Co-host System Research Articles

9-(9-Alkylcarbazol-3-yl)-3-(methoxypyridin-3-yl)carbazoles as host materials for very efficient OLEDs

Four derivatives of 9-(9-alkylcarbazol-3-yl)-3-(methoxypyridin-3-yl)carbazoles (HM1-HM4) have been synthesized from key starting compounds: 9-alkyl-3-iodocarbazoles and corresponding 3-(methoxypyridin-3-yl)-9H-carbazoles by using Ullmann coupling reactions. The objective materials have very high thermal stabilities (temperatures of 5 % weight loss 371–387 °C) and can form amorphous layers, also having rather high glass transition temperatures in the region of 89–97 °C. Triplet energy gaps of the four compounds were about 2.7–2.8 eV, making them appropriate for use as host materials in green phosphorescent OLEDs with Ir(ppy)3 guest. Additionally, a composite host system incorporating a synthesized compound of HM series and bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine was developed. The devices with HM2 or HM4 demonstrated the best characteristics whether the emitting layer was a single host or a co-host system, indicating that both compounds could facilitate ideal energy transfer and achieve carrier balance in the device architectures. Notably, the device using 9-(9-butylcarbazol-3-yl)-3-(2-methoxypyridin-3-yl)carbazole (HM2) host outperformed the other devices, achieving peak efficiencies of 16.9 % (58.3 cd/A and 65.0 lm/W) with maximum luminance exceeding 241100 cd/m2.

Exciplex-Forming Cohost Systems with 2,3-Dicyanopyrazinophenanthrene-based Acceptors to Achieve Efficient Near Infrared OLEDs.

Two new 2,3-dicyanopyrazinophenanthrene-based acceptors (A) p-QCN and m-QCN were synthesized to blend with a donor (D) CPTBF for the exciplex formation. The energy levels of p-QCN and m-QCN are modulated by the peripheral substituents 4- and 3-benzonitrile, respectively. Exciplex-forming blends were identified by the observation of the red-shifted emissions from various D : A blends with higher ratios of donor for suppressing the aggregation of acceptor. The two-component relaxation processes observed by time-resolved photoluminescence support the thermally activated delayed fluorescence (TADF) character of the exciplex-forming blends. The device employing CPTBF : p-QCN and (2 : 1) and CPTBF : m-QCN (2 : 1) blend as the emitting layer (EML) gave EQEmax of 1.76 % and 5.12 %, and electroluminescence (EL) λmax of 629 nm and 618 nm, respectively. The device efficiency can be further improved to 4.32 % and 5.57 % with CPTBF : p-QCN and (4 : 1) and CPTBF : m-QCN (4 : 1) as the EML, which is consistent with their improved photoluminescence quantum yields (PLQYs). A new fluorescent emitter BPBBT with photoluminescence (PL) λmax of 726 nm and a high PLQY of 67 % was synthesized and utilized as the dopant of CPTBF : m-QCN (4 : 1) cohost system. The device employing CPTBF : m-QCN (4 : 1): 5 wt.% BPBBT as the EML gave an EQEmax of 5.02 % and EL λmax centered at 735 nm, however, the weak residual exciplex emission remains. By reducing the donor ratio, the exciplex emission can be completely transferred to BPBBT and the corresponding device with CPTBF : m-QCN (2 : 1): 5 wt.% BPBBT as the EML can achieve EL λmax of 743 nm and EQEmax of 4.79 %. This work manifests the high efficiency near infrared (NIR) OLED can be realized by triplet excitons harvesting of exciplex-forming cohost system, followed by the effective energy transfer to an NIR fluorescent dopant.

High-efficiency all fluorescence white OLEDs with high color rendering index by manipulating excitons in co-host recombination layers.

