Highly efficient inverted top emitting organic light emitting diodes using a horizontally oriented green phosphorescent emitter

We report a highly efficient phosphorescent green inverted top emitting organic light emitting diode doped with Ir(ppy)₂tmd [bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate)] as the horizontally oriented emitter in an exciplex forming co-host system. The device showed a maximum current efficiency of 120.7 cd/A, a maximum external quantum efficiency (EQE) of 27.6% and the power efficiency of 85.9 lm/W at 1,000 cd/m². Moreover the efficiency roll off was small long-lasting to 20,000 cd/m² with EQE’s and current efficiencies of 26.0% and 113.7 cd/A at 10,000 cd/m² and 24.5% and 107.6 cd/A at 20,000 cd/m², respectively. Optical analysis of the efficiencies and emission spectra of the device is also reported.     

High efficiency green phosphorescent organic light-emitting diodes with a low roll-off at high brightness

Highly efficient green phosphorescent organic light-emitting diodes (PHOLEDs) with low efficiency roll-off at high brightness have been demonstrated with a novel iridium complex. The host material 1,3-bis(carbazol-9-yl)benzene (mCP) with high triplet energy is also used as the hole transporting layer to avoid carrier accumulation near the exciton formation interface and reduce exciton quenching. It provides a new approach for easily fabricating PHOLED with high triplet energy emitter. Moreover, the hole blocking layer is extended into the light emitting layer to form a co-host, realizing better control of the carrier balance and broader recombination zone. As a consequence, a maximum external quantum efficiency of 20.8% and current efficiency of 72.9 cd/A have been achieved, and maintain to 17.4% and 60.7 cd/A even at 10,000 cd/m², respectively.     

Improved property in organic light-emitting diode utilizing two Al/Alq3 layers

We reported on the fabrication of organic light-emitting devices (OLEDs) utilizing the two Al/Alq₃ layers and two electrodes. This novel green device with structure of Al(110 nm)/tris(8-hydroxyquinoline) aluminum (Alq₃)(65 nm)/Al(110 nm)/Alq₃(50 nm)/N,N′-dipheny1-N, N′-bis-(3-methy1phyeny1)-1, 1′-bipheny1-4, 4′-diamine (TPD)(60 nm)/ITO(60 nm)/Glass. TPD were used as holes transporting layer (HTL), and Alq₃ was used as electron transporting layer (ETL), at the same time, Alq₃ was also used as emitting layer (EL), Al and ITO were used as cathode and anode, respectively. The results showed that the device containing the two Al/Alq₃ layers and two electrodes had a higher brightness and electroluminescent efficiency than the device without this layer. At current density of 14 mA/cm², the brightness of the device with the two Al/Alq₃ layers reach 3693 cd/m², which is higher than the 2537 cd/m² of the Al/Alq₃/TPD:Alq₃/ITO/Glass device and the 1504.0 cd/m² of the Al/Alq₃/TPD/ITO/Glass. Turn-on voltage of the device with two Al/Alq₃ layers was 7 V, which is lower than the others.     

Color stable and low driving voltage white organic light-emitting diodes with low efficiency roll-off achieved by selective hole transport buffer layers

White organic light-emitting diodes (WOLEDs) showing high color stability, low operating voltage, high efficiency and low efficiency roll-off by adopting different hole transport buffer layers which also behaves as electron/exciton blocking layers (EBL) have been developed. The characteristics of WOLEDs based on blue–green and orange phosphors could be easily manipulated by hole transport buffer layer, which tailors charge carrier transportation and energy transfer. Our WOLEDs show low operating voltages, 100 cd/m² at 3.2 V, 1000 cd/m² at 3.7 V and 10000 cd/m² at 4.8 V, respectively, and achieve a current efficiency of 35.0 cd/A, a power efficiency of 29.0 lm/W at a brightness of 1000 cd/m², and a low efficiency roll-off 8.7% calculated from the maximum efficiency value to that of 5000 cd/m².     

