High-performance red organic light-emitting devices based on an exciplex system with thermally activated delayed fluorescence characteristic

To realize high-performance red organic light-emitting diodes (OLEDs), a novel exciplex system Tris-PCz:B4PyPPM with thermally activated delayed fluorescence (TADF) characteristic was developed as the host of both red fluorescent and phosphorescent emitters. Due to the blue-green PL spectrum and suitable triplet energy level of 2.53 eV, the exciplex system matches the requirements of both red fluorescent and phosphorescent dopants. With an optimized mixing molar ratio of 5:5, Tris-PCz:B4PyPPM exciplex system shows outstanding host performance in both the red fluorescent and phosphorescent devices. The red fluorescent device based on DCJTB exhibits a low turn-on voltage of 2.3 V and an extremely high maximum external quantum efficiency (EQE) of 9.3%. And the red phosphorescent device with the Ir(MDQ)₂acac as the emitter realizes a maximum EQE of 20.3%. These remarkable results prove the feasibility of TADF exciplex systems simultaneously as the hosts of both high-performance fluorescent and phosphorescent red devices.     

A host material with a small singlet–triplet exchange energy for phosphorescent organic light-emitting diodes: Guest,host, and exciplex emission

A host material containing a triazine core and three phenylcarbazole arms, called 2,4,6-tris(3-(carbazol-9-yl)phenyl)-triazine (TCPZ), was developed for phosphorescent organic light-emitting diodes (OLEDs). Ultra-low driving voltages were achieved by utilizing TCPZ as the host due to its decreased singlet–triplet exchange energy (ΔE_ST) and low-lying lowest unoccupied molecular orbital (LUMO) energy level. Interaction between the RGB triplet emitters and TCPZ were studied in both photoluminescent and electroluminescent processes. Transient photoluminescence (PL) measurement of the co-deposited film of fac-tris(2-phenylpyridine) iridium (Ir(PPy)₃):TCPZ exhibits a shoulder at 565 nm whose lifetime is about two times longer than that of the Ir(PPy)₃ triplet excitons and can be attributed to the triplet exciplex formed between Ir(PPy)₃ and TCPZ. Such exciplex was also found for the green phosphorescent OLED, giving the most efficient phosphorescent OLED with triplet exciplex emission hitherto. Different from the PL process, a broad featureless band with a maximum at 535 nm was found for the OLED based on an EML of iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C²′)picolinate (FIrpic):TCPZ, which can be attributed to the emission from the singlet excited state of TCPZ formed by direct hole-electron recombination. A multi-emitting-layer white OLED was also fabricated by utilizing FIrpic and tris(1-phenylisoquinolinolato-C²,N)iridium(III) (Ir(piq)₃) as the complementary triplet emitters and TCPZ as the host. Different from most of ever reported white OLEDs fabricated with blue/red complementary triplet emitters that exhibit color rendering index (CRI) lower than 70, a high CRI of 82 is achieved due to the combination of blue and red phosphorescence emissions from FIrpic and Ir(piq)₃, and the emerging green fluorescence emission from TCPZ.     

Highly efficient single-layer organic light-emitting devices using cationic iridium complex as host

Highly efficient single-layer organic light-emitting devices (OLEDs) based on blended cationic Ir complexes as emitting layer have been demonstrated using narrow band gap cationic Ir complex [Ir(Meppy)₂(pybm)](PF₆) (C1) as guest and wide band gap cationic Ir complex [Ir(dfppy)₂(tzpy-cn)](PF₆) (C2) as host. As compared with single cationic Ir complex emitting layer, these host–guest systems exhibit highly enhanced efficiencies, with maximum luminous efficiency of 25.7 cd/A, external quantum efficiency of 8.6%, which are nearly 3-folds of those of pure C1-based device. Compared with a multilayer host-free device containing C1 as emitting layer and TPBI as electron-transporting and hole-blocking layer, the above single-layer devices also show 2-folds enhancement efficiencies. The high efficiencies achieved in these host–guest systems are among the highest values reported for ionic Ir complexes-based solid-state light-emitting devices. In addition, a white-similar emission with CIE of (0.36, 0.47) has also been achieved with luminous efficiency of 4.2 cd/A as the C1 concentration is 0.1 wt.%. The results demonstrate that the ionic Ir complexes-based host–guest system provides a new approach to achieve highly efficient OLEDs upon single-layer device structure and solution-processing technique.     

