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.     

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.     

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.     

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.     

Design,fabrication and application of organic power converters: Driving light-emitting electrochemical cells from the AC mains

The design, fabrication and operation of a range of functional power converter circuits, based on diode-configured organic field-effect transistors as the rectifying unit and capable of transforming a high AC input voltage to a selectable DC voltage, are presented. The converter functionality is demonstrated by selecting and tuning its constituents so that it can effectively drive a low-voltage organic electronic device, a light-emitting electrochemical cell (LEC), when connected to high-voltage AC mains. It is established that the preferred converter circuit for this task comprises an organic full-wave rectifier and a regulation resistor but is void of a smoothing capacitor, and that such a circuit connected to the AC mains (230 V, 50 Hz) successfully can drive an LEC to bright luminance (360 cd m⁻²) and high efficiency (6.4 cd A⁻¹).     

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.     

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.     

Carbazole/oligocarbazoles substituted silanes as wide bandgap host materials for solution-processable electrophosphorescent devices

We have designed and synthesized a series of organic wide bandgap materials, namely DCzSiCz, DDCzSi and DTCzSi, by incorporating carbazole/oligocarbazoles via a silicon-bridged linkage mode. All the materials show good thermal stability and excellent solution-processibility. Their HOMOs and LUMOs could be tuned to facilitate the efficient carriers injection by the incorporated carbazole/oligocarbazoles, while their singlet and triplet energy levels still maintain high levels, all above 3.44 eV and 2.87 eV, respectively. High efficient blue electrophosphorescent devices with low turn-on voltage are realized using DCzSiCz, DDCzSi and DTCzSi as hosts for FIrpic through solution-processable method. Among them, DCzSiCz-based device demonstrates the best performance, showing a maximum brightness of 6600 cd m⁻² at 11 V and maximum luminous efficiency of 8.40 cd A⁻¹ at 5 V.     

High efficiency blue PhOLEDs using spiro-annulated triphenylamine/fluorene hybrids as host materials with high triplet energy,high HOMO level and high Tg

Two spiro-annulated triphenylamine/fluorene oligomers, namely 4′-(9,9′-spirobifluoren-4-yl)-10-phenyl-10H-spiro[acridine-9,9′-fluorene] (NSF-SF), and 4,4′-di(spiro(triphenylamine-9,9′-fluorene)-2-yl)-spiro(triphenylamine-9,9′-fluorene) (NSF-NSF), are designed and synthesized. Their thermal, electrochemical and photophysical properties were investigated. The introduction of spiro-annulated triphenylamine moieties assurances the high HOMO energy levels of NSF-NSF and NSF-SF at −5.31 eV and −5.33 eV, respectively, which accordingly facilitates the hole injection from nearby hole-transporting layer. Meanwhile, the perpendicular arrangement of the spiro-conformation and the full ortho-linkage effectively prevents the extension of the π-conjugation and consequently guarantees their high triplet energies of 2.83 eV. Phosphorescent organic light-emitting devices (PhOLEDs) with the configurations of ITO/MoO₃/TAPC/EML/TmPyPB/LiF/Al were fabricated by using the two compounds as host materials and bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^2′]iridium(III) picolate (FIrpic) as the dopant. The turn-on voltage of the device B based on NSF-NSF was 2.8 V. Simultaneously, the device exhibited excellent performance with the maximum current efficiency of 41 cd A⁻¹, the maximum power efficiency of 42 lm W⁻¹ and the maximum external quantum efficiency (EQE) of 19.1%. At a high brightness of 1000 cd m⁻², the device remained EQE of 16.2% and the roll-off value of external quantum efficiency is 15%.     

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.     

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.     

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.     

Cadmium-free quantum dots based violet light-emitting diodes: High-efficiency and brightness via optimization of organic hole transport layers

Bright and efficient violet quantum dot (QD) based light-emitting diodes (QD-LEDs) with heavy-metal-free ZnSe/ZnS have been demonstrated by choosing different hole transport layers, including poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB), and poly-N-vinylcarbazole (PVK). Violet QD-LEDs with maximum luminance of about 930 cd/m², the maximum current efficiency of 0.18 cd/A, and the peak EQE of 1.02% when poly-TPD was used as HTL. Higher brightness and low turn-on voltage (3.8 V) violet QD-LEDs could be fabricated when TFB was used as hole transport material. Although the maximum luminance could reach up to 2691 cd/m², the devices exhibited only low current efficiency (∼0.51 cd/A) and EQE (∼2.88%). If PVK is used as hole transport material, highly efficient violet QD-LEDs can be fabricated with lower maximum luminance and higher turn-on voltages compared with counterpart using TFB. Therefore, TFB and PVK mixture in a certain proportion has been used as HTL, turn-on voltage, brightness, and efficiency all have been improved greatly. The QD-LEDs is fabricated with 7.39% of EQE and 2856 cd/m² of maximum brightness with narrow FWHM less than 21 nm. These results represent significant improvements in the performance of heavy-metal-free violet QD-LEDs in terms of efficiency, brightness, and color purity.     

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     

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.     

