Highly efficient inverted phosphorescent organic light-emitting devices with Li-doped ZnO nanoparticles

Inverted organic light-emitting devices (OLEDs) have attracted much attention due to their superior characteristics such as high stability, low brightness drop, and low driving voltage in display applications. To improve the charge balance that has been known as a critical issue in inverted OLEDs, Li-doped ZnO nanoparticle electron injection layer was investigated. Hexagonal wurtzite-structured Li-doped ZnO nanoparticles were synthesized using zinc acetate dihydrate, ammonium hydroxide pentahydrate, and lithium chloride. The particle size of the nanoparticles decreases with Li doping, exhibiting particle sizes of 4.1, 3.7, and 3.3 nm for the undoped, 10 % and 15 % Li-doped nanoparticles, respectively. The band gap, conduction band minimum and valence band maximum energy, photoluminescence in the visible region, surface roughness, and electrical conduction characteristics of the Li-doped ZnO nanoparticles were investigated. The inverted phosphorescent devices were prepared using the synthesized nanoparticles. The inverted devices with Li-doped nanoparticles exhibited higher external quantum efficiency (EQE) due to better charge balance. The maximum EQE of 18.2 % was achieved in the 15 % Li-doped device.