Degenerately Doped Semi-Crystalline Polymers for High Performance Thermoelectrics

Thermoelectric (TE) energy conversion in conjugated polymers is considered a promising approach for low-energy harvesting and self-powered temperature sensing. To enhance the TE performance, it is necessary to understand the relationship between the Seebeck coefficient (α) and electrical conductivity (σ). Typical doped polymers exhibit α–σ relationship that is distinct from that of inorganic materials due to their large structural and energetic disorder, which prevents them from achieving the maximum TE power factor (PF = α²σ). Here, an ideal α–σ relationship in the Kang–Snyder model following a transport parameter s = 1 is demonstrated with two degenerately doped semi-crystalline polymers, poly[(4,4′-(bis(hexyldecylsulfanyl)methylene)cyclopenta[2,1-b:3,4-b′]dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTSBT) and poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) using a sequential doping method. The results allow the realization of the PFs reaching theoretic maxima (i.e., 112.01 µW m⁻¹ K⁻² for PPDT2FBT and 49.80 µW m⁻¹ K⁻² for PCPDTSBT) and close to metallic behavior in heavily doped films. Additionally, it is shown that the PF maxima appear when the doping state switches from non-degenerate to degenerate. Strategies towards an optimal α–σ relationship enable optimization of the PF and provide an understanding of the charge transport of doped polymers.