Elimination of defect-induced color instabilities in polymer light-emitting devices

We demonstrate that utilizing a blue light-emitting oligo(ethylene oxide)-grafted conjugated poly(p-phenylene) (PPP-OR11) as an active layer in a light-emitting electrochemical cell (LEC) preserves a color-stable blue light emission. This is in contrast to the polymer light-emitting diodes (PLEDs) constructed using the same emitting polymer without ionic species, which exhibits a rapid red shifting of the emission to a distinct green. We attribute the color shift to various degradation processes caused by oxidative degradation and aluminum deposition-induced defects in PPP-OR11. The analysis of the degradation processes in the PLEDs shows that the color stability in the LEC is caused by a controlled shift of the recombination zone from the defects including cathode/polymer interface to an undoped intrinsic zone between the p- and n-type regions of the active layer in the LECs, which qualifies the LEC type devices as a cost-effective approach to the single layer color-stable blue emitting devices. (C) 2005 American Institute of Physics.