Synthetic Metals 161 (2012) 2618–2622 Contents lists available at ScienceDirect Synthetic Metals j o ur nal homep ag e: www.elsevier.com/locate/synmet Inkjet printed perylene diimide based OTFTs: Effect of the solvent mixture and the printing parameters on film morphology Immacolata Angelica Grimaldi a,b,∗ , Mario Barra b , Anna De Girolamo Del Mauro a , Fausta Loffredo a , Antonio Cassinese b , Fulvia Villani a , Carla Minarini a a Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Portici Research Center, Piazzale Enrico Fermi 1, 80055 Portici, Naples, Italy b CNR-SPIN and Department of Physics Science, University of Naples Federico II, Piazzale Tecchio, 80125 Naples, Italy a r t i c l e i n f o Article history: Received 6 May 2011 Received in revised form 15 July 2011 Accepted 1 August 2011 Available online 15 September 2011 Keywords: Inkjet printing Organic thin film transistor n-Type organic semiconductor a b s t r a c t In the present work, we report the influence of the solvents on the morphology and the uniformity of inkjet printed n-type (electron-transporting) perylene diimide (PDI-8CN2) semiconductor films on SiO 2 substrates. In particular, a solvent mixture composed by o-dichlorobenzene and chloroform was employed and the semiconductor crystalline structure was investigated as a function of the mixing ratio of the component solvents. For each mixture composition, the printing parameters such as substrate temperature and drop overlapping degree, were optimized to improve the reproducibility of the deposi- tion process and the structural quality of the final films. Organic thin film transistors were fabricated and electrically characterized. The electrical measurements suggest that for the devices with larger active areas, the solvent mixture approach improves the performances of OTFTs in comparison with the use of pure o-dichlorobenzene solution. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Recently, organic thin film transistors (OTFTs) have received much attention for the fabrication of low cost, flexible and large area electronic circuitry [1]. Both p-type and n-type OTFTs are required for the development of organic complementary metal oxide semiconductor (CMOS) circuits [2–4], which are able to pro- vide electronic components with superior performances in terms of low static power consumption and higher noise margins. Different from p-channel semiconductors which have been widely studied for more than 20 years and exhibit excellent field-effect charac- teristics [5,6], today n-channel semiconductors are less commonly employed because of the low air stability and limited processing capability. Indeed, despite many experimental works carried out over the past few years, nowadays the number of n-channel com- pounds able to operate in ambient conditions with good charge mobility () remains very limited. Among them, cyanated perylene carboxylic diimide derivatives are certainly the most promising n- channel candidates owing to the high  values (0.1–1 cm 2 /V s) [7,8], low sensitivity to oxygen and moisture [9,10], remarkable environ- ∗ Corresponding author at: Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Portici Research Center, Piazzale Enrico