Highly efficient inverted organic light-emitting devices by tuning particle size of ZnO nanoparticles

Inverted organic light-emitting devices (OLEDs) have been extensively researched because they can provide many advantages such as low driving voltage and high environmental stability in display applications. However, the electron injection is a critical issue in the inverted OLEDs because the work function of the ITO cathode is too high to inject electrons from the cathode into the adjacent organic layer. In this paper, we enhanced the electron injection by tuning the particle size of ZnO nanoparticles (NPs). Hexagonal wurtzite ZnO NPs with different size were synthesized by the solution precipitation method. As the particle size increases, the photoluminescence spectrum in the visible region shifts to the long wavelength due to the narrowing of band gap. The electrical resistivity decreases with increasing the particle size. The inverted phosphorescent OLEDs with a tris(2phenylpyridine)iridium(III) emitter have been fabricated using ZnO NPs with different size. As the particle size increases from 3.0 to 6.2 nm, the enhanced injection of electrons from the ZnO NPs to the organic electron transport layer results in the decrease of the driving voltage and the increase of the maximum external quantum efficiency (EQE). The maximum EQE of 26.02 % was achieved in the inverted OLEDs with 6.2 nm ZnO NPs.