Microstructure Evolution and Device Performance in Solution- Processed Polymeric Field-Effect Transistors: The Key Role of the First Monolayer Suhao Wang, † Adam Kiersnowski, †,‡ Wojciech Pisula,* ,† and Klaus Mü llen* ,† † Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ‡ Polymer Engineering and Technology Division, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland *S Supporting Information ABSTRACT: Probing the role of the first monolayer in the evolution of the film polymer microstructure is essential for the fundamental understanding of the charge carrier transport in polymeric field-effect transistors (FETs). The monolayer and its subsequent microstructure of a conjugated polymer [poly(2,5-bis(3-alkylthiophen-2- yl)thieno[3,2-b]thiophene), PBTTT] film were fabricated via solution deposition by tuning the dip-coating speed and were then studied as accumulation and transporting layers in FETs. Investigation of the microstructure of the layers prepared at different coating velocities revealed that the monolayer serves as an important base for further development of the film. Significant improvement of the charge carrier transport occurs only at a critical multilayer network density that establishes the required percolation pathways for the charge carriers. Finally, at a low dip- coating speed, the polymer chains are uniaxially oriented, yielding pronounced structural anisotropy and high charge carrier mobilities of 1.3 cm 2 V −1 s −1 in the alignment direction. D uring the past decade, organic field-effect transistors (OFETs) have attracted considerable attention because of their potential applications in large-area, low-cost, flexible electronic devices, such as flexible electronic paper, radio- frequency identification (RFID) tags, postage stamps, and backplane circuitry for active-matrix displays. 1 Particularly, ultrathin-film OFETs with few semiconducting monolayers are of vital importance, because charge carriers are directly transported to conduction channels without diffusion through a dense film. For small conjugated molecules 2a−i and very recently for a conjugated polymer, 2j it has been proven that the main charge carrier transport in transistors occurs in a few molecular layers near the dielectric surface. This is also in line with the theory that predicts a high density of charges and thus high charge carrier mobility in the first few nanometers of the active film. 2k Therefore, particular emphasis was put on the molecular order within this thin accumulation layer processed, for example, by vacuum deposition, 2b,f Langmuir−Blodgett deposition, 2c and electrostatic force-based self-assembly. 2d Little is known, however, about the influence of solution processing (considered to be the future process in roll-to-roll fabrication of electronic devices) on the molecular organization in ultrathin films after solvent evaporation. There have been only few studies on conjugated polymers in ultrathin-film FETs, consisting of multilayers of poly(3- hexylthiophene) 3a,b,d and polydiacetylene. 3c Both cases showed a performance far inferior to the corresponding thick-film OFETs, proving that potential applications are a long way off. The solution processing of conjugated polymers into one single monolayer and its subsequent layers directly on the surface in a FET channel is a great challenge and therefore has been rarely reported to date. Especially, technical questions concerning precise bottom-up solution growth of a conjugated polymer from monolayer to multilayer still need to be answered. This would allow a fundamental study of the role of the first monolayer on the evolution of the bulk polymer microstructure and the charge carrier transport in the transistor. This communication reports the early stages of polymer film formation that is precisely controlled via a facile dip-coating process by tuning the pulling speed of the substrate from the solution reservoir. As a model compound, the well-known high- performance p-type polymer poly(2,5-bis(3-alkylthiophen-2- yl)thieno[3,2-b]thiophene) (PBTTT, Scheme 1) was used. 1d A monolayer and the subsequent microstructure of a conjugated polymer on a rigid OFET surface were successfully obtained from solution. We prove that the first monolayer has essential importance for the bulk microstructure evolution, whereby a critical multilayer network is necessary for creating the required percolation pathways for the charge carriers in thin-film polymer OFETs. During the dip-coating process, the pulling speed was gradually changed and had a great impact on the growth of Received: December 13, 2011 Published: February 21, 2012 Scheme 1. Molecular Structure of PBTTT Communication pubs.acs.org/JACS © 2012 American Chemical Society 4015 dx.doi.org/10.1021/ja211630w | J. Am. Chem. Soc. 2012, 134, 4015−4018