Understanding of the Intrinsic Difference between n- and p-Doping Propagation in Polymer Light-Emitting Electrochemical Cells: A Numerical Modeling Approach

Distinct doping propagation characteristics between p-doping and n-doping in light-emitting electrochemical cells (LECs) have been highlighted by intensive reports. Typically, there are significant differences in the doping speeds between p-doping and n-doping, with the former exhibiting a sawtooth frontier and the latter displaying a more uniform frontier profile. In addition, experimental observations demonstrate a uniform motion instead of the theoretically suggested accelerated electrochemical doping frontier propagation. Therefore, there is an urgent need to establish a quantitative model that delves into the underlying mechanisms responsible for doping propagation in LECs. In this study, four variables were selected to investigate the detailed mechanism of electrochemical doping propagation: temperature, voltage, and concentrations of salt and solid electrolyte. Fluorescence imaging revealed that the n-doping and p-doping propagations behaved contrarily with increasing temperature and voltage. By numerically fitting the doping propagation frontier, equations were derived to describe the relationship between the speed of electrochemical doping propagation and temperature/voltage. The underlying mechanisms were elucidated, indicating that anions undergo motion through the cooperative effects of electric field drift and concentration diffusion, while cation transport strongly relies on poly(ethylene oxide) (PEO) segmental motions. In other words, the movement of anions within the electrolyte is characterized by a greater degree of freedom, whereas the motion of cations is significantly dependent on the segmental motions of PEO. The resulting equations were well-fitted with experimental data, providing a solid foundation for further theoretical investigations into electrochemical doping in various devices.