All fluorescence white organic light-emitting diodes (WOLEDs) based on thermally activated delayed fluorescence (TADF) emitters are an attractive route to realize highly efficient and high color quality white light sources. However, harvesting triplet excitons in these devices remains a formidable challenge, particularly for WOLEDs involving conventional fluorescent emitters. Herein, we report a universal design strategy based on a co-host system and a cascaded exciton transfer configuration. The co-host system furnishes a broad and charge-balanced exciton generation zone, which simultaneously endows the devices with low efficiency roll-off and good color stability. A yellow TADF layer is put forward as an intermediate sensitizer layer between the blue TADF light-emitting layer (EML) and the red fluorescence EML, which not only constructs an efficient cascaded Förster energy transfer route but also blocks the triplet exciton loss channel through Dexter energy transfer. With the proposed design strategy, three-color all fluorescence WOLEDs reach a maximum external quantum efficiency (EQE) of 22.4% with a remarkable color rendering index (CRI) of 92 and CIE coordinates of (0.37, 0.40). Detailed optical simulation confirms the high exciton utilization efficiency. Finally, by introducing an efficient blue emitter 5Cz-TRZ, a maximum EQE of 30.1% is achieved with CIE coordinates of (0.42, 0.42) and a CRI of 84 at 1000 cd m-2. These outstanding results demonstrate the great potential of all fluorescence WOLEDs in solid-state lighting and display panels.

Improving excitons utilization of blue thermally activated delayed fluorescence emitter to achieve highly efficient solution-processed hybrid white organic light-emitting diodes

5-(5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)-10,15-diphenyl-10,15-dihydro-5H-diindolo[3,2-a:3′,2′-c]carbazole (DBA-DI) with high triplet energy was utilized as efficient blue emission dopant to construct solution-processed thermally activated delayed fluorescence-phosphorescence (TADF-phosphorescence) hybrid white organic light-emitting diodes (WOLEDs). Meanwhile, a series of TADF-phosphorescence hybrid WOLEDs composed of bis(2-phenylquinoline) (2,2,6,6-tetramethylheptane-3,5-dionate)iridium(III) (PQ2Ir(dpm)), tris(2-(4-tolyl)-phenylpyridine)iridium (Ir(mppy)3) and DBA-DI are successfully developed by introducing the co-host system consist of poly(N-vinylcarbazole) (PVK) and 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) with high color rendering index (CRI) and improving the utilization of triplet excitons. Furthermore, energy transfer processes among different emitters were investigated in detail based on absorbance spectra, photoluminescence and transient photoluminescence spectra. Ultimately, the Commission Internationale de I'Eclairage coordinates of the optimum solution-processed WOLEDs in warm-white emission region are (0.41, 0.41) and high CRI is 82. Furthermore, the same device realized outstanding efficiencies of 32.14 cd A−1, 21.00 lm W−1 and 15.1% at high brightness of 1000 cd m−2.

Highly Efficient Solution-Processed Blue Phosphorescent Organic Light-Emitting Diodes Based on Co-Dopant and Co-Host System.

The low-lying HOMO level of the blue emitter and the interfacial miscibility of organic materials result in inferior hole injection, and long exciton lifetime leads to triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), so the efficiencies of blue phosphorescent organic light-emitting diodes (PhOLEDs) are still unsatisfactory. Herein, we design co-host and co-dopant structures to improve the efficiency of blue PhOLEDs by means of solution processing. TcTa acts as hole transport ladder due to its high-lying HOMO level, and bipolar mCPPO1 helps to balance carriers’ distribution and weaken TPA. Besides the efficient FIr6, which acts as the dominant blue dopant, FCNIrPic was introduced as the second dopant, whose higher HOMO level accelerates hole injection and high triplet energy facilitates energy transfer. An interesting phenomenon caused by microcavity effect between anode and cathode was observed. With increasing thickness of ETL, peak position of electroluminescence (EL) spectrum red shifts gradually. Once the thickness of ETL exceeded 140 nm, emission peak blue-shifts went back to its original position. Finally, the maximum current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of blue phosphorescent organic light-emitting diode (PhOLED) went up to 20.47 cd/A, 11.96 lm/W, and 11.62%, respectively.

Highly efficient solution-processed white organic light-emitting diodes based on a co-host system by controlling energy transfer among different emitters

Efficient solution-processed white organic light-emitting diodes with FIrpic, Ir(mppy)3 and (MDQ)2Ir(acac) as the blue, green and red emitters, respectively, were realized by employing TcTa and CzSi as hosts.