Silver grid transparent conducting electrodes for organic light emitting diodes

Polymer organic light emitting diodes (OLEDs) were fabricated using thin silver hexagonal grids replacing indium tin oxide (ITO) as the transparent conducting electrodes (TCE). Previous literature has assumed that thick metal grids (several hundred nanometres thick) with a lower sheet resistance (<10 Ω/□) and a similar light transmission (>80%) compared to thinner grids would lead to OLEDs with better performance than when thinner metal grid lines are used. This assumption is critically examined using OLEDs on various metal grids with different thicknesses and studying their performances. The experimental results show that a 20 nm thick silver grid TCE resulted in more efficient OLEDs with higher luminance (10 cd/A and 1460 cd/m² at 6.5 V) than a 111 nm thick silver grid TCE (5 cd/A and 159 cd/m² at 6.5 V). Furthermore, the 20 nm thick silver grid OLED has a higher luminous efficiency than the ITO OLED (6 cd/A and 1540 cd/m² at 6.5 V) at low voltages. The data shows that thinner metal grid TCEs (about 20 nm) make the most efficient OLEDs, contrary to previous expectations.     

Efficient nondoped organic light-emitting diodes with Cu complex emitter

In Cu^I complex based organic light emitting diodes (OLEDs) a host matrix is traditionally thought to be required to achieve high efficiency. Herein, it is found that the device ITO/MoO₃ (1 nm)/4,4′-N,N′-dicarbazole-biphenyl (CBP, 35 nm)/[Cu(μ-I)dppb]₂ (dppb = 1,2-bis[diphenylphosphino]benzene, 20 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBi, 65 nm)/LiF (1 nm)/Al (100 nm) with a vacuum thermal evaporated nondoped Cu^I complex emissive layer (EML) showed external quantum efficiency and current efficiency of 8.0% and 24.3 cd/A at a brightness of 100 cd/m², respectively, which are comparable to the maximum efficiencies reported in an optimized doped OLED with the same emitter, higher efficiency than the OLED with a [Cu(μ-I)dppb]₂:CBP EML, and much higher efficiencies than the nondoped OLED with a bis(2-phenylpyridine)(acetylacetonate)iridium [Ir(ppy)₂(acac)] EML. A series of reference films and single carrier devices were fabricated and studied to understand the difference between Cu^I and Ir^III complex based nondoped OLEDs.     

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes on glass and flexible glass substrates

Stacked inverted top-emitting green electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated on glass and flexible glass substrates. A single-unit OLED is shown to have a current efficacy of 46.8 cd/A at a luminance of 1215 cd/m². When two of these OLEDs are stacked, the double-unit OLED exhibits a current efficacy more than twice that of the single-unit OLED, with a current efficacy of 97.8 cd/A at a luminance of 1119 cd/m². With the addition of an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1′-biphenyl)-4,4′-diamine (α-NPD) on top of the semitransparent gold anode, the double-unit stacked OLED achieves a maximum current efficacy of 205 cd/A at a luminance of 103 cd/m², maintaining a high current efficacy of 200 cd/A at a luminance of 1011 cd/m². These stacked inverted OLED combine the advantages of inverted OLEDs with the benefits of having a stacked architecture.     

Efficient blue emitters based on 1,3,5-triazine for nondoped organic light emitting diode applications

Two novel efficient blue emitters (TTT-1, TTT-2) containing 1,3,5-triazine, thiophene and triphenylamine have been designed and synthesized. Organic light emitting diodes (OLEDs) using these new triazine derivatives as emissive layers, ITO/TAPC (60 nm)/TTT-1 (Device A) or TTT-2 (Device B) (40 nm)/TPBi (60 nm)/LiF (1 nm)/Al (100 nm), were fabricated and tested. The OLEDs exhibited good performances with low turn-on voltage of 3 V, maximum luminance of ca. 8990 cd/m² for TTT-1 and 15,980 cd/m² for TTT-2, and maximum luminance efficiency of 4.7 cd/A for TTT-1 and 4.0 cd/A for TTT-2, respectively.     

