Chang Seoul

Enhanced electron injection in organic light-emitting devices using Al/LiF electrodes

AbstractThe temperature dependence of the current-voltage-luminescence characteristics in organic light-emitting diodes (OLEDs) withvarying thickness of LiF layers are studied to understand the mechanism of the enhanced electron injection by inserting a thininsulating LiF layer at the tris(8-hydroxyquinoline) aluminum (Alq 3 )–Al interfaces. At room temperature, the LiF/Al cathodeenhances the electron injection and the quantum e†ciency (QE) of the electroluminescence (EL), implying that the LiF thin layerlowers the electron-injection barrier. However, at low temperatures it is observed that the injection-limited current dominates andthe barrier height for the electron injection in the device with LiF/Al appears to be similar with the Al only device. Thus, our resultssuggest that at low temperatures the insertion of LiF does not cause a significant band bending of Alq 3 or reduction of the Al workfunction. O 2001 Elsevier Science B.V. All rights reserved. PACS: 78.60.Fi; 85.60.JbKeywords: Organic light-emitting diodes; Electroluminescence; Al/LiF electrodes

Blue Electroluminescence and Dynamics of Charge Carrier Recombination in a Vacuum-Deposited poly(p-phenylene) Thin Film

Abstract We have studied the electroluminescence (EL) and the dynamics of charge carrier recombination in the vacuum-deposited poly(p-phenylene) (PPP) thin film that shows a bright blue EL emission at about 450 nm. The current–voltage–luminescence (I–V–L) characteristics are systematically studied in the temperature range between 12 and 325 K in the light-emitting devices of ITO/PPP/Al and the bilayer devices with hole transporting layers of aromatic diamine (TPD) and poly(9-vinylcarbazole) (PVK). The EL quantum efficiency (QE) of ITO/PPP/Al and ITO/PVK/PPP/Al is almost temperature-independent, indicating carrier injection via tunnelling mechanism. However, ITO/TPD/PPP/Al shows the increasing QE with decreasing temperature and the power-law I–V–L dependence, characteristics of a space-charge-limited current in a trap-filled insulator. From the time delay between the onset of the voltage pulse and that of the EL we can estimate the charge carrier mobility of μ≈4×10−6 cm2/V s in ITO/PPP/Al and μ≈5×10−5 cm2/V s in ITO/PVK/PPP/Al.

Control of resonant wavelength from organic light-emitting materials by use of a Fabry-Perot microcavity structure.

We report the fabrication of Fabry-Perot microcavity structures with the organic light-emitting material tris-(8-hydroxyquinoline) aluminum (Alq3) and derive their optical properties by measuring their photoluminescence (PL) and absorption. Silver and a TiO2-SiO2 multilayer were used as metal and dielectric reflectors, respectively, in a Fabry-Perot microcavity structure. Three types of microcavity were prepared: type A consisted of [air[Ag[Alq3]Ag]glass]; type B, of [air[dielectric[Alq3]dielectric]glass]; and type C, of [air[Ag[Alq2]dielectric]glass]. A bare Alq3 film of [air[Alq3]glass] had its PL peak near 514 nm, and its full width at half-maximum (FWHM) was 80 nm. The broad FWHM of a bare Alq3 film was reduced to 15-27.5, 7-10.5, and 16-16.6 nm for microcavity types A, B, and C, respectively. Also, we could control the PL peak of the microcavity structure by changing the spacer thickness, the amount of phase change on reflection, and the angle of incidence.

Preparation of light-emitting devices with poly(p-phenylenevinylene): effects of thermal elimination conditions and polymer layer thickness on device performance

Abstract The optimum thermal elimination conditions for poly( p -phenylene vinylene) light-emitting diodes were sought. The precursor films should be heated to 230°C and kept at this temperature for 5 min under a N 2 flow of 50 ml/min. By the heat treatment the degree of conversion to PPV was about 70%. A method determining the degree of conversion to PPV was proposed with IR measurement. About 8% of the THT resides in the fully converted PPV. A high external quantum efficiency of 0.0078% was achieved for the ITO/partially converted PPV/Al devices. The optimum thickness for the partially converted PPV layer as a electroluminescent was about 150 nm.

Effect of molar mass on electroluminescence of poly(p-phenylene)

Oligo(p-phenylene)s (OPP) with different degree of polymerisations (DP) of 6,8,10,and poly(p-phenylene)(PPP) of DP ¼ 27; 63 were synthesized. Thin-films were formed for four OPPs (DP ¼ 5; 6; 8; 10) by thermal vacuum deposition. Two PPP samples (DP ¼ 27; 63) are used in the powder form. All the samples were semicrystalline. Infra-red (IR), UV‐VIS absorption, photoluminescence (PL) and electroluminescence (EL) spectra were measured and discussed in terms of the molecular-mass. All the optical peaks shifted to lower wavelengths in proportion to the inverse of the DP. The quantum yield of the ITO/OPP/Al electroluminescent device (ELD) increased with the increase of DP up to DP ¼ 10, the highest molar mass p-phenylene compound which can be thermally evaporated without loss of original DP. # 2002 Elsevier Science B.V. All rights reserved.

