Zhi-Lin Zhang

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.

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.

Study of blue organic light emitting diode by inserting a red dye ultra thin layer at the emitting layer

Experiments of inserting an ultra thin layer of DCJTB at three different levels in a blue emitter layer of a blue device, ITO/CuPc/NPB/TBADN/Alq/LiF/Al, were carried out. The spectra of these devices indicate that the recombination zone is chiefly located near the NPB/TBADN interface, implying that TBADN is capable of electron transport. The inserted DCJTB at TBADN/Alq interface has no contribution to emission, leaving only the blue emission from TBADN, even at high voltage. However, it improved the stability of the device; the half lifetime was prolonged twice more and the rise of applied voltage during ageing was suppressed from 3 V to 0.4 V.

Effect of a SiNx insulator on device properties of pentacene-TFTs with a low-cost copper source/drain electrode

We fabricated different SiNx insulators at a 150–350 °C deposition temperature by plasma-enhanced chemical vapor deposition (PECVD) and investigated the effect of SiNx insulator on the device properties of pentacene-TFTs with a low-cost copper source/drain electrode. A pentacene-TFT with a SiNx insulator at a deposition temperature of 300 °C exhibited the highest saturation mobility of 0.29 cm2 (V s)−1, the lowest threshold voltage of −8.9 V and an on/off current ratio of 5.3 × 106. The best performance of the device with a SiNx insulator at a 300 °C deposition temperature can be attributed to the electrical properties and a relatively large water contact angle of insulator, smooth insulator surface and better crystallinity of the pentacene active layer. The present organic thin-film transistors (OTFTs) with the PECVD SiNx insulator and the low-cost Cu electrode could be a significant step toward the commercialization of OTFT technology.

Effect of hafnium doping on density of states in dual-target magnetron co-sputtering HfZnSnO thin film transistors

This study investigates the effect of hafnium doping on the density of states (DOSs) in HfZnSnO thin film transistors fabricated by dual-target magnetron co-sputtering system. The DOSs is extracted by temperature-dependent field-effect measurements, and they decrease from 1.1 × 1017 to 4.6 × 1016 eV/cm3 with increasing the hafnium concentrations. The behavior of DOSs for the increasing hafnium concentration HfZnSnO thin film transistors can be confirmed by both the reduction of ΔVT under bias stress and the trapping charges calculated by capacitance voltage measurements. It suggests that the reduction in DOSs due to the hafnium doping is closely related with the bias stability and thermal stability.

Nitrogen anion doping as a strategy to suppress negative gate-bias illumination instability of ZnSnO thin film transistor

This work presents a strategy of nitrogen anion doping to suppress negative gate-bias illumination instability. The electrical performance and negative gate-bias illumination stability of the ZnSnON thin film transistors (TFTs) are investigated. Compared with ZnSnO-TFT, ZnSnON-TFT has a 53% decrease in the threshold voltage shift under negative bias illumination stress and electrical performance also progresses obviously. The stability improvement of ZnSnON-TFT is attributed to the reduction in ionized oxygen vacancy defects and the photodesorption of oxygen-related molecules. It suggests that anion doping can provide an effective solution to the adverse tradeoff between field effect mobility and negative bias illumination stability.

Temperature stress on a thin film transistor with a novel BaZnSnO semiconductor using a solution process

We have fabricated novel BaZnSnO-TFT using a solution process and investigated the electrical performance and temperature stability. BaZnSnO-TFT shows an improved field-effect mobility of 3.2 cm2 V−1 s−1, a subthreshold swing of 0.61 V per decade and an on/off current ratio of 2 × 107 compared to those of ZnSnO-TFT. Density of state distribution of BaZnSnO and ZnSnO semiconductor has been extracted from electrical measurements. BaZnSnO-TFT shows an improved electrical performance and temperature stability due to smaller oxygen vacancies, less bulk trap density and interface state density.