Organic Light‐Emitting Diodes with 30% External Quantum Efficiency Based on a Horizontally Oriented Emitter

High‐efficiency phosphorescent organic light‐emitting diodes (OLEDs) doped with Ir(ppy)₂(acac) [bis(2‐phenylpyridine)iridium(III)‐acetylacetonate] in an exciplex forming co‐host have been optically analyzed. This emitter has a preferred orientation with the horizontal to vertical dipole ratio of 0.77:0.23 as compared to 0.67:0.33 in the isotropic case. Theoretical analysis based on the orientation factor (Θ, the ratio of the horizontal dipoles to total dipoles) and the photoluminescence quantum yield (q_PL) of the emitter predicts that the maximum external quantum efficiency (EQE) of the OLEDs with this emitter is about 30%, which matches very well with the experimental data, indicating that the electrical loss of the OLEDs is negligible and the device structure can be utilized as a platform to demonstrate the validity of optical modeling. Based on the results, the maximum EQE achievable for a certain emitting dye in a host can be predicted by just measuring q_PL and Θ in a neat film on glass without the need to fabricate devices, which offers a universal plot of the maximum EQE as a function of q_PL and Θ.     

Highly Efficient Red Phosphorescent OLEDs based on Non‐Conjugated Silicon‐Cored Spirobifluorene Derivative Doped with Ir‐Complexes

A novel host material containing silicon‐cored spirobifluorene derivative (SBP‐TS‐PSB), is designed, synthesized, and characterized for red phosphorescent organic light‐emitting diodes (OLEDs). The SBP‐TS‐PSB has excellent thermal and morphological stabilities and exhibits high electroluminescence (EL) efficiency as a host for the red phosphorescent OLEDs. The electrophosphorescence properties of the devices using SBP‐TS‐PSB as the host and red phosphorescent iridium (III) complexes as the emitter are investigated and these devices exhibit higher EL performances compared with the reference devices with 4,4′‐N,N′‐dicarbazole‐biphenyl (CBP) as a host material; for example, a (piq)₂Ir(acac)‐doped SBP‐TS‐PSB device shows maximum external quantum efficiency of η_ext = 14.6%, power efficiency of 10.3 lm W^?1 and Commission International de L'Eclairage color coordinates (0.68, 0.32) at J = 1.5 mA cm^?2, while the device with the CBP host shows maximum η_ext = 12.1%. These high performances can be mainly explained by efficient triplet energy transfer from the host to the guests and improved charge balance attributable to the bipolar characteristics of the spirobifluorene group.     

Hydrogen-Bond-Assisted Exciplex Emitters Realizing Improved Efficiencies and Stabilities in Organic Light Emitting Diodes

Exciplex emitters have been extensively studied owing to their natural thermally activated delayed fluorescence characteristic, and many efforts have been made to improve their performance in organic light emitting diodes (OLEDs). In this work, the authors propose a novel strategy by introducing intermolecular hydrogen bond (HB) between electron-donating and electron-accepting constituting molecules (D and A) to suppress non-radiative transition of exciplex emitters and thus improve their efficiencies and stabilities in the OLEDs. Accordingly, three exciplex emitters are constructed by using 1,3-di(10H-phenoxazin-10-yl)benzene (13PXZB) as donor and 4,6-bis(3,5-di(pyridin-4-yl)phenyl)-2-methylpyrimidine (B4PyMPM), 4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PyMPM), and 4,6-bis(3,5-di(pyridin-2-yl)phenyl)-2-methylpyrimidine (B2PyMPM) as acceptors. With the most intermolecular HBs, 13PXZB:B4PyMPM shows the highest photoluminescence quantum yield (69.6%) and the lowest rate constant of non-radiative process of triplet excitons (3.4 × 10⁵ S⁻¹). And the OLED using 13PXZB:B4PyMPM as the emitter successfully exhibits a much higher external quantum efficiency of 14.6% than other contrastive devices. Moreover, the half lifetime of 13PXZB:B4PyMPM is 4.8 and 8.2 times higher than those of 13PXZB:B3PyMPM and 13PXZB:B2PyMPM in the devices. This work not only demonstrates that intermolecular HBs between D and A molecules can improve the performance of exciplex emitters, but also paves a new route to develop efficient and stable exciplex emitters.     

Color Tuning of Avobenzone Boron Difluoride as an Emitter to Achieve Full‐Color Emission

Organic light‐emitting diodes (OLEDs) displaying a wide range of emission colors with emission peaks from 450 to 665 nm using a single emitting material, avobenzone boron difluoride (AVB‐BF₂), are reported. Color tuning is achieved by controlling the aggregation of AVB‐BF₂ and the formation of a “triadic” exciplex of an AVB‐BF₂ dimer and a host molecule. Various electroluminescent devices containing AVB‐BF₂ cover the whole visible light spectrum and a white‐emitting device with CIE coordinates of (0.35, 0.37) is obtained with a single emitting material in a single emissive layer. Furthermore, an exceptionally high external quantum efficiency of nearly 13% is achieved for a green‐emitting OLED because AVB‐BF₂ exhibits thermally activated delayed fluorescence by forming the exciplex.     

