The light‐emitting electrochemical cell (LEC) allows for energy‐ and cost‐efficient printing and coating fabrication of its entire device structure, including both electrodes and the single‐layer active material. This attractive fabrication opportunity is enabled by the electrochemical action of mobile ions in the active material. However, a related and up to now overlooked issue is that such solution‐fabricated LECs commonly comprise electrode/active‐material interfaces that are open for transfer of the mobile ions, and it is herein demonstrated that a majority of the mobile anions in a common spray‐coated active material can transfer into a spray‐coated poly(3,4‐ethylenedioxythiophene):poly(styrene‐sulfonate) (PEDOT:PSS) positive electrode during LEC operation. Since it is well established that the mobile ion concentration in the active material has a profound influence on the LEC performance, this significant ion transfer is an important factor that should be considered in the design of low‐cost LEC devices that deliver high performance.

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