Enhancements in the Mechanical Stretchability and Thermoelectric Properties of PEDOT:PSS for Flexible Electronics Applications

Intrinsically conducting polymers can have important application in flexible electronic devices and systems owing to their high mechanical flexibility. Among them, poly(3,4-ethylenedioxythiophene):-polystyrenesulfonate (PEDOT:PSS) is particularly important, because it can be dispersed in water or polar organic solvents while most of conducting polymers are intractable and insoluble. In addition, PEDOT:PSS can have high transparency in the visible range, excellent thermal stability, and high conductivity. Thus, it can have important applications in many areas, such as in electronic devices, peculiarly optoelectronic devices like light-emitting diodes (LEDs), solar cells and photodetectors, and energy storage devices including batteries and supercapacitors. Although PEDOT:PSS can have high conductivity and excellent biocompatibility, its stretchability is limited. Stretchable conductors are crucial for stretchable electronic systems. The stretchability of PEDOT:PSS can be enhanced by blending with a soft polymer or elastomer or plasticization. The blends of PEDOT:PSS with waterborne polyurethane (WPU) can have a conductivity of similar to 80 S/cm and an elongation at break of >30%. We demonstrated their applications as stretchable electromagnetic shielding, stretchable thermotherapy, and compliant electrodes of soft robots. We also found that D-sorbitol that is biocompatible and a solid at room temperature can serve as a plasticizer of PEDOT:PSS. It can enhance the conductivity of PEDOT:PSS to >1000 S/cm and the elongation at break to >60%. The plasticization mechanism is related to the hydrogen bond formation between D-sorbitol and PSSH of PEDOT:PSS. PEDOT:PSS with high thermoelectric properties can be used for flexible thermoelectric generators. It should have both high conductivity and a high Seebeck coefficient. But conductivity and the Seebeck coefficient are interdependent. The thermoelectric properties of PEDOT:PSS can be enhanced by dedoping or energy filtering. PEDOT:PSS with a power factor of 332 mu W/(m K-2) can be attained through the successive treatments with acid and base, because the acid treatment can enhance its conductivity and the base treatment can enhance its Seebeck coefficient by dedoping PEDOT:PSS. Dedoping PEDOT:PSS by bases is more effective in enhancing the Seebeck coefficient than dedoping by reducing agents. Coating a layer of an ionic liquid on the acid-then-base treated PEDOT:PSS can enhance the Seebeck coefficient up to 70 mu V/K, power factor to 750 mu W/(m K-2), and figure of merit (ZT) to 0.75. The ZT value is comparable to the best inorganic thermoelectric materials at room temperature. The enhancement in the Seebeck coefficient by ionic liquids is ascribed to the energy filtering owing to the ion accumulations at the two ends of the ionic liquid layer under a temperature gradient. This can be obtained by coating a layer of polyelectrolyte as well. Moreover, coating a layer of Rhodamine 101 that is a zwitterion with a high intrinsic dipole moment can also enhance the Seebeck coefficient of PEDOT:PSS. Because the Seebeck coefficient enhancement is consistent with the work function decrease of PEDOT:PSS, we proposed the surface energy filtering mechanism for the Rhodamine 101 effect. The intrinsic dipole moment and interfacial dipole moment of Rhodamine 101 can block the charge carriers with low energy and thus increase the Seebeck coefficient of PEDOT:PSS.