Highly Efficient TADF OLEDs: How the Emitter–Host Interaction Controls Both the Excited State Species and Electrical Properties of the Devices to Achieve Near 100% Triplet Harvesting and High Efficiency

New emitters that can harvest both singlet and triplet excited states to give 100% internal conversion of charge into light, are required to replace Ir based phosphors in organic light emitting diodes (OLEDs). Molecules that have a charge transfer (CT) excited state can potentially achieve this through the mechanism of thermally activated delayed fluorescence (TADF). Here, it is shown that a D–A charge transfer molecule in the solid state, can emit not only via an intramolecular charge transfer (ICT) excited state, but also from exciplex states, formed between the molecule and the host material. OLEDs based on a previously studied D–A–D molecule in a host TAPC achieves >14% external electroluminescence yield and shows nearly 100% efficient triplet harvesting. In these devices, it is unambiguously established that the triplet states are harvested via TADF, but more interestingly, these results are found to be independent of whether the emitter is the ICT state or the D–A–D/host exciplex.     

Efficient iridium(III) based, true-blue emitting phosphorescent OLEDS employing both double emission and double buffer layers

New device architectures and efficient iridium based phosphors were simultaneously developed for fabrication of true-blue phosphorescent organic light-emitting devices (OLEDs). To fully explore the potential of these true-blue-emitting phosphors, we employed a device architecture that incorporates both double-emitting layers (one with hole-transport and the second with electron-transport materials) and double buffer layers for efficient exciton confinement. In addition, the parent, true-blue emitting heteroleptic Ir^III complex Ir1 was synthesized by incorporating one 4,6-difluorophenyl-2-pyridyl cyclometalate (dfppy) together with two 3-(trifluoromethyl)-5-pyridyl pyrazolates (fppz), while others derivatives Ir2–Ir4 were prepared by addition of alkyl substituent at the pyridyl sites. Electrophosphorescence with efficiencies up to 13.7% photon/electron and 20.4 cd/A, and with adequate CIE_x,y color coordinates of (0.157, 0.189) were successfully achieved.     

Solution-processed organic light-emitting diodes with emission from a doublet exciton; using (2,4,6-trichlorophenyl)methyl as emitter

Neutral-π radical based open shell molecules foster new potential in light emitting diodes because of their theoretical near-equity quantum efficiencies. In this study, we report organic light emitting diodes (OLEDs) based on a novel open shell molecule (2,4,6-trichlorophenyl)methyl (TTM) as the electroluminescent layer. The singly occupied molecular orbital (SOMO) level and optical bandgap of TTM was calculated using cyclic voltammetry and UV–visible absorption respectively. Thermogravimetric analysis showed a stable molecule capable of sublimation. Two decidedly different approaches, thermal evaporation and solution processing, were employed to deposit TTM:host blend thin films for OLED device fabrication. OLED devices fabricated via thermal evaporation and solution processing yielded external quantum efficiencies of 2.78% and 0.28% and luminances of 330 cdm^-2 and 78 cdm^-2 respectively. Further, the effect of doping ratios of the host materials on OLED device performance were investigated and optimal ratios were established. We report for the first time solution processed open shell organic molecules for light emitting diode applications. Our results elucidate the potential for low-cost and high efficiency optoelectronic devices.     

Low Roll‐Off and High Efficiency Orange Organic Light Emitting Diodes with Controlled Co‐Doping of Green and Red Phosphorescent Dopants in an Exciplex Forming Co‐Host

An exciplex forming co‐host is introduced in order to fabricate orange organic light‐emitting diodes (OLEDs) with high efficiency, low driving voltage and an extremely low efficiency roll‐off, by the co‐doping of green and red emitting phosphorescence dyes in the host. The orange OLEDs achieves a low turn‐on voltage of 2.4 V, which is equivalent to the triplet energy gap of the phosphorescent‐green emitting dopant, and a very high external quantum efficiency (EQE) of 25.0%. Moreover, the OLEDs show low efficiency roll‐off with an EQE of over 21% at 10 000 cdm^?2. The device displays a very good orange color (CIE of (0.501, 0.478) at 1000 cdm^?2) with very little color shift with increasing luminance. The transient electroluminescence of the OLEDs indicate that both energy transfer and direct charge trapping takes place in the devices.     

