Threshold Lowering and by Intensity and Efficiency Enhancement by Dopants in Polymer Emitting Diodes

A new method to increase the luminance and quantum efficiency of polymer light emitting diodes with a lower threshold voltage has been reported. The threshold voltage, luminance and quantum efficiency have been significantly improved by doping certain dopants with a lower highest occupied molecular orbital (HOMO) level into the hole transporting layer. A high performance device has been achieved by addition of the perylene and triphenylamine as a dopant into poly(N-vinylcarbazole). The luminance and quantum efficiency increase by 2-3 times in comparison with the undoped device, reaching 10000cd/m² in luminance and 0.58% in quantum efficiency, while threshold voltage is reduced to one half value. The energy diagram has been obtained by measuring the HOMO levels and band gap values. Based on this, the carriers injection and balance between electrons and holes as well as the action of dopant are discussed.