Tuneable thermal expansion of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate

Linear coefficient of thermal expansion is calculated for a mixture of poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate (PEDOT:PSS) using density functional theory and the Debye–Grüneisen model. The linear coefficient of thermal expansion is a key factor in thermal management (thermal conductivity, thermal stress and thermal fatigue) of microelectronic and energy devices, being common applications of the conjugated polymeric PEDOT:PSS system. The obtained value of 53 × 10 −6 K −1 at room temperature can be rationalised based on the electronic structure analysis. The PEDOT and PSS units are bonded by a dipole–dipole interaction between S in PEDOT and H in PSS. A C–C bond in a benzene ring (PSS) or thiophene (PEDOT) is up to 13 times stronger than the S–H bond. By adjusting the population of the S–H bonds by deprotonating PSS, the linear coefficient of thermal expansion can be enhanced by 57%. This allows for tuning the thermal properties of PEDOT:PSS in cutting-edge devices.