High efficiency warm white phosphorescent organic light emitting devices based on blue light emission from a bipolar mixed-host

In this study, we demonstrate a high-efficiency and low turn-on voltage warm white phosphorescent organic light emitting devices (PH-WOLEDs) based on a blue mixed-host emission layer (EML) and an orange ultrathin layer. The device has a simple structure and would simplify the fabrication process and reduce fabrication costs. The concept is based on the design a high-efficiency blue mixed-host EML, using an electron-transport material, 4,6-Bis(3,5-di(pyridin-4-yl) phenyl)-2-(3-(pyridin-3-yl) phenyl) pyrimidine (B4PYMPM) to enhance the carrier balance ability of the hole-transport material 1,3-Bis(carbazol-9-yl) benzene (MCP) which operates as the mixed-host and when the MCP: B4PYMPM ratio in the mixed-film was 4:1 got better effects. Based on the blue EML, we realized WOLEDs, characterized by a peak power efficiency of 71.3 lm/W at 3.1 V and a low turn-on voltage of 2.65 V. The mixed-host blue EML exhibited a much higher performance compared to the MCP host. Stable warm white light emission with Commission International de L'Eclairage (CIE) coordinates from (0.37, 0.45) to (0.38, 0.47) for a luminance value ranging from 1000 to 10,000 cd/m² was obtained.     

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/m² 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.     

发光性能稳定的磷光与荧光复合型白光OLED

使用荧光染料TBPe和Ir(ppy)_2acac 、R-4B两种光染料,采用蓝/红绿双发光层的结构,并结合TPBi对空穴的有效限制作用 ,制备了结构为ITO/MoO₃(X nm)/ADN:(2%)TBPe(30 nm)/CBP:Ir(ppy)_2acac(14%):R-4B(2%)(5nm)/TPBi(10 nm)/Alq₃(30nm)/LiF(1nm )/Al(100nm)的磷光与荧光复合的白光OLED,其中,MoO₃的厚 分别为0、15、20、30和40nm,通过改变MoO₃的厚度调控载流子的注入能力,使用空穴阻挡层提高光效; 通过测量其电压、电流、亮度、色坐标和电致发光(EL)光谱等参数,研究不同厚度的MoO ₃对器件发光性能的影响。结果表明,在MoO₃厚为20nm的情况下,器件的效率滚降 最为平缓。在电压分别 为8、9、10、11、12和13V时,器件的色坐标分别为 (0.31,0.33)、(0.30,0.33)、(0.29,0.33)、(0.29,0.33)、(0.29,0.33)和(0.29, 0.33),具有较高的稳定性,原因为采用 蓝/红绿双发光层结构更有利于蓝光的 出射,且使用ADN主体材料掺杂蓝色荧光染料TBPe作为蓝光发光层降低三重态-三重态 湮灭几率。 研究还发现,在电压为11V、器件的亮度为9744cd/m²和电流密度为11.50mA/cm²时,最大器件的电流效率为 7.0cd/A。     

A host material containing tetraphenylsilane for phosphorescent OLEDs with high efficiency and operational stability

A host material containing tetraphenylsilane moiety, 9-(4-triphenylsilanyl-(1,1′,4,1′′)-terphenyl-4′′-yl)-9H-carbazole (TSTC), was synthesized for green phosphorescent organic light emitting diodes. The tetraphenylsilane moiety was introduced to provide high triplet energy level, thermal and chemical stability, and glassy properties leading to high efficiency and operational stability of the devices. Ir(ppy)₃ based OLEDs using the TSTC host and DTBT (2,4-diphenyl-6-(4′-triphenylsilanyl-biphenyl-4-yl)-1,3,5-triazine) hole blocking layer (HBL) resulted in the maximum external quantum efficiency of 19.8% and the power efficiency of 59.4 lm/W. High operational stability with a half lifetime of 160,000 h at an initial luminance of 100 cd/m² was achieved from an electrophosphorescent device using TSTC host and BAlq HBL.     

