In-situ Measurement of the Swelling, Mechanical Property, Damage, and Doping Kinetics of Poly(3,4-propylenedioxythiophene) Electrochromic Polymers

The doping of electrochromic polymers involves rich coupling between mechanics and electrochemistry. The drastic changes in the microstructure and properties originate from the electron transport and ion intercalation in the polymers. In our recent work, we quantify the breathing strain, the evolution of the mechanical properties, materials degradation and damage, and the doping kinetics of poly(3,4-propylenedioxythiophene) (PProDOT) thin films in-situ using customized environmental nanoindentation and optical microscopy. Upon oxidation, a breathing volumetric strain of 12–33% is persistent in different sets of electrolytes of various solvents, salts, and salt molarities. Also, the elastic modulus, hardness, and yield strength decrease by a factor of two. Heavily cycled PProDOT films show reduced volumetric strain and accumulated mechanical damage of channel cracks and dysfunctional regions of slow and inhomogeneous electrochromic switching. Interfacial engineering techniques are demonstrated to increase the lifetime of thin film electrochromic electrodes by two orders of magnitude. As the swelling-induced stress may bias the doping kinetics, we observe that an applied pressure as low as 2.8 MPa significantly retards the electrochromic switching in a designed moving front setup. Our work presents a systematic characterization of mechanical behavior and doping kinetics in a model electrochromic polymer and informs the mechanical and electrochemical reliability of organic electrochromic devices.