The role of triplet states in the interfacial energy transfer in perovskite light-emitting diodes (PeLEDs) has so far not been clarified because of the complex exciton recombination and decay dynamics. This work aims to study this issue and accordingly proposes a novel interfacial-engineering strategy for efficient sky-blue PeLEDs. To this end, bis[2-(diphenylphosphino)phenyl]ether oxide with a high triplet energy level is introduced into sky-blue PeLEDs. It effectively reduces undesirable exciton transfer from the perovskite emission layer to the electron-transport layer, largely suppresses exciton quenching at the interface, and simultaneously passivates defects at the perovskite surfaces. As a result of the multichannel energy-loss reduction, sky-blue PeLED that emits at 488 nm is achieved with a peak external quantum efficiency of 10.17% and a maximum brightness of 6728.41 cd m-2. This work thus provides indirect evidence for the triplet mechanism of blue emission of mixed-halide perovskites and sheds new light on a promising way of boosting the performance of blue PeLEDs.