Xue-Yin Jiang

High performance ZnO-thin-film transistor with Ta2O5 dielectrics fabricated at room temperature

The authors report on the fabrication of low-driven-voltage and high mobility ZnO thin-film transistor with sputtering Ta2O5 film as the dielectric. The device shows a field effect mobility of 60.4 cm2/V s, a threshold voltage of 1.1 V, an on/off ratio of 1.22×107, and a subthreshold swing of 0.23 V/decade. The high mobility partially resulted from the fringing-electric-field effect due to the undefined active layer. Therefore, considering our device geometry, the actual mobility is about 40.5 cm2/V s. We contribute the high performance to the proper dielectric thickness, smooth insulator surface, and relatively low trap state densities in the insulator/channel interface.

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.

C-9 Fluorenyl Substituted Anthracenes: A Promising New Family of Blue Luminescent Materials

Syntheses, optical, and electrochemical properties of novel C-9 fluorenyl substituted anthracenes linked by a tetrahedral sp(3)-hybridized carbon atom are reported for blue light emitting materials. Remarkably, an unoptimized organic light-emitting diode based on 1-fold fluorene-functionalized anthracene 3 exhibits a radiance of 4100 cd/m(2) at 12 V and a maximum EL efficiency of 1.36 cd/A with color purity CIE x, y (0.157, 0.082), which is very close to the National Television System Committee standard blue.

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 8500 cd/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 11000 cd/m2 and 3.3%, respectively. Foster energy transfer and especially a carrier trapping mechanism are considered to dominate in the process of electroluminescence.

MoOx modified Ag anode for top-emitting organic light-emitting devices

Efficient top-emitting organic light-emitting devices (TOLEDs) using a thin MoOx layer modified Ag as the effective hole-injection anode are demonstrated. With tris-(8-hydroxy quinoline)aluminum as emitting layer and trilayer LiF∕Al∕Ag as semitransparent cathode, the Ag∕MoOx based TOLED shows a tune-on voltage of 2.67V and a maximum current efficiency of 7.27cd∕A, which are much better than those (3.92V, 6.12cd∕A) obtained from Ag∕Ag2O based TOLED and those (5.25V, 3.5cd∕A) obtained from the corresponding bottom-emitting organic light-emitting devices. Contact potential difference measurement shows that the work function of Ag∕MoOx is higher than those of Ag∕Ag2O and ozone-treated indium tin oxide, leading to a stronger hole injection. The good performance of Ag∕MoOx based TOLED is attributed to the efficient hole injection from the Ag∕MoOx anode as well as a microcavity effect.

Stability enhancement of organic electroluminescent diode through buffer layer or rubrene doping in hole-transporting layer

Abstract The stability of organic electroluminescent devices is significantly improved by inserting a buffer layer between ITO and the holetransporting layer or by doping rubrene in the hole layer. The durabilities of the improved devices increase by a factor of about 10. The reasons for the improvements are discussed based on tunnelling theory and the energy-level diagram of the device.

Elucidation of carrier injection and recombination characteristics with impedance and capacitance in organic light-emitting diodes and the frequency effects

Impedance (Z), phase (φ) and capacitance (C) versus bias voltage (V) characteristics are studied to clarify carrier injection and recombination characteristics in organic light-emitting diodes (OLEDs). The Z–V transition starts at a characteristic voltage (Vc), which is strongly frequency dependent, i.e. Vc shifts to a high voltage with increasing measuring frequency. The electron–hole recombination starts at a voltage above Vc revealed by the φ–V and C–V transitions, which correspond to a phase approaching 0, a sharp rise in current density and a decrease in capacitance. Hole injection starts at a low Vc and corresponds to charge carrier accumulation and a slight rise in capacitance. Cole–Cole impedance plots illustrate that the interfacial resistance corresponds to the impedance at ultrahigh frequencies and shows bias independence, while the impedance at low frequencies represents the sum of interfacial resistance and organic stacks, and exhibits considerable bias dependence.

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.

Colour tunability of blue top-emitting organic light-emitting devices with single-mode resonance and improved performance by using C60 capping layer and dual emission layer

p-di(p-N,N-diphenyl-amino-styryl)benzene (DSA-Ph)-based blue top-emitting organic light-emitting devices (TOLEDs) with low bias and single-mode resonant emission were fabricated. Microcavity optimization was demonstrated by altering the hole-transport-layer thickness. C60 was demonstrated to be an effective capping layer and improved the luminance by a factor of 1.6 in between the maximum and minimum transmittance of top contact. For sky-blue TOLED, the luminance was enhanced by 50% by using a dual emission layer composed of DSA-Ph doped 2-(t-butyl)-9,10-di(2-naphthyl)anthracene and DSA-Ph doped tris(8-hydroquinoline) aluminium (Alq3). The improvement was attributed to the energy transfer from Alq3 to DSA-Ph and DSA-Ph directly harvesting carriers in the emission layer of Alq3 : DSA-Ph.

Performance enhancement of organic thin-film transistors using WO3-modified drain/source electrodes

Pentacene-based organic thin film transistors (OTFTs) with WO3/Au as source/drain (S/D) electrodes were fabricated. In comparison with the pentacene-based OTFT having Au-only S/D electrodes, the performance of a device having WO3/Au S/D electrodes has been considerably improved. The saturation mobility increased from 0.39 to 2.8 cm2 (V s)?1, the threshold voltage reduced from ?21.3 to ?11.2 V, the on/off ratio shifted from 6.7 ? 105 to 1.5 ? 107 and the threshold swing varied from 3.75 to 1.82 V dec?1. The improvement was attributed to (i) the significant reduction of contact resistance by using the WO3 interlayer and (ii) the strong heat radiation during the thermal evaporation of WO3. Our results indicated that using WO3/Au as S/D electrodes is an effective approach to improving pentacene-based OTFTs performance.