Tuning electron injection/transporting properties of 9,10-diphenylanthracene based electron transporters via optimizing the number of peripheral pyridine for highly efficient fluorescent OLEDs

By incorporating different number of pyridine rings to the periphery of the 9,10-diphenylanthracene (DPA) core, four new pyridine-containing DPA derivatives, 3-(4-(10-phenylanthracen-9-yl)phenyl)pyridine (AnPy), 9,10-bis(4-(pyridin-3-yl)phenyl)anthracene (AnDPy), 3,3'-((2-(pyridin-3-yl)anthracene-9,10-diyl)bis(4,1-phenylene))dipyridine (AnTPy), 3,3'-(9,10-bis(4-(pyridin-3-yl)phenyl)anthracene-2,6-diyl)dipyridine (AnFPy) were designed and synthesized as electron transporters. Their photophysical properties, energy levels and electron mobilities can be readily regulated through tuning the quantity of the pyridine ring. Through optimizing electron injection/transporting properties, AnTPy exhibits not only a suitable lowest unoccupied molecular orbital (LUMO) energy level for electron injection into light-emitting layer (EML), but also a relatively high electron mobility of around 10⁻³ cm² V⁻¹ s⁻¹, which is about two orders of magnitude higher than that of the widely used material Alq₃. As expected, the blue fluorescent OLEDs with AnPy, AnTPy and AnFPy as an electron-transporting layer (ETL) exhibited superior performance compared to that using Alq₃, remarkably lowering the driving voltages and improving efficiencies. In particular, the device with AnTPy as an ETL showed a maximum current efficiency of 14.4 cd A⁻¹, a maximum power efficiency of 12.1 lm W⁻¹, a maximum external quantum efficiency (EQE) of 8.15% and low efficiency roll-off even at an illumination-relevant luminance of 10,000 cd m⁻². These results clearly demonstrated that tuning electron injection/transporting properties by optimizing the number of peripheral electron-withdrawing groups was an efficient strategy to achieve high-performance ETMs.     

Alternating current-driven,white field-induced polymer electroluminescent devices with high power efficiency

A solution-processed, all-phosphor, three-color (i.e., blue, green, and red), alternating current-driven white field-induced polymer electroluminescent device (WFIPEL), with low operational voltage, high luminance, high efficiency, high color-rendering index (CRI), and excellent color-stability, was demonstrated. The devices employed poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) [P(VDF–TrFE–CFE)] dielectric modified by single-walled carbon nanotubes (SWNTs) to further improve the dielectric characteristics, as the insulating layer. This significantly lowers the driving voltage of the device. Moreover, hole-generation layer and electron-transporting layer with high conductivity were used to more efficiently form and confine excitons in the emissive layer. The resulting WFIPEL devices show significant improvements in performance as compared to previous reports. Specifically, the devices exhibit a low turn-on voltage of 10 V, a maximum luminance of 7210 cd m⁻², a maximum current efficiency and power efficiency of 33.8 cd A⁻¹ and 10.5 lm W⁻¹, and a CRI of 82. The power efficiency is even 10 times higher than the highest previous report (1 lm W⁻¹).     

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.     

Highly efficient blue polyfluorenes using blending materials as hole transport layer

Efficient blue polyfluorenes have been generated by incorporating the hole transport material N-([1,1′-biphenyl]-4-yl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)- phenyl)-9H-fluoren-2-amine (BCFN) into poly(9,9-dioctylfluorene) (PFO) as an emissive layer. BCFN has an appropriate highest occupied molecular orbital (HOMO) energy level and high hole transport/electron barrier properties, which can effectively reduce the hole injection barrier and improve the charge carrier injection and transport. These properties resulted in a significant improvement in the electroluminescent (EL) performance of PFO. To further improve the EL performance of PFO, the blend hole transport layer, PVK [Poly(N-vinylcarbazole)]:BCFN with weight ratio of 3:7, was inserted between the PEDOT:PSS and the emissive layer. The blend hole transport layer effectively reduced exciton quenching and markedly decreased the hole injected barrier. A maximum luminous efficiency (LE_max) of 4.31 cd A⁻¹ was obtained with the CIE coordinates of (0.17, 0.13). The device maintained a LE_max of 4.27 cd A⁻¹ at a luminance of 1000 cd m⁻². In addition, stable EL spectra were obtained and were nearly identical when the applied voltage was increased from 5 to 11 V. These results indicate that blending the appropriate hole transport material can be an efficient method to improve device performance based on the large band gap of blue-lighting materials.     

Constructing a novel single-layer white organic light-emitting device through a new sky-blue fluorescent bipolar host

A novel device concept was realized for simple single-layer small-molecule white organic light emitting devices. The single organic active layer here is simply comprised of a newly synthesized sky-blue fluorescent bipolar host (TPASO) and a common orange phosphorescent dopant. Suppressed singlet Föster energy transfer induced by a low-concentration doping and spontaneous high- to low-lying triplet energy transfer, respectively, lead to sky-blue fluorescence from TPASO and orange phosphorescence from the dopant. The resulting two-organic-component device exhibits a low turn-on voltage of 2.4 V, maximum current/power efficiencies up to 11.27 ± 0.02 cd A⁻¹ and 14.15 ± 0.03 lm W⁻¹, and a warm-white CIE coordinate of (0.42, 0.45) at 1000 cd m⁻².