Yoshiaki Tsubata

Recent progress of high performance polymer OLED and OPV materials for organic printed electronics

Abstract The development of organic printed electronics has been expanding to a variety of applications and is expected to bring innovations to our future life. Along with this trend, high performance organic materials with cost-efficient fabrication processes and specific features such as thin, light weight, bendable, and low power consumption are required. A variety of organic materials have been investigated in the development of this field. The basic guidelines for material design and the recent progress of polymer-based organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs) are reported.

29.1: Invited Paper: Recent Progress in Light-Emitting Polymers for Full Color OLEDs

We have significantly improved the efficiency, lifetime and hue of red, green, blue (RGB) light-emitting polymers (LEPs) through intensive studies on degradation and charge balance in polymer organic light-emitting diodes (OLEDs). The degradation of LEPs during device operation is shown to result from decreased emissive material fluorescence efficiency and a loss of charge-carrier balance. Laser photolysis results show that low-energy sites are generated in the LEP during driving, leading to the quenching of blue and green emission, but not red emission. Charge carrier balance also shifts due to decreased electron injection compared with hole injection. Taking these degradation mechanisms, we have focused on copolymerization of various functional units to stabilize the photoluminescence (PL) intensity and the balance of charge carriers. In this paper, we present the degradation mechanism and performance of our latest RGB LEPs.

Novel blue light-emitting polymers for PLED

We have been researching blue-emitting polymers and developed some novel polymers with blue fluorescence, which are not fluorene copolymers. Our previous works showed the dependence of EL efficiency on molecular weight of poly(p-phenylene vinylene) derivatives. We concluded that polymers must be synthesized by the method which gave larger molecular weight in order to achieve higher performance. Firstly, we modified the synthetic method of polyarylenes and confirm that Mw of ca.1,000 to over 1,000,000 is obtainable depending on the condition. Polymers having moderately large Mw showed good solubility in solvents and film-forming properties. PLEDs using these polymers had longer lifetime as well as higher EL efficiency. Secondly, we understood the importance of charge balance between electrons and holes, by using polyfluorene derivatives as model polymers. We measured electron, hole and total currents by electron-only, hole-only and usual bipolar devices respectively. We found out that the hole mobility could be changed from 10-7cm2/Vs to 10-3cm2/Vs depending on the content of hole transporting units. In addition, electron injection from a cathode seemed to be assisted by trapped holes. We utilized these results in order to design our blue polymers. Finally, we obtained a new blue-emitting polymer, which showed longer device lifetime among non-fluorene polymers.