Ultra high-efficiency multi-photon emission blue phosphorescent OLEDs with external quantum efficiency exceeding 40%

We developed high-efficiency multi-photon emission (MPE) blue phosphorescent OLEDs with external quantum efficiency exceeding 40% at 100 cd m⁻². In these MPE devices, we used a blue phosphorescent emitter, FIrpic and pyridine-containing electron-transporters, B3PyPB and B3PyMPM, B4PyMPM. We also used a well-known electron-transporter, BCP for comparison. We used a combination of TAPC/MoO₃/Al/Liq layers as the charge-generation layer unit. An optimized MPE device showed an extremely high current efficiency of over 90 cd A⁻¹ and a high power efficiency of over 40 lm W⁻¹ at 100 cd m⁻² without any outcoupling enhancement.     

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).     

Novel diarylborane–phenanthroimidazole hybrid bipolar host materials for high-performance red,yellow and green electrophosphorescent devices

In this work, two novel bipolar host materials p-BPPI and m-BPPI containing phenanthroimidazole/dimesitylborane (Mes₂B) with para- and meta-linkage have been designed, synthesized and characterized. The appending Mes₂B moiety improves the thermal stability, electrochemical stability and carrier injection/transport ability of both target compounds. The test results of time-of-flight (TOF) and single-carrier devices show that both the new hosts possess bipolar charge-transporting characteristics. As a result, series of highly efficient green (66.3 cd A⁻¹, 63.1 lm W⁻¹, 18.2%), yellow (55.2 cd A⁻¹, 66.6 lm W⁻¹, 14.5%) and red (20.1 cd A⁻¹, 20.4 lm W⁻¹, 13.5%) PhOLEDs are achieved by using them as the universal host materials. The results indicate that bipolar host p-BPPI and m-BPPI have high potential in fabricating various color OLEDs for displays and lighting applications. Our study further enriches the selection of D and A group for phosphorescent host materials. The relationship between molecular structures and optoelectronic properties is discussed experimentally and theoretically.     

High-efficiency deep blue fluorescent emitters based on phenanthro[9,10-d]imidazole substituted carbazole and their applications in organic light emitting diodes

A series of highly efficient deep blue emitters comprising of carbazole and phenanthro[9,10-d]imidazole moieties are designed and synthesized. These compounds present deep blue emission, narrow FWHM, high quantum yields, high thermal and morphological stabilities. Among them, the design strategy of 2:1 ratio of phenanthro[9,10-d]imidazole and carbazole unit affords M2 with more balanced carrier injection and transporting properties. OLEDs using M2 as emitting layer is observed to deliver a truly deep blue CIE of y < 0.06 with a highest external quantum efficiency of 3.02%. By taking the full advantage of these deep blue emitters, they are further served as excellent hosts for fluorescent and phosphorescent dyes. High-performance green phosphorescent device based on M2/Ir(ppy)₃ is attained with a maximum current efficiency of 33.35 cd A⁻¹, a power efficiency of 22.99 lm W⁻¹ and a maximum external quantum efficiency of 9.47%. When doped with an orange fluorescent material, upon careful tuning the doping proportion, the two-emitting-component white OLED is successfully fabricated with a maximum current efficiency of 5.53 cd A⁻¹ and CIE coordinates of (0.313, 0.305). Both the non-doped and doped devices exhibited high operational stability with negligible efficiency roll-off over the broad current density range.     

Non-interlayer and color stable WOLEDs with mixed host and incorporating a new orange phosphorescent iridium complex

In this work, we have synthesized a new phosphorescent iridium complex (Bppya)₂Ir(acac), as an orange dopant for organic light-emitting diodes (OLEDs). The device achieved an external quantum efficiency (EQE) of 22.4%, a current efficiency of 49.5 cd A⁻¹ and a power efficiency of 38.9 lm W⁻¹, which are among the highest values for orange OLEDs. Furthermore, color stable and high efficiency phosphorescent white OLED (WOLED) was demonstrated by utilizing a mixed-host in the emissive layers (EMLs) composed of a blue phosphor and the new orange phosphorescent emitter, as well as by avoiding the use of interlayers that commonly exist between different EMLs in WOLEDs. Our WOLED presented decent white emission with maximum efficiencies of 25.3 cd A⁻¹ and 12.6%. Furthermore, the 1931 Commission Internationale de l’Eclairage color coordinates exhibited extremely small variation of (0.3940 ± 0.0102, 0.4323 ± 0.0046) in a wide luminance range from 49 to 38,035 cd m⁻² when driving voltages increased from 4 V to 12 V. The root cause for this excellent color stability is the utilization of the mixed-host to obtain bipolar transport properties in EMLs as well as the eliminating of interlayer between different EMLs, which, on one hand, effectively broaden the exciton recombination zone; on the other hand, reduce the accumulation of triplet excitons at the emissive interface.     

