Eric Howard Klingenberg

Enhanced lifetime of polymer light-emitting diodes using poly(thieno[3,4-b]thiophene)-based conductive polymers

Poly(thieno[3,4-b]thiophene) (PTT) based transparent conductive polymers have been developed for hole injection layer (HIL) applications in polymer light-emitting devices. Using a second generation material that uses a poly(perfluoroethylene-perfluoroethersulfonic acid) as polymeric dispersant/counter-ion (PTT:PFFSA), significant improvement (up to 6 times with LUMATION Green 1304 as the light emitting layer) in PLED lifetime has been achieved compared to the lifetime of first generation PTT:PSSA (PTT:poly(styrene sulfonic acid)) based devices. Compared with the work function of 5.2 eV for PTT:PSSA film, PTT:PFFSA films have a higher work function of 5.5~5.7 eV. Interestingly, we found that the resistivity of PTT:PFFSA films is dependent on the film preparation conditions such as annealing temperature and time; while the work function of PTT:PFFSA films is independent on the film annealing conditions. We also studied the dependence of the device leakage current and lifetime on the preparation conditions of PTT:PFFSA based HIL films. Higher HIL annealing temperature results in higher device leakage current mainly due to the lower resistivity of the PTT:PFFSA HIL film prepared under such condition. However, the device lifetime is almost independent on the annealing temperature in the studied temperature range from 130 to 210°C using dispersions with PFFSA-to-TT ratios of 12:1 and 18:1.