Organic light-emitting devices based on vacuum-deposited poly(p-phenylene) thin film

Abstract Organic light-emitting device with poly( p -phenylene) (PPP) is prepared by vacuum deposition with various heating rates. The structure, morphology, photoluminescence (PL), electroluminescence (EL), and current ( I )–voltage ( V )–EL characteristics have been studied by IR spectroscopy, atomic force microscopy (AFM) and spectrofluorophotometer. The IR spectrum gave an evidence that the vacuum-deposited PPP (vd-PPP) chain involves 8–9 p -phenylene rings, which is known to be an effective conjugation length. The onset of the EL occurs at the electric field of about 7×10 7 V/m. I – V dependence fits very well with the Fowler–Nordheim (FN) tunneling formula. The results suggest that charge carrier injection is a tunneling process through an energy barrier of about 0.6–0.8 eV in ITO/PPP/Al devices.

Polymer light-emitting diodes based on poly(3-hexyl thiophene)

Poly(3-hexyl thiophene)(P3HT) and poly(3-dodecyl thiophene)(P3DT) were polymerized by oxidative coupling with ferric chloride. The P3HT light-emitting device emitted red light and it could be observable in the ordinary indoor light. The device had the turn-on electric field of about 3×107 V/m. The maximum electroluminescene (EL) intensity was obtained when the thickness of polymer layer was about 130 nm in ITO/P3HT/Al device. The maximum external quantum yield was 0.002%. The maximum luminance was 21 cd/m2. The EL intensity decreases with increasing the crystallinity of the polymer layer. By using the oriented poly(3-alkyl thiophene)(PAT) layer as an electroluminescent layer in the ITO/polymer/Al light-emitting devices, the polarized EL light emission was observed. The EL intensity ratio of parallel to perpendicular direction to the stretch direction for P3HT was about 1.40.

Polymer light-emitting devices based on plasma-polymerized benzene and plasma-polymerized naphthalene

Polymer light-emitting devices (PLED) were fabricated utilizing plasma-polymerized benzene (PPB) and plasma-polymerized naphthalene (PPN) as an electroluminescent (EL) emitting layer. The plasma polymerization is well suited for forming the transparent, sturdy thin film for EL polymer layers. For the ITO/PPB/Al and ITO/PPN/Al devices, the turn-on voltage of the device was at 12V and 6V, respectively. The luminance of the PPB device reached 6 cdm -2 at 10 V, whereas the PPN device reached 11 cd m -2 at 14 V. The external quantum efficency was 0.0035% for the PPB device and 0.0056% for the PPN device. The dense crosslinked structure formed by the plasma polymerization makes the EL device relatively stable during operation.

Optical properties of Fabry–Perot microcavity with organic light emitting materials

Abstract We investigated the optical properties of the tris(8-hydroxyquinoline)aluminum (Alq 3 ) organic film with Fabry–Perot microcavity by measuring photoluminescence (PL) and transmittance. We have simulated the phase change on reflection as a function of wavelength. The Fabry–Perot microcavity structures were designed according to the simulation results and the resonant wavelength corresponding to the maximum of PL spectrum of a bare Alq 3 film. These structures were fabricated in three types of microcavities, such as type A [air|metal|Alq 3 |metal|glass], type B [air|dielectric|Alq 3 |dielectric|glass], and type C [air|metal|Alq 3 |dielectric|glass]. A bare Alq 3 layer on glass, [air|Alq 3 |glass], showed a PL peak around 514 nm and its full width at half maximum (FWHM) was about 80 nm. The broad FWHM of the bare Alq 3 film was reduced to 15–27.5, 7–10.5 and 16–16.6 nm for three types by cavity effects. Also, the control of the resonant wavelength can be achieved by the spacer length as well as the phase change on reflection on mirror.

Effect of Silver Nanoparticles in the Hole Injection Layer on the Performance of Organic Light Emitting Diodes

The configuration of the OLED device is like this : ITO/PEDOT:PSS(Ag nanoparticle) /NPB/Alq 3 /Al The effect of Ag nanoparticles in the hole injection layer on the performance of Organic Light Emitting Diodes(OLED) was investigated. The hole mobility in the hole injection layer seems to be increased by Ag nanoparticles. The increased current in the hole injection layer increased the emission efficiency of the OLED. Initially, we try to incorporate silver nanoparticles(diameter:30nm) directly into the hole injection layer. However the Ag nanoparticles are aggregated into big particles, ( size 1∼2 μm). We can confirm this behavior by the optical microscope. Thus, PEDOT:PSS/Ag nanoparticles solutions were filtered before spin-coating and big particles were eliminated by this process. The several silver nanoparticles concentration were selected (0.05, 0.1, 0.25 wt % of the silver nanoparticles was mixed with PEDOT;PSS solutions). Theses solutions were spin-coated on the ITO layer with 3000 rpm for 60 secs. The spin coated Ag, PEDOT:PSS solutions were heat treated at 100 °C in 30 min. The big Ag agglomerate was not observed in the optical microscope. Then the thermal evaporation of the NPB and Alq 3 layer was conducted. The current-voltage relation was measured for the device and compared with the OLED device without Ag particle. The lumninance of the devices are also measured. The Ag nanoparticle increases the deriving current and thus increases the quantum efficiency of the OLED.