Ligand memory effect in purple quantum dot LEDs

We report a ligand-dependent, trap state memory effect in a purple quantum dot light emitting diode. The measured, bistable current-voltage characteristics show a high conductance state and a low conductance state. Thereby, a memory window with an ON state and an OFF state is achieved, which can be reversed by applying a negative bias. The height of the memory window depends strongly on the length of the ligand passivating the quantum dot (QD). Together with a 5 nm thick aluminum film under the hole transport layer, charges accumulate at the charge transport/quantum dot ligand interface, leading to an increase in conductance during the forward voltage sweep. During a reverse voltage sweep, previously injected charge carriers recombine until the active layer is free of charge carriers. Voltage-capacitance measurements confirm the changes in the charge carrier population within the active layer and demonstrate that they depend on both the direction of the bias sweep and ligands surrounding the QD.