Gi-Wook Kang

White-light-emitting devices based on organic multilayer structure ☆

Abstract We report an efficient white-light-emitting devices using vacuum-deposited film of a hole-transporting layer of 4,4 ′ bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (α-NPD), a bluish-green emitting layer of bis[2-(2-hydroxyphenyl)benzothiazolate] zinc, a Zn(BTZ) 2 , Zn(BTZ) 2 doped with a red fluorescent dye of 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[i, j]quinolizin-8-yl)vinyl]-4H-pyran (DCM2), and an electron-transporting layer of tris(8-hydroxyquinoline) aluminum (Alq 3 ). The incomplete energy transfer from Zn(BTZ) 2 to DCM2 results in the emission of two complementary colors (blue and orange). Controlling the thickness of a Zn(BTZ) 2 layer which acts as a bluish-green emitter and a hole blocking layer and the concentration of DCM2 enables tailoring of the spectral output of the device to have a balanced white emission. A device with the structure of ITO/α-NPD (50 nm)/Zn(BTZ) 2 (4 nm)/Zn(BTZ) 2 :DCM2 (10 nm, 0.8%)/Alq 3 (30 nm)/LiF(0.5 nm)/Al shows an external quantum efficiency (QE) of about 0.5%, a luminous efficiency of 0.27 lm/W, and the CIE chromaticity coordinates of (0.36, 0.33) at luminance 100 cd/m 2 .

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.

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.

Photovoltaic Properties of Multilayer Heterojunction Organic Solar Cells

We report the photovoltaic properties of the multilayer organic solar cells fabricated with the vacuum deposition of copper phthalocyanine (Cu-PC) as a p-type layer and fullerene (C 60 ) as an n-type layer. The insertion of the bathocuproine (BCP) layer as an exciton-blocking layer and a thin (0.5 nm) LiF film between BCP and Al significantly enhances the efficiency of the cells. The power conversion efficiency is calculated as about 0.8% for the ITO/Cu-PC/C 60 /BCP/LiF/Al cell upon illumination of about 30 mW/cm 2 .

Efficient blue electroluminescence from 9,10-diphenylanthracene

We report efficient blue electroluminescence (EL) devices fabricated using 9,10-diphenylanthracene (DPA) as an active emitting material. Although DPA exhibits fluorescence in the blue spectral region with high quantum efficiency, its EL efficiency has been previously reported poor. By doping DPA into a bathocuproine (BCP) layer that acts as a hole-blocking layer, we can successfully fabricate very efficient blue EL devices with the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of (0.145, 0.195). The devices show a luminous efficiency of 2.9 cd/A at 200 cd/m2 and a maximum luminance of about 10344 cd/m2 at 16.6 V.

Electroluminescence of bilayer light-emitting diodes of poly(vinylcarbazole) and vacuum-deposited poly(p-phenylene) thin films

We present electroluminescence (EL) studies of bilayer organic light-emitting diodes formed from poly(N- vinylcarbazole) (PVK) and the vacuum-deposited poly(p- phenylene) (PPP). The ITO/PPP/Al devices show EL spectra with a blue emission peak around 450 nm, very similar with the photoluminescence (PL) spectra of PPP. However, the EL spectra of the ITO/PVK/PPP/Al devices show a broad shoulder around 550 nm, which is dependent upon the bias voltage, as well as the peaks at the shorter-wavelength region originating from the PPP layer. The shoulder around 550 nm becomes more pronounced at the low bias voltage while the EL peaks originating from PPP become stronger with increasing bias voltages. This behavior can be accounted by exciplex formation at the interface between PVK and PPP. The current- voltage-luminescence characteristics and the quantum efficiency of both devices are almost independent of temperature in the temperature range between 15 and 300 K, indicating a tunneling mechanism for charge carrier injection.