Continuously Graded Doped Semiconducting Polymers Enhance Thermoelectric Cooling

Spatial control of thermoelectric (TE) material properties through functional grading is a promising strategy in improving cooling performance. Notably, studies on organic-based functionally graded materials for thermoelectrics have been limited compared to their inorganic-based counterparts. In this Letter, we demonstrate how the inherent processability of semiconducting polymers coupled with molecular doping provides a facile approach in fabricating continuously graded (CG) thin films beneficial for thermoelectric (Peltier) cooling. We achieve CG thin films with 1D profiles in conductivity (sigma) and Seebeck coefficient (alpha) through spatial compositional control of the molecular p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane in semiconducting poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno [3,2-b]thiophene]. Using the experimentally derived sigma and alpha spatial profiles, linear constitutive relations coupled with conservation of charge and energy are used to model the cooling performance of the CG thin films. In comparison to their equivalent uniform conditions, the CG thin films yield higher cooling temperature (Delta T-c = T-H - T-c) and higher coefficient of performance. The enhanced performance arises from efficient redistribution of the Joule heating and Peltier cooling effects. Moreover, the model calculations reveal that the magnitude of the sigma profile and the slope of the alpha profile are specific attributes leading to the enhanced cooling in CG thin films. Overall, this study highlights a simple yet powerful strategy to improve the cooling performance of thermoelectric materials through functionally graded doped semiconducting polymers.