Polymer blends in organic LEDs offer the advantage of processibility coupled with flexibility with respect to the chromophores which can be incorporated into their structures. The polymer matrix is also though to have a positive effect on the quantum efficiency and other LED performance indicators. Two‐layer devices have also been shown recently to yield improved performances. Here, a two‐layer device based on polymer blends is reported.

Transient electroluminescence (EL) from organic bilayer light emitting diodes addressed by a rectangular voltage pulse often features an overshoot when the voltage is switched off. Experimental results are presented for a variety of hole transporting layers in contact with an oxadiazole layer simultaneously acting both as a blockade for holes and as an electron transport layer. The overshoot occurs in spin coated yet not in vapor deposited samples. A model is developed to rationalize charge recombination under the premise (i) of an interfacial layer of finite thickness between hole and electron transport layers in which both transport molecules coexist and (ii) of interfacial energy barriers impeding both hole and electron passage. It predicts the occurrence of an EL overshoot due to the recombination of stored electrons and holes under the action of their mutual space charge field when the external voltage is switched off. The temporal pattern of the predicted transient EL signal is in good agreement with experiment.

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The electroluminescent properties of 5′-[4-[bis(4-ethylphenyl)amino]phenyl] -N,N,N′,N′-tetrakis(4-ethylphenyl) -[1,1′:3′,1″-terphenyl]-4,4″-diamine (pEFTP) were investigated in a vapor deposited layer and dispersed in a polymer matrix. Blue–violet electroluminescence was observed, after applying voltages beyond 12 V for the single layer devices and 22 V for the double layer devices. The electroluminescence (EL) spectrum shows two maxima, of which one corresponds to that of the photoluminescence spectrum. Either direct radiative recombination of the hole and the electron residing at two neighboring molecules or phosphorescence is causing this red shifted electroluminescence maximum. Transient electroluminescence measurements allow us to estimate the mobilities of the charge carriers in the different transport layers. The occurrence of an EL overshoot after switching off a voltage pulse, confirms the importance of detrapping and interfacial phenomena in the radiative recombination in pEFTP.