Investigating the operation mechanism of light-emitting electrochemical cells through operando observations of spin states

Light-emitting electrochemical cells (LECs) are next-generation devices that are flexible, emit light and have several advantages over organic light-emitting diodes, such as a simpler structure and lower cost. However, the operation mechanism of LECs remains unknown from a microscopic viewpoint. Here, we perform an operando microscopic investigation of LECs with Super Yellow, a typical light-emitting material, by observing the spin states of electrically doped charges using electron spin resonance. The operando electron spin resonance and light emission increase as the voltage applied to the LECs increases. Through density functional theory, we determine that the origin of the electron spin resonance increases to be from electrochemically doped holes and electrons in Super Yellow. We find that the doping progress correlates with the luminance increase, suggesting that electrochemically doped charges are distributed over the light-emitting layer as the operation mechanism. Moreover, we deduce the molecular orientation of electrochemically charge-doped Super Yellow.