A phosphorescent OLED with an efficiency roll-off lower than 1% at 10 000 cd m−2 achieved by reducing the carrier mobility of the donors in an exciplex co-host system

A phosphorescent OLED with an external quantum efficiency of 22.31% and an extremely low efficiency roll-off of 0.67% at 10 000 cd m−2 was achieved by reducing the carrier mobility of the donors in an exciplex co-host system.

Highly efficient blue and white phosphorescent organic light-emitting diodes with low-efficiency roll-off utilizing thermally activated delayed fluorescent-based co-host architecture

Highly efficient blue phosphorescent organic light-emitting diodes (PhOLEDs) with reduced efficiency roll-off are still challenging. A blue phosphor of (cyanato-κO)[3,5-difluoro-2-(2-pyridinyl-κN)phenyl-κC][[1,1'-(1,3-phenylene-κC2)bis[3-butyl-2,3-dihydro-1H-imidazolato-κC2]](5-)]-iridium complex is utilized as the blue emitter to fabricate highly efficient blue PhOLEDs. Especially, it is first utilized for white PhOLEDs with superior device performance. The realized blue PhOLED employing a novel TADF co-host system achieves maximum power efficiency (PE) of 43.3 lm/W and the PE still remains 26.3 lm/W at 1000 cd/m2, which are 1.9 and 2.5 times higher than that of blue PhOLED using conventional phosphor single-host structure. Furthermore, the current efficiency (CE) roll-off of the blue PhOLED with TADF co-host architecture is only 2.2% at 1000 cd/m2, which can be assigned to the balanced charge carriers injection and transport. The final white device obtained the maximum CE of 46.1 cd/A, PE of 48.1 lm/W, and a low roll-off of 1.9% at 1000 cd/m2. We also systemically investigate the energy transfer mechanism and exciton distribution of the devices. It was found that our proposed device structure could effectively promote the Förster energy transfer mechanism and the even distribution of excitons. We firmly believe that our work may bring light to the future development of highly efficient PhOLEDs with simultaneous low roll-off.

Study on the difference in exciton generation processes for a single host and exciplex-type co-host

We distinctly reveal the difference in the exciton generation processes in phosphorescent organic light-emitting devices with an exciplex-type co-host and a single host. Excitons in the co-host consisting of 4,4,4-tris(N-carbazolyl)-triphenylamine and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene are created via efficient energy transfer from the exciplex to the phosphorescent dopant. In contrast, excitons in the single host of 4,4,4-tris(N-carbazolyl)-triphenylamine are formed by the combination of holes and electrons trapped by the phosphorescent dopants. The optimized device utilizing the co-host system exhibits highly superior performance relative to the single-host device. The maximum external quantum efficiency and maximum luminance are 14.88% and 90,700cd/m2 for the co-host device, being 1.6 times and 3.6 times the maximum external efficiency and maximum luminance for the single-host device, respectively. Significantly, the critical current density, evaluating the device efficiency roll-off characteristic, is as high as 327.8mA/cm2, which is highly superior to 120.8mA/cm2 for the single-host device, indicating the notable alleviation in efficiency roll-off for the co-host device. The significant improvement in device performance is attributed to eliminating the exciton quenching resulting from the captured holes and the efficient energy transfer from the exciplex-type co-host to the phosphorescent emitter incurred by the reverse intersystem crossing process.

