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

Vishnu Vijayakumar; Elena Zaborova; Laure Biniek; Huiyan Zeng; Laurent Herrmann; Alain Carvalho; Olivier Boyron; Nicolas Leclerc; Martin Brinkmann

doi:10.1021/acsami.8b17594

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

ACS Appl Mater Interfaces. 2019 Feb 6;11(5):4942-4953. doi: 10.1021/acsami.8b17594. Epub 2019 Jan 28.

Authors

Vishnu Vijayakumar¹, Elena Zaborova^{2

3}, Laure Biniek¹, Huiyan Zeng¹, Laurent Herrmann¹, Alain Carvalho¹, Olivier Boyron⁴, Nicolas Leclerc³, Martin Brinkmann¹

Affiliations

¹ Université de Strasbourg, CNRS, ICS UPR 22 , F-67000 Strasbourg , France.
² CiNaM, UMR 7325, Université Aix Marseille , Campus de Luminy, Case 913 , 13288 Marseille Cedex 9, France.
³ Université de Strasbourg, CNRS, ICPEES UMR 7515 , F-67087 Strasbourg , France.
⁴ Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2) , Université de Lyon 1, CPE Lyon, CNRS UMR 5265 , Bat 308F, 43 bd du 11 Novembre 1918 , 69616 Villeurbanne , France.

PMID: 30644706
DOI: 10.1021/acsami.8b17594

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.

Keywords: conducting polymers; doping; organic thermoelectric; structure; thin films.