Doping and Thermoelectric Behaviors of Donor-Acceptor Polymers with Extended Planar Backbone

Controlling the Thermoelectric Performance of Doped Naphthobisthiadiazole-Based Donor-Acceptor Conjugated Polymers through Backbone Engineering

This study investigates backbone engineering and evaluates the thermoelectric properties of FeCl3-doped naphthobisthiadiazole (NTz)-based donor-acceptor (D-A) conjugated polymer films. The NTz acceptor unit is coupled with three distinct donor units, namely dialkylated terthiophene (3T), dialkylated quaterthiophene (4T), and dialkylated bisthienyl thienothiophene (2T-TT) to yield copolymers designated as PNTz3T, PNTz4T, and PNTzTT. The difference in donor units leads to diverse molecule stacking and electronic properties, which can be systematically discovered via the three polymers. The linear structure of PNTz4T enables an orderly arrangement of side chains, thereby promoting dopant intercalation for enhanced carrier concentration. Additionally, this linear structure leads to an edge-on stacking mode, thereby improving the in-plane carrier mobility. As a result, the doped PNTz4T exhibits the highest electrical conductivity (σ) of 88.3 S cm-1 along with a Seebeck coefficient (S) of 62.2 µV K-1, thereby achieving the highest power factor (PF) of 34.2 µW m-1 K-2. These results highlight the relationship between the molecular design, microstructure, and doping effects in manipulating the thermoelectric performance of doped NTz-based D-A polymers.

Unravelling Disorder Effects on Thermoelectric Properties of Semicrystalline Polymers in a Wide Range of Doping Levels

Thermoelectric (TE) performance of a specific semicrystalline polymer is studied experimentally only in a limited range of doping levels with molecular doping methods. The doping level is finely controlled via in situ electrochemical doping in a wide range of carrier concentrations with an electrolyte ([PMIM]⁺ [TFSI]^- )-gated organic electrochemical transistor system. Then, the charge generation/transport and TE properties of four p-type semicrystalline polymers are analyzed and their dynamic changes of crystalline morphologies and local density of states (DOS) during electrochemical doping are compared. These polymers are synthesized based on poly[(2,5-bis(2-alkyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophene-2-yl)benzo[c][1,2,5]thiadiazole)] by varying side chains: With oligoethylene glycol (OEG) substituents, facile p-doping is achieved because of easy penetration of TFSI^- ions into the polymer matrix. However, the charge transport is hindered with longer OEG chains length because of the enhanced insulation. Therefore, with the shortest OEG substituents the electrical conductivity (30.1 S cm^-1 ) and power factor (2.88 µW m^-1 K^-2 ) are optimized. It is observed that all polymers exhibit p- to n-type transition in Seebeck coefficients in heavily doped states, which can be achieved by electrochemical doping. These TE behaviors are interpreted based on the relation between the localized DOS band structure and molecular packing structure during electrochemical doping.