Investigating Bulk-to-Interface Doping Relaxation in Light-Emitting Electrochemical Cells via Displacement Current Measurements

Recently, we proposed an extended displacement current measurement (DCM) technique to study the dynamic properties of light-emitting electrochemical cells (LECs) and demonstrated the interrelated transient changes in electrical and optical properties during the relaxation of electrochemical doping (ECD) [Adv. Opt. Mater. 2018, 6, 1800318]. In the present work, this extended DCM method was employed for Super Yellow-based LECs with two typical active layer thicknesses of approximately 60 and 120 nm, and the relaxation processes of the resulting devices were investigated. In the thick-film device, the deterioration of the luminous efficiency was dominated by the optical processes of self-absorption and exciton–polaron quenching, whereas in the thin-film device, the deterioration was facilitated by the electronic process of carrier injection. The carrier balance factor was critical to the luminous efficiency of the thin-film device under reverse-bias operation, although this was not the case for the thick-film device. The results indicate that the ECD relaxation propagates from the bulk to the interface and confirm that the active layer thickness is a major factor in the maintenance of efficient carrier injection. The extended DCM method is a promising approach for analyzing the dynamic and complex properties of LECs.