Hole-injection properties of annealed polythiophene films to replace PEDOT–PSS in multilayered OLED systems

Hole-injection properties of annealed poly(alkoxy- and alkylthiophene) films in OLEDs were studied. Among them, annealed poly(3,3′-dihexyloxy-2,2′-bithiophene) (aPHOBT) film exhibited good hole-injection properties and a triple-layered OLED with the structure ITO/aPHOBT/PVK/Alq3/Mg–Ag (device I) showed much higher performance than a double-layered device without the aPHOBT layer (device II, ITO/PVK/Alq3/Mg–Ag). Device I was slightly inferior to a device having a PEDOT–PSS layer as the hole injector (device III, ITO/PEDOT– PSS/PVK/Alq3/Mg–Ag) in the low-intermediate region of the applied voltage (6–11 V), but gave comparable luminance to III when the applied voltage exceeded 11 V.     

Cyclometalated iridium(III) complexes with 5-acetyl-2-phenypyridine derived ligands for red phosphorescent OLEDs

A series of new iridium complexes with 5-acetyl-2-phenylpyridine derivatives as ligands was developed. The complexes exhibited high EL performance when applied to OLEDs. These materials showed red emission with a peak at 575–636 nm. In particular, one of the devices in this study showed a maximum luminous efficiency, maximum power efficiency, external quantum efficiency and CIE coordinates of 29.0 cd/A, 6.13 lm/W, 8.86% at 20 mA/cm² and (0.57, 0.43) at 10 V, respectively. In addition, a deep red OLED with CIE coordinates of (0.67, 0.32) at 10 V exhibited a maximum luminous efficiency, maximum power efficiency and external quantum efficiency of 5.61 cd/A, 1.02 lm/W and 5.35% at 20 mA/cm², respectively.     

Poly(3-pentylmethoxythiophene)/Alq3 heterostructure light emitting diodes

The electroluminescence efficiency of an organic light emitting diode (OLED) based on poly(3-pentylmethoxythiophene) (P5OMe) is improved by two orders of magnitude by employing the heterostructure architecture realized with a thin layer of tris(8-hydroxy)quinoline aluminium (Alq3), which acts as both electron transport and emitting layer. The emission can be tuned from the red to the green spectral region by varying the applied voltage. An additional emission observed both in photoluminescence and electroluminescence at the polymer/Alq3 interface is tentatively assigned to exciplex emission.     

3,12-Dimethoxy-7,8-dicyano-[5]helicene as a novel emissive material for organic light-emitting diode

3,12-Dimethoxy-7,8-dicyano-[5]helicene (DDH) was introduced as a novel emissive material for organic light-emitting diode. It shown good thermal stability and no glass transition temperature was observed. The LUMO, HOMO and energy band gap (?3.3, ?5.9 and 2.6 eV) of this compound were determined using cyclic voltammetry technique. Fluorescence quantum yield of DDH in chloroform is 0.27. The turn-on voltage of OLEDs with a configuration of ITO/PEDOT:PSS/DDH/Ca/Al was not a function of DDH thickness in a range of 60–100 nm. The best OLED, in which DDH thickness was 100 nm, exhibited a turn-on voltage of 3.7 V with maximum brightness of 1587 cd/m² at 8.0 V and 281 mA/cm². The maximum current efficiency and power efficiency were 0.64 cd/A and 0.29 lm/W, respectively. The CIE coordinates of the OLED electroluminescence, however, appeared to depend on the applied voltage as they were (0.38,0.47) at 5.0 V and (0.51,0.44) at 8.0 V.     

Small molecule host system for solution-processed red phosphorescent OLEDs

We demonstrate high efficiency solution-processed red phosphorescent OLEDs with small molecule mixed host systems. 2-TNATA (4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine):TPBI (2,2′,2″-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole)) and m-MTDATA (4,4′,4″-tri-(N-3-methylphenyl-N-phenylamino)triphenylamine):TPBI host systems are reported as good soluble mixed host systems. A doping level of 3% bis(2-phenylquinoline)(acetylacetonate)iridium (Ir(phq)₂acac) dopant in the 2-TNATA:TPBI (1:1 ratio) mixed host produces the best quantum efficiency and driving voltage. This fabricated red phosphorescent OLED has a driving voltage of 5.2 V and maximum current and power efficiencies of 17.8 cd/A and 11.3 lm/W, respectively. Minimal electron or hole trapping in the phosphorescent dopant molecules and prevention of self quenching by the low doping technique appear to be the key reasons for good device performance.     

