Wenqing Zhu

Electron mobility of 4,7-diphyenyl-1,10-phenanthroline estimated by using space-charge-limited currents

The electron mobility of 4,7-diphyenyl-1,10-phenanthroline (BPhen) at various thicknesses (50–300nm) has been estimated by using space-charge-limited current measurements. The measured bulk mobility is in excellent agreement with results from time-of-flight method. It has been observed that the electron mobility of BPhen approaches its true value when the thickness is more than 150nm. The estimated electron mobility of BPhen at 300nm is found to be 3.4×10−4cm2∕Vs (at 0.3MV∕cm) with weak dependence on electric field. For thickness typical of organic light-emitting devices, the electron mobility of BPhen is also investigated.

Highly efficient pure blue electroluminescence from 1,4-bis[2-(3-N-ethylcarbazoryl)vinyl]benzene

An efficient blue organic light-emitting diode with 1,4-bis[2-(3-N-ethylcarbazoryl)vinyl]benzene doped into 4,4′-N,N′-dicarbazole-biphyenyl is reported. Maximum luminance and external quantum efficiency are 8500u2009cd/m2 and 2.6%. CIE-1931 coordinates are x=0.15, y=0.16. The device performance was further improved by introducing bis(2-methyl-8quinolinato)4-phenylphenolate aluminum to assist electron injection. The maximum luminance and quantum efficiency reached 11000u2009cd/m2 and 3.3%, respectively. Foster energy transfer and especially a carrier trapping mechanism are considered to dominate in the process of electroluminescence.

White-emitting organic diode with a doped blocking layer between hole- and electron-transporting layers

A new white-emitting organic diode was realized simply by inserting a doped hole-blocking layer between the hole-transporting layer (HTL) and the electron-transporting layer (ETL). The structure of this device is ITO/CuPc/NPB/blocking layer:rubrene/Alq/MgAg. Copper phthalocyanine (CuPc) was used as a buffer layer; N,N´-bis-(1-naphthyl)-N,N´-diphenyl-1.1´-biphenyl-4-4´-diamine (NPB) was the HTL; the tris(8-quinolinolato)aluminium complex (Alq) was the ETL; and the trimer of N-arylbenzimidazole (TPBi), 2-(4-biphenylyl-5-(4-tertbutylphenyl)-1,2,3-oxadiazole (PBD) or the 1,2,4-triazole derivative (TAZ) were used as the blocking layers, in which rubrene is doped. The emission spectrum of this device covers a wide range of the visible region and can be sensitively adjusted by the concentration of rubrene. The white emission with CIE (Commission International de lEclairage) coordinates x = 0.31, y = 0.32, a maximum luminance of 8635 cd m-2 and maximum luminous efficiency 1.39 lm w-1 (4.9 V) were obtained in the device with a concentration of 1.5% rubrene in TPBi.

White OLED with high stability and low driving voltage based on a novel buffer layer MoOx

White organic light emitting diodes (WOLEDs) with copper phthalocyanine (CuPc), 4,4,4-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA), tungsten oxide (WO3) and molybdenum oxide (MoOx) as buffer layers have been investigated. The MoOx based device shows superior performance with low driving voltage, high power efficiency and much longer lifetime than those with other buffer layers. For the Cell using MoOx as buffer layer and 4,7-diphenyl-1,10-phenanthroline (Bphen) as electron transporting layer (ETL), at the luminance of 1000 cd m−2, the driving voltage is 4.9 V, which is 4.2 V, 2 V and 0.7 V lower than that of the devices using CuPc (Cell-CuPc), m-MTDATA (Cell-m-MTDATA) and WO3 (Cell-WO3) as buffer layers, respectively. Its power efficiency is 7.67 Lm W−1, which is 2.37 times higher than that of Cell-CuPc and a little higher than that of Cell-m-MTDATA. The projected half-life under the initial luminance of 100 cd m−2 is 55 260 h, which is more than 4.6 times longer than that of Cell-m-MTDATA and Cell-CuPc. The superior performance of Cell-MoOx is attributed to its high hole injection ability and the stable interface between MoOx and organic material. The work function of MoOx has been measured by the contact potential difference method. The J–V curves of hole-only devices indicate that a small hole injection barrier between MoOx/N,-bis(naphthalene-1-y1)-N, N-bis(phenyl)-benzidine (NPB) leads to a strong hole injection, resulting in a low driving voltage and a high stability.

Organic thin-film field-effect transistors with MoO3/Al electrode and OTS/SiO2 bilayer gate insulator

An organic thin-film transistor (OTFTs) having OTS/SiO2 bilayer gate insulator and MoO3/Al electrode configuration between gate insulator and source-drain (S-D) electrodes has been investigated. Thermally grown SiO2 layer is used as the OTFT gate dielectric and copper phthalocyanine (CuPc) for an active layer. We have found that using silane coupling agents, octadecyltrichlorosilane (OTS) on SiO2, surface energy of SiO2 gate dielectric is reduced; consequently, the device performance has been improved significantly. This OTS/SiO2 bilayer gate insulator configuration increases the field-effect mobility, reduces the threshold voltage and improves the on/off ratios simultaneously. The device with MoO3/Al electrode has similar source-drain current (IDS) compared to the device with Au electrode at same gate voltage. Our results indicate that using double-layer of insulator and modified electrode is an effective way to improve OTFT performance.

