Processing and doping of thick polymer active layers for flexible organic thermoelectric modules

While the majority of research on organic thermoelectric generators has focused on individual devices with organic films having thicknesses of several hundred nanometers (nano-films), films with micrometer-scale thicknesses (micro-films) provide a longer thermal conduction path that results in a larger temperature gradient and higher thermoelectric voltages in modules. In this study, the properties of solution-processed nano-and micro-films of the p-type semiconductor P3HT doped with two different dopants, F-4-TCNQ and Fe3+-tos(3)center dot 6H(2)O, were investigated. While doping with F-4-TCNQ resulted in high electrical conductivity only in nano-films, doping with Fe3+-tos(3)center dot 6H(2)O from a 25 mM solution yielded power factors of up to similar to 30 mu Wm(-1) K-2 with a conductivity of 55.4 Scm(-1) in micro-films. Changes in the molecular packing were compared based on X-ray diffraction, and the best operational stability in air was found for the Fe3+-tos(3)center dot 6H(2)O-doped micro-films. Using Fe3+-tos(3)center dot 6H(2)O as dopant, flexible thermoelectric modules with solution-processed micro-films patterned by a photo-etching technique that does not require alignment and assembly of individual devices were demonstrated, exhibiting a maximum power output of 1.94 nWK(-2) for a uni-leg module with 48 elements. Analysis of the flexible module performance showed that the performance is limited by the contact resistance, which must be taken into consideration when optimizing module structure. (c) 2016 Elsevier B.V. All rights reserved.