Charge transport and structural properties of functionally graded conjugated polymer thin films for organic thermoelectrics

Semiconducting polymers demonstrate great potential for low-cost, light-weight, and flexible organic electronic devices such as field-effect transistors and organic photovoltaics. One emerging application involves organic thermoelectrics (OTEs), which interconverts heat and electricity. Functionally graded materials (FGMs) provides a pathway to improve performance of thermoelectric devices by locally tuning the material properties across operational temperature gradient. However, FGMs have not been investigated in the field of organic materials. The solution processability of semiconducting polymers provides the opportunity to test different functionally graded models. Here, we report on a double-segmented polythiophene-based conjugated polymer thin film (~30nm) through compositional control of molecular dopant F₄TCNQ. By introducing the dopant from the vapor phase, we managed to spatially control TE properties of our segmented film. Macroscopic in-plane charge transport properties were measured within and across segments. The effective Seebeck coefficient matches known models. This preliminary study on functionally graded organic thermoelectric materials provides guidelines to further development on more complex FGMs.