host materials for red, green and blue high-efficiency single-layer phosphorescent organic light-emitting diodes

As simplifying the device structure is a key step for the future of organic electronic, we report herein high efficiency universal host materials for red, green and blue Single-Layer Phosphorescent OLEDs.

Efficient and stable single-layer organic light-emitting diodes based on thermally activated delayed fluorescence

From a design, optimization and fabrication perspective, an organic light-emitting diode consisting of only one single layer of a neat semiconductor would be highly attractive. Here, we demonstrate an efficient and stable organic light-emitting diode based on a single layer of a neat thermally activated delayed fluorescence emitter. By employing ohmic electron and hole contacts, charge injection is efficient and the absence of heterojunctions results in an exceptionally low operating voltage of 2.9 V at a luminance of 10,000 cd m−2. Balanced electron and hole transport results in a maximum external quantum efficiency of 19% at 500 cd m−2 and a broadened emission zone, which greatly improves the operational stability, allowing a lifetime to 50% of the initial luminance of 1,880 h for an initial luminance of 1,000 cd m−2. As a result, this single-layer concept combines high power efficiency with long lifetime in a simplified architecture, rivalling and even exceeding the performance of complex multilayer devices. Long-lived, efficient organic light-emitting diodes based on a simple design of a single layer of an active light-emitting medium sandwiched between two contacts and no additional charge injection and transport layers are reported.

High efficient Light-Emitting Electrochemical Cells based on ionic liquids 1,2,3-triazolium

Abstract The Light-Emitting Electrochemical Cell (LEEC) is one of the simplest kind of electroluminescent solid-state devices. Generally, they are fabricated with only one emitting material layer (EML), so-called active emission layer, that consist of an emitter material mixed with an ionic electrolyte. Apart from the electrolyte, their structure is similar to that of a single layer organic light-emitting diode (OLED). Not only LEECs assemble most of OLEDs applications and their technological advantages, but they also bring additional ones such as (i) low dependence of the electrodes work function, (ii) low dependence to the EML thickness, (iii) low operation voltage and high brightness and finally (iv) LEECs can be printed or sprayed in large areas. To investigate the performance of ionic liquid (IL) based devices we synthesized four organic salts that were compared with devices using four inorganic salts commonly used as electrolyte for LEECs. All devices were fabricated with a commercial poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as emitter polymer. As a result, we showed that the best device performance was achieved with the Ionic Liquid 1-dodecyl-4-(hydroxymethyl)-3-methyl-1H-1,2,3-triazol-3-ium iodide (dohmtI) presenting a current efficiency of 3.6 cd/A, an external quantum efficiency (EQE) of 1.4% as well as a luminance of 3.2 × 104 cd/m2. Despite the use of MEH-PPV, a polymer not so efficient when compared to the state-of-art of the novel polymers employed nowadays, our realization showed these new ILs have been able to provide the specific requirements of the LEEC devices.

Highly efficient phosphorescent organic light-emitting diodes based on single-layer structure.

We demonstrated highly efficient single-layer organic light-emitting diodes (OLEDs) based on graded-composition architecture. The single-layer OLEDs comprise graded-composition from a mostly hole-transport material at anode to a mostly electron-transport material at cathode. The resulting device with low turn-on voltage of 2.85V shows maximum power efficiency and current efficiency of 44.8lm/W and 42.7cd/A, respectively. It is found that the emission dopant plays an important role to obtain more balanced charge carriers, causing the recombination zone slightly shift to the center of the device. The results of our work suggest that the graded-composition structure represents a path to achieving highly efficient single-layer OLEDs.

Performance enhancement of single layer organic light-emitting diodes using chlorinated indium tin oxide as the anode.

Effective green single-layer organic light-emitting diodes (OLEDs) are reported with fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as a dopant and chlorinated indium tin oxide (Cl–ITO) as a transparent anode. The work function of the chlorinated ITO is manipulated to be ∼5.3 eV from ∼4.7 eV for bared ITO. The improvement in anode workfunction allows the direct hole injection into the HOMO of the phosphorescent dopant. As a result, the green phosphorescent OLEDs with simple single layers can deliver a current efficiency (CE) and external quantum efficiency (EQE) as high as 33.48 cd A−1 and 10.1%, respectively.

