Transparent organic light-emitting devices with LiF/Yb:Ag cathode

Highly transparent organic light-emitting devices (TOLEDs) based on LiF/Yb:Ag cathode are reported. The device with a structure of Indium tin oxide/N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biph-enyl-4, 4′-diamine (50 nm)/tris (8-hydroxyquinoline) aluminum (50 nm)/LiF(0.5 nm)/Yb:Ag (15 nm, volume ratio 1:1) shows about the same luminances and spectra from both sides. At a current density of 20 mA/cm², the luminance and electroluminescent efficiency of the top/bottom side are 165/167 cd/m² and 0.825/0.835 cd/A, respectively. We attribute the characteristics to the high transmittance and low reflectivity of Yb:Ag cathode. The results suggest that LiF/Yb:Ag electrode can be used as an effective and stable cathode in TOLEDs or top-emitting OLEDs.     

Top emission organic light-emitting devices with CsCl capping layer

We have developed top emission organic light-emitting devices using a CsCl capping layer on top of semitransparent Ca/Ag cathode. By using a CsCl capping layer, the transmittance of top electrode can be improved by 93%. While the electrical conduction characteristic of device is not influenced by the capping layer, the current efficiency increases with increasing the transmittance of Ca/Ag/CsCl cathode. For example, as the transmittance of top electrode increases from 55 to 91% by varying CsCl thickness, the current efficiency of green fluorescent top-emitting device increases from 8 to 18 cd/A.     

Low-cost and reliable thin film encapsulation for organic light emitting diodes using magnesium fluoride and zinc sulfide

We report highly efficient gas diffusion barriers for organic light emitting diodes (OLEDs) with an encapsulation structure composed of alternating magnesium fluoride (MgF₂) and zinc sulfide (ZnS) layers grown by vacuum thermal deposition. The half lifetime of yellow OLEDs under an initial luminance of 2000 cd/m² with rubrene as an emitter reached 245 h using three pairs of MgF₂/ZnS layers. The device lifetime was obviously improved using MgF₂ and ZnS as passivation layers before UV-cured epoxy seal without desiccant with the lifetime for the initial luminance dropping to 56% being over 500 h. This simple and inexpensive encapsulation method can potentially be applied to top-emitting OLEDs due to good light transmission characteristic of the passivation film.     

Electro-optical properties of poly(1-(difluorophenyl)-2-(4-alkylcyclohexyl phenyl)acetylene) organic light-emitting diodes

For organic light-emitting diodes (OLEDs) applications, we have investigated novel polymers, using substituted polyacetylenes (PA), poly(1-(fluorophenyl)-2-(alkylcyclohexylphenyl)acetylene) (PDPA-nF) (n = 1 or n = 2) which exhibit air stability, better solubility in common organic solvent and higher luminescence than polyacetylene. In this study, we have used poly[(1-(3,4-difluorophenyl)-2-(4-pentylcyclohexylphenyl)acetylene)] (PDPA-2F) as an emitter in OLEDs and their performance was determined by measuring the current-voltage-luminance characteristic. The devices have a maximum brightness of 827 candela (cd)/m² at 12 V and a maximum current efficiency of 0.78 cd/A at 9 V with a maximum luminescence at 536 nm. Influence of the metal electrode on the charge injection was studied using several cathode configurations (Ca, Al and Au) for the devices. Furthermore, the charge injection and transport processes were correlated to the presence of traps inside the polymer, determined by deep level transient spectroscopy (DLTS).     

Top-emitting organic light-emitting device with high efficiency and low voltage using a silver-silver microcavity

Top-emitting organic light-emitting devices (TOLEDs) with high efficiency and low driving voltage using silver-silver microcavity structure were demonstrated. With tris(8-hydroquinoline) aluminum as emission layer, the Ag-Ag based TOLED showed a maximum luminous efficiency of 9.21 cd/A which was much higher than conventional Ag-Al/Ag based TOLED (4.21 cd/A) and corresponding bottom-emitting OLED (3.77 cd/A). The Ag-Ag based TOLED also exhibited low driving voltage and thereby an enhanced power efficiency of 4.25 lm/W (at 40 mA/cm²). The excellent performance of Ag-Ag based TOLED was attributed to the significant microcavity effect and efficient carrier injection from Ag electrode.     

