The complexes [Cu(dnbp)(DPEphos)]+(X–) (dnbp and DPEphos are 2,9-di-n-butyl-1,10-phenanthroline and bis[2-(diphenylphosphino)phenyl]ether, respectively, and X– is BF4–, ClO4–, or PF6–) can form high-quality films with photoluminescence quantum yields of up to 71 ± 7 %. Their electroluminescent properties are studied using the device structure indium tin oxide (ITO)/complex/metal cathode. The devices emit green light efficiently, with an emission maximum of 523 nm, and work in the mode of light-emitting electrochemical cells. The response time of the devices greatly depends on the driving voltage, the counterions, and the thickness of the complex film. After pre-biasing at 25 V for 40 s, the devices turn on instantly, with a turn-on voltage of ca. 2.9 V. A current efficiency of 56 cd A–1 and an external quantum efficiency of 16 % are realized with Al as the cathode. Using a low-work-function metal as the cathode can significantly enhance the brightness of the device almost without affecting the turn-on voltage and current efficiency. With a Ca cathode, a brightness of 150 cd m–2 at 6 V and 4100 cd m–2 at 25 V is demonstrated. The electroluminescent performance of these types of complexes is among the best so far for transition metal complexes with counterions.