Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

During the last few years, transition metal oxides (TMO) such as molybdenum tri-oxide (MoO3), vanadium pent-oxide (V2O5) or tungsten tri-oxide (WO3) have been extensively studied because of their exceptional electronic properties for charge injection and extraction in organic electronic devices. These unique properties have led to the performance enhancement of several types of devices and to a variety of novel applications. TMOs have been used to realize efficient and long-term stable p-type doping of wide band gap organic materials, charge-generation junctions for stacked organic light emitting diodes (OLED), sputtering buffer layers for semi-transparent devices, and organic photovoltaic (OPV) cells with improved charge extraction, enhanced power conversion efficiency and substantially improved long term stability. Energetics in general play a key role in advancing device structure and performance in organic electronics; however, the literature provides a very inconsistent picture of the electronic structure of TMOs and the resulting interpretation of their role as functional constituents in organic electronics. With this review we intend to clarify some of the existing misconceptions. An overview of TMO-based device architectures ranging from transparent OLEDs to tandem OPV cells is also given. Various TMO film deposition methods are reviewed, addressing vacuum evaporation and recent approaches for solution-based processing. The specific properties of the resulting materials and their role as functional layers in organic devices are discussed.

Transition Metal Oxides for Organic Electronics: Energetics, Device Physics and Applications

In order to attain low drive voltage and high power conversion efficiency in tandem OLEDs, the mechanism for carrier generation/separation at the intermediate connecting parts, which are interposed between emission units, has been examined, and a new intermediate connector has been proposed. Voltage increase due to the provision of the new intermediate connector is minimized, and marked decrease in drive voltage of the tandem OLEDs has been attained.

60.2: Intermediate Connector with Suppressed Voltage Loss for White Tandem OLEDs

WOLEDs offer new design opportunities in practical solid-state lighting and could play a significant role in reducing global energy consumption. Obtaining white light from organic LEDs is a considerable challenge. Alongside the development of new materials with improved color stability and balanced charge transport properties, major issues involve the fabrication of large-area devices and the development of low-cost manufacturing technology. This Review will describe the types of materials (small molecules and polymers) that have been used to fabricate WOLEDs. A range of device architectures are presented and appraised.

Recent advances in white organic light-emitting materials and devices (WOLEDs).

Recently, organic fluorescent molecules harnessing the excited-state intramolecular proton transfer (ESIPT) process are drawing great attention due to their unique photophysical properties which facilitate novel optoelectronic applications. After a brief introduction to the ESIPT process and related photo-physical properties, molecular design strategies towards tailored emission are discussed in relation to their theoretical aspects. Subsequently, recent studies on advanced ESIPT molecules and their optoelectronic applications are surveyed, particularly focusing on chemical sensors, fluorescence imaging, proton transfer lasers, and organic light-emitting diodes (OLEDs).

Advanced Organic Optoelectronic Materials: Harnessing Excited-State Intramolecular Proton Transfer (ESIPT) Process

Two types of tandem organic light-emitting diodes (OLEDs) with white-light emission have been developed by using Mg:Alq3∕WO3 as the interconnecting layer. While the Commission Internationale d’Eclairage (CIE) coordinates of the tandem device with individual blue- and yellow-emitting OLEDs was sensitive to the viewing angle and the operating time, the tandem device connecting two white-emitting OLEDs was considerably less. At an optimal WO3 thickness of 5nm, the tandem two-unit device produced three higher luminance efficiency than that expected of a single-unit device. A maximum efficiency of 22cd∕A was achieved by the tandem device comprised of two white-fluorescent OLEDs, and the projected half-life under the initial luminance of 100cd∕m2 was over 80000h.

https://ir.nctu.edu.tw:443/bitstream/11536/12955/1/000234118900093.pdf

Highly efficient white organic electroluminescent devices based on tandem architecture

