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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 we can replicate the measured luminance transients by an optical model which considers these electrode-dependent effects. 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.

A light-emitting electrochemical cell (LEC) comprises mobile ions in its active material, which enable for in situ formation of a p-n junction by electrochemical doping. The position of this emissive p-n junction in the interelectrode gap is important, because it determines whether the emission is affected by constructive or destructive interference. An appealing LEC feature is that the entire device can be fabricated by low-cost solution-based printing and coating. Here, we show, somewhat unexpectedly, that the replacement of conventional vacuum-deposited indium-tin-oxide (ITO) for the positive anode with solution-processed poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) can result in an increase in the peak light-emission output by 75%. We demonstrate that this emission increase is due to that the p-n junction shifts from a position of destructive interference in the center of the interelectrode gap with ITO to a position of constructive interference closer to the anode with PEDOT:PSS. We rationalize the anodic p-n junction shift by significant anion transfer into the soft and porous PEDOT:PSS electrode during LEC operation, which is prohibited for the ITO electrode because of its compact and hard nature. Our study, thus, contributes with important design criteria for the attainment of efficient light emission from solution-processed LEC devices.