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.     

Molecular engineering of “A-D-D-A” dual-emitting-cores emitters with thermally activated delayed fluorescence and aggregation-induced emission characteristics

Endowing thermally activated delayed fluorescence (TADF) emitter with aggregation-induced emission (AIE) peculiarity is of great significance for realizing more promising commercial applications. Herein, two new dual-emitting-cores emitters with a structure of acceptor-donor-donor-acceptor (A-D-D-A), namely 2DBT-BZ-2Cz and 2DFT-BZ-2Cz, were designed and synthesized to explore their luminescence trait. The emitters, adopting dual carbazole as donor segments and dual phenyl ketone in peripheral skeleton as electron acceptor units, were featured with small singlet (S₁)–triplet (T₁) splitting energy (ΔE_ST) of 0.02 eV and 0.01 eV. The efficient thermally activated delayed fluorescence (TADF) characteristics and aggregation-induced emission property make them suitable for nondoped OLED devices. The solution-processed green OLEDs based on 2DBT-BZ-2Cz demonstrated greater device performance with current efficiency of 20.7 cd A⁻¹ and maximum luminescence of 10,000 cd m⁻². This work thus provides the direction to explore luminogens of dual-emitting-cores with TADF and AIE features as promising candidates in solid state lighting.     

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.     

Efficient,color-stable and high color-rendering-index white organic light-emitting diodes employing full thermally activated delayed fluorescence system

High efficiency, high color rendering index (CRI) and excellent color-stability are important requirements for high-performance white organic light emitting diodes (WOLEDs). To realize these issues for the WOLEDs based on the full thermally activated delayed fluorescence (TADF) system, we constructed WOLED devices by employing CDBP:PO-T2T exciplex as the host, while 2CzPN and AnbTPA as blue and red dopants, respectively. We carefully optimize the device to realize balanced carrier transporting property of each emitting layer and good exciton confinement in the device. As a result, the WOLED achieves stable white emission with a high CRI of 82, a CIE coordinate of (0.33, 0.38) at a luminance of 1000 cd m⁻², and a small CIE variation value of (0.00, 0.02) in the luminance range of 100–3000 cd m⁻². It also demonstrates high maximum forward-viewing external quantum efficiency of 19.2%, current efficiency of 36.7 cd A⁻¹ and power efficiency of 46.2 lm W⁻¹. Our work presents a novel and useful approach to develop highly efficient, color-stable and high-CRI WOLEDs through the full TADF mechanism.     

Highly efficient and stable blue quantum-dot light-emitting diodes based on polyfluorenes with carbazole pendent groups as hole-transporting materials

Highly efficient and stable blue quantum-dot light-emitting diodes (QD-LEDs) have been realized by using poly (9,9-bis(N-(2′-ethylhexyl)-carbazole-3-yl)-2,7-fluorene) (PFCz) as hole-transporting layers (HTLs). Due to the carbazole units as substituents at the 9-position of polyfluorene, PFCz shows higher hole mobility and better electrochemical stability than poly (N-vinlycarbazole) (PVK). As a result, the maximum current efficiency (CE) and external quantum efficiency (EQE) of the blue QD-LEDs increased from 4.32 cd A⁻¹ to 7.9% for PVK HTL to 7.38 cd A⁻¹ and 12.61% for PFCz HTL, respectively. Furthermore, the PFCz-based blue QD-LED exhibited lower turn-on voltage and longer device lifetime than the PVK-based device. The improvement performance of blue QD-LED should be attributed to the conjugated fluorene backbone and the substituents of the carbazole active sites, thus enhancing hole mobility and electrochemical stability. This result demonstrates that polyfluorenes with pendent carbazole groups is a promising hole-transporting materials for improving performance of blue QD-LEDs.     

