Preparation of π-Conjugated Polymer Ultrathin Films and their Application to Organic Nanodevices

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π-Conjugated polymers have received much attention for the application to electroluminescent devices, field-effect transistors and molecular electric devices because of their versatile electronic and photonic properties, structural flexibility and easy processability. These polymer films are generally formed by casting, dipping, spin-coating and ink-jet method. However, it is difficult to control film thickness at a molecular level and molecular orientation. It is necessary to control the thickness of ultrathin films for preparation of organic nano-devices. The Langmuir-Blodgett (LB) technique has attractive features to fabricate ultrathin films at a molecular level with molecular orientation and ordering. However, π-conjugated polymers have less ability to form monolayer on a water surface due to strong π-π interaction between aromatic main chains.To overcome the problem, we try to fabricate mixed LB films of π-conjugated polymer and an amphiphilic polymer. We have found that poly(N-d … More odecylacrylamide) (pDDA) has an ability to form a stable monolayer on the water surface and to fabricate an excellent LB film. In this study, the monolayer behavior of the mixtures of pDDA and poly(3-hexylthiophene) (PHT) or poly[2-[2',5'-bis[2''-ethylhexyloxyl]phenyl]-1,4-phenylene vinylenel (BEHP-PPV) on the water surface was investigated and conductivity of their mixed LB films was evaluated by conductivity measurement using interdigitated array (IDA) electrodes. We have found that π-conjugated polymers are well miscible with pDDA and the mixtures can form a stable monolayer on a water surface and LB film on substrates by the LB technique. The π-conjugated polymer ultrathin conducting film was fabricated by LB method using the mixture of pDDA and π-conjugated polymers. The conductivity of the mixed PHT LB film becomes 3 S/cm with doping, which is about three orders of magnitude higher than that of the undoping. It is expected to fabricate molecular electronic devices using the mixed LB films.Performance of the mixed PHT LB films on hole-injection for organic electroluminescent (EL) devices was also investigated. The matching of the energy potential between the work function of the ITO anode and HOMO energy level of the hole-injection materials is needed whereas the thickness of the hole-injection layer would not be needed as an ideal hole-injection layer. Onset voltage of EL devices using PHT LB films decreased with increasing number of layers due to reduction of energy barrier of hole injection from indium-tin-oxide (ITO) anode to PHT films. Quantum efficiency was improved at the monolayer PHT LB film EL devices. We demonstrated that the mixed PHT LB films could function as a hole-injection layer for EL devices. Less