The influence of the phase morphology on the optoelectronic properties of light-emitting electrochemical cells

We report on the morphological aspects of thin films prepared from a blue-green light-emitting conjugated polymer, (methyl-substituted ladder-type poly(p-phenylene, mLPP)), blended with a solid-state electrolyte composed either by a crown ether, dicyclohexano-18-crown-6 (DCH18C6), or a high-molecular-weight poly(ethylene oxide) (HMWPEO), and a Li salt, lithium trifluoromethanesulfonate (LiCF3SO3, Li triflate (LiTf)), as they have been successfully applied in light-emitting electrochemical cells (LECs). The surface morphologies of the blend layers were investigated using atomic force microscopy (AFM) in tapping mode, and the ion distribution was probed using X-ray analysis by means of energy-dispersive X-ray spectrometry (EDXS) in the scanning electron microscope (SEM). We show that the two different phase-separation processes, the complexation tendencies of the ionic species as well as the ionic transport numbers, have tremendous influence on the performances of the corresponding LECs, revealing either rectifying or symmetric optoelectronic characteristics in forward and reverse bias directions. This opens up new possibilities for tuning the optoelectronic properties of ion-supported organic electronic devices.