Improving the performance of doped pi-conjugated polymers for use in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) represent a promising technology for large, flexible, lightweight, flat-panel displays(1-3). Such devices consis t of one or several semiconducting organic layer(s) sandwiched between two electrodes. When an electric field is applied, electrons are injected by th e cathode into the lowest unoccupied molecular orbital of the adjacent mole cules (simultaneously, holes are injected by the anode into the highest occ upied molecular orbital). The two types of carriers migrate towards each ot her and a fraction of them recombine to form excitons, some of which decay radiatively to the ground state by spontaneous emission. Doped pi-conjugate d polymer layers improve the injection of holes in OLED devices(4-9); this is thought to result from the more favourable work function of these inject ion layers compared with the more commonly used layer material (indium tin oxide). Here we demonstrate that by increasing the doping level of such pol ymers, the barrier to hole injection can be continuously reduced. The use o f combinatorial devices allows us to quickly screen for the optimum doping level. We apply this concept in OLED devices with hole-limited electrolumin escence (such as polyfluorene-based systems(10-12)), finding that it is pos sible to significantly reduce the operating voltage while improving the lig ht output and efficiency.