Management of Host–Guest Triplet Exciton Distribution for Stable,High-Efficiency,Low Roll-Off Solution-Processed Blue Organic Light-Emitting Diodes by Employing Triplet-Energy-Mediated Hosts

High-quality hosts are indispensable for simultaneously realizing stable, high efficiency, and low roll-off blue solution-processed organic light-emitting diodes (OLEDs). Herein, three solution processable bipolar hosts with successively reduced triplet energies approaching the T₁ state of thermally activated delayed fluorescence (TADF) emitter are developed and evaluated for high-performance blue OLED devices. The smaller T₁ energy gap between host and guest allows the quenching of long-lived triplet excitons to reduce exciton concentration inside the device, and thus suppresses singlet-triplet and triplet-triplet annihilations. Triplet-energy-mediated hosts with high enough T₁ and better charge balance in device facilitate high exciton utilization efficiency and uniform triplet exciton distribution among host and TADF guest. Benefited from these synergetic factors, a high maximum external quantum efficiency (EQE_max) of 20.8%, long operational lifetime (T₅₀ of 398.3 h @ 500 cd m⁻²), and negligible efficiency roll-off (EQE of 20.1% @ 1000 cd m⁻²) are achieved for bluish-green TADF OLEDs. Additionally introducing a narrowband emission multiple-resonance TADF material as terminal emitter to accelerate exciton dynamic and improve exciton utilization, a higher EQE_max of 23.1%, suppressed roll-off and extended lifetime of 456.3 h are achieved for the sky-blue sensitized OLEDs at the same brightness.     

Indolo[3,2,1-jk]carbazole based planarized CBP derivatives as host materials for PhOLEDs with low efficiency roll-off

Three novel planarized CPB derivatives (ICzCz, ICzPCz, ICzICz) have been synthesized and characterized concerning applications as host materials for PhOLEDs. The incorporation of fully planar indolo[3,2,1-jk]carbazole (ICz) in the CBP scaffold has been systematically investigated, revealing a significant impact on molecular properties, such as improved thermal stability (t_g > 110 °C), high triplet energies (E_T > 2.81 eV) and charge transport properties. Employing the newly developed materials as host materials, efficient green PhOLEDs (CE_max: 60.1 cd A⁻¹, PE_max: 42.1 lm W⁻¹, EQE_max: 15.9%) with a remarkably low efficiency roll-off of 5% at 1000 cd m⁻² as well as blue PhOLEDs (ICzCz) with a high PE of 26.1 lm W⁻¹ have been realized. Hence, the first comprehensive report on the application of ICz as integral building block for electroluminescent materials is presented, establishing this particular structural motive as versatile structural motive in this field.     

The synthesis,crystal structures,aggregation-induced emission and electroluminescence properties of two novel green-yellow emitters based on carbazole-substituted diphenylethene and dimesitylboron

Introducing the hole-transporting carbazole moiety into an aggregation-induced emissive tetraarylethene skeleton and attaching electron-transporting dimesitylboron groups to the periphery, we obtain two novel electroluminescent materials. Their structures are fully characterized by elemental analysis, mass spectrometry, NMR spectroscopy and X-ray crystallography. Furthermore, their thermal, electrochemical, as well as photophysical properties including AIE-behavior are systematically investigated not only by experimental methods but also by DFT computation. Thereby, we show that the two compounds possess high thermal and electrochemical stability with a remarkable AIE-behavior. X-ray crystal analyses aided by DFT calculations provide insights in the origin of the luminescent properties and AIE features. Ultimately, two non-doped OLEDs (Device A and Device B) were fabricated by using PDPBCE and BDPBCE as light-emitting layer, respectively. Device A showed yellowish-green light with a turn-on voltage of 3.8 V, a maximum brightness of 59130 cd m⁻² and a maximum current efficiency of 6.43 cd A⁻¹. Device B exhibited greenish-yellow light with a turn-on voltage of 3.0 V, a maximum brightness of 67,500 cd m⁻² and a maximum current efficiency of 11.2 cd A⁻¹.     

Solution processed single-emissive-layer white organic light-emitting diodes based on fluorene host: Balanced consideration for color quality and electroluminescent efficiency

