We report on the photophysics of the pristine oligo(ethylene oxide) side-chain grafted polymer PPP-OR11 and the polymer blended with the lithium salt lithiumtriflate. The side-chains render the polymer soluble in common organic solvents and in addition provide ionic conductivity, which is important for the application of the polymer as mixed ionic-electronic conductor for instance in light-emitting electrochemical cells (LECs). The optoelectronic properties of the polymer were studied for two types of light-emitting devices, first in light emitting diodes and secondly in LECs. From these investigations it is evident that in polymer light-emitting diodes (PLEDs) several degradation processes caused by defects on the PPP backbone deteriorate the color stability. These defects are induced either by the oxidation of the polymer or the aluminum deposition process upon device fabrication. Contrarily, LECs fabricated from the same polymer provide color stable blue emission. The color stability of the LEC can be explained by the fact that the recombination zone is shifted from the cathode/polymer interface in PLEDs to the non-doped intrinsic zone between the p- and n-type regions of the LEC, avoiding emission from aluminum evaporation induced defects.