Effect of annealing ZnO on the performance of inverted polymer light-emitting diodes based on SAM/ZnO as an electron injection layer

An efficient electron injection/hole blocking layer, N-[3-(trimethoxysily)propyl]ethylenediamine that was grafted onto zinc oxide (ZnO/PEDA-TMS), was used in inverted polymer light-emitting diodes (PLEDs). PEDA-TMS induced a strong dipole directed towards ZnO, which was enhanced by vacuum energy level shifts of ultra-violet photoelectron spectroscopic observations, reducing the barrier height between ZnO and the emissive layer. The effect of annealing ZnO on the performance of PLEDs was analyzed using an electron-only device and capacitance-voltage (C-V) characterization. Following annealing at 300 degrees C, ZnO provided effective electron injection and blocked holes, which enhanced the recombination of the electrons and holes. The eletroluminescence efficiency of an inverted PLED with a ZnO/PEDA-TMS layer is comparable to that of a conventional device. The electron injection mechanism of the inverted device can be explained by C-V behaviors, which are correlated with the threshold voltage and turn-on voltage. (C) 2011 Elsevier B.V. All rights reserved.