Effect of Alkyl Side Chain Length on Doping Kinetics, Thermopower, and Charge Transport Properties in Highly Oriented F₄TCNQ-Doped PBTTT Films

Abstract

Doping of polymer semiconductors such as poly­(2,5-bis­(3-alkylthiophen-2-yl)­thieno­[3,2-b]­thiophene) (PBTTT) with acceptor molecules such as 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is widely used to tune the charge transport and thermoelectric (TE) properties in thin films. However, the mechanism of dopant insertion in the polymer matrix, insertion kinetics, and the ultimate doping levels reached have been investigated only marginally. This contribution addresses the effect of alkyl side chain length on the doping mechanism of a series of PBTTTs with linear side chains ranging from n-octyl to n-octyldecyl. The study focuses on thin films oriented by high-temperature rubbing and sequentially doped in F4TCNQ solution. Structure–property correlations are established as a function of side chain length by a combination of transmission electron microscopy, polarized UV–vis–NIR spectroscopy, and charge transport/thermopower measurements. Intercalation of F4TCNQ into the layers of side chains results in the expansion of the lattice along the side chains and the contraction along the π-stacking direction for all polymers. The extent of lattice expansion decreases with the increasing side chain length. UV–vis–NIR spectroscopy demonstrates integer charge transfer for all investigated PBTTTs. The doping kinetics and the final doping level depend on both the side chain length and packing. Highly disordered n-octyl and crystalline n-octyldecyl side chain layers tend to hamper dopant diffusion in the side chain layers contrary to n-dodecyl side chains that can host the highest proportion of dopants. Consequently, the best TE properties are observed for C12-PBTTT films. Alignment of the polymers significantly enhances the TE performance by increasing the charge conductivity and the thermopower along the rubbing direction. Aligned films of C12-PBTTT show charge conductivities of 193 S cm–1 along the rubbing direction and power factors of approximately 100 μW m–1 K–2 versus a few μW m–1 K–2 for nonoriented films.

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

“…[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. [25] However, those values remain well below the best conductivities reported for doped PA or even PEDOT-Tos. [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.…”

“…[24] The fact that both crystalline and amorphous phases are doped with FeCl 3 explains, at least in part, the higher overall conductivity relatively to F 4 TCNQ. [24,25] As seen hereafter, the higher overall oxidation of P3HT and C 12 -PBTTT chains achieved upon FeCl 3 doping accounts for the improved charge conductivities. Energy Mater.…”

“…[32,33] To confirm this stoichiometry, we further analyzed our samples using energy-dispersive X-ray spectroscopy-scanning transmission electron microscopy (EDX-STEM). [24,25,34,35] First, one can see that there is a uniform distribution of both S and Fe atoms in the doped layers, indicating that doping is uniform in the samples.…”

“…For both polymers, the unit cell expands along the alkyl side chain direction (d 100 ) and the π-stacking repeat (020) is reduced, demonstrating that dopant molecules penetrate inside the layers of alkyl side chains of the crystalline phase and that the change in lattice parameters correlates with the amount of incorporated dopants. [24,25] The main difference between C 12 -PBTTT and P3HT resides in the evolution of the side chain spacing. [24,25] The main difference between C 12 -PBTTT and P3HT resides in the evolution of the side chain spacing.…”

“…In the aligned C 12 -PBTTT films, the conductivity increases by several orders of magnitude from 150 S cm −1 at 0.1 × 10 −3 m to 2.2 10 5 S cm −1 at 5 × 10 −3 m. The highest conductivities of C 12 -PBTTT surpass the 10 5 S cm −1 reported for stretch-oriented bulk films of polyacetylene doped with iodine. [25] For the highly conducting C 12 -PBTTT films, one can estimate the charge carrier density based on the structural data indicating that the unit cell contains two FeCl 4 − anions per unit cell, which translates to a maximum of two charge carriers per unit cell, i.e., a carrier density of ≈9 × 10 20 cm −3 . Indeed, C 12 -PBTTT films doped directly with a 10 × 10 −3 m solution reach only a 10 4 S cm −1 conductivity.…”