Highly efficient inverted organic light-emitting devices adopting solution-processed double electron-injection layers

Highly efficient inverted green fluorescent and phosphorescent organic light-emitting diodes (OLEDs) are achieved by ZnO modified with solution-processed Cs₂CO₃ doped 4,7-diphenyl-1,10-phenanthroline (BPhen). It is found that the n-doped interlayer greatly enhances the electron injection efficiency, significantly improving the electroluminescence efficiency of inverted OLEDs. Under the optimal Cs₂CO₃ doping concentration of 10 wt%, the inverted green fluorescent OLED with solution-processed double-stacked Cs₂CO₃:BPhen/ZnO electron-injection layers (EILs) shows a current efficiency of 5.65 ± 0.09 cd/A, which is about eightfold higher than that of the device with single ZnO EIL. Furthermore, the inverted green phosphorescent OLED was also developed showing a low turn-on voltage of 3.4 V, a maximum current efficiency of 52.5 ± 4.1 cd/A and a maximum external quantum efficiency of 14.3 ± 1.2%. The results show that constructing solution-processed double-stacked ZnO/Cs₂CO₃:BPhen EILs is a promising strategy to achieve highly efficient and low-cost inverted OLEDs.