Development of high charge mobility bipolar organic semiconductor materials by University of Tokyo

News June 9th, University of Tokyo and the Japan Science and Technology Agency (JST), the research team developed the amorphous material as the charge transfer rate reached the "top level" (the research group) of the polar organic semiconductor materials.

The Benzodifuran derivative (left), released in 2007, was developed for the structure of bipolar CZBDF

Voltage - organic EL element external quantum efficiency properties of the intermediate layer doping after pigment homojunctions (left) and the light emitting element case (right photo)

The bipolar material is called "CZBDF", which was developed on the basis of the derivatives of the condensed electron conjugated compounds Benzodifuran, which contain oxygen atoms in the research group, in 2007. Benzodifuran derivative is a kind of P type semiconductor material with high hole mobility in amorphous films. By replacing the "amine" site of the Benzodifuran derivative with carbazole, a highly charged polar material CZBDF was developed. In addition, CZBDF was used to produce a homogeneous organic EL element, and successfully achieved the simultaneous use of fluorescence and phosphorescence of EL luminescence and blue, green, red 3 primary colors EL luminescence.

The charge mobility of the amorphous CZBDF thin film, the hole is 3.7 x 10-3cm2/Vs, the electron is about 4.4 * 10-3cm2 / Vs. The mobility of the two kinds of charge is very high and the numerical equilibrium is good. These values are measured using time-of-flight (TOF:Time Of Flight) method (electric field strength of 2.5 x 105V/cm).

In addition, the team also uses the development of the bipolar material CZBDF, the vacuum evaporation method for the production of a homogeneous organic EL element. In particular, the ITO (indium tin oxide) on the glass substrate is used as an anode, and an organic thin film with a thickness of 150 to 200nm is formed by vacuum evaporation on the anode, and a Al metal (cathode) is formed.

The organic thin film was made of CZBDF as the main material. In the range of 30nm from the anode, the P doping was carried out by evaporation with an inorganic oxidant V2O5 (five oxidation of two vanadium). In the range of 20nm from the cathode, the N doping was carried out by evaporation with the reducing agent (CS). In this way, the charge from the electrode to the CZBDF can be easily injected and transported.

In the middle layer (50 ~ 100nm) of undoped oxidant and reducing agent, blue, green fluorescent pigment and red phosphorescent pigment were respectively added to realize the luminescence of the 3 primary colors. When the luminance is 60 thousand CD / m2, the green fluorescent element shows a high external quantum efficiency of 4.2%.

The trial of organic EL element has been able to achieve 3 primary light-emitting efficiency, that is due to the nature of the following CZBDF. (1) is polar high balance and high mobility; (2) CZBDF is a wide bandgap semiconductor material, HOMO (the highest occupied molecular orbital) and LUMO (LUMO) the energy difference between the very large (about 3eV); (3) the efficient charge sealed luminous pigment.

At present, the organic EL element is mainly composed of 5 to 6 different types of organic thin films. The University of Tokyo research team using the homojunction type organic EL element to achieve a simple 3 color light and efficient luminescence, so you can look forward to this development of organic EL display and lighting with low cost and high efficiency. In addition, in the future also intends to organic EL also has a multi-layer structure of organic thin-film solar cells and other fields to promote.

The results were published in May 25, 2009 in the online edition of the German science and technology magazine Advanced Materials