Study of flexible organic electroluminescent devices with PEDOT:PSS anodes by impedance measurements

Purpose – Purpose This study aims to apply the impedance spectroscopy (IS) for analyzing the electrical behavior and extracting the equivalent circuit of single-layer flexible organic light-emitting diodes (OLEDs) with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) anode. Design/methodology/approach – The preliminary ultraviolet (UV) treatment of the flexible substrate of polyethylene terephthalate (PET) influenced the conductivity of PEDOT:PSS anodes. Findings – The IS showed that the OLED with UV-treated PET/PEDOT:PSS anodes had lower values of the contact resistance and higher value of the interface capacitance. Originality/value – The obtained data were used for modeling of flexible OLEDs with polymeric anodes and calculation of important display parameters such as pixel refresh ratio, signal delays and energy losses due to contact resistances. These parameters were compared for PEDOT:PSS anodes deposited on PET treated and non-treated by UV.

MoO3 as p-type dopant for Alq3-based p–i–n homojunction organic light-emitting diodes

Using high-work-function material MoO3 as a p-type dopant, efficient single-layer hybrid organic light-emitting diodes (OLEDs) with the p–i–n homojunction structure are investigated. When MoO3 and Cs2CO3 are doped into the conventional emitting/electron-transport material tris-(8-hydroxyquinoline) aluminum (Alq3), respectively, a significant increase in p- and n-type conductivities is observed compared to that of intrinsic Alq3 thin films. With optimal doping, the hole and electron mobilities in Alq3:MoO3 and Alq3:Cs2CO3 films was estimated to be 9.76×10−6 and 1.26×10−4cm2/Vs, respectively, which is about one order of magnitude higher than that of the undoped device. The p–i–n OLEDs outperform undoped (i–i–i) and single-dopant (p–i–i and i–i–n) OLEDs; they have the lowest turn-on voltage (4.3V at 1cd/m2), highest maximum luminance (5860cd/m2 at 11.4V), and highest luminous efficiency (2.53cd/A at 100mA/cm2). These values are better than those for bilayer heterojunction OLEDs using the same emitting layer. The increase in conductivity can be attributed to the charge transfer process between the Alq3 host and the dopant. Due to the change of carrier concentration in the Alq3 films, the Fermi level of Alq3 is close to the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels upon p- and n-type doping, respectively, and the carrier injection efficiency can thus be enhanced because of the lower carrier injection barrier. The carriers move closer to the center energy levels of the HOMO or LUMO distributions, which increases the hopping rate for charge transport and results in an increase of charge carrier mobility. The electrons are the majority charge carriers, and both the holes and electrons can be dramatically injected in high numbers and then efficiently recombined in the p–i–n OLEDs. As a result, the improved conductivity characteristics as well as the appropriate energy levels of the doped layers result in improved electroluminescent performance of the p–i–n homojunction OLEDs.

Highly efficient non-doped single-layer blue organic light-emitting diodes based on light-emitting conjugated polymers containing trifluoren-2-ylamine and dibenzothiophene-S,S-dioxide

A series of blue-light-emitting conjugated polymers based on electron-donating trifluoren-2-ylamine and electron-deficient dibenzothiophene-S,S-dioxide were synthesized via typical Suzuki coupling reactions. Their thermal, photophysical and electrochemical properties were fully studied. Owing to the excellent thermal stability, film-forming ability as well as the favorable energy levels of the polymers, highly efficient non-doped single-layer blue devices by using these polymers as an emitting layer (EML) were achieved. Among them, the device based on PSFOFN1 showed the highest maximum current efficiency (CEmax) of 1.99cd/A. Moreover, the device presented small efficiency roll-off with current efficiency (CE) of 1.97cd/A at high brightness up to 100cd/m2. In addition, the Commission Internationale de L’Eclairage (CIE) coordinates of all the devices are very close to the deep blue region. Our findings suggest the potential applications of the polymers in full-color flat-panel displays and organic lighting.