Solution-processed small molecule thin films and their light-emitting devices

Thin films of N,N′-bis-(3-Naphthyl)-N,N′-biphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB), tris-(8-hydroxyquinoline)-aluminum (Alq₃) and their blends prepared by spin-coating process were investigated. Experimental results revealed that the NPB films prepared by spin-coating process have smoother surface than that of Alq₃, which was attributed to their different molecular structures. Organic light-emitting devices (OLEDs) with emitting layer prepared by spin-coating the blends of NPB and Alq₃ exhibited a maximum luminance and a current efficiency over 10,000 cd/m² and 3.8 cd/A respectively, and when 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-[l]benzopyrano[6,7,8-ij]quinolizin-11-one was doped in, a current efficiency of 8 cd/A can be obtained. Comparative device performance to the vapor-deposited OLEDs suggested that solution-process could be an alternative route for the fabrication of OLEDs based on Alq₃.     

Efficient fluorescence from 9,10-bis(m-tolylphenylamino)anthracene doped into a blue matrix in Si-based top-emitting organic light-emitting diode

We demonstrate an efficient Si-based top-emitting organic light-emitting diode using a fluorescent emitting system composed of a green dye of 9,10-bis(m-tolylphenylamino)anthracene (TPA) doped into a blue matrix of 9,9′,10,10′-tetraphenyl-2,2′-bianthracene (TPBA). The device shows green emission with a maximum luminous efficiency of 3.3 cd/A and a power efficiency of 2.3 lm/W. The maximum luminance reaches 1.3 × 10⁴ cd/m² at a driving voltage of 12 V. The excellent performances partially resulted from effective energy transfer in the emitting system of [TPBA: 2 wt.% TPA]. In addition, the emission spectra exhibit negligible variation with increasing viewing angles, indicating a weak microcavity effect because of low reflectance of Si anode.     

Semitransparent passive matrix organic light-emitting displays

There has been an increased interest in developing top emission organic light-emitting diodes (OLEDs) that are able to emit light from both sides of the OLED display. One important application of the top emission device structure is to achieve monolithic integration of a top-emitting OLED on a polycrystalline or amorphous silicon thin film transistors used in active matrix displays. A high performance dual-sided top-emitting polymer OLED developed in this work exhibited a total luminous efficiency of ∼5.0 cd/A at 4.0 V, which is comparable to that observed for a control device having bottom emission structure. A laser ablation technology was developed to define the pixels. The cathode separation was achieved without using the conventional reverse trapezoid type separators that are normally used for pixellated OLED displays. A prototype of semitransparent polymer light-emitting passive matrix display has a matrix of 100 × 32 with a display area of 32.25 mm by 11.15 mm.     

Fabrication and optical modulation of top-emitting yellow light polymer light-emitting diodes on the FR4 board

Top-emitting yellow light polymer light-emitting diodes (PLEDs) were fabricated on the FR4 board for future application in optical interconnects. A 100 μm thick glass plate with sputtered Ag films on the back side for light reflection was bonded to the board for smooth surface. The PLED had a structure of indium tin oxide /poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate)/phenyl substituted polypa raphenylene-vinylene/Ba/Ag and reached the brightness of 4528 cd/m² with a corresponding current efficiency of 7 cd/A. A small AC signal imposed onto a DC bias was employed to characterize the electroluminescence delay time and hole mobility in the PDY-132 super yellow films.     