Highly power efficient organic light-emitting diodes based on p-doped and novel n-doped carrier transport layers

The power efficiency of organic light-emitting diodes was significantly improved by introducing a novel n-doping (47- diphyenyl-1, 10-phenanthroline: 33 wt% 8-hydroxy-quinolinato lithium) layer as an electron transport layer and a p-doping layer composed of 4, 4, 4-tris (3-methylphenylphenylamono) triphenylamine (m-MTDATA) and tetrafluro-tetracyano-quinodimethane (F4-TCNQ) as a hole transport layer. Hole-only and electron-only devices were demonstrated to observe an improvement in the conductivity of the transport layers. With this strategy, we demonstrated that the power efficiency was enhanced by ~100%, luminous efficiency was enhanced by ~54% while driving voltage was reduced by 32% as compared with the control device. We obtained a power efficiency of 4.44 Lm W−1, which is the best value so far reported for tris (8- hydroxyquinolinato) aluminium (Alq3)-based emitters. This improvement was ascribed to the improved conductivity of the transport layers and to the better charge balance in the emission zone.

Highly efficient and stable white organic light emitting diode with triply doped structure

Abstract A triply doped white organic light emitting diode with red and blue dyes in the light emitting layer and a green dye in another layer is proposed. The device structure was CuPc(12xa0nm)/NPB(40xa0nm)/ADN:DCJTB(0.2%):TBPe(1%)(50xa0nm)/Alq:C545(0.5%)(12xa0nm)/LiF(4xa0nm)/Al. Here copper phthalocyanine (CuPc) is a buffer layer, N,N′-di(naphthalene-1-y1)-N,N′-dipheyl-benzidine (NPB) is a hole transporting layer, 9,10-di-(2-naphthyl) anthracene (ADN) is blue emitting layer, tris (8-quinolinolato)aluminium complex (Alq) is an electron transporting layer, 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidy1-9-enyl)-4H- pyran (DCJTB), 2,5,8,11-tetra-butylperylene (TBPe), Coumarin6 and deveriative (C545) are red, blue and green dyes, respectively. This device shows a luminance of 21200xa0cd/m2 at driving current of 400xa0mA/cm2 and 1026xa0cd/m2 at 20xa0mA/cm2. Its efficiency is 6xa0cd/A and 3.11xa0Lm/W. It also shows a higher operating stability: the half lifetime is 22,245xa0h at an initial luminance of 100xa0cd/m2, while the driving voltage increased only 0.3xa0V.

Decay mechanisms of a blue organic light emitting diode

A blue organic light-emitting diode employing perylene as light emitting dopant and 9,10-bis(3’5’-diaryl)phenyl anthracene (DPA) as host has been studied for its decay mechanisms. The device structure is ITO(indium tin oxide)∕CuPc(copper phthalocyanine)∕NPD(α-naphthylphenylbiphenyl diamine)∕DPA:perylene∕Alq3 (8-hydroxy-quinoline aluminum)∕MgAg. In this device, CuPc and NPD are used as hole injection and transporting layers, DPA as a blue host, perylene as a blue emitting dopant, Alq3 as an electron transport layer, MgAg as cathode, respectively. A luminance of 4359cd∕m2 at 15V and a current efficiency of 3cd∕A at 5V have been achieved. The breakdown of the interfaces in the device is found to be one of the factors for the decay and the decomposition of the light emitter is not significantly studied by current–voltage–luminance, photoluminescence, and electroluminescence measurements. The lifetime is not intrinsic for this type of device.

Electron injection and transport mechanism in organic devices based on electron transport materials

Electron injection and transport in organic devices based on electron transport (ET) materials, such as 4,7- diphyenyl-1,10-phenanthroline (Bathophenanthroline BPhen), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (Bathocuproine BCP) and bipyridyl oxadiazole compound 1,3-bis [2-(2,2-bipyridin-6-yl)-1,3,4-oxadiazol-5-yl]benzene (Bpy-OXD), have been reported. The devices are composed of ITO/ET materials (BPhen, BCP Bpy-OXD)/cathodes, where cathodes = Au, Al and Ca. Current–voltage characteristics of each ET material are performed as a function of cathodes. We have found that Ca and Al exhibit quite different J–V characteristics compared with the gold (Au) cathode. The current is more than one order of magnitude higher for the Al cathode and more than three orders of magnitude higher for Ca compared with that of the Au cathode at ~8u2009V for all ET materials. This is because of the relatively low energy barrier at the organic/metal interface for Ca and Al cathodes. Electron-only devices with the Au cathode show that the electron transfer limitation is located at the organic/cathode interface and the Fowler–Nordheim mechanism is qualitatively consistent with experimental data at high voltages. With Ca and Al cathodes, electron conduction is preponderant and is bulk limited. A power law dependence J ~ Vm with m > 2 is consistent with the model of trap-charge limited conduction. The total electron trap density is estimated to be ~5 × 1018u2009cm−3. The critical voltage (Vc) is found to be ~45u2009V and is almost independent of the materials.

Spectral studies of white organic light-emitting devices based on multi-emitting layers

Abstract White organic light emitting devices (WOLEDs) with an RBG stacked multilayer structure were demonstrated. In RGB stacked OLEDs, blue emitting, 2-t-butyl-9,10-di-(2-naphthyl)anthracene (TBADN) doped with p-bis(p-N,N-diphenyl-amono-styryl)benzene (DSA-Ph), green emitting, tris-(8-hydroxyquinoline)aluminum (Alq) doped with C545, and red emitting, tris-[8-hydroxyquinoline]aluminum (Alq) doped with 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)- 4H-pyran (DCJTB), were used. By adjusting the order and thickness of emitting layer in RBG structure, we got a white OLED with current efficiency of 5.60xa0cd/A and Commission Internationale De L’Eclairage (CIE) coordinates of (0.34, 0.34) at 200xa0mA/cm2. Its maximum luminance was 20,700xa0cd/m2 at current density of 400xa0mA/cm2. The results have been explained on the basis of the theory of excitons generation and diffusion. According to the theory of excitons generation and diffusion, an equation has been set up which relates EL spectra to the thickness of every layer and to the exciton diffusion length.