Cost-effective fabrication of white organic light-emitting diodes (WOLED) is meaningful toward commercial application of environment-friendly solid-state lighting sources. Electroluminescent efficiency and color quality are two opposite performance characteristics facing solution processed WOLEDs requiring balanced consideration. Herein, a recently synthesized molecule of 4,4’-(9,9’-(1,3-phenylene)bis(9H-fluorene-9,9-diyl))bis(N,N-diphenylaniline) (DTPAFB) is introduced as a host material for solution processed all-phosphor WOLEDs, embracing four well-known molecules which are blue iridium (III) bis(2-(4,6-difluorophenyl)pyridinato-N,C²)(picolinate) (FIrpic), green iridium (III) bis[2-(2-pyridinyl-N)phenyl-C](2,4-pentanedionato-O²,O⁴) [Ir(ppy)₂(acac)], and orange iridium (III) bis(2-phenyl-benzothiazole-C²,N)(acetylacetonate) [Ir(bt)₂(acac)] plus a home-made red phosphor of iridium (III) tris(1-(2,6-dimethylphenoxy)-4-(4-chlorophenyl)phthalazine) [Ir(MPCPPZ)₃]. Illumination quality white light with high brightness, high efficiency, suitable correlated color temperature (CCT), high color-rendering index (CRI), and stable electroluminescent (EL) emission is obtained. A stable white emission with a CRI over 70, Commission Internationale de L'Eclairage (CIE) of (0.37, 0.42), and high EL efficiency of 19.6 lm W⁻¹ at high luminance of 2000 cd m⁻² for blue/orange complementary color WOLEDs is demonstrated. The optimized red/green/blue three primary color WOLEDs show improved CRI up to 81, moderate high efficiency of 25.8 cd A⁻¹, 14.4 lm W⁻¹, and EQE of 13.9%. Furthermore, the red/green/blue/orange four primary color WOLEDs show the optional balance between color quality and EL efficiency with high CRI of around 81–83 and medium CCT of 3755–3929 K which is warm and soft to human eyes. At an illumination relevant luminance of 1000 cd m⁻², the total power efficiency reaches 33.6 lm W⁻¹, and still remains 30.2 lm W⁻¹ at 3000 cd m⁻², approaching the efficiency of state-of-the-art fluorescent-tube (40–70 lm W⁻¹), potentially suitable as an environment-friendly solid-state lighting source. This work indicates that developing high performance host materials and highly efficient phosphors and carefully combining them with common phosphors is an effective way toward high performance WOLEDs.     

High performance doping-free WOLEDs based on rationally designed asymmetric orange-red Ir(III) emitter with balanced charge mobility

Doping-free organic light-emitting diodes (OLEDs) have attracted continuous attention owing to reduced phase separation, better repeatability, and low cost. Despite demonstrating great potential for white OLEDs (WOLEDs), development of phosphorescent materials capable of achieving high performance with low voltage, high luminance, and low efficiency roll-off simultaneously, still remains a significant challenge. Herein, we design three orange-red Ir(III) phosphors employing functionalized 1,2-diphenylbenzimidazole as main ligands. Clear relationship between structures and electroluminescence (EL)-performances has been established by comprehensively studying their emission properties and intrinsic carrier transporting abilities. Designed phosphor SFIrbiq with spirobifluorene moiety showing negligible intermolecular interactions and balanced carrier transporting ability, not only achieves favorable monochromatic doping-free device but also high-performance doping-free WOLEDs. Optimized WOLED realizes low voltages (2.5 V at 1 cd m⁻², 3.3 V at 100 cd m⁻², and 4.2 V at 1000 cd m⁻²), maximum brightness of 34 505 cd m⁻² and efficiencies of 24.2 cd A⁻¹, 21.7 lm W⁻¹, 10.3%. Such doping-free hybrid WOLED also achieves low efficiency roll-off of 5% for external quantum efficiency (EQE) at 1000 cd m⁻². The device performance can be further improved by employing doping-free all-phosphorescent device structure, achieving maximum efficiencies of 33.3 cd A⁻¹, 32.4 lm W⁻¹, and 16.9%. The results are promising among reported doping-free three-color WOLEDs, paving a feasible way to development of efficient Ir(III) phosphors and doping-free WOLEDs.     

Carbazole-pyridine pyrroloquinoxaline/benzothiadiazine 1,1-dioxide based bipolar hosts for efficient red PhOLEDs

Two novel bipolar hosts Cbz-Py-PQ and Cbz-Py-SA have been designed, synthesized, and eventually successfully used for fabrication of red phosphorescent organic light-emitting diodes (PhOLEDs). Considering higher hole mobility than that of electron mobility in most of the bipolar host with 1:1 donor: acceptor ratio, herein we have made it 1:2 by linking carbazole (donor core) to pyrroloquinoxaline/benzothiadiazine 1,1-dioxide (acceptor core) through pyridine (acceptor core) featuring donor-acceptor-acceptor (D-A-A) architecture. Structure-property-performance relationship have been realized through evaluation of thermal, photophysical and electrochemical properties of both the molecules. Cbz-Py-PQ hosted red PhOLED revealed maximum efficiencies of 16.4%, 9.6 cd A⁻¹ and 9.4 lm W⁻¹ with maximum luminance of 20753 cd m⁻² at 11.0 V.     

