HOMO–HOMO Electron Transfer: An Elegant Strategy for p‐Type Doping of Polymer Semiconductors toward Thermoelectric Applications

Abstract

Doped semiconductor polymers are gaining huge interest as materials in future energy conversion applications such as low-power polymeric thermoelectrics (TEs), because they are light weight, flexible, printable, and suitable for large area applications like wearable technologies. [1-4] The basic challenge in TE, however, lies in efficient doping of the organic semiconductors (OSCs), because OSCs have extremely low intrinsic charge carrier concentrations and hence very low electrical conductivities in the range of 10 −6 to 10 −12 S cm −1. Molecular doping, [5] commonly used to increase the electrical conductivities of OSCs, involves the addition of a redox active organic or inorganic molecule as dopant. These dopants are capable of accepting (for p-type doping) or donating electrons to OSCs (for n-type doping), thereby generating free holes or electrons in OSCs. For p-type doping, acceptor dopants such as I 2 , [6] FeCl 3 , [7] molybdenum tris(1,2-bis(trifluoromethyl) ethane-1,2-dithiolene) (Mo(tfd) 3), [8] tetrafluorotetracyano-quinodimethane (F 4 TCNQ) and

A Solution‐Processable Pristine PEDOT Exhibiting Excellent Conductivity, Charge Carrier Mobility, and Thermal Stability in the Doped State

“…(−5.3 eV, chemical structure shown in Figure a). [ 27,38,39 ] We used this kind of electron transfer doping method due to the high stability of the doped state. [ 27 ] PEDOT‐C 6 C 8 1 (3 kg mol −1 ) displays slight in situ doped behavior in the pristine state, which could not be chemically de‐doped fully by treatment with the reducing agent sodium dithionite.…”

“…[ 27,38,39 ] We used this kind of electron transfer doping method due to the high stability of the doped state. [ 27 ] PEDOT‐C 6 C 8 1 (3 kg mol −1 ) displays slight in situ doped behavior in the pristine state, which could not be chemically de‐doped fully by treatment with the reducing agent sodium dithionite. Therefore, PEDOT‐C 6 C 8 2 was selected for further chemical doping experiments and for a detailed consequent study of the electronic properties.…”

“…With increasing dopant concentration from 0.5 up to 1.25 mol%, E F continues to shift closer to the VBM of PEDOT‐C 6 C 8 2, decreasing the hole injection barrier; concurrently, SECO also moves towards the lower binding energy region, shifting down the work function value, as typically observed in p‐doped systems. [ 27 ] These changes are plotted in Figure 3d and the absolute values are collected in Table S7, Supporting Information. Further increase in the dopant concentration up to 2.5 mol% practically coalesce the polymer VBM with E F , thus reducing the hole injection barrier down to 130 meV.…”

“…This is an impressive value for a p‐type polymer with a remarkably low dopant content (2.5 mol%), which emphasizes the facile and controlled oxidative nature of PEDOT‐C 6 C 8 , as well the efficient doping ability of Spiro‐MeOTAD(TFSI) 2 . [ 27 ]…”

“…The influence of doping on charge carrier mobility was studied using the negative differential susceptance (−Δ B ) method, applying impedance spectroscopy experiments. [ 27 ] To summarize, our strategy offers a route to synthesize soluble pristine PEDOT polymers with controlled molecular weights, which allow any degree of doping in solution in a well‐controlled fashion, and the doped state was found to be remarkably stable, retaining 80% of the initial value for 12 h in ambient atmosphere after annealing under nitrogen at 100 °C.…”