Performance enhancement by sol-gel processed Ni-doped ZnO layer in InP-based quantum dot light-emitting diodes

We report high-performance inverted red cadmium-free indium phosphide (InP) based QLED devices with Ni-doped ZnO as an electron transport layer (ETL). Using Ni-doping in ZnO, the conductivity and conduction band minimum of ZnO can be modulated, which helps in enhancement of charge balance in the QLED devices. The optimized devices with NiZnO-3% ETL exhibited a maximum current efficiency (CE) and external quantum efficiency (EQE) of 4.9 cd/A and 5.0%. By introducing ZnS interlayer at ETL/QD interface, the exciton quenching is suppressed furthermore. The device with NiZnO-3%/ZnS interlayer revealed a maximum CE and EQE of 10.4 cd/A and 10.6%, which are almost 2.1-fold improved compared to those with reference NiZnO-3% ETL devices. The device also demonstrated a long operational lifetime (LT₇₀) of 710 h at 1000 cd/m². The predicted half-lifetime (LT₅₀) is 164,048 h at 100 cd/m². Our results indicate that sol-gel Ni-doped ZnO serves a good ETL to attain high efficiency and long lifetime cadmium-free InP-based QLED devices.