The use of robust, inorganic charge-transport materials is always desired in quantum-dot light emitting diodes (QLEDs) because they are expected to allow higher stability and lower cost than their organic counterparts. We achieve here an all-inorganic QLED with excellent efficiency by modifying the solution-processed NiO (s-NiO) surface with an ultrathin Al2O3 passivating layer. Both transient resolution photoluminescence and X-ray photoelectron spectroscopy measurements demonstrate that the Al2O3 layer can effectively passivate NiOOH on the s-NiO surface, thereby suppressing exciton quenching. This improves the highest efficiency of the QLED without an Al2O3 layer by over 800% to a current efficiency (external quantum efficiency) of 34.1 cd A-1 (8.1%), making it the best-performing all-inorganic QLED.