Improving charge balance in quantum‐dot light‐emitting diodes by using copper cathode

The effect of the cathode work‐function on the performance of colloidal quantum‐dot light‐emitting diodes (QD‐LEDs) is investigated in this work. In typical QD‐LEDs, electron injection is more efficient than the hole‐injection. The imbalance of carrier injection leads to a low device performance because the excess electrons, either charging the QDs and consequently enhancing the nonradiative Auger recombination, or leaking to the counter electrode without recombination, greatly reduces the efficiency of the devices. By using cathode with high work‐function such as copper (Cu), electron injection is more difficult. As a result, the charge balance in QD‐LEDs is greatly improved, which consequently enhances the efficiency of the devices. At a luminance of 1000 cd m⁻², devices with Cu cathode exhibit a current efficiency of 10.4 cd A⁻¹ and an external quantum efficiency of 6.9%, which is remarkably higher than that of 5.7 cd A⁻¹ and 3.8% for the devices with Al cathode.