Charge Transport, Morphological Properties and Cooling Performance of Functionally Graded Semiconducting Polymer Thin Films as Organic Thermoelectrics

Molecularly doped semiconducting polymers have demonstrated great potential in organic thermoelectrics (TEs) for thermal energy management. While functionally graded materials (FGMs) where transport properties are spatially controlled have been proven to improve TE device performance, experimentally fabricating FGMs has been a challenging task. In this work, we utilize the facile processability to modulate electronic properties through molecular doping of conjugated polymers to fabricate and characterize FG thin films. We leverage sequential vapor doping of poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) to fabricate one form of FGMs: continuously graded thin films. We observe the presence of a 5mm gradient across which doping level and transport properties vary continuously. We predict TE cooling performance based on our experimental results where cold side temperature (T_c) and coefficient of performance are calculated through linear constitutive relations coupled with conservation of charge and energy. The results demonstrate that T_c of graded samples are significantly improved compared to that of uniform profile. This study provides guidelines to further development on more complex FGMs.