Effect of solution aging on morphology and electrical characteristics of regioregular P3HT FETs fabricated by spin coating and spray coating U. Bielecka 1 , P. Lutsyk ⇑ , K. Janus, J. Sworakowski, W. Bartkowiak Institute of Physical and Theoretical Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370 Wroclaw, Poland article info Article history: Received 27 April 2011 Received in revised form 23 June 2011 Accepted 25 June 2011 Available online 28 July 2011 Keywords: OFET Poly(3-hexylthiophene) Nanofibers Spray coating abstract The paper reports on a comparative study of poly(3-hexylthiophene) (P3HT) films as well as P3HT organic field effect transistors (OFETs) fabricated by the spin coating and spray coating techniques, using fresh and aged (for 2 months) chloroform solutions. The films obtained from aged solutions contain nanostructured fibers of the polymer. The presence of nanofibers in P3HT thin films affects the performance of the OFETs: charge carrier mobil- ities in the samples deposited from the aged solutions appeared over one order of magni- tude higher than those fabricated using fresh solutions. Particularly high mobilities, exceeding 10 2 cm 2 V 1 s 1 , were found in spin coated samples deposited from aged solu- tions. Additionally, the latter samples exhibit a certain degree of anisotropy, associated with the action of centrifugal force during the deposition. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Since 1986, when the first organic field effect transistor (OFET) based on a polythiophene derivative was fabricated [1], this family has been considered promising materials for organic printed electronics, especially for OFETs. In par- ticular, regioregular poly(3-hexylthiophene) (P3HT) has attracted much attention due to its high charge carrier mobility, and favorable physical and chemical properties. Similar to the whole family of polyalkylthiophenes, P3HT exhibits a strong tendency towards crystallization and for- mation of nanofibers [2]. The aggregation affects the mobil- ity of charge carriers; in general, the mobility is strongly correlated with the material morphology, which in turn de- pends on the method of deposition, processing conditions [3], dopants [4], macromolecular parameters of material [5,6] and substrate used [7]. The highest values of charge carrier mobility in OFETs, l FET , have been obtained in films with an expanded nanofibril network formed [8]. The mechanism of formation of nanofibers has been discussed in several papers and monographs (see e.g., [9–11]). Their growth was found dependent on several fac- tors: solvent, temperature, concentration of the solution, etc. In particular, kinetics of processes occurring during long-term storage of P3HT solutions (‘‘aging’’), and the resulting molecular organization of P3HT are crucially dependent on solvent used in preparation of the solution (e.g., [8,12–14]). The effect of various solvents on aging of polythiophene solutions has been reported in several pa- pers (e.g., [3,9,12,15]). ‘‘Ripening’’ of the solution manifests itself in change of color, from orange to purple to brown [3]. Additional peaks, red-shifted with respect to the main absorption band, develop in UV–Vis absorption spectra of the solutions during aging, vindicating the aggregation of P3HT molecules. AFM images of thin films deposited from solutions at different stages of aging evidence the formation and growth of fiber-like aggregates [3,9,11,16], accompanied by an increase in l FET [3,16]. The 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.06.027 ⇑ Corresponding author. Permanent address: Institute of Physics, National Academy of Sciences of Ukraine, pr. Nauky 46, 03680 Kyiv, Ukraine. E-mail address: petro.lutsyk@pwr.wroc.pl (P. Lutsyk). 1 Present address: Institute of Macromolecular Chemistry AS CR, v. v. i., Heyrovsky ´ Sq. 2, 162 06 Prague 6, Czech Republic. Organic Electronics 12 (2011) 1768–1776 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel