A Planar Cyclopentadithiophene-Benzothiadiazole-Based Copolymer with sp²-Hybridized Bis(alkylsulfanyl)methylene Substituents for Organic Thermoelectric Devices

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 sp²-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 poly[(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 ∼1 order higher than that (0.65 S cm^-1) of PCPDTBT, and the best power factor is measured to be 7.73 μW 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 sp²-hybridized olefinic substituents to modulate interchain packing, crystalline morphology, and the resulting electrical properties.