Understanding “efficiency roll‐off” (i.e., the drop in emission efficiency with increasing current) is critical if efficient and bright emissive technologies are to be rationally designed. Emerging light‐emitting electrochemical cells (LECs) can be cost‐ and energy‐efficiently fabricated by ambient‐air printing by virtue of the in situ formation of a p‐n junction doping structure. However, this in situ doping transformation renders a meaningful efficiency analysis challenging. Herein, a method for separation and quantification of major LEC loss factors, notably the outcoupling efficiency and exciton quenching, is presented. Specifically, the position of the emissive p‐n junction in common singlet‐exciton emitting LECs is measured to shift markedly with increasing current, and the influence of this shift on the outcoupling efficiency is quantified. It is further verified that the LEC‐characteristic high electrochemical‐doping concentration renders singlet‐polaron quenching (SPQ) significant already at low drive current density, but also that SPQ increases super‐linearly with increasing current, because of increasing polaron density in the p‐n junction region. This results in that SPQ dominates singlet‐singlet quenching for relevant current densities, and significantly contributes to the efficiency roll‐off. This method for deciphering the LEC efficiency roll‐off can contribute to a rational realization of all‐printed LEC devices that are efficient at highluminance. The dynamic doping operation of light‐emitting electrochemical cells (LECs) renders the understanding of the efficiency roll‐off challenging. Herein, a method for quantification of major LEC loss factors, notably the outcoupling efficiency and exciton quenching, is presented. It reveals that singlet‐polaron quenching strongly affects the efficiency roll‐off in singlet‐emitting LECs because of increasing polaron concentration in the p‐n junction with increasing current .