Achieving above 30% external quantum efficiency for inverted phosphorescence organic light-emitting diodes based on ultrathin emitting layer

High efficiency inverted phosphorescence organic light-emitting diodes (PhOLEDs) based on ultrathin undoped and doped emitting layer (EML) have been developed. Compared to conventional device, the inverted PhOLED with 0.5 nm undoped EML exhibits significantly larger external quantum efficiency (EQE), due to effective energy transfer from the excited host to the emitter. According to the atomic force microscopy image of EML, the 0.5 nm emitter sandwiched by two hosts can be considered as the emitter doped in two hosts. The inverted device with intentionally doped ultrathin EML (1.5 nm) exhibits the maximum EQE of 31.1%, which is attributed to optimized charge balance and preferred horizontal orientation of emitter. However, such inverted device has large efficiency roll-off at high brightness because of triplet–triplet annihilation process within the ultrathin EML. This can be improved by broadening the doped EML. The inverted device with 10.5 nm doped EML has about EQE of 20 % at 10,000 cd/m². It is expected that our inverted PhOLED will promote development of high efficiency active-matrix organic light-emitting diodes based on the n-type Indium Gallium Zinc Oxide thin film transistor.     

Bright,efficient, deep blue-emissive polymer light-emitting diodes of suitable hole-transport layer and cathode design

This study reports the fabrication of efficient deep blue-emissive polymer light-emitting diodes (PLEDs), incorporating a polyfluorene derivative of nonsymmetric and bulky aromatic groups at C-9 position as the light-emissive layer. Another poly(fluorene-co-triphenylamine) (PFO-TPA) derivative of the highest occupied molecular orbital level, −5.3 eV, is used as the hole-injection and -transport layer in the anode part. The thermally crosslinking of styryl groups in PFO-TPA inhibits the solvation of an interlayer in constructing the multilayer device architecture of PLEDs. While applying a cesium carbonate (Cs₂CO₃)/Aluminum (Al) cathode rather than Calcium (Ca)/Al, the device has the superior performance (i.e. one order of magnitude higher). Experimental results indicate that the interfacial reactions at the polymer/Ca junction, as characterized in this study, significantly degrade the luminescence properties and the device performance. Moreover, Cs₂CO₃/Al is a highly favorable cathode in fabricating polyflourene-based PLEDs. The device of the optimal configuration has a decent deep blue emission centered at 430–450 nm of the Commission Internationale de l’Eclairage chromaticity coordinates, (0.15, 0.14), with a maximum brightness of 35054.2 cd/m² and luminous efficiency of 14.0 cd/A (at 2975.0 cd/m²).     

Novel 4-(2,2-diphenyl-vinyl)-phenyl substituted pyrene derivatives as efficient emitters for organic light-emitting diodes

Two pyrene derivatives (PVPP and TPVPP) bearing 4-(2,2-diphenyl-vinyl)-phenyl side unit were developed for organic light-emitting diodes. Their photophysical, thermal, electrochemical, and electroluminescent properties as well as the film morphologies have been investigated in detail. Both of them exhibit high solid-state quantum yields, good thermal stability, and high glass-transition temperatures in the range of 127–150 °C. In particular, it is found that multiple side units can significantly affect the electrochemical properties to improve the electron injection. The LUMO energy level of TPVPP is −2.76 eV, which is very close to that of commonly used electron transport materials. The maximum luminance of OLED devices using TPVPP as an emitter layer is 103835 cd/m² with a maximum current efficiency of 5.19 cd/A and a maximum power efficiency of 3.38 lm/W.     

Synthesis and optoelectronic properties of novel fluorene-bridged dinuclear cyclometalated iridium (III) complex with A–D–A framework in the single-emissive-layer WOLEDs

