Impact of the electrode material on the performance of light-emitting electrochemical cells

TL;DR

This paper investigates how electrode material properties influence light-emitting electrochemical cell performance, revealing that electrode work function affects exciton generation zones and overall luminance, contrary to previous assumptions of independence.

Contribution

It demonstrates experimentally and theoretically that electrode work function impacts LEC performance by affecting ion concentration and exciton generation, providing new design criteria.

Findings

Electrode work function influences exciton generation zone.

Ion concentration in EDLs depends on electrode work function.

Optical modeling can replicate electrode-dependent luminance transients.

Abstract

Light-emitting electrochemical cells (LECs) are promising candidates for fully solution-processed lighting applications because they can comprise a single active-material layer and air-stable electrodes. While their performance is often claimed to be independent of the electrode material selection due to the in-situ formation of electric double layers (EDLs), we demonstrate conceptually and experimentally that this understanding needs to be modified. Specifically, the exciton generation zone is observed to be affected by the electrode work function. We rationalize this finding by proposing that the ion concentration in the injection-facilitating EDLs depends on the offset between the electrode work function and the respective semiconductor orbital, which in turn influences the number of ions available for electrochemical doping and hence shifts the exciton generation zone. Further, we investigate the effects of the electrode selection on exciton losses to surface plasmon polaritons and discuss the impact of cavity effects on the exciton density. We conclude by showing that the measured electrode-dependent LEC luminance transients can be replicated by an optical model that considers these electrode-dependent effects to calculate the attained light outcoupling of the LEC stack. As such, our findings provide rational design criteria considering the electrode materials, the active-material thickness, and its composition in concert to achieve optimum LEC performance.