Carbazole-bridged triphenylamine-bipyridine bipolar hosts for high-efficiency low roll-off multi-color PhOLEDs

Three new bipolar molecules composed of carbazole, triarylamine, and bipyridine were synthesized and utilized as host materials in multi-color phosphorescent OLEDs (PhOLEDs). These carbazole-based materials comprise a hole-transport triarylamine at C3 and an electron-transport 2,4′- or 4,4′-bipyridine at N9. The different bipyridine isomers and linking topology of the bipyridine with respect to carbazole N9 not only allows fine-tuning of physical properties but also imparts conformational change which subsequently affects molecular packing and carrier transport properties in the solid state. PhOLEDs were fabricated using green [(ppy)₂Ir(acac)], yellow [(bt)₂Ir(acac)], and red [(mpq)₂Ir(acac)] as doped emitters, which showed low driving voltage, high external quantum efficiency (EQE), and extremely low efficiency roll-off. Among these new bipolar materials, the 2Cz-44Bpy-hosted device doping with 10% (ppy)₂Ir(acac) as green emitting layer showed a high EQE of 22% (79.8 cd A⁻¹) and power efficiency (PE) of 102.5 lm W⁻¹ at a practical brightness of 100 cd m⁻². In addition, the device showed limited efficiency roll-off (21.6% EQE) and low driving voltage (2.8 V) at a practical brightness of 1000 cd m⁻².     

Aggregation induced intermolecular charge transfer in simple nonconjugated donor–acceptor system

The exploration of exciplex for organic light-emitting diodes (OLEDs) has been fleetly developed. However, many of them confront with the problems like phase separation and poor solubility, hampering their utilization in solution process. Hence, a series of soluble exciplex luminophores with the simple architecture of D-spacer-A (mCP-6C-TRZ, phCz-6C-TRZ and 2phCz-6C-TRZ) are synthesized and characterized, in which, the alkyl chain as ample spacer breaks the molecular backbone conjugation, induces intermolecular charge transfer process instead of intramolecular charge transfer in solid state. These materials are endowed with narrowed singlet−triplet splitting energy (ΔE_ST), efficient reverse intersystem crossing (RISC) process, and distinct thermally activated delayed fluorescence (TADF) characteristics. In view of their high triplet energy level (E_T) and bipolar carrier transport ability, where efficient exciplexes are applied as the host, the solution-processed phosphorescence devices realize a low efficiency roll-off of 7.0% at 1000 cd m⁻², high luminance, current efficiency (CE) and external quantum efficiency (EQE) of 25,990 cd m⁻², 20.0 cd A⁻¹ and 6.7%, respectively. These results offer a promising tactic to the establishment of exciplex with TADF feature as host for fabricating efficient solution processed OLEDs.     

Enhanced efficiency and brightness in organic light- emitting devices with MoO3 as hole-injection layer

The organic light-emitting devices (OLEDs) using 4,4’,4’’-tris{N-(3-methylphenyl)-N-phenylamin}triphenylamine (m-MTDATA) and MoO3 or 1,3,5-triazo-2,4,6-triphosphorine-2,2,4,4,6,6-tetrachloride (TAPC) and MoO3 as the hole-injection layer (HIL) were fabricated. MoO3 can be expected to be a good injection layer material and thus enhance the emission performance of OLED. The highest occupied molecular (HOMO) of MoO3 is between those of m-MTDATA or TAPC and N,N’-bis-(1-naphthyl)-N,N’-diphenyl-1,1’-biphenyl-4,4’-diamine (NPB), which reduces the hole-injection barrier and improves the luminance of the OLEDs. The current efficiency is improved compared with that of the device without the MoO3 layer. The highest luminous efficiency of the device with 2-nm-thick MoO3 as HIL is achieved as 5.27 cd/A at 10 V, which is nearly 1.2 times larger than that of the device without it. Moreover, the highest current efficiency and power efficiency of the device with the structure indium-tin oxide (ITO)/TAPC (40 nm)/MoO3 (2 nm)/TcTa:Ir(ppy)3 (10%, 10 nm)/ tris-(8-hydroxyquinoline) aluminium (Alq) (60 nm)/LiF (1 nm)/Al are achieved as 37.15 cd/A and 41.23 lm/W at 3.2 V and 2.8 V, respectively.     

Indenocarbazole based bipolar host materials for highly efficient yellow phosphorescent organic light emitting diodes

We report bipolar host materials with robust indenocarbazole and biphenyl moiety as hole-electron-transporting unit for phosphorescent yellow organic light-emitting diodes (OLEDs). New host materials demonstrated an excellent morphological stability with high glass transition temperature of 207 °C. Simultaneously, it also revealed appropriate triplet energy of about 2.6 eV for ideal triplet energy transfer to yellow phosphorescent dopant. A phosphorescent yellow OLED with new host ICBP1 (and ICBP2) and conventional yellow dopant iridium(III)bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C2′)acetylacetonate (Ir(tptpy)₂acac) shows a low driving voltage of 3.4 (and 3.6 V) at 1000 cd/m², and maximum external quantum efficiency as high as 26.4%. Such efficient performance of phosphorescent yellow OLEDs is attributed to a good charge balance and high electron transport properties of host materials.