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.     

Highly efficient,solution-processed orange–red phosphorescent OLEDs by using new iridium phosphor with thieno[3,2-c]pyridine derivative as cyclometalating ligand

A new orange iridium phosphor of (EtPy)₂Ir(acac) with thieno[3,2-c]pyridine derivative as cyclometalating ligand was designed and synthesized. The combination of thieno[3,2-c]pyridine with rigid fluorene moiety enlarged the π conjugation of ligand, and consequently caused the peak emission of (EtPy)₂Ir(acac) red-shift to 588 nm. By using (EtPy)₂Ir(acac) as the orange phosphor, the fully solution-processed PhOLEDs were fabricated with the following device configuration: ITO/PEDOT:PSS/PVK: PBD: (EtPy)₂Ir(acac)/CsF/Al. With PEDOT:PSS 8000 as the hole-injecting material, the orange device achieved a maximum current efficiency of 13.4 cd A⁻¹, a maximum power efficiency of 5.9 lm W⁻¹ and a maximum external quantum efficiency (EQE) of 11.2% with a CIE coordinate of (0.62, 0.38) that falls into the orange–red region. Moreover, at high luminance of 1000 cd m⁻², the device still remained high current efficiency of 8.7 cd A⁻¹ and EQE of 7.3%. To the best of our knowledge, these efficiencies were among the highest ever reported for solution-processed orange–red PhOLEDs.     

Organic light-emitting devices with double-block layer

We report on the fabrication of organic light-emitting devices (OLEDs) using double-block layers on the electron transport layer and emitting layer. The current efficiency of the organic light-emitting diode is improved by 43% to 9.16 cd A⁻¹ as compared to the device with a single host of Alq₃ as the electron transport layer. The maximum luminance is over 23 750 cd m⁻² at the bias of 18 V and the current of 338.3 mA cm⁻², which is 33% higher than the single host Alq₃ device without block layer. Using a step-by-step procedure to smooth electron injection and transport, the energy levels introduced by the insertion layers are an effective method of improving the luminance characteristics.     

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%.     

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.     

Efficient blue and bluish-green iridium phosphors: Fine-tuning emissions of FIrpic by halogen substitution on pyridine-containing ligands

Three new iridium compounds, 4-F-FIrpic, 4-Cl-FIrpic and 4-Br-FIrpic, were designed and synthesized by introducing the F, Cl and Br atoms to the 4-position of pyridine ring in the frame of sky-blue emitter, FIrpic. Adding F atom stabilizes the HOMO level of FIrpic but keeps the LUMO level of FIrpic almost unchanged, which consequently broadens the HOMO–LUMO gap of FIrpic and finely tunes the emission to 465 nm of 4-F-FIrpic from 470 nm of FIrpic. In contrast, introducing of Cl and Br atoms simultaneously lowers the HOMO and LUMO levels of FIrpic, which brings about the squeeze of HOMO–LUMO gap in FIrpic and makes the emissions of 4-Cl-FIrpic and 4-Br-FIrpic red-shift to 475 and 479 nm, respectively. The phosphorescent organic light-emitting devices using the three iridium compounds as dopants were fabricated with the following configuration: ITO/MoO₃/TAPC/TCTA:dopants/Tm/LiF/Al. The device based on 4-F-FIrpic showed a blue emission with the Commission Internationale de L’Eclairage coordinate of (0.15, 0.28), and revealed rather high efficiencies, with maximum current efficiency of 29 cd A⁻¹, power efficiency of 29 lm W⁻¹ and external quantum efficiency of 14.6%.     