Optimization of emitting efficiency in organic LED cells using Ir complex

Multilayer organic light-emitting devices (OLED) with phosphorescent guest emitter, tris(2-phenylpyridine) iridium doped in a host 4,4′-N,N′-dicarbazol-biphenyl layer, were prepared. We optimized the cell structure paying special attention to the multiple reflection at the multilayers’ interfaces and succeeded in improving the luminance efficiency. Our method consists of adjusting optical distances between emission sites and dominant reflective surfaces, organic/cathode and ITO/glass interfaces. The device with the 8.7 wt.% guest emitter exhibited external quantum efficiency and power luminous efficiency of 14.9% and 43.4 lm/W, respectively at the luminance of 100 cd/m² driven at the voltage of 4.2 V. In addition, we investigated the emission site in the electrophosphorescent cells and recalculated the external quantum efficiency by the actual emission pattern.     

High efficiency and long lifetime in organic light-emitting diodes using bilayer electron injection structure

A lithium quinolate-based electron injection structure was developed to improve luminance efficiency and lifetime of organic light-emitting diodes (OLEDs). A electron injection material based on Li complex, 8-hydroxyquinolinato lithium (Liq), was introduced as an electron injection material for OLEDs and the efficiency and lifetime of OLEDs were investigated according to the structure of the electron injection layer. A bilayer electron injection structure, a mixed layer of tris(8-hydroxyquinoline) aluminium (Alq₃) and Liq and a Liq layer, showed high efficiency of 11.6 cd/A compared with 9.8 cd/A for lithium fluoride (LiF). In addition, the extrapolated lifetime of OLED with the bilayer electron injection structure was improved by 40% at 1000 cd/m².     

Pure red electrophosphorescent organic light-emitting diodes based on a new iridium complex

A new iridium complex was synthesized and demonstrated a saturated red light emission in organic light-emitting diodes (OLEDs). The maximum brightness of 2800 cd/m² and the external quantum efficiency of 5.5% were achieved in multilayer OLEDs. The peak wavelength of the emission was found to be at 677 nm with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.71, 0.27).     

Organic interlayer for high power efficiency in organic light-emitting diodes

The power efficiency of organic light-emitting diodes (OLEDs) was improved using hexaazatriphenylene-hexacarbonitrile (HAT) as the interlayer between hole injection layer and hole transport layer for efficient hole injection. The hole injection was enhanced and the driving voltage was lowered by the HAT interlayer. A high power efficiency of 73.3 lm/W was obtained from the green phosphorescent organic light-emitting diode with the HAT interlayer.     

Highly efficient energy transfer to a novel organic dye in OLED devices

The neutral 4,4-difluoro-8-(2,2′:6′,2″-terpyridine-4′-yl)-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (Boditerpy) molecule was synthesized and incorporated as dopant (<1%) in double-layer organic light emitting diodes (OLEDs) with the configuration ITO/α-NPD(60 nm)/Alq₃(60 nm):Boditerpy (0.4 nm)/LiF(0.02 nm)/Al(80 nm). This device exhibits green emission with a brightness of 545 cd/m² at 8 V and a maximum power efficiency of 0.9 lm/W. A full quantitative energy transfer process is indicated by a complete quenching of light emission from Alq₃ in photoluminescence. However, I–V characteristics indicate some losses during the charge transfer processes in OLED configuration     

Undoped yellow-emitting organic light-emitting diodes from a phenothiazine-based derivative