To explore the influence of push–pull chromophores on properties of emitter in organic light-emitting devices (OLEDs), an acceptor–donor–acceptor (A–D–A)-based dinuclear iridium (III) complex of (dfppy)₄Ir₂(dipic-FL) was synthesized via Suzuki coupling reaction, in which dfppy is 2-(2,4-difluorophenyl)pyridine and dipic-FL is 2,7-di(5-pyridyl-2-carboxyl)-9,9-dioctyl-9H-fluorene. An intense emission peak at about 480 nm resulting from the (dfppy)₂Ir(pic) chromophore and a weak long-wavelength emission band at 580–660 nm attributed to intramolecular charge transfer transition were exhibited for (dfppy)₄Ir₂(dipic-FL) in dichloromethane solution. But a remarkably hypsochromic photoluminescence profile with an intense characteristical emission peak at 422 nm was observed, which is attributed to the intraligand (IL) π–π^∗ excited states in its thin film. White emission with a maximum luminance of 1040 cd/m² and current efficiency of 1.2 cd/A was obtained in its single-emissive-layer (SEL) OLEDs with a configuration of ITO/PEDOT:PSS/(dfppy)₄Ir₂(dipic-FL) (10 wt%):TCTA/TPBi/LiF/Al. To our knowledge, this is one of the best examples in term of dinuclear iridium complex as single dopant in the high-performance white-emitting SEL-OLEDs.     

Unmodified small-molecule organic light-emitting diodes by blade coating

Blade coating with substrate heating and hot wind is demonstrated to be a general platform for multi-layer deposition of unmodified small-molecule organic semiconductors. Most unmodified small molecules, originally designed for vacuum evaporation, can be blade coated while the solubility is above 0.5 wt.%. High uniformity is achieved for scale over 5 cm. Orange devices by evaporation and blade coating are compared with 4,4′-bis(carbazol-9-yl)biphenyl (CBP) as the host, iridium(III) bis(4-(4-t-butylphenyl)thieno[3,2-c]pyridinato-N,C²′)acetylacetonate (PO-01-TB) as the emitter. The efficiency difference is within 10%. When 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) is used as the host, the current efficiencies are 40 cd/A for orange, 32 cd/A for green, and 20 cd/A for blue. The optimized organic light-emitting diodes (OLED) structure developed for vacuum deposition can therefore be exactly copied by the low cost blade coating method in solution.     

Soldering of solution-processed organic vertical transistor and light-emitting diode on separate glass substrates by tin micro-balls

The vertical organic space-charge-limited transistor made of P3HT and small-molecule phosphorescent organic light-emitting diode (OLED) are made on two separate glass substrate by blade coating, then soldered vertically together by tin balls with 40 μm diameter. The soldering is done by hot wind of 150 °C for 5 min Contact resistance is only 10 Ω. The vertical transistor is annealed at 150 °C for 5 min before soldering to enhance the output current up to 25 mA/cm² and give high thermal stability. Both OLED and the annealed vertical transistor are not affected by the soldering process. The vertical transistor has 1/4 of the OLED area and turns on the bottom-emission white OLED up to 300 cd/m² and orange OLED up to 600 cd/m². The entire operation is within 8 V. OLED and transistor array can therefore be made on separate glass substrates then soldered together to form the display.     

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.     

Great improvement of operation-lifetime for all-solution OLEDs with mixed hosts by blade coating

We investigated some effective device designs and fabrication methods for long operation-lifetime all-solution-processed Phosphorescent OLEDs (PhOLEDs) and fluorescent OLEDs with mixed-hosts system and thin Poly [(9, 9-dioctylfluorenyl-2, 7-diyl)-co-(4, 4′-(N-(4-sec-butylphenyl) diphenylamine)] (TFB). The all-solution-processed green PhOLEDs had high current efficiency (30.3 cd/A) and long operation-lifetime. The best half-lifetime of green PhOLEDs with thin HTL, MH-hosts EML and optimized deposition was 310 h at an initial luminance 1000 cd/m², 250 h at an initial luminance 500 cd/m² for green PhOLEDs with thin HTL, and MH-hosts EML, and the lifetime of triple layer PhOLEDs device was only 0.5 h for the same materials. The red PhOLEDs exhibited a high current efficiency (10.93 cd/A) and half-lifetime with 157.9 h at an initial luminance 500 cd/m². For the blue fluorescent OLEDs, the thin polymer TFB, mixed-hosts EML, double EMLs and optimization deposition yield a high current efficiency (5.68 cd/A) and long operation-lifetime with 117.7 h at an initial luminance 500 cd/m². Single host fluorescent device had half-lifetime of 73.5 h only at an initial luminance 100 cd/m². Finally, by doping red emitter Rubrene into stable blue device, we achieved soft yellow OLEDs with high efficiency (10.87 cd/A) and 8 fold improvement operation-lifetime (1200 h). We believe that such all-solution-processed OLEDs which showed greatly improved operational lifetimes would be suitable for the indoor supportive lighting with natural colors.     

