Potential of Electrochemical Charge Injection for Quantum Dot Light-Emitting Devices

The efficiency of quantum dot (QD) light-emitting diodes is limited by inefficient hole injection into the valence levels of the QDs. Electrochemical doping, where mobile ions form electrical double layers (EDLs) at electrodes, offers a route to removing injection barriers. While QD light-emitting electrochemical cells (QLECs) have shown promise, prior studies relied on additional charge injection layers, complicating the study of charge injection into QDs. In this work, devices with a simple ITO/QD active layer/Al structure were fabricated using highly photoluminescent ligand-exchanged CdSe/CdS/ZnS QDs, poly-(ethylene oxide), and lithium trifluoromethanesulfonate as electrolyte. We show that the dense QD films in these QLECs can be electrochemically doped, transport charges, and exhibit electroluminescence. Symmetrical cyclic voltammograms and operando photoluminescence measurements prove that these devices function as electrochemically doped LECs. Spectroelectrochemical experiments on separately n- and p-doped QD films indicate that hole injection remains the primary limitation in QLEC performance. These findings demonstrate that using EDLs to facilitate charge injection in QD light-emitting devices is promising, but significant challenges remain to be solved before electron and hole injections are balanced.