The single layer and multilayer undoped light-emitting devices were fabricated using a new soluble phenothiazine-based derivative, poly(3,7-N-octyl phenothiozinyl terephthalylidene) (POPTP). Through the optimization of device structures, the multilayer device has a maximum luminance of 1203 cd/m² at the bias voltage of 9.3 V, using 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a hole-blocking layer and tris-(8-hydroxyquinoline)aluminium (Alq₃) as a electron-injection/transporting layer. The Commision International de L’Eclairage (CIE) coordinates stabilized at (x, y) = (0.46, 0.53) at various bias voltages. Additionally, the dominant wavelength (λ_D) of around 575 nm and the color purity of approximately 100% indicated a pure yellow emission property. Therefore, POPTP is a stable candidate material with a pure yellow emission for the undoped organic light-emitting diodes (OLEDs).     

Fabrication and characterization of electro-phosphorescent organic light-emitting devices with a ferromagnetic cathode for observation of spin injection effect

Organic light-emitting devices (OLEDs) with a phosphorescent molecule Ir(ppy)₃ as the emitter and with an Fe cathode as the spin injector were fabricated for the observation of the large degree of circular polarization. The OLED structure was a glass-substrate/ITO/α-NPD/CBP doped with Ir(ppy)₃/BCP/Al-oxide/Fe/Al. The mixing ratio of CBP and Ir(ppy)₃ in the emissive layer was optimized for high luminescence efficiency. The OLEDs showed circular polarization, and the maximum degree of circular polarization of the OLEDs was 0.4% at the applied magnetic field of 1.6 T at room temperature. On the other hand, no circular polarization was observed from the OLEDs with an Al cathode.     

Synthesis, characterization, and electroluminescent properties of iridium complex containing 4-phenybenzoquinoline ligand

A phosphorescent iridium(III) complex Ir(PBQ)₂(acac) (PBQ: 4-phenylbenzoquinoline, acac: acetylacetone) was designed and synthesized, and the single crystal of this complex was obtained. This complex shows well optoelectronic properties. The organic light emitting devices (OLEDs) based on this complex were successfully fabricated with the device configuration of ITO/NPB or TCTA (40 nm)/Ir-complex: CBP (7%, 30 nm)/BCP (15 nm)/Alq (30 nm)/LiF (1 nm)/Al (100 nm). Using TCTA as the hole-transporting material, the device gives an extremely high external quantum efficiency of 14.6% at 5.0 V, a brightness of 61,693 cd/m² at 16.0 V, and a power efficiency of 37.0 lm/W at 3.5 V.     

High-performance organic light emitting diodes fabricated with a ruthenium oxide hole injection layer

We report enhanced hole injection using an RuO_x layer between indium tin oxide anodes and 4,4′-bis[N-(1-naphtyl)-N-phenyl-amino]biphenyl in organic light emitting diodes (OLEDs). The operation voltage of OLEDs at a current density of 100 mA/cm² decreased from 17 V to 14 V and the maximum luminance value increased from 120 cd/m² to 2500 cd/m² upon transformation of the Ru layer to RuO_x by surface treatment using O₂ plasma. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.4 eV as the Ru layer was transformed to RuOx. Thus, the hole injection energy barrier was lowered, reducing the turn-on voltage and increasing the quantum efficiency of the OLEDs.     

White organic light-emitting diodes via mixing exciplex and electroplex emissions

The authors report the fabrication of white organic light-emitting devices and discuss their electroluminescence (EL) properties. The device structure is ITO/TPD (50 nm)/BCP (8 nm)/Rubrene (0.5 nm)/BCP (10 nm)/Alq₃ (20 nm)/LiF (1 nm)/Al. In the EL spectra of this device, two new emissions peaking at 590 and 630 nm have been observed. These two emissions should be attributed to triplet exciplex and electroplex occurring at TPD/BCP interface. White emission was obtained based on this device under 12 V driving voltage, the Commission Internationale de l’Eclairage (CIE) coordinates arrives to (0.31, 0.33).     

