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 °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.