In this paper, we report on the fabrication of multilayer organic light emitting diodes (OLEDs) with high electroluminescent (EL) yield achieved by integrating two units of green-emissive devices in series. The architecture of the device used in the experiment is Indium–Tin–Oxid (ITO)/CuPc/NPB/C545T:Alq3/Alq3/Mg:Alq3/WO3/NPB/C545T:Alq3/Alq3/LiF/Al. We found the efficiency of the two-unit devices can be controllable by the thickness of WO3. The two-unit devices with 1-nm-thick WO3 produces the luminance efficiency of 49.2 cd/A at 20 mA/cm2, which is around four times that of the controlled single-unit device (ITO/CuPc/NPB/C545T:Alq3/Alq3/LiF/Al). Compared with reported research data, the "amplification effect" discovered in our device is a rather unexpected result. The external quantum efficiency of 12.6%, with near-saturated Commission Internationale d'Eclairage coordinates (CIEx=0.27, CIEy=0.68), is one of the best ever reported for a fluorescent dye-doped OLED. We also demonstrate that the electron injection layer of Mg:Alq3 is a necessary component for the enhancement of EL efficiency. These results may prove to be an effective method to enhance the efficiency as well as the lifetime of current OLEDs.

https://ir.nctu.edu.tw:443/bitstream/11536/26396/1/000224579000099.pdf

High-Efficiency Organic Electroluminescent Device with Multiple Emitting Units

The carrier distribution and defects have been investigated in InAs/GaAs quantum dots by cross-sectional transmission electron microscopy (XTEM), capacitance–voltage, and deep level transient spectroscopy. Carrier confinement is found for 1.1- and 2.3-monolayer-(ML)-thick InAs samples. For 2.3 ML sample, XTEM images show the presence of defect-free self-assembled quantum dots. With further increase of the InAs thickness to 3.4 ML, significant carrier depletion caused by the relaxation is observed. In contrast to 1.1 and 2.3 ML samples in which no traps are detected, two broad traps and three discrete traps at 0.54, 0.40, and 0.34 eV are observed in 3.4 ML sample. The traps at 0.54 and 0.34 eV are found to be similar to the traps observed in relaxed In0.2Ga0.8As/GaAs single quantum well structures. By comparing with the XTEM images, the trap at 0.54 eV is identified to be the relaxation-induced dislocation trap in the GaAs layer.

https://ir.nctu.edu.tw:443/bitstream/11536/30136/1/000165069500029.pdf

Carrier distribution and relaxation-induced defects of InAs/GaAs quantum dots

The effect of tungsten oxide (WO3) incorporation into 1,4-bis[N-(1-naphthyl)-N′-phenylamino]-4,4′ diamine (NPB) layer is investigated in NPB-tris(8-hydroxyquinoline)aluminium heterojunction organic light-emitting diodes. The admittance spectroscopy studies show that increasing the WO3 volume percentage from 0% to 16% can increase the hole concentration of the NBP layer from 1.97×1014to1.90×1017cm−3 and decrease the activation energy of the resistance of the NPB layer from 0.354to0.176eV. Thus, this incorporation reduces the Ohmic loss and increases the band bending in the NBP layer near the interface, resulting in an improved hole injection via tunneling through a narrow depletion region.

https://ir.nctu.edu.tw:443/bitstream/11536/11798/1/000240384000096.pdf

Study of hole concentration of 1,4-bis[N-(1-naphthyl)-N-'-phenylamino]-4,4(') diamine doped with tungsten oxide by admittance spectroscopy

The GaAs sample under study is a n-low temperature-i-p structure grown by molecular beam epitaxy with a low-temperature (LT) layer grown at 300 °C and annealed at 620 °C for 1 h. Admittance measurements on this sample reveal a negative capacitance at low frequency. This work analyzes the origin of the negative capacitance and its corresponding frequency-dependent conductance by combining two current components: charging–discharging current and the inertial conducting current. Analysis results indicate that the activation energies and time constants of both current components closely resemble each other and should correspond to the same trap. Based on the results presented herein, we can conclude that the negative capacitance at low frequency provides evidence of a generation-recombination center with an activation energy of 0.77 eV in the LT layer.

Low frequency negative capacitance behavior of molecular beam epitaxial GaAs n-low temperature-i-p structure with low temperature layer grown at a low temperature

Jenn-Fang Chen

Papers

Highly efficient white organic electroluminescent devices based on tandem architecture

High-Efficiency Organic Electroluminescent Device with Multiple Emitting Units

Carrier distribution and relaxation-induced defects of InAs/GaAs quantum dots

Study of hole concentration of 1,4-bis[N-(1-naphthyl)-N-'-phenylamino]-4,4(') diamine doped with tungsten oxide by admittance spectroscopy

Low frequency negative capacitance behavior of molecular beam epitaxial GaAs n-low temperature-i-p structure with low temperature layer grown at a low temperature