Ching Wan Tang

Organic electroluminescent diodes

A novel electroluminescent device is constructed using organic materials as the emitting elements. The diode has a double‐layer structure of organic thin films, prepared by vapor deposition. Efficient injection of holes and electrons is provided from an indium‐tin‐oxide anode and an alloyed Mg:Ag cathode. Electron‐hole recombination and green electroluminescent emission are confined near the organic interface region. High external quantum efficiency (1% photon/electron), luminous efficiency (1.5 lm/W), and brightness (>1000 cd/m2) are achievable at a driving voltage below 10 V.

Two-layer organic photovoltaic cell

A thin‐film, two‐layer organic photovoltaiccell has been fabricated from copper phthalocyanine and a perylene tetracarboxylic derivative. A power conversion efficiency of about 1% has been achieved under simulated AM2 illumination. A novel feature of the device is that the charge‐generation efficiency is relatively independent of the bias voltage, resulting in cells with fill factor values as high as 0.65. The interface between the two organic materials, rather than the electrode/organic contacts, is crucial in determining the photovoltaicproperties of the cell.

Electroluminescence of doped organic thin films

Electroluminescent (EL)devices are constructed using multilayer organic thin films. The basic structure consists of a hole‐transport layer and a luminescent layer. The hole‐transport layer is an amorphous diamine film in which the only mobile carrier is the hole. The luminescent layer consists of a host material, 8‐hydroxyquinoline aluminum (Alq), which predominantly transports electrons. High radiance has been achieved at an operating voltage of less than 10 V. By doping the Alq layer with highly fluorescent molecules, the EL efficiency has been improved by about a factor of 2 in comparison with the undoped cell. Representative dopants are coumarins and DCMs. The ELquantum efficiency of the doped system is about 2.5%, photon/electron. The EL colors can be readily tuned from the blue‐green to orange‐red by a suitable choice of dopants as well as by changing the concentration of the dopant. In the doped system the electron‐hole recombination and emission zones can be confined to about 50 A near the hole‐transport interface. In the undoped Alq, the EL emission zone is considerably larger due to excitondiffusion. The multilayerdopedEL structure offers a simple means for the direct determination of excitondiffusion length.

Organic electroluminescent devices with improved stability

Highly stable organic electroluminescent devices based on vapor‐deposited Alq thin films have been achieved. The improvement in stability is derived from several factors including: (1) a multilayer thin‐film structure with a CuPc stabilized hole‐injection contact, (2) a hole‐transport diamine layer using a naphthyl‐substituted benzidine derivative, and (3) an ac drive wave form. These emissive devices have shown an operational half‐lifetime of about 4000 h from an initial luminance of 510 cd/m2.

Enhanced electron injection in organic electroluminescence devices using an Al/LiF electrode

A bilayer is used as an electrode for organic electroluminescent devices. The bilayer consists of an ultrathin LiF layer adjacent to an electron-transporting layer and an aluminum outerlayer. Devices with the bilayer electrode showed enhanced electron injection and high electroluminescence efficiency as compared with a Mg0.9Ag0.1 cathode. Similar effects were observed when replacing MgO for LiF. The improvements are attributed to band bending of the organic layer in contact with the dielectrics.

Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy

We used ultraviolet and x‐ray photoelectron spectroscopy (XPS) and (UPS) techniques to directly measure absolute values of vacuum work function of indium tin oxide (ITO) thin films. We obtained a work function of 4.4–4.5 eV which is lower than the commonly cited value. These values do not change substantially by heating and Ar ion sputtering. The atomic concentrations of each element in ITO, measured with XPS, are also quite stable under heat treatment and ion sputtering.

Doped organic electroluminescent devices with improved stability

Remarkable improvement in stability has been demonstrated in an organic electroluminescent (EL) device using a doped emitter consisting of 8-hydroxyquinoline aluminum (Alq) as the host and N,N-dimethylquinacridone (DMQA) as the emissive dopant. A luminance half-life on the order of about 7500 h has been achieved in the DMQA/Alq EL device, operating under a constant current of 20 mA/cm2 and starting at a high luminance of 1400 cd/m2. The improved stability was attributed to the elimination of intermolecular hydrogen bonding between the dopant molecules.

Anthracene derivatives for stable blue-emitting organic electroluminescence devices

A class of anthracene derivative which is suitable used as emitting materials for producing efficient and stable blue emission for full color organic electroluminescence (EL) devices has been developed. Multilayer organic EL devices using these fluorescent materials as an emitting layer produced blue emissions with good chromaticity and luminous efficiency as high as 3.5 cd/A. The half life of 4000 h of blue emission EL device with initial light output 700 cd/m2 has been achieved.

High-efficiency tandem organic light-emitting diodes

Tandem organic light-emitting diodes (OLEDs), with multiple electroluminescent (EL) units connected electrically in series, have been fabricated. Using an optically transparent doped organic “p-n” junction as the connecting unit between adjacent EL units, excellent light out-coupling and carrier-injection properties have been realized. The luminous efficiency is found to scale almost linearly with the number of EL units in the stack, giving values as high as 32 or 136 cd/A for a three-unit tandem OLED using a fluorescent or a phosphorescent emitter, respectively.

CHARACTERIZATION OF TREATED INDIUM-TIN-OXIDE SURFACES USED IN ELECTROLUMINESCENT DEVICES

The influence of oxidative and reductive treatments of indium–tin–oxide (ITO) on the performance of electroluminescent devices is presented. The improvement in device performance is correlated with the surface chemical composition and work function. The work function is shown to be largely determined by the surface oxygen concentration. Oxygen-glow discharge or ultraviolet–ozone treatments increase the surface oxygen concentration and work function in a strongly correlated manner. High temperature, vacuum annealing reduces both the surface oxygen and work function. With oxidation the occupied, density of states (DOS) at the Fermi level is also greatly reduced. This process is reversible by vacuum annealing and it appears that the oxygen concentration, work function, and DOS can be cycled by repeated oxygen treatments and annealing. These observations are interpreted in terms of the well-known, bulk properties of ITO.