The nature of the electrical conduction and light emitting efficiency in organic semiconductors layers: The case of [m-MTDATA] – [NPB] – Alq3 OLED

In this work, a detailed study on the influence of different layers in an Alq3 [aluminium-tris(8-hydroxyquinoline)] based OLED is made. Two hole transport layers have been used: m-MTDATA [4,4′,4″-tris (3-methyl-phenylphenylamino) triphenylamine] and NPB [N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4″-diamine]. It is found that the efficiency (at 520 nm) can increase up to one order of magnitude when we consider a three layer device instead of a conventional two layer. The driving voltage decreases in the three layer device, but the output electroluminescence spectra (in green spectral region) tend to be broaden towards high energies. The current–voltage behavior follows a space charge limited current behavior. The influence of the traps states and/or a Poole-Frenkel like field mobility dependence is discussed. The electroluminescence data fit to a large green band (one main green band and a small orange band) and for the three layer device a small blue band appears. Finally, a model involving an individual layer contribution with the corresponding trap levels is proposed.