Two trans-1-(9-anthryl)-2-phenylethene derivatives as blue-green emitting materials for highly bright organic light-emitting diodes application

1-(9-Anthryl)-2-phenylethene (t-APE) is a blue-green material with high fluorescence quantum yield (Ф_f 0.44). However, it is easily crystallized. Herein, Two asymmetric blue-green emitting materials based on t-APE, (E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-1-yl)anthracene (6) and (E)-9-(4-(2-(anthracen-9-yl)vinyl)phenyl)-10-(naphthalen-2-yl)anthracene (7) were firstly designed and synthesized. The two compounds possess high thermal stability, morphological durability, and bipolar characteristics. The non-doped blue-green organic light-emitting diodes (OLEDs) using 6 and 7 as emitting layers showed emission at 495 nm, full width at half maximum of 80 nm, maximum brightness of 13,814, 10,579 cd m⁻², maximum current efficiency of 3.62, 7.16 cd A⁻¹, and Commission Internationale de L'Eclairage (CIE) coordinate of (0.20, 0.43), respectively. Furthermore, when employing 6 and 7 as blue-green emitting layers and rubrene doped in tris-(8-hydroxyquinolinato)aluminum (Alq3) as the orange emitting layers to fabricate white OLEDs (WOLEDs), the WOLEDs exhibit a maximum brightness of 10,984, 14,652 cd m⁻², maximum current efficiency of 2.04, 2.70 cd A⁻¹, and CIE coordinate of (0.30, 0.40), (0.37, 0.47), Color Rendering Index (CRI) of 65, 60, stable EL spectra, respectively. This study demonstrates that the t-APE-type derivatives have the excellent properties for the emitting materials of OLEDs.     

Ultraefficient Non-Doped Deep Blue Fluorescent OLED: Achieving a High EQE of 10.17% at 1000 cd m−2 with CIEy<0.08

The pursuit for efficient deep blue material is an ever-increasing issue in organic optoelectronics field. It is a long-standing challenge to achieve high external quantum efficiency (EQE) exceed 10% at brightness of 1000 cd m⁻² with a Commission International de L'Eclairage (CIE_y) <0.08 in non-doped organic light-emitting diodes (OLEDs). Herein, this study reports a deep blue luminogen, PPITPh, by bonding phenanthro[9,10-d]imidazole moiety with m-terphenyl group via benzene bridge. The non-doped OLED based on PPITPh exhibits an exceptionally high EQE of 11.83% with a CIE coordinate of (0.15, 0.07). The EQE still maintains 10.17% at the brightness of 1000 cd m⁻², and even at a brightness as high as 10000 cd m⁻², an EQE of 7.5% is still remained, representing the record-high result among non-doped deep-blue OLEDs at 1000 cd m⁻². The unprecedented device performance is attributed to the reversed intersystem crossing process through hot exciton mechanism. Besides, the maximum EQE of orange phosphorescent OLED with PPITPh as host is 32.02%, and remains 31.17% at the brightness of 1000 cd m⁻². Such minimal efficiency roll-off demonstrates that PPITPh is also an excellent phosphorescent host material. The result offers a new design strategy for the enrichment of high-efficiency deep blue luminogen.     

Novel benzothiadiazine 1,1-dioxide based bipolar host materials for efficient red phosphorescent organic light emitting diodes

In this work, three novel bipolar host materials TPA-SA, 3CBZ-SA and 4CBZ-SA have been designed and synthesized by incorporating triphenyl amine and carbazole as donor and benzothiadiazine 1,1-dioxide as an acceptor. These molecules exhibit moderately high triplet energies and bipolar carrier transport characteristics (ambipolarity) which is useful for the exothermic energy transfer to the dopants and also for the balanced carrier injection/transport in the emissive layers. These materials exhibited good performances in PhOLEDs and furnished external quantum efficiency in the range of 10.0–15.0%. Notably, a red phosphorescent device using TPA-SA as the host doped with Ir(pq)₂(acac) exhibited a maximum EQE, power efficiency and current efficiency of 15.0%, 16.0 lm/W, and 25.3 cd A⁻¹, respectively.     