Organic light-emitting diode based on a carbazole compound

A carbazole compound was synthesized by Knovenagel condensation and characterized by the measurements of ¹H NMR, IR and melting point. A multilayer organic light-emitting diode (OLED) using this compound as an active layer was fabricated by vacuum-deposition. This OLED showed a turn-on voltage of approximately 4.5 V and a maximum luminance of 910 cd/m². Additionally, the maximum luminous efficiency was found as 0.95 cd/A, at this point the device luminance was measured as 146 cd/m² at an operating voltage of 7 V. The coordinate value of CIE 1931 was calculated as (x, y) = (0.3843, 0.5345) from the electroluminescence (EL) spectrum, which suggested that the device can emit a yellow-green light.     

Improved electron injection in organic LED with lithium quinolate/aluminium cathode

Lithium quinolate (Liq), covered with aluminium, was used as an electron injection layer in a double layer organic light emitting diode consisting of NPD as the hole transport layer and Alq as the emitting layer resulting in lower turn on voltage and increased power efficiency. The driving voltage required to achieve a luminance of 100 cd/m² decreased from 5.8 V for the Ca/Al to 4.2 V when Liq/Al was used, improving device power efficiency from 2.3 to 4.1 lm/W. The performance tolerance to layer thickness of Liq devices is also better than that of the devices with lithium fluoride (LiF). Due to the highly insulating nature of LiF, it can only be used when deposited as an ultra-thin layer, while the Liq can be deposited into layers as thick as 5 nm without significantly affecting the EL properties. An Liq electron injecting layer has also been tried in combination with Ca, Mg and Ag cathodes. Our experiments support the assumption that free lithium is released from lithium quinolate, as in the case of lithium fluoride, when Liq is over coated with active metals such as Al.     

Effects of annealing of poly(3-hexylthiophene) film on the performance of double-layered EL devices of ITO/polymer/Alq3/Mg–Ag

Double layer devices with a structure of ITO/pHT/Alq3/Mg–Ag (ITO = indium tin oxide, pHT = regio-regular or random poly(3-hexylthiophene), Alq3 = tris(8-hydroxyquinoline)aluminium) were fabricated. The device with a random pHT film emitted a green-yellow light in all voltage region, while that having a regio-regular pHT film exhibited a color change from green to red by applying the bias voltage higher than 15 V. Annealing the pHT films prepared on ITO at 200 °C for 1 h in nitrogen, prior to vapor-deposition of the Alq3 layer, improved the device performance with lowering the onset bias voltage by 2–3 V. The EL colors and spectra were also affected by annealing. X-ray reflectivity measurements before and after annealing the pHT film on ITO indicated increased density of the pHT layer and structural changes in the pHT/ITO interface by annealing, which seems to be responsible for the improved EL device performance.     

Space-charge limited conduction with a field and temperature dependent mobility in Alq light-emitting devices

Electrical transport in organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinolato)aluminium (Alq) is investigated as a function of temperature and organic layer thickness. It is shown that the thickness dependence of the current provides a unique criterion to discriminate between (1) injection limited behavior, (2) trap-charge limited conduction with an exponential trap distribution and a field independent mobility, and (3) trap-free space charge limited conduction with a field and temperature dependent mobility. The observed thickness and temperature dependent current–voltage characteristics are found to be in excellent agreement with trap-free SCLC with a hopping type charge carrier mobility.     

Low voltage organic light emitting diode using p–i–n structure

Efficient n-type doping has been achieved by doping Liq in electron transport material Alq₃. Detailed investigation of current density–voltage characteristics of electron only devices with different doping concentrations of Liq in Alq₃ has been performed. An increase in current density by two orders of magnitude has been achieved with 33 wt% of Liq doped in Alq₃. Organic light emitting diode with p–i–n structure was fabricated using F₄-TCNQ doped α-NPD as hole transport layer, Ir(ppy)₃ doped CBP as emitting layer and 33 wt% Liq doped Alq₃ as electron transport layer. Comparison of OLEDs fabricated using undoped Alq₃ and 33 wt% Liq doped Alq₃ as electron transport layer shows reduction in turn on voltage from 5 to 2.5 V and enhancement of power efficiency from 5.8 to 10.6 lm/W at 5 V.