Electrical and Thermoelectric Properties of a Poly(3-(2-octyldodecyl)thiophene)/Poly(3-octylthiophene)/2,3,5,6-tetrafluoro-7,7',8,8'-tetracyanoquinodimethane Viscoelastic Polymer Blend Doping System

Abstract

Organic thermoelectric materials have attracted significant research interest due to their unique advantages. Chemical doping serves as a key strategy for enhancing the thermoelectric performance of conductive polymers by modulating their electronic structures and physical properties. In this study, we investigated the electrical and thermoelectric properties of blend-doped composites comprising poly(3-(2-octyldodecyl)thiophene) (P3ODT, with branched alkyl side chains) and poly(3-octylthiophene) (P3OT, with linear alkyl side chains), doped with 2,3,5,6-tetrafluoro-7,7',8,8'-tetracyanoquinodimethane (F₄-TCNQ). The results demonstrated that incorporating P3ODT enhanced the flexibility and surface quality of the material. As the proportion of P3ODT increased in the test groups, the number of conductive complexes decreases, leading to a reduction in electrical conductivity. But the Seebeck coefficient of the blend-doped material initially rose continuously, reaching a maximum of 62.67 μV K⁻¹ due to optimized carrier energy filtering while it declined when the P3ODT:P3OT ratio reached 4:6. Notably, despite the lower conductivity, the thermoelectric performance peaked at a P3ODT:P3OT ratio of 1:9, achieving a power factor (PF) of 0.69 μW m⁻¹ K⁻². Regarding the doping mechanism, F₄-TCNQ effectively induced the aggregation of P3OT and the generation of polarons/bipolarons. However, its doping reaction with P3ODT was limited, likely due to the steric hindrance caused by bulky alkyl side chains of P3ODT. Nevertheless, the alkyl side chains enhanced carrier mobility through backbone planarization which compensated for the reduced carrier concentration and conductivity. These findings suggested that blending P3ODT as a second conjugated polymer with tailored side chains can offer an effective strategy to improve the thermoelectric properties of flexible organic materials, providing valuable molecular design principles for performance optimization.