Efficient red- and orange-light-emitting diodes realized by excitation energy transfer from blue-light-emitting conjugated polymers

The emission color of polymer-light-emitting diodes (PLED's) can be tuned by doping the active polymer layer with certain dyes or other fluorescent polymers. We demonstrate the realization of red-orange PLED's by doping a blue-light-emitting ladder-type poly(paraphenylene) ($m$-LPPP) with a new red-light-emitting polymer poly(perylene-co-diethynylbenzene) (PPDB). For PPDB concentrations far below 1 wt % the photo- (PL) and the electroluminescence (EL) spectrum of the $m$-LPPP/PPDB blend is already dominated by the PPDB emission because of an excitation energy transfer from $m$-LPPP to PPDB. The reason for this efficient energy transfer can be explained by the energetic position of the highest occupied and lowest unoccupied states in PPDB relative to the corresponding levels in $m$-LPPP. Besides the change in the emission color by doping PPDB into $m$-LPPP, a significant increase in the PL quantum efficiency (up to 41%) and even a more pronounced improvement of the external EL quantum efficiency of these PLED's (up to 1.6%) is observed.