Optical and electroluminescent properties of samarium complex-based organic light-emitting diodes

The optical and electroluminescent (EL) properties of newly synthesized tris(hexafluoroacethylacetonato)(phenanthroline)samarium(III)mono-methanol [Sm(hfa)₃(phen)₂MeOH]-based organic light-emitting diodes (OLEDs) were investigated. The as-prepared photoluminescence (PL) spectrum of Sm(hfa)₃(phen)₂MeOH-doped PMMA film exhibits the peaks at the wavelength around 564, 598, 645 and 710 nm which correspond to the ⁴G_5/2 → ⁷H5_/2, ⁴G_7/2 → ⁷H_7/2, ⁴G_5/2 → ⁷H_9/2 and ⁴G_5/2 → ⁷H_11/2 transitions of the Sm³⁺ ion, respectively. The best organic light-emitting device performance is obtained for a device using 8 wt.% Sm(hfa)₃(phen)₂MeOH and 40 wt.% 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4,-oxadiazole doped in poly(N-vinylcarbazole) as a emitting layer. The optimal device exhibits maximum luminance of 135 candela (cd)/m² at the current density of 0.16 A/cm², with current efficiency of 0.1 cd/A at the current density of 0.08 A/cm². The EL spectrum from optimal device has the Commission Internationale De L'Eclairage (CIE) coordinate of (0.60, 0.36).     

Synthesis and electroluminescent properties of blue emitting materials based on arylamine-substituted diphenylvinylbiphenyl derivatives for organic light-emitting diodes

This paper reports the synthesis and electroluminescent properties of a series of blue emitting materials with arylamine and diphenylvinylbiphenyl groups for applications to efficient blue organic light-emitting diodes (OLEDs). All devices exhibited blue electroluminescence with electroluminescent properties that were quite sensitive to the structural features of the dopants in the emitting layers. In particular, the device using dopant 4 exhibited sky-blue emission with a maximum luminance, luminance efficiency, power efficiency, external quantum efficiency and CIE coordinates of 39,000 cd/m², 12.3 cd/A, 7.45 lm/W, 7.71% at 20 mA/cm² and (x = 0.17, y = 0.31) at 8 V, respectively. In addition, a blue OLED using dopant 2 with CIE coordinates (x = 0.16, y = 0.18) at 8 V exhibited a luminous efficiency, power efficiency and external quantum efficiency of 4.39 cd/A, 2.46 lm/W and 2.97% at 20 mA/cm², respectively.     

A novel fluorinated europium ternary complex for highly efficient pure red electroluminescence

A novel fluorine-functionalized europium(III) ternary complex, i.e., Eu(DBM)₃(BFPP), in which DBM was dibenzoylmethane and BFPP 2, 3-bis(4-fluorophenyl)pyrazino[2,3-f] [1,10]phenanthroline, was designed, synthesized and characterized. The complex emits the characteristic red emission of trivalent europium ion due to the ⁵D₀ → ⁷F_j (j = 0–4) transitions under photo excitation with good luminescent quantum efficiency (0.55) and exhibits high thermal stability (387 °C). The organic light-emitting diodes (OLEDs) employing the complex as a dopant emitter with the structures of ITO/TPD (40 nm)/CBP:Eu-complex (30 nm)/Bphen (10 nm)/Alq₃ (20 nm)/LiF (1 nm)/Al (150 nm) were successfully fabricated. The 4 wt.% Eu(DBM)₃(BFPP) doped device exhibited the maximum luminance of 1766 cd/m² and a peak current efficiency of 4.6 cd/A, corresponding to the high external quantum efficiency of 2.27%.     

Efficient orange-red organic light-emitting diodes using 9,10-bis[4-(di-4-tert-butylphenylamino)styryl] anthracene as a fluorescent orange-red emitter

The authors have demonstrated efficient orange-red organic lighting diodes (OLEDs) using a new fluorescent orange-red material, 9,10-bis[4-(di-4-tert-buthylphenylamino)styryl]anthracene (ATBTPA). The optimized orange-red OLED using ATBTPA achieved a maximum external quantum efficiency (EQE) of 3.78%, a current efficiency (CE) of 9.47 cd/A, and Commision Internationale de L'Eclairage (CIE_x,y) coordinates of (0.51, and 0.48) at 1.61 mA/cm² in comparison with orange-red OLED using (5,6,11,12)-tetraphenyl-naphthacene (rubrene) which showed a maximum EQE of 1.65%, CE of 4.94 cd/A, and CIE_x,y coordinates of (0.50, and 0.49) at 0.61 mA/cm², respectively. The optimized orange-red device using ATBTPA showed higher efficiency of two times the orange-red device using rubrene due to the efficient Förster singlet energy transfer from MADN to ATBTPA in comparison with that from MADN to rubrene. This study clearly suggests that ATBTPA is an excellent fluorescent orange-red material for efficient WOLEDs.     

