Impact of Morphology on Charge Carrier Transport and Thermoelectric Properties of N‐Type FBDOPV‐Based Polymers

The impact of the chemical structure and molecular order on the charge transport properties of two donor-acceptor copolymers in their neutral and doped states is investigated. Both polymers comprise 3,7-bis((E)-7-fluoro-1-(2-octyl-dodecyl)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b ']difuran-2,6-dione (FBDOPV) as electron-accepting unit, copolymerized with 9,9-dioctyl-fluorene (P(FBDOPV-F)) or with 3-dodecyl-2,2 '-bithiophene (P(FBDOPV-2T-C-12)). These copolymers possess an amorphous and semi-crystalline nature, respectively, and exhibit remarkable electron mobilities of 0.065 and 0.25 cm(2) V-1 s(-1) in field effect transistors. However, after chemical n-doping with 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl)dimethylamine (N-DMBI), electrical conductivities four orders of magnitude higher can be achieved for P(FBDOPV-2T-C-12) (sigma = 0.042 S cm(-1)). More charge-transfer complexes are formed between P(FBDOPV-F) and N-DMBI, but the highly localized polaronic states poorly contribute to the charge transport. Doped P(FBDOPV-2T-C-12) exhibits a negative Seebeck coefficient of -265 mu V K-1 and a thermoelectric power factor (PF) of 0.30 mu W m(-1) K-2 at 303 K which increases to 0.72 mu W m(-1) K-2 at 388 K. The in-plane thermal conductivity (kappa(||) = 0.53 W m(-1) K-1) on the same micrometer-thick solution-processed film is measured, resulting in a figure of merit (ZT) of 5.0 x 10(-4) at 388 K. The results provide important design guidelines to improve the doping efficiency and thermoelectric properties of n-type organic semiconductors.