Enhanced Electrical Conductivity of Molecularly p-Doped Poly(3-hexylthiophene) through Understanding the Correlation with Solid-State Order

Abstract

Molecular p-doping of the conjugated polymer poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is a widely studied model system. Underlying structure–property relationships are poorly understood because processing and doping are often carried out simultaneously. Here, we exploit doping from the vapor phase, which allows us to disentangle the influence of processing and doping. Through this approach, we are able to establish how the electrical conductivity varies with regard to a series of predefined structural parameters. We demonstrate that improving the degree of solid-state order, which we control through the choice of processing solvent and regioregularity, strongly increases the electrical conductivity. As a result, we achieve a value of up to 12.7 S cm–1 for P3HT:F4TCNQ. We determine the F4TCNQ anion concentration and find that the number of (bound + mobile) charge carriers of about 10–4 mol cm–3 is not influenced by the degree of solid-state order. Thus, the observed increase in electrical conductivity by almost 2 orders of magnitude can be attributed to an increase in charge-carrier mobility to more than 10–1 cm2 V–1 s–1. Surprisingly, in contrast to charge transport in undoped P3HT, we find that the molecular weight of the polymer does not strongly influence the electrical conductivity, which highlights the need for studies that elucidate structure–property relationships of strongly doped conjugated polymers.

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

“…For a large part, the synthetic method is to blame: it requires the use of in situ poly merization techniques with a low control over the polymerization process and finally the structure of the polymer thin films. [21][22][23] Vapor-phase doping of P3HT and PBTTT with F 4 TCNQ leads to conductivities of 12.7 and 200 S cm −1 , respectively. [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).…”

“…The only improvements in charge transport were obtained via control of secondary crystallization, e.g., by using cosolvents such as ethylene glycol, that helps extract the nonconducting polystyrene sulfonate (PSS) from the bulk of the PEDOT-PSS films. [22,23] Controlled doping of wellcrystallized and oriented conjugated polymer can further lead to anisotropic conducting polymer films. [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.…”

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

“…Sequential doping can be carried out by either depositing the dopant onto the polymer from the vapour phase ('vapour doping'), 15,18 or by bringing the polymer in contact with an orthogonal solvent that dissolves the dopant.…”

“…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 .…”

“…[18][19][20] By tuning the regioregularity and processing solvent the nanostructure of P3HT could be altered leading to a much higher electrical conductivity. For example, by changing the polymer processing solvent we were able to increase the electrical conductivity from 0.01 to 13 S cm…”