Enhancement of the luminance efficiency for top-emitting DB-PPV polymer light-emitting diodes with Ca/Ag cathode

The enhancement of the luminance efficiency of the top-emitting 2,3-dibutoxy-1,4-polyphenylene vinylene (DB-PPV) polymer light-emitting diodes (PLEDs) with Ca/Ag cathode has been investigated in this work. The luminance efficiency of 2.9 cd A⁻¹ (at 10 mA) for the thickness of Ca/Ag cathode of 10/15 nm is achieved by our devices. In this paper, partially oxidized CaO_x film is proposed to have the capability of hole-blocking as well as the buffer layer on condition of the suitable Ag thickness of Ca/Ag cathode. The tuning ability of colors by simply adjusting the thickness of the DB-PPV emitting layer is available for the devices.     

Flexible top-emitting organic light-emitting diodes using semi-transparent multi-metal layers

In this paper, the improved device performance of top-emitting organic light-emitting diodes (TEOLEDs) with a thin multi-metal layer stack of nickel/silver/nickel (Ni/Ag/Ni) and aluminum/silver/aluminum (Al/Ag/Al) that were used as the anode and cathode on a flexible substrate is discussed. In particular, Indium-Tin-Oxide (ITO) as an anode electrode has been used recently even though it has some problems for flexible devices. Therefore we suggested that a thin multi-metal layer electrode as a new anode is fabricated instead of ITO anode. It was verified that the ITO-free TEOLEDs showed an enhanced probability of the recombination of the electrons and holes through an improved electron/hole charge balance. We also analyzed the optical and electrical characteristics using the current density, luminance, luminance efficiency, external quantum efficiency (EQE), CIE x, y coordinates, and EL spectra of flexible TEOLED devices were characterized. ITO-free, flexible, green-emitting OLEDs with a low cost and a simple process were demonstrated.     

Synthesis and electroluminescent properties of blue-emitting t-butylated bis(diarylaminoaryl)anthracenes for OLEDs

A new series of blue fluorescent emitters based on t-butylated bis(diarylaminoaryl) anthracenes were synthesized and their electroluminescent properties investigated. Into these blue materials, t-butyl groups were introduced to both prevent molecular aggregation between the blue emitters through steric hindrance and reduce self-quenching. As such, this would contribute to overall improvement in OLED efficiency. To explore the electroluminescent properties of these materials, multilayered OLEDs were fabricated into a device structure of: ITO/NPB(50 nm)/blue emitters doped in ADN(30 nm)/Alq₃(20 nm)/Liq(2 nm)/Al(100 nm). All devices showed efficient blue emissions. In particular, one device exhibited highly efficient sky blue emissions with a maximum luminance of 11,060 cd/m² at 12.0 V and respective luminous and power efficiencies of 6.59 cd/A and 2.58 lm/W at 20 mA/cm². The peak wavelength of the electroluminescence was 468 nm with CIEx,y coordinates of (0.159, 0.198) at 12.0 V. In addition, a deep blue device with CIEx,y coordinates of (0.159, 0.151) at 12.0 V showed a luminous efficiency of 4.2 cd/A and power efficiency of 1.66 lm/W at 20 mA/cm².     

