Exploring Electronic and Excitonic Processes toward Efficient Deep-Red CuInS2/ZnS Quantum-Dot Light-Emitting Diodes

The electroluminescence mechanisms in the Cd-free CuInS2/ZnS quantum dot-based light-emitting diodes (QLEDs) are systematically investigated through transient electroluminescence measurements. The results demonstrate that the characteristics of hole transporting layers (HTLs) determine the QLEDs to be activated by the direct charge injection or the energy transfer. Moreover, both the energy level alignment between the HTL and quantum dot and the carrier mobility properties of the HTLs are critical factors to affect the device performance. By choosing the suitable HTL, such as 4,4′-bis(9-carbazolyl)-2,2′-biphenyl, highly efficient deep-red (emission peak at ∼650 nm) CuInS2/ZnS QLEDs based on the single HTL can be obtained with a peak current efficiency and luminance of ∼2.0 cd/A and nearby 3000 cd/m2, respectively.