Solution-processed multiple exciplexes via spirofluorene and S-triazine moieties for red thermally activated delayed fluorescence emissive layer OLEDs

A series of novel binary and ternary components of the exciplexes as the cohosts for a red thermally activated delayed fluorescence (TADF) dopant were investigated in the solution-processed OLEDs, where 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl] phenylene (OXD-7) as a conventional acceptor, and 1,3-bis(carbazol-9-yl)benzene (mCP) as a conventional donor were respectively mixed with two molecules containing spirofluorene and s-triazine moieties (TDP-TRZ or DTDP-TRZ) with excellent thermal stability and high electron mobility as the second acceptors. Particularly, the power efficiencies of the devices with the exciplexes are generally enhanced via this strategy of host engineering. The designed devices could achieve a percentage increase of 179% in the power efficiency, compared with the reference device with single-component host, mainly owing to the synergistic effects of electron block, balanced injection of charge carriers and efficient exciton harvesting. The working mechanism of energy transfer in binary and ternary components of the exciplexes hosted red TADF OLEDs is studied. This work provides a novel device design philosophy with the multiple exciplexes cohosts for solution-processed TADF OLEDs, which would help to simplify the fabrication processing, lower the cost, and popularize OLED technology. • A novel efficient binary components exciplexes co-host system applied in TADF emitter s-OLEDs. • A novel solution-processed ternary components exciplexes co-host with red TADF dopant luminescent system is successfully fabricated. • The power efficiency of TADF s-OLED binary exciplexes co-host exhibited a percentage increase of 179% compared with the reference device. • The synergistic effects of electron block, balanced injection of charge carriers and efficient exciton harvesting occurred.

Lead-Free Halide Light-Emitting Diodes with External Quantum Efficiency Exceeding 7% Using Host–Dopant Strategy

Lead-free halide light-emitting diodes (LEDs) are fabricated using nontoxic and earth-abundant CsCu2I3 with a strong yellow emission at a peak wavelength of 568 nm. CsCu2I3-based host–dopant emitters are formed by vacuum thermal evaporation (VTE) film codeposition process instead of the commonly used solution-based film deposition process. Using the VTE process, extremely thin (30 nm) host–dopant emitters have successfully been formed with the CsCu2I3 dopant and various organic host molecules. A bright yellow emission with a photoluminescence quantum yield value of 84.8% is achieved in the 0.5% CsCu2I3-doped halide emitter film due to the successful spatial localization of charge carriers and excitons using an organic host with appropriate energy levels to CsCu2I3. With the further enhancement in charge balance using the cohost system, a record-breaking lead-free halide LED has been fabricated with an EQE of 7.4%. The lead-free halide LEDs are also highly stable in the device operation with LT70 of 20 h at 100 cd/m2.

Structural effect of phenylcarbazole-based molecules on the exciplex-forming co-host system to achieve highly efficient phosphorescent OLEDs with low efficiency roll-off

The structural effect of phenylcarbazole-based molecules on the exciplex-forming co-host system was investigated to achieve a green phosphorescent OLED with a high external quantum efficiency of 31.5%.

Solution-Processable Csp3-Annulated Hosts for High-Efficiency Deep Red Phosphorescent OLEDs.

In this report, a solution-processable cohost system incorporating N,N'-di(naphtalene-1-yl)-N,N'-diphenylbenzidine (NPB) and Csp3-annulated phenylquinoline derivatives, including spiro[indeno[1,2-b]quinoline-11,8'-indolo[3,2,1-de]acridine] (IAIQ), 10-phenyl-10H-spiro[acridine-9,11'-indeno[1,2-b]quinoline] (PAIQ) and 3,3'-(11H-indeno[1,2-b]quinoline-11,11-diyl)bis(N-phenyl-N-(m-tolyl)aniline) (m-TPA-DPIQ), is developed for highly efficient saturated red phosphorescent organic light emitting diodes (OLEDs). IAIQ, PAIQ, and m-TPA-DPIQ, designed with the increase of molecular flexibility, are systematically investigated. Solution-processable devices based on the efficient phosphorescent emitter bis[2-(3,5-dimethylphenyl)isoquinolinato](2,8-dimethyl-4,6-nonanedionato)Iridium [Ir(mpiq)2divm] are successfully fabricated, and give electroluminescent peaks at 634-636 nm with Commission Internationale de L'Eclairage coordinates of (0.70, 0.30). Under optimized conditions, the devices incorporating IAIQ, PAIQ, and m-TPA-DPIQ exhibit high external quantum efficiency with the maximum value at 25.1%, 23.4%, and 23.3%, respectively, and all exceeding 18% at the luminance of 1000 cd/m2. In application, the supersaturated red devices with excellent performance could facilitate the development of wet-made displays. The newly developed Csp3-annulated host materials with their excitonic properties also showoff the tactic to construct cohost system for high-quality phosphorescent OLEDs.

