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

Abstract

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 o C using dispersions with PFFSA-to-TT ratios of 12:1 and 18:1.

Electromodulated doping of the hole transport layer in a small molecule organic light-emitting diode

“…This results in the incorporation of indium into the active layer of the device and leading to serious concerns about the device lifetime and reliability. 15 These issues have driven the development of polymer HILs 16,17 with higher work functions and less acidity to give improved device performance and stability. Choudhury et al studied small molecule OLEDs with a HIL based on a conducting polymer polythienothiophene (PTT) doped with poly(perfluoroethyleneperfluoroethersulfonic acid) (PFFSA) in OLEDs.…”