温度对单层有机电致发光器件电流平衡因子的影响

温度对单层有机电致发光器件电流平衡因子的影响

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.

High morphology stability and ambipolar transporting host for use in blue phosphorescent single-layer organic light-emitting diodes

The authors report a small molecule host of 2,7-bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino)phenyl]-9-phenylfluorene (POAPF) doped with 8wt% iridium(III)-bis[(4,6-difluorophenyl)pyridinato-N,C2′]picolinate (FIrpic) for use in efficient and single-layer blue phosphorescent organic light-emitting diodes (PHOLEDs) exhibiting a maximum external quantum efficiency of ∼20.3% at brightness of 100cd/m2. The high performance of such single layer PHOLEDs is attributed to the POAPF host’s high morphological stability, suitable triplet energy level, and equal charge carrier mobilities of hole and electron to form the broad carrier recombination zone in the emitting layer, thus reducing the triplet-triplet annihilation and resulting in a slight efficiency roll off of 0.5% from the brightness of 1 and 1000cd/m2. This work also systematically investigated the arrangement of the POAPF:FIrpic recombination zone for optimizing the performance of the single layer PHOLED.

Solution-processible brilliantly luminescent Eu(III) complexes with host-featured phosphine oxide ligands for monochromic red-light-emitting diodes.

A series of solution-processible electroluminescent (EL) Eu(3+) complexes were constructed with a self-host strategy, in which neutral ligands were employed as functionalized bidentate phosphine oxide (PO) ligands named DPEPOArn (DPEPO = bis(2-(diphenylphosphino)phenyl) ether oxide). The solubility of these complexes was dramatically improved owing to the increased ratios of organic components. This further enhanced the antenna effect of these ligands in both singlet and triplet energy-transfer processes to support high photoluminescent quantum yields (PLQYs) up to 86 % for their Eu(3+) complexes, which is outstanding among conjugated Eu(3+) complexes. Density function theory (DFT) simulations and electrochemical analysis further verified the contributions of DPEPOArn to the carrier injecting/transporting ability of the complexes. In this sense, these functionalized PO ligands served as hosts in optoelectronic processes, which rendered the self-host feature of their Eu(3+) complexes. With the enhanced electrical properties, the spin-coated single-layer organic light-emitting diodes (OLEDs) of these complexes achieved improved low driving voltages, such as onset voltages about 6 V, compared to their Eu(3+)-contained red-emitting polymeric analogues. [Eu(DBM)3DPEPODPNA2] (DBM = 1,3-diphenylpropane-1,3-dione, DPNA = diphenylnaphthylamine) with the most enhanced electrical properties and suitable frontier molecular orbital (FMO) and triplet state locations endowed its devices with the biggest maximum luminance of >90 cd m(-2) and the highest EL efficiencies. This work verified the potential of small molecular EL Eu(3+) complexes for solution-processed OLEDs through rational function integrations.

Numerical study of the influence of applied voltage on the current balance factor of single layer organic light-emitting diodes

Current balance factor (CBF) value, the ratio of the recombination current density and the total current density of a device, has an important function in fluorescence-based organic light-emitting diodes (OLEDs), as well as in the performance of the organic electrophosphorescent devices. This paper investigates the influence of the applied voltage of a device on the CBF value of single layer OLED based on the numerical model of a bipolar single layer OLED with organic layer trap free and without doping. Results show that the largest CBF value can be achieved when the electron injection barrier (ϕn) is equal to the hole injection barrier (ϕp) in the lower voltage region at any instance. The largest CBF in the higher voltage region can be achieved in the case of ϕn > ϕp under the condition of electron mobility (μ0n) > hole mobility (μ0p), whereas the result for the case of μ0n < μ0p, is opposite. The largest CBF when μ0n = μ0p can be achieved in the case of ϕn = ϕp in the entire region of the applied voltage. In addition, the CBF value of the device increases with increasing applied voltage. The results obtained in this paper can present an in-depth understanding of the OLED working mechanism and help in the future fabrication of high efficiency OLEDs.