Harnessing a New Co-Host System and Low Concentration of New TADF Emitters Equipped with Trifluoromethyl- and Cyano-Substituted Benzene as Core for High-Efficiency Blue OLEDs.

A strategic approach combining a new co-host system and low concentration of new thermally activated delayed fluorescence (TADF) emitters to make efficient blue TADF organic light-emitting diode (OLED) was developed. The benchmark TADF molecule, 4CzIPN, was adopted as a probe to examine the feasibility of a co-host composing of a hole transporter SimCP and an electron transporter oCF3-T2T. As a result, a sky blue device with 1 wt % 4CzIPN doped in SimCP:oCF3-T2T co-host exhibited 100% energy transfer and achieved a high external quantum efficiency (EQE) up to 26.1%. Importantly, this device showed a limited efficiency rolloff with an EQE of 24% at 1000 cd m-2. To further shift the emission toward blue, three new TADF molecules, 4CzIPN-CF3, 3CzIPN-H-CF3, and 3CzIPN-CF3, modified either by lowering the electron-withdrawing ability of the acceptor group or by reducing the number of carbazole donors of 4CzIPN, have been synthesized and characterized. Among them, 4CzIPN-CF3 and 3CzIPN-H-CF3 display hypsochromic shift emissions compared to that of 4CzIPN. These new compounds were then explored for their potential applications as TADF emitters. Blue TADF OLEDs with 1 wt % of 4CzIPN-CF3 and 3CzIPN-H-CF3 dispersed in SimCP:oCF3-T2T co-host achieved EQEs of 23.1 and 16.5% and retained high EQEs of 20.9 and 14.7% at 1000 cd m-2, respectively.

Enhanced near-infrared electroluminescence from a neodymium complex in organic light-emitting diodes with a solution-processed exciplex host

We report enhanced near-infrared (NIR) electroluminescence from a Nd3+-complex with thenoyltrifluoroacetone and 1,10-phenanthroline ligands. The NIR-emitting complex was blended into an exciplex-forming co-host system comprising 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene as the electron transport material and 4,4′,4″-tris(carbazol-9-yl)triphenylamine as the hole transport material in solution-processed small molecule organic light-emitting diodes (OLEDs). This binary ambipolar host system favors direct charge trapping and exciton formation on the Nd3+-complex molecules. Efficient energy transfer from the singlet and triplet exciplexes formed between the host molecules to the Nd3+ ions contributes to the enhanced luminescence efficiency. The photoluminescence quantum yield of this blend is 1.2%, and the optimized OLED shows a maximum electroluminescence external quantum efficiency of 0.034%. The device also exhibits a low efficiency roll-off of only 12% over a current density range of 100 mA/cm2, due to the reduced triplet-polaron annihilation.

Low energy consumption phosphorescent organic light-emitting diodes using phenyl anthracenone derivatives as the host featuring bipolar and thermally activated delayed fluorescence.

A novel host material featuring the characteristics of bipolarity and thermally activated delayed fluorescence, 10-(4-(5,5-dimethylbenzofuro[3,2-c]acridin-13(5H)-yl)phenyl)-10-phenylanthracen-9(10H)-one (DphAn-5BzAc), has been designed and synthesized. By employing this material as the host of green emitter Ir(ppy)2acac, we have fabricated phosphorescent organic light-emitting diodes (PhOLEDs) with two hosting schemes, which are the single host system consisting of DhAn-5BzAc and the co-host system with 1,3-bis(carbazolyl)benzene (mCP). We found that the co-host based PhOLED achieved very low energy consumption values at high brightnesses, which were only 0.5, 5.9 and 94.0 mW m−2 at 100, 1000 and 10 000 cd m−2, respectively. The extremely low energy consumption for DhAn-based PhOLEDs were attributed to the excellent bipolar transport properties and thermally activated delayed fluorescence characteristics.

