Highly Efficient Electroluminescence from Green‐Light‐Emitting Electrochemical Cells Based on CuI Complexes

Abstract The complexes Cu(dnbp)(DPEphos) + (X – ) (dnbp and DPEphos are 2,9‐di‐ n ‐butyl‐1,10‐phenanthroline and bis2‐(diphenylphosphino)phenylether, respectively, and X – is BF 4 – , ClO 4 – , or PF 6 – ) 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.