Degenerately Doped Semi-Crystalline Polymers for High Performance Thermoelectrics

Abstract

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 (alpha) and electrical conductivity (sigma). Typical doped polymers exhibit alpha-sigma 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 = alpha(2)sigma). Here, an ideal alpha-sigma 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 mu W m(-1) K-2 for PPDT2FBT and 49.80 mu W 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 alpha-sigma relationship enable optimization of the PF and provide an understanding of the charge transport of doped polymers.