Transient overshoot and storage of charge carriers on ligands in quantum dot LEDs

Quantum dot light emitting diodes (QLEDs) emitting at 410 nm were studied by time-resolved electroluminescence measurements. A transient overshoot after voltage turn-off was seen, which is attributed to the accumulation and storage of charge carriers at the ligand-quantum dot interface. Shorter ligands showed a faster rise time and prevented the formation of an overshoot, whereas longer ligands caused the storage of charges, responded slower, and showed an overshoot. When the external voltage was switched off, the electric field between the injected and stored electrons and holes led to the occurrence of the overshoot. Applying a dual voltage pulse avoided this overshoot and instead a delayed luminescence was observed. As the accumulated charges were immobile and trapped in shallow states, a reverse pulse was applied to fully deplete the emissive layer. Because the transient overshoot disappeared after the device had been turned on, this can be used as a measure of the degradation of QLEDs. The ligands constitute a major obstacle for an efficient and long lasting blue/purple quantum dot emitter.