Light-emitting devices with a photoluminescent quinquethiophene derivative as an emitting material

Electrical and electroluminescence measurements were carried out on single- and double-layer light emitting devices where a functionalized quinquethiophene, having a high photoluminescence quantum yield in its solid state (∼11%), was used as the emitting molecule. The thiophene derivative was used together with N,N′-dipheny-N,N′-bis(3-metylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), the well-studied hole injecting and transporting molecule, and with Alq3 as the electron transporting and emitting material. The deposition techniques for the device construction were both vacuum sublimation of the pure compounds and spinning of the concentrated solutions of pure or mixed compounds. Electroluminescence spectra show that the thiophene derivative is the only emitting species when put together with TPD, both in the single- and double-layer devices. Its orange emission is mixed with the green Alq3 emission in the double layer devices where a vacuum-deposited Alq3 layer is in contact with the cathode, the orange/green intensity ratio being dependent on the applied voltage. These light emitting devices are characterized by an extremely low onset voltage (2 Volt) and a fairly good electroluminescence external quantum efficiency (EQE=0.7%).