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(-7) cm(2)/Vs to 10(-3) cm(2)/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.