Solution-processed ZnO nanocrystals in thin-film light-emitting diodes for printed electronics

Thin-film light-emitting diodes (LEDs) containing solution-processed ZnO nanocrystals (NCs) were prepared as printed electronics. The electroluminescent (EL) properties of thin-film LEDs were investigated along with the structural and photoluminescence (PL) properties of the ZnO NCs. Scanning electron microscope and x-ray diffraction studies revealed that the crystal sizes D were ranged from 5–11 nm, and can be controlled by varying growth time tG in the Zn2+/OH− solution at 40 °C. The time evolution of D was analyzed using Lifshitz–Slyozov–Wagner theory, showing that growth is limited by diffusion. The results of PL studies indicated that increases in the peak energies in the ultraviolet (UV) region could be attributed to the quantum-size effects on the exciton emission in the NCs with a small D, the ZnO surfaces became sufficiently passivated as D increases. Printed layers containing well-passivated ZnO NCs with different D of 8–11 nm were used as emission layers in thin-film LEDs together with pentacene hole transport layers. The current-voltage characteristics were analyzed using the trapped-charge-limited current mechanism. EL spectral measurements revealed the presence of weak UV emission that increased slightly as D decreased.