Precise regulation of the emissive layer for ultra-high performance white organic light-emitting diodes in an exciplex forming co-host system

Ultra-high performance WOLEDs were achieved by solving the trade-off between charge carrier trapping and energy transfer in electroluminescence processes.

Exciplex-Forming Cohost for High Efficiency and High Stability Phosphorescent Organic Light-Emitting Diodes.

An exciplex forming cohost system is employed to achieve a highly efficient organic light-emitting diode (OLED) with good electroluminescent lifetime. The exciplex is formed at the interfacial contact of a conventional star-shaped carbazole hole-transporting material, 4,4',4″-tris(N-carbazolyl)-triphenylamine (TCTA), and a triazine electron-transporting material, 2,4,6-tris[3-(1H-pyrazol-1-yl)phenyl]-1,3,5-triazine (3P-T2T). The excellent combination of TCTA and 3P-T2T is applied as the cohost of a common green phosphorescent emitter with almost zero energy loss. When Ir(ppy)2(acac) is dispersed in such exciplex cohost system, OLED device with maximum external quantum efficiency of 29.6%, the ultrahigh power efficiency of 147.3 lm/W, and current efficiency of 107 cd/A were successfully achieved. More importantly, the OLED device showed a low-efficiency roll-off and an operational lifetime (τ80) of ∼1020 min with the initial brightness of 2000 cd/m2, which is 56 times longer than the reference device. The significant difference of device stability was attributed to the degradation of exciplex system for energy transfer process, which was investigated by the photoluminescence aging measurement at room temperature and 100 K, respectively.

The Role of Charge Balance and Excited State Levels on Device Performance of Exciplex-based Phosphorescent Organic Light Emitting Diodes

The design of novel exciplex-forming co-host materials provides new opportunities to achieve high device performance of organic light emitting diodes (OLEDs), including high efficiency, low driving voltage and low efficiency roll-off. Here, we report a comprehensive study of exciplex-forming co-host system in OLEDs including the change of co-host materials, mixing composition of exciplex in the device to improve the performance. We investigate various exciplex systems using 5-(3–4,6-diphenyl-1,3,5-triazin-2-yl)phenyl-3,9-diphenyl-9H-carbazole, 5-(3–4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9-phenyl-9H-3,9′-bicarbazole, and 2-(3-(6,9-diphenyl-9H-carbazol-4-yl)phenyl)-4-phenylbenzo[4,5]thieno[3,2-d]pyrimidine, as electron transporting (ET: electron acceptor) hosts and 9,9′-dipenyl-9H, 9′H-3,3′-bicarbazole and 9-([1,1′-biphenyl]-4-yl)-9′-phenyl-9H,9′H-3,3′-bicarbazole as hole transporting (HT: electron donor) hosts. As a result, a very high current efficiency of 105.1 cd/A at 103 cd/m2 and an extremely long device lifetime of 739 hrs (t95: time after 5% decrease of luminance) are achieved which is one of the best performance in OLEDs. Systematic approach, controlling mixing ratio of HT to ET host materials is suggested to select the component of two host system using energy band matching and charge balance optimization method. Furthermore, our analysis on exciton stability also reveal that lifetime of OLEDs have close relationship with two parameters; singlet energy level difference of HT and ET host and difference of singlet and triplet energy level in exciplex.

Triplet exciton confinement for enhanced fluorescent organic light-emitting diodes using a co-host system

In this work, the co-host system within an emitting layer (EML) consists of the host and triplet managing (TM) host materials. A set of EML structures was fabricated with various concentrations of the TM host (0, 10, 30, 50, and 70%). The TM host triplet energy level is lower than the energy levels of the host and the guest, which leads to a reduction in the triplet exciton density and the singlet-triplet annihilation of the guest. Blue fluorescent organic light-emitting diodes exhibit a maximum luminous efficiency (LE) and an external quantum efficiency (EQE) of 9.74 cd/A and 4.92%, respectively. In addition, the efficiency roll-off ratios of the LE and the EQE are 14.25 and 13.16%, respectively.