Balancing electron and hole currents in single layer poly(9,9-dioctylfluorene) light emitting diodes

Poly(9,9-dioctylfluorene) (PFO) exhibits very good, non-dispersive hole transport but very poor electron transport. To achieve the maximum efficiency in a PFO light emitting diode it is important to balance the electron and hole currents. Here we report three schemes to achieve this in single layer devices. Firstly, by using different treatments to change the work function of the indium tin oxide anode contact, the hole current can be varied by up to 4 orders of magnitude, thus allowing it to be adjusted to the same level as the electron current. Secondly, the hole mobility can be decreased by doping PFO with a hole trapping, emissive material. Upon the addition of 5% by weight of a red-emitting tetraphenylporphyrin, hole transport in PFO becomes as highly dispersive as electron transport, resulting in a decrease in the current for a given applied bias but an increase in the electroluminescent efficiency. Thirdly, the electron mobility can be increased by doping PFO with an emissive, electron transporting material. The electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (BT) exhibits strong but dispersive electron transport. PFO devices doped with BT show very high efficiencies, high peak brightnesses and very low turn on voltages.