Field-emission lighting tube with CNT film cathode

A novel cylinder-shaped structure field emission lighting tube (FELT) was fabricated. As an emission cathode the carbon nanotubes (CNTs) film were grown directly on the tungsten wire by pyrolysis of iron FePc, and were mounted by using elastic assembling technology. The model of electric field of this FELT was established. The electrons emitting characteristics of electronic field was measured. FELT can reach a static brightness of ∼1.6×10³ cd/m² and can be operated at 180 V. The turn-on field is 0.9 V/μm. When electronic field E=2.0 V/μm, the current density J=320 A/m².     

High performance top-emitting and transparent white organic light-emitting diodes based on Al/Cu/TcTa transparent electrodes for active matrix displays and lighting applications

It is challenging to obtain broadband emission covering as much of the visible light spectrum as possible in top-emitting white organic light-emitting diodes (TEWOLEDs) due to the well known microcavity effects. In this work, we achieved TEWOLED with three separate peak and negligible angular dependence by employing a high transmittance stack cathode Al (2 nm)/Cu (18)/TcTa (60 nm). The TEWOLED shows an efficiency of 25.6 cd/A, 20.1 Lm/W at 1000 cd/m², and low voltage of 4.2 V for 1222 cd/m². Synchronously, we achieved transparent white organic light-emitting diode (TWOLED) using this high transmittance stack cathode, the TWOLED exhibits similar spectrum and comparable luminance from both sides, and the maximum total efficiencies of the TWOLED are 28.6 cd/A, 24.9 Lm/W.     

Efficient fluorescent white organic light-emitting devices based on a ultrathin 5,6,11,12-tetraphenylnaphthacene layer

White organic light-emitting devices (OLEDs) were fabricated using a ultrathin layer 5,6,11,12-tetraphenylnaphthacene as the yellow light-emitting layer and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-ph) doped in 2-methyl-9,10-di(2-naphthyl)anthracene (MADN) matrix as the blue light-emitting layer. The thickness of rubrene ultrathin layer will seriously affect the device performance, and the device with 1 nm rubrene achieves the best performance, with the maximum luminance of 33,152 cd/m² at 11 V and the maximum current efficiency of 8.69 cd/A at 7 V.     

Synthesis and luminescence properties of lithium,zinc and scandium 1-(2-pyridyl)naphtholates

1-(2-Pyridyl)naphth-2-ol (pynH) was synthesized from 2-bromopyridine and 1-bromo-2-hydroxynaphthalene and structurally characterized. This ligand and the known 2-(2-pyridyl)phenol (ppH) ligand were reacted with LiN(SiMe₃)₂, ZnEt₂ and Sc[N(SiMe₃)₂]₃ to prepare the new luminescent complexes Li(pp), Li(pyn), Zn(pyn)₂, Sc(pp)₃, Sc(pyn)₃ and known Zn(pp)₂. Photoluminescent (PL) spectra of the compounds contained a single broad band with a maximum at 447–473 nm. The OLED devices with a configuration of ITO/TPD/complex/Bath/Yb gave blue–green emission. The emission spectra of these devices resembled the PL spectra; however, the bands of electroluminescence (EL) were shifted 20–40 nm to the long-wavelength side. A maximum current efficiency 15.3 cd/A and a power efficiency 8.12 lm/W at 100 cd/m² were measured for the device with the zinc luminophore Zn(pp)₂, whereas the highest luminance of 8300 cd/m² at 22.5 V was observed with the device with the scandium complex Sc(pp)₃. DFT calculations showed that the latter complex exhibited the lowest HOMO and the highest LUMO energy levels compared with the other investigated compounds. The calculated trends with respect to the influence of the metal and the ligand on the LUMO–HOMO gap agree well with the shifts of the electronic transitions observed in the PL spectra of the complexes.