A Planar Cyclopentadithiophene-Benzothiadiazole-Based Copolymer with sp(2)-Hybridized Bis(alkylsulfanyl)methylene Substituents for Organic Thermoelectric Devices

Abstract

A semicrystalline p-type thermoelectric conjugated polymer based on a polymer backbone of cyclopentadithiophene and benzothiadiazole, poly[(4,4'-(bis-(hexyldecylsulfanyl)methylene)cyclopenta[2,1-b:3,4-b']- dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTSBT), is designed and synthesized by replacing normal alkyl side-chains with bis(alkylsulfanyl)methylene substituents. The sp(2)-hybridized olefinic bis(alkylsulfanyl)methylene side-chains and the sulfur-sulfur (S-S) chalcogen interactions extend a chain planarity with strong interchain packing, which is confirmed by density functional calculations and morphological studies, i.e., grazing incidence X-ray scattering measurement. The doping, electrical, morphological, and thermoelectric characteristics of PCPDTSBT are investigated by comparison with those of p oly [(4,4'-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4--b']dithiophene)-alt-(benzo[c][1,2,5]thiadiazole)] (PCPDTBT) with ethylhexyl side-chains. Upon doping with a Lewis acid, B(C6F5)(3), the maximum electrical conductivity (7.47 S cm(-1)) of PCPDTSBT is similar to 1 order higher than that (0.65 S cm(-1)) of PCPDTBT, and the best power factor is measured to be 7.73 mu W m(-1) K-2 for PCPDTSBT with doping 9 mol % of B(C6F5)(3). The Seebeck coefficient-electrical conductivity relation is analyzed by using a charge transport model for polymers, suggesting that the doped PCPDTSBT film has superb charge transport property based on a high crystallinity with olefinic side-chains. This study emphasizes the importance of side-chain engineering by using the sp(2)-hybridized olefinic substituents to modulate interchain packing, crystalline morphology, and the resulting electrical properties.