In this work, the authors have demonstrated a high external quantum efficiency (EQE) in deep-blue organic light-emitting diodes (OLEDs) based on a new cerium(III) complex Ce-1 as the emitter, which can achieve 100% exciton utilization efficiency (EUE). Coupled with its short excited-state lifetime, adjustable emission spectrum, and low cost, cerium(III) complex has the potential to develop deep-blue OLEDs with high efficiency and long-term stability, and also to expand into the field of full-color OLEDs.