Doping High‐Mobility Donor–Acceptor Copolymer Semiconductors with an Organic Salt for High‐Performance Thermoelectric Materials

Abstract

Organic semiconductors (OSCs) are attractive for fabrication of thermoelectric devices with low cost, large area, low toxicity, and high flexibility. In order to achieve high‐performance organic thermoelectric devices (OTEs), it is essential to develop OSCs with high conductivity (σ ), large Seebeck coefficient (S), and low thermal conductivity (κ ). It is equally important to explore efficient dopants matching the need of thermoelectric devices. The thermoelectric performance of a high‐mobility donor–acceptor (D–A) polymer semiconductor, which is doped by an organic salt, is studied. Both a high p‐type electrical conductivity approaching 4 S cm−1 and an excellent power factor (PF) of 7 µW K−2 m−1 are obtained, which are among the highest reported values for polymer semiconductors. Temperature‐dependent conductivity, Seebeck coefficient and power factor of the doped materials are systematically investigated. Detailed analysis on the results of thermoelectric measurements has revealed a hopping transport in the materials, which verifies the empirical relationship: S ∝ σ−1/4 and PF ∝ σ1/2. The results demonstrate that D–A copolymer semiconductors with proper combination of dopants have great potential for fabricating high‐performance thermoelectric devices.

An Organic Borate Salt with Superior p‐Doping Capability for Organic Semiconductors

“…[62] The EPR measurements did not show any evidence for the presence of additional dopant-based paramagnetic species, in line with the fact that the Mes 2 B + [B(C 6 F 5 ) 4 ] − salt cation and anion are diamagnetic, as similarly reported for [B(C 6 F 5 ) 4 ] − with a trityl cation. [16,40] This observation, however, already points toward a doping mechanism that does not yield the Mes 2 B neutral radical, at least not in the films. The determined number of spins, shown in Figure 4b, in the doped samples therefore provides a direct estimate of the number of charge carriers on P3HT.…”

“…[9][10][11] These doping strategies were recently instrumental for achieving high hole [12] and electron [13] mobility in transistors, enabling coherent charge transport in polymers, [14] boosting the power conversion efficiency in organic solar cells, [15] and demonstrating high-performance thermoelectric organic materials. [16] The nature of charge carriers formed upon doping conjugated polymers with a nondegenerate ground state, like the most widely studied poly(3-hexylthiophene) (P3HT), has been a widely discussed topic. While many studies favored the occurrence of mainly positive polarons (cation segment on a polymer chain), others suggested the formation of mainly positive bipolarons (dication segment on a polymer chain).…”

“…More recently, novel dopants based on organic salts have emerged, [16,[38][39][40][41] providing additional possibilities for doping procedures, beyond the neutral single-component molecular dopants mentioned above. Despite the opportunities offered by such doping approaches, comparably little is known about the fundamental doping processes, the expected charge-exchange reactions, and, notably, the fate in thin films of that salt ion, which is not further needed once doping has occurred, and thus the overall charge balance.…”

Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers

“…[ 26 ] We note that our doped PTB7 films had conductivities ≥ 1 S cm −1 , similar to that reported previously for both PTB7 [ 28 ] and other doped push–pull conjugated polymers. [ 6,8,31 ]…”

Doped Vertical Organic Field‐Effect Transistors Demonstrating Superior Bias‐Stress Stability

“…Trityl tetrakis(pentafluorophenyl) borate (TrTPFB), an organic salt dopant that has been reported by us previously was used for p‐doping of PDVT‐10 (see Figure S11, Supporting Information, for doping evidence). [ 51,52 ] The doping was realized by blending the PDVT‐10 solution with TrTPFB solutions at different doping ratios (Figure 5d). In our experiments, we varied the doping ratio from 0.125 to 1 mol% to fabricate doped VOFETs.…”