Synthesis and blue electroluminescent properties of zinc (II) [2-(2-hydroxyphenyl)benzoxazole]

This study reports on the properties of organic light-emitting diodes (OLEDs) with zinc (II) [2-(2-hydroxyphenyl)benzoxazole] as a hole-blocking layer. OLEDs devices are prepared in a conventional OLEDs structure (i.e., anode/HTL/EL/HBL/cathode and anode/HTL/HBL/EL/cathode). The luminescence efficiencies and the turn-on voltage are significantly affected by the existence of the hole-blocking layer. This is attributed to an excellent hole-blocking property, which is in turn due to the high HOMO energy level (6.5 eV). The device showed luminous efficiency 2.46 lm/W at 5 V. The maximum luminance of about 10,000 cd/m² is obtained, and the turn-on voltage (2.5 V) is affected by the existence of the hole-blocking layer.     

Highly efficient orange–red electroluminescence from a single layer MEH-PPV-POSS:CdS0.75Se0.25 hybrid PLED

In this study, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] end capped with polyhedral oligomeric silsesquioxanes (MEH-PPV-POSS): cadmium sulfide selenide quantum dots (CdS_0.75Se_0.25 QDs) nanocomposites based OLEDs were fabricated. By the addition of CdS_0.75Se_0.25 QDs into the polymer active layer, a considerable enhancement was observed in terms of hole and electron injection in devices. Additionally, the presence of QDs reduced the interchain interaction of polymer that resulted in narrower electroluminescence (EL) spectrum. The device structure of ITO/PEDOT: PSS/MEH-PPV-POSS: 25 wt% CdS_0.75Se_0.25/Ca (40 nm)/Al demonstrated the best performance with a brightness of 8672 cd/m² at 10 V, current efficiency of 2.5 cd/A at 8 V, and an EQE of 0.55% at 150 mA/cm².     

Surface potential behaviors of UV treated of Ag anode for high-performance T-OLED by nanotribology

A high reflective bottom anode is essential for high-performance top-emitting organic light emitting devices (OLEDs). Ag has high reflectivity for visible light but its electronic properties are not ideal for anodes of OLEDs. Thus, we investigated the UV treatment effect of Ag film with a thin silver oxide layer at the surface for application of top-emitting OLEDs. In this study, we measured the change of potential difference between Ag film and ITO (indium tin oxide) films, according to UV treatment time and the mechanical properties by KPFM (Kelvin Probe Force Microscope) method and nano-indenter system, respectively. The KPFM and nano-indenter results show that the differences of surface potential and surface volume change according to UV treatment time.     

Photo/electroluminescence properties of an europium (III) complex doped in 4,4′-N,N′-dicarbazole-biphenyl matrix

The photoluminescence properties of one europium complex Eu(TFNB)₃Phen (TFNB = 4,4,4-trifluoro-1-(naphthyl)-1,3-butanedione, Phen = 1,10-phenanthroline) doped in a hole-transporting material CBP (4,4′-N,N′-dicarbazole-biphenyl) films were studied. A series of organic light-emitting devices (OLEDs) using Eu(TFNB)₃Phen as the emitter were fabricated with a multilayer structure of indium tin oxide, 250 Ω/square)/TPD (N,N′-diphenyl-N,N′-bis(3-methyllphenyl)-(1,1′-biphenyl)-4,4′-diamine, 50 nm)/Eu(TFNB)₃phen (x): CBP (4,4′-N,N′-dicarbazole-biphenyl, 45 nm)/BCP (2,9-dimethyl-4,7-diphenyl-l,10 phenanthroline, 20 nm)/AlQ (tris(8-hydroxy-quinoline) aluminium, 30 nm)/LiF (1 nm)/Al (100 nm), where x is the weight percentage of Eu(TFNB)₃phen doped in the CBP matrix (1-6%). A red emission at 612 nm with a half bandwidth of 3 nm, characteristic of Eu(III) ion, was observed with all devices. The device with a 3% dopant concentration shows the maximum luminance up to 1169 cd/m² (18 V) and the device with a 5% dopant concentration exhibits a current efficiency of 4.46 cd/A and power efficiency of 2.03 lm/W. The mechanism of the electroluminescence was also discussed.