Stable and efficient blue fluorescent organic light-emitting diode by blade coating with or without electron-transport layer

Multi-layer small-molecule blue fluorescent organic light-emitting diode (OLED) is fabricated by blade coating. The emission layer is based on a mixed host of 1-(7-(9,9′-bianthracen-10-yl)-9,9-dioctyl-9H-fluoren-2-yl)pyrene (PT-404) and electron-transport material 2,7-Bis(diphenylphosphoryl)-9,9′ -spirobifluorene (SPPO13), and the blue guest emitter is 4-4′-(1E,1′E)-2,2′-(naphthalene-2,6-diyl)bis(ethane-2,1-diyl)bis(N,N-bis(4-hexyl- Phenyl) aniline) (Blue D). A hole-transport layer of Poly-(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4-(N-(4-sec-butylphenyl)) diphenylamine) (TFB) is added on top of PEDOT: PSS anode. The electrons are blocked away from TFB by a layer of pure host emission layer of PT-404 between TFB and the mixed –host emission layer. For the device with the electron transport layer of Tris(8-hydroxyquinolinato)aluminum (Alq₃) blade-coated over the emission layer the efficiency and lifetime at initial brightness of 500 cd m⁻² are 7.5 cd A⁻¹ and 150 h for Alq₃/CsF/Al cathode. When the Alq₃/CsF/Al is replaced by simply CsF/Al over the mixed-host emission layer the efficiency and lifetime are 6.4 cd A⁻¹ and 300 h (2 times longer than that of the Alq₃/CsF/Al cathode). The lifetime depends on the electron-hole balance tuned by the mixed-host blending ratio as well as the electron injection from the cathode. This work shows good stability is possible for all-solution-processed blue OLED.     

Improved efficiency,stable spectra and low efficiency roll-off achieved simultaneously in white phosphorescent organic light-emitting diodes by strategic exciton management

Despite their merits of high efficiency and environmental friendliness, phosphor based white organic light-emitting diodes (WOLEDs) for commercial applications still face tough challenges of efficiency roll-off and color stability. Herein, we fabricated high-efficiency phosphor WOLEDs with extremely low roll-off and stable white emission by employing mixed spacer layer between the two complementary emissions as well as mixed host in the orange layer. The strategic exciton management in our proposed device structure greatly balanced the transport of charge carriers due to the excellent exciton manipulation of the mixed spacer, and significantly suppressed the exciton quenching owing to the extended exciton recombination region, which significantly minimized the efficiency roll-off of the fabricated WOLEDs. The resulting phosphor WOLED exhibited the maximum current efficiency (CE) and power efficiency (PE) of 47.5 cd A⁻¹ and 44.7 lm W⁻¹, respectively, and the CE still had 43.1 cd A⁻¹ at 5000 cd m⁻², showing a suppressed efficiency roll-off of only 9.2%. Additionally, the device achieved fairly stable spectra over a wide range of luminance with suitable CIE coordinates for indoor lighting and outdoor displays.     

Simplified and high-efficiency warm/cold phosphorescent white organic light-emitting diodes based on interfacial exciplex co-host

Possessing the reverse intersystem crossing (RISC) process, exciplex system has vast potential to enhance the efficiency of the white organic light-emitting diodes (WOLEDs). Nevertheless, general structures of the emitting layer always employ triple-doping in a long range (20–30 nm) which is complicated on fabrication progress. In this paper, based on the interfacial exciplex co-host, a flexible and simplified structure design is proposed to realize both warm and cold phosphorescent WOLEDs. In the two devices, with strategically locating the ultrathin orange phosphorescent emitting layers at two sides of the blue phosphorescent emitting layer (2 nm), respectively, multiple energy transfer channels are created to carry out highly efficient exciton utilization. Owing to the different energy transfer mechanisms, different organic emission ratios are obtained in two WOLEDs. The cold WOLEDs exhibited superior maximum external quantum efficiency (EQE), current efficiency (CE) and power efficiency (PE) of 28.37%, 72.17 cd A⁻¹ and 87.17 lm W⁻¹, respectively. Also, the warm WOLEDs showed high values as EQE of 23.80%, CE of 67.70 cd A⁻¹ and PE of 81.10 lm W⁻¹. Furthermore, both the devices presented rather stable color output in the luminance range from 2000 cd m⁻² to 10000 cd m.⁻²     

Modulating Backbone Conjugation of Polymeric Thermally Activated Delayed Fluorescence Emitters for High-Efficiency Blue OLEDs