Ultra-simple hybrid white organic light-emitting diodes with high efficiency and CRI trade-off: Fabrication and emission-mechanism analysis

We present a simple hybrid white organic light-emitting diodes (WOLED) consisting of only two layers, i.e., a hole-transporting layer and an emitting layer. The emitting layer is formed by simply co-doping a green phosphor and a red phosphor in bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp₂), which acts as the blue emitter, electron-transport material, and high triplet energy host for the phosphors, i.e., a multifunctional chromophore. This simple device exhibits a maximum power and quantum efficiency of 46.8 lm W⁻¹ and 16.5%, respectively, with a good CRI up to 90. The versatile experimental techniques are performed to gain a deep understanding of the emission mechanism. We believe that this simple design concept can provide a new avenue for achieving ultrahigh performance WOLEDs for lighting application.     

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.     

Highly efficient phosphorescent organic light-emitting diodes using a homoleptic iridium(III) complex as a sky-blue dopant

Homoleptic triscyclometalated iridium(III) complex Ir(dbi)₃ was used as a dopant for sky blue phosphorescent organic light-emitting diodes (PHOLEDs). Its photophysical, thermal, electrochemical properties as well as the device performances were investigated. Ir(dbi)₃ exhibited high quantum yield of 0.52 in solution at room temperature. A maximum current efficiency and external quantum efficiency (EQE) of 61.5 cd A⁻¹ and 23.1% were obtained, which are the highest ever reported for blue homoleptic iridium complexes. High efficiencies of 53.5 cd A⁻¹ and 20.1% EQE were achieved even at the luminance of 1000 cd m⁻².     

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 ITO-free flexible white organic light-emitting diodes based on multi-cavity technology

The technology of white organic light-emitting diodes (WOLEDs) is attracting growing interest due to their potential application in indoor lighting. Nevertheless the simultaneous achievement of high luminous efficacy (LE), high color rendering index (CRI), very low manufacturing costs and compatibility with flexible thin substrates is still a great challenge. Indeed, very high efficiency devices show usually low values of CRI, not suitable for lighting applications, and use expensive indium tin oxide (ITO) electrodes which are not compatible with low cost and/or flexible products. Here we show a novel low cost ITO-free WOLED structure based on a multi-cavity architecture with increased photonic mode density and still broad white emission spectrum, which allows for simultaneous optimization of all device characteristics. Without using out-coupling optics or high refractive index substrates, CRI of 85 and LE as high as 33 lm W⁻¹ and 14 lm W⁻¹ have been demonstrated on ITO-free glass and flexible substrates, respectively.     

Gravure printed flexible small-molecule organic light emitting diodes

Small-molecule based flexible organic light-emitting diodes (SMOLEDs) were fabricated by gravure printing. In order to modify rheological properties of the functional ink, the green emitter was embedded into an ultrahigh molecular weight polystyrene (UHMW-PS) matrix. The viscosity of the ink was characterized as a function of the small molecule:UHMW-PS weight ratio and solvent type. The gravure printed SMOLEDs exhibited a maximum luminance of 850 cd m⁻², a maximum efficiency of up to 7.7 cd A⁻¹, and turn on voltage of ∼3.5 V. The gravure printed SM:UHMW-PS device exhibits ∼67% higher luminance efficiency comparing to the spin-coated pristine SM device.     

The influence of thermal annealing to the characteristics of AlGaInP/GaInP multiple quantum wells light-emitting diode wafers

The influence of thermal annealing to the characteristics of AlGaInP/GaInP multiple quantum wells (MQWs) light-emitting diode wafers was studied by means of electrochemical capacitance-voltage (ECV) and photoluminescence (PL). Compared with the sample unannealed, the hole carrier concentration of p-GaP layer increased from 5.5×10¹⁸ to 6.5×10¹⁸ cm⁻³, and the hole carrier concentration of p-AlGaInP layer increased from 6.0×10¹⁷ to 1.1×10¹⁸ cm⁻³, after wafer was annealed at 460 °C for 15 min in nitrogen. The hole carrier concentrations of p-GaP layers and p-AlGaInP layers did not obviously change when the annealing temperature varied from 460 to 700 °C. However, after the sample was annealed under 780 °C for 15 min, the hole carrier concentration of p-GaP layer and p-AlGaInP layer decreased to 8×10¹⁷ and 1.7×10¹⁷ cm⁻³, respectively. At the same time, the diffusion of Mg atoms was observed.     

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