Electrical characterization and investigation of the piezoresistive effect of PEDOT:PSS thin films

Abstract

The field of organic electronics is recently emerging in modern electrical applications. Organic light emitting diodes have been developed and are implemented in commercially available products. The novel materials are also used in sensor applications, utilizing their intrinsic physical, chemical and electrical characteristics. Poly(3,4-ethylenedioxythiophene): poly(styrene-sulfonic acid) (PEDOT:PSS) is one of the most successful organic conductive materials. Developed as antistatic coating, it is now used in other fields as well such as in electro-optical devices as transparent electrodes. One of the reasons for its widely spread use is that water-based dispersions in high quality are available. In addition, it is considered highly stable, resisting degradation under typical ambient conditions. For this work, the usability of PEDOT:PSS as active layer for electromechanical sensor applications was investigated. The electrical properties of the material were characterized including temperature dependencies and environmental influences. A piezoresistive effect with negative sign was found. It is small in magnitude and of the same order as the change in resistance due to geometrical effects. The piezoresistive effect is temperature dependent and increasing in magnitude with higher temperatures. An average longitudinal piezoresistive coefficient pi_l of -5.6x10-10 Pa-1 at room temperature has been evaluated. The transverse effect under the same conditions is opposite in sign and two thirds in magnitude of the lateral effect. The hole mobility of PEDOT:PSS follows an Arrhenius function and thus the resistivity has a negative temperature coefficient. Some other thermally induced effects have been observed such as de-doping of the material resulting in an irreversibly lowered conductivity. Due to the low thermal conductivity of the substrate material used, Joule heating of the samples played an important role during the characterization and was utilized to investigate the temperature dependencies. The change of resistance caused by an applied stress to the sample is small, with a gage factor smaller than one.