A Versatile Method to Fabricate Highly In‐Plane Aligned Conducting Polymer Films with Anisotropic Charge Transport and Thermoelectric Properties: The Key Role of Alkyl Side Chain Layers on the Doping Mechanism

Abstract

A general method is proposed to produce oriented and highly crystalline conducting polymer layers. It combines the controlled orientation/crystallization of polymer films by high-temperature rubbing with a soft-doping method based on spin-coating a solution of dopants in an orthogonal solvent. Doping rubbed films of regioregular poly(3-alkylthiophene)s and poly(2,5-bis(3dodecylthiophen-2-yl)thieno[3,2-b]thiophene) with 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F 4 TCNQ) yields highly oriented conducting polymer films that display polarized UV-visible-near-infrared (NIR) absorption, anisotropy in charge transport, and thermoelectric properties. Transmission electron microscopy and polarized UV-vis-NIR spectroscopy help understand and clarify the structure of the films and the doping mechanism. F 4 TCNQ − anions are incorporated into the layers of side chains and orient with their long molecular axis perpendicular to the polymer chains. The ordering of dopant molecules depends closely on the length and packing of the alkyl side chains. Increasing the dopant concentration results in a continuous variation of unit cell parameters of the doped phase. The high orientation results in anisotropic charge conductivity (σ) and thermoelectric properties that are both enhanced in the direction of the polymer chains (σ = 22 ± 5 S cm −1 and S = 60 ± 2 µV K −1 ). The method of fabrication of such highly oriented conducting polymer films is versatile and is applicable to a large palette of semiconducting polymers.

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

“…An example is the recent work by Brinkmann et al, who studied sequentially-doped P3HT lms that had been uniaxially aligned through rubbing. 17 Upon F4TCNQ doping a concomitant increase in electrical conductivity and Seebeck coefficient was observed, with maximum values of s ¼ 22 S cm À1 for a ¼ 60 mV K À1 , which we rationalise with improved connectivity along the direction of orientation. …”

“…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).…”

“…Subsequently, P3HT can be doped in a precise manner, which allows to study the interplay of charge-carrier density, nanostructure and electrical properties. [17][18][19] Moreover, sequential doping can lead to a signicantly higher electrical conductivity above 10 S cm À1 .…”

Bringing Conducting Polymers to High Order: Toward Conductivities beyond 105 S cm−1 and Thermoelectric Power Factors of 2 mW m−1 K−2

“…[12,13] In strong contrast, advanced control of crystallization, crystal orientation, and alignment has been demonstrated for PSCs such as regioregular poly(3-hexylthiophene) (P3HT) or poly(2,5-bis(3-dodecyl-2-thienyl)thieno [3,2-b]thiophene) (C 12 -PBTTT) (see Figure 1a). [24] Solution doping of rubbed P3HT films with F 4 TCNQ afforded aligned conducting polymers with conductivities of the order of 22 S cm −1 , whereas vapor-phase doping of highly crystalline P3HT afforded conductivities of 12.7 S cm −1 . [14][15][16][17][18][19][20] It is therefore natural to take benefit of the high structural control gained on polymer semiconductors such as P3HT or PBTTT to further improve their charge transport and thermoelectric properties after doping.…”

“…[18,19] Numerous methods including self-seeded growth, blade coating, epitaxy, or hightemperature rubbing help control precisely the crystal dimensions and their orientation in thin films, resulting in diverse morphologies: single crystals, spherulites, and aligned crystalline lamellae. [22,24] More recently, the impact of the alkyl side chains on the efficiency of doping, its kinetics and the ultimate TE properties, was demonstrated for the family of PBTTT with alkyl side chains from C 8 to C 18 . Doping of thin films of P3HT or PBTTT with fluoroalkylsilanes (FTSs) or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) has led to high conductivities typically of the order of a few hundreds to 1000 S cm −1 .…”