Keith Douglas Campbell

Tuning the properties of polythienothiophene conductive polymers for hole injection layer application

It has been demonstrated that Air Products(R) HIL (hole injection layer) material based on the conducting polymer polythienothiophene (PTT) and poly(perfluoroethylene-perfluoroethersulfonic acid) (PFFSA) dramatically improves the lifetime of polymer light emitting diodes. Compared with other conductive polymer HILs, PTT based HILs have some unique properties. The resistivity of PTT:PFFSA films is sensitive to the annealing temperature. The resistivity dependence on annealing temperature is not favorable for certain applications (e.g., in passive matrix display applications, where too low a resistivity after annealing can lead to cross-talking), or from the point view of process control. We have found that raising the pH of PTT:PFFSA dispersions can suppress the resistivity sensitivity to annealing conditions. At the same time, raising the pH of PTT:PFFSA dispersions also lowers the work function of PTT:PFFSA films. When LumationTM Green 1304 light emitting polymer is used as the emitting layer, all PTT:PFFSA based devices showed lifetime that is several times longer than that of PTT:PSSA based devices. Among the PTT:PFFSA dispersions, pH adjusted ones show a lower leakage current, lower efficiency and shorter device lifetime compared with the original dispersion. We have also explored the application of PTT:PFFSA in small molecule devices. Longer device lifetime has been obtained in devices using PTT:PFFSA as HIL and aluminum tris(8-hydroxyquinoline) (Alq3) as emitter compared with devices using copper phthalocyanine (CuPc) as HIL. We have also found that hole injection from PTT:PFFSA into hole transport materials commonly used in small molecule devices is very efficient.