Blue conjugated polymers-based OLEDs with both high efficiency and low efficiency roll-off are under big challenge. Herein, a strategy of local conjugation is proposed to construct high-efficiency blue-emitting conjugated polymers, in which the conjugation degree of polymeric backbones is adjusted by inserting different spacers. In this way, the energy level of triplet state and the energy transfer direction of the polymeric main-chains can be effectively regulated. Benefiting from such fine regulation, the prepared alternative copolymers Alt-PB36 with local conjugated main-chains can better suppress the accumulation of long-lived triplet excitons comparing with the complete conjugated polymers. The higher PLQY of Alt-PB36 also verifies the effective energy transfer from the polymeric main-chains to the TADF units. Accordingly, Alt-PB36 based solution-processed OLEDs achieve an EQE_max of 11.6% and a very low efficiency roll-off of 2.8% at 100 cd m⁻² and 15.2% at 500 cd m⁻². This result represents the best efficiency among blue light-emitting conjugated polymer-based OLEDs so far under high luminance.     

Triphenyl phosphine oxide-bridged bipolar host materials for green and red phosphorescent organic light-emitting diodes

Two wide band gap functional compounds of phenylbis(4-(spiro [fluorene-9,9'-xanthen]-2-yl)phenyl)phosphine oxide (2SFOPO) and (4-(9-ethyl-9H- carbazol-3-yl)phenyl)(phenyl)(4-(spiro[fluorene-9,9′-xanthen]-2-yl)phenyl)phosphine oxide (SFOPO-CZ) were designed, synthesized and characterized. Their thermal, photophysical, electrochemical properties and device applications were further investigated to correlate the chemical structure of bipolar host materials with the electroluminescent performance for phosphorescent organic light-emitting diodes (PhOLEDs). Both of them show high thermal stability with glass transition temperatures in a range of 105–122 °C and thermal decomposition temperatures at 5% weight loss in a range of 406–494 °C. The optical band gaps of compound 2SFOPO and SFOPO-CZ in CH₂Cl₂ solution are 3.46 and 3.35 eV, and their triplet energy levels are 2.51 eV and 2.52 eV, respectively. The high photoluminescent quantum efficiency of emissive layer of doped green device up to 50% is obtained. Employing the developed materials, efficient green and red PhOLED in simple device configurations have been demonstrated. As a result, the green PhOLEDs of compound SFOPO-CZ doped with tris(2-phenylpyridine) iridium shows electroluminescent performance with a maximum current efficiency (CE_max) of 52.83 cd A⁻¹, maximum luminance of 34,604 cd/m², maximum power efficiency (PE_max) of 39.50 lm W⁻¹ and maximum external quantum efficiency (EQE_max) of 14.1%. The red PhOLED hosted by compound 2SFOPO with bis(2-phenylpyridine)(acetylacetonato) iridium(III) as the guest exhibits a CE_max of 20.99 cd A⁻¹, maximum luminance of 33,032 cd/m², PE_max of 20.72 lm W⁻¹ and EQE_max of 14.0%. Compound SFOPO-CZ exhibits better green device performance, while compound 2SFOPO shows better red device performance in PhOLEDs.     

Triarylphosphine oxide–phenanthroline molecular conjugate as a promising doped electron-transport layer for organic light-emitting diodes

Over the past three decades, transparent high electron mobility molecular materials have attracted intensive research efforts for organic light-emitting diodes as electron-transport layer for the sake of low working voltage, high power efficiency and operational stability. However, developing high-performing electron-transport materials presents a demanding challenge owing to difficulties in synthesis, purification and/or processing. In this contribution, we show that n-doping a simple and facilely available phenanthroline derivative, namely 3-(6-diphenylphosphinylnaphth-2-yl)-1,10-phenanthroline Phen-NaDPO with a high T_g of 116 °C, is capable of greatly increasing the electron conductivity up to 3.3 × 10⁻⁴ S m⁻¹. The characterization of the blue sky fluorescent and green phosphorescent OLEDs involving this doped electron-transport layer Phen-NaDPO:50% wt Cs₂CO₃ revealed comparable performances to the analogue BPhen (T_g ≈ 66 °C) OLEDs. For instance, the resulting sky blue fluorescent OLEDs provided ca. 15 cd/A, 13 lm/W @1000 cd m⁻² & t₉₅ ≈ 167 h @1000 cd m⁻². The present finding shows that the doped Phen-NaDPO may be a robust electron-transport material for optoelectronics.