Donor-acceptor conjugated copolymer with high thermoelectric performance: A case study of the oxidation process within chemical doping

Abstract

The doping process and thermoelectric properties of donor-acceptor (D-A) type copolymers are investigated with the representative poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b] dithiophene]3-fluoro-2-[(2-ethylhexyl)-carbonyl]thieno[3,4-b]thiophenediyl)) (PTB7-Th). The PTB7-Th is doped by FeCl3 and only polarons are induced in its doped films. The results reveal that the electron-rich donor units within PTB7-Th lose electrons preferentially at the initial stage of the oxidation and then the acceptor units begin to be oxidized at a high doping concentration. The energy levels of polarons and the Fermi level of the doped PTB7-Th remain almost unchange with different doping levels. However, the morphology of the PTB7-Th films could be deteriorated as the doping levels are improved, which is one of the main reasons for the decrease of electrical conductivity at the later stage of doping. The best electrical conductivity and power factor areobtained to be 42.3 S⋅cm−1 and 33.9 μW⋅mK−2, respectively, in the doped PTB7-Th film at room temperature. The power factor is further improved to 38.3 μW⋅mK−2 at 75 °C. This work may provide meaningful experience for development of D-A type thermoelectric copolymers and may further improve the doping efficiency.

Counterion Control and the Spectral Signatures of Polarons, Coupled Polarons, and Bipolarons in Doped P3HT Films

“…[ 9–11 ] The resulting allowed bipolaronic optical transition, referred to as BP1, is bluer than the P1 transition of the single polaron and typically occurs between 0.8 and 1.1 eV; this transition has been observed in many highly‐doped conjugated polymers, including poly(3,4‐ethylenedioxythiophene) (PEDOT) and derivatives, [ 15–18 ] P3HT and derivatives, [ 19–22 ] as well as other conjugated polymers. [ 23–27 ]…”

“…All of this leads to the main questions that we address in this paper: what are the electronic structure and spectral signatures of bipolarons in doped conjugated polymer films? Why do most experiments suggest that the BP1 absorption band lies to the blue of the P1 transition, [ 15–26 ] while the experiments in Figure 1 [ 28 ] and the 2‐particle Holstein model [ 29 ] suggest that BP1 lies to the red of P1? Why are bipolarons readily observed in electrochemically‐doped films, but rarely observed in chemically‐doped films?…”

“…[1][2][3][4][5] To improve their performance in many of these applications, The resulting allowed bipolaronic optical transition, referred to as BP1, is bluer than the P1 transition of the single polaron and typically occurs between 0.8 and 1.1 eV; this transition has been observed in many highly-doped conjugated polymers, including poly(3,4-ethylenedioxythiophene) (PEDOT) and derivatives, [15][16][17][18] P3HT and derivatives, [19][20][21][22] as well as other conjugated polymers. [23][24][25][26][27] Despite all this support for the way polarons and bipolarons behave in doped conjugated polymers, Enengl et al recently have proposed a different picture. [28] As shown in Figure 1, these researchers monitored both the visible/NIR (panel b) and mid-infrared (panel a) absorption spectrum of P3HT films during electrochemical doping.…”

Optimizing the doping efficiency and thermoelectric properties of isoindigo-based conjugated polymers using side chain engineering

“…[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Inherently conjugated polymers exhibit relatively low s, and chemical doping, which occurs alongside the charge transfer between the host polymer and dopant molecules, is indispensable for increasing the carrier concentration in doped polymer films. The dopants (usually FeCl 3 [26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] and 7,7,8,8-tetracyanoquinodimethane (TCNQ) [42][43][44][45][46][47][48][49][50][51][52][53][54] derivatives used for p-doping) tend to aggregate from phase separation, and the charge transport of the pristine polymers is interrupted, resulting in a low s and thus low PF value. Consequently, the doping effici...…”

“…Inherently conjugated polymers exhibit relatively low σ , and chemical doping, which occurs alongside the charge transfer between the host polymer and dopant molecules, is indispensable for increasing the carrier concentration in doped polymer films. The dopants (usually FeCl 3 26–41 and 7,7,8,8-tetracyanoquinodimethane (TCNQ) 42–54 derivatives used for p-doping) tend to aggregate from phase separation, and the charge transport of the pristine polymers is interrupted, resulting in a low σ and thus low PF value. Consequently, the doping efficiency ( η d ) significantly depends on the miscibility between polymer and dopant as well as their corresponding microstructures.…”