Water‐Induced Modulation of Bipolaron Formation in N-type Polymeric Mixed Conductors

N-type organic mixed ion-electron conductors (OMIECs) are a unique class of organic materials, capable of both transporting and coupling cations and electrons. While important for various devices operating in an aqueous electrolyte, understanding n-type mixed conduction remains challenging due to a lack of comprehensive data, which impedes the rational design of high performance electronic materials. Using a diverse array of in situ spectroscopy techniques, including an electrochemical spectro-gravimetric tool, Raman spectroscopy, and grazing-incidence wide angle X-Ray scattering, we investigate the electrochemical doping of two polymeric n-type OMIECs with differing oligoether side chain length. We find that the polymer films swell drastically during electrochemical reduction, with electrolyte uptake scaling proportionally to the length of the polar side chains, ultimately disrupting the crystalline structure. Electrolyte ingress in the film has two important consequences. First, the extent of water uptake during reduction governs the nature of polaronic species formed at the same doping voltage, which we studied by varying aqueous salt concentration and using ionic liquid electrolytes. Second, swelling regulates oxygen reduction reaction rates, revealing the importance of side-chain chemistry in controlling the electrochemical side reactions of the OMIECs. This study underscores the critical role of water, traditionally perceived as a passive element, in influencing the optoelectronic and electrochemical properties of OMIEC films and suggests in situ electrolyte permeation as a crucial material specification when designing n-type devices for electrochromic displays, energy storage, and catalysis.

n-type conjugated polymers Organic electrochemical transistor (OECT) Bipolaron