Polar Side Chains Enhance Processability, Electrical Conductivity, and Thermal Stability of a Molecularly p‐Doped Polythiophene

Abstract

Molecular doping of organic semiconductors is critical for optimizing a range of optoelectronic devices such as field‐effect transistors, solar cells, and thermoelectric generators. However, many dopant:polymer pairs suffer from poor solubility in common organic solvents, which leads to a suboptimal solid‐state nanostructure and hence low electrical conductivity. A further drawback is the poor thermal stability through sublimation of the dopant. The use of oligo ethylene glycol side chains is demonstrated to significantly improve the processability of the conjugated polymer p(g42T‐T)—a polythiophene—in polar aprotic solvents, which facilitates coprocessing of dopant:polymer pairs from the same solution at room temperature. The use of common molecular dopants such as 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) and 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) is explored. Doping of p(g42T‐T) with F4TCNQ results in an electrical conductivity of up to 100 S cm−1. Moreover, the increased compatibility of the polar dopant F4TCNQ with the oligo ethylene glycol functionalized polythiophene results in a high degree of thermal stability at up to 150 °C.

Conductive, Solution‐Processed Dioxythiophene Copolymers for Thermoelectric and Transparent Electrode Applications

“…[39] Using blade coating as the optimized casting method for PE 2 , various dopants were then tested (via solution doping for the same time period) and the thermoelectric properties were compared, as shown in Figure 2. [9,10,13,15] Once AgPF 6 was chosen as the best of this set of dopants for generating high conductivity PE 2 films, the final optimization was to vary the doping level by changing the dopant concentration and doping time. Iron(III) and Ag(I) based dopants were then investigated due to their relatively low cost, commercial availability, and previous use in doping conjugated polymer films to achieve high σ values.…”

“…[5,6] Recently, conductivity values of ≈10-350 S cm −1 have been reported [7][8][9][10][11][12] for charge neutral conjugated polymers (soluble in organic solvents) with film microstructures that favor high charge carrier mobilities that were subsequently oxidatively doped in solution to yield films with long range order. [16,17] More readily doped materials including a glycolated polythiophene [15] and an alkylated bithiophene-alt-biEDOT polymer [10] (both doped with F 4 TCNQ) were reported with σ values of 100 and 140 S cm −1 , respectively. [9,11,[13][14][15] Beyond this, the crystalline domains in the many of these materials complicate doping.…”

“…Many of these materials are difficult to dope due to their high oxidation potentials and either require specially designed dopants or are only tested with one or two common dopants (such as ferric chloride or F 4 TCNQ). [9,11,[13][14][15] Beyond this, the crystalline domains in the many of these materials complicate doping. It has been shown that chemical and electrochemical doping occurs at different rates in the amorphous and crystalline regions and that the lamellar and pi-stacking of these materials change as ions enter the crystalline domains.…”

Influence of crystallinity on the thermoelectric power factor of P3HT vapour-doped with F4TCNQ

“…4). 6,11,17,[28][29][30][31][32][33][34][35][36][37] The variation in Seebeck coefficient with electrical conductivity follows the empirical trend described by eqn (2) (Fig. 4, top).…”

Molecular Doping of a Water‐Soluble Polythiophene Derivative

“…In general, the doping efficiency is low. A high molecular doping ratio (MDR) per monomer unit is required, typically between 5 and 30%, to achieve sufficient doping effects . The high concentration of impurities and the charged species in the polymer matrix can cause defects, distortion or aggregation, which disturb the self‐assembly behavior.…”