Design of Donor-Acceptor Polymer Semiconductors for Optimizing Combinations with Dopants to Maximize Thermoelectric Performance

Abstract

Several conjugated polymers have been designed for applications in thermoelectrics (TEs) to improve doping efficiency and charge transport. However, optimizing the compatibility of the polymer with molecular dopants through judicious design to achieve high TE performance remains a challenge. By designing a novel donor-acceptor (D-A)-based conjugated polymer, poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-co-(4,7-bis(4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-2-yl)-5,6-difluorobenzo[c][1,2,5] thiadiazole) (PBDTC2FBT), compatibility with molecular dopants was maximized, resulting in enhanced TE properties. PBDTC2FBT was designed to include two types of electron donor building blocks with one acceptor in a repeat unit; this structure formed local short-range ordered crystals. Two molecular doping processes employing FeCl3 and tris(pentafluorophenyl)borane (BCF)-water complexes generated polaronic charge carriers in PBDTC2FBT while minimizing crystal deterioration. The transport behavior of the charge carriers in PBDTC2FBT differed depending on the dopant. Studies using the temperature-dependent electrical conductivity and Kang-Snyder charge transport models indicated that the weak Coulomb binding energy with the dopant counterion in BCF-doped PBDTC2FBT facilitated charge transport compared with that in the FeCl3-doped sample at the low doping region. Finally, BCF-doped PBDTC2FBT exhibited an optimized power factor of 35.1 μW m-1 K-2, which is extremely close to the theoretical maximum (36.3 μW m-1 K-2) that can be achieved with PBDTC2FBT. This study provides useful guidelines for maximizing the TE performance of D-A conjugated polymers through judicious design.