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

• Thermoelectric properties of nano- and micro-films of P3HT were investigated.
• F4-TCNQ and Fe3+-tos3·6H2O were used as dopants and compared.
• Fe3+-tos3·6H2O was found to be a better dopant for micro-films of P3HT.
• Photo-etching was used to fabricate solution-processed flexible modules.
• Maximum power output from optimized flexible module was ∼2.5 μW at ΔT = 35 K.

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, F4-TCNQ and Fe3+-tos3·6H2O, were investigated. While doping with F4-TCNQ resulted in high electrical conductivity only in nano-films, doping with Fe3+-tos3·6H2O from a 25 mM solution yielded power factors of up to ∼30 μWm−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+-tos3·6H2O-doped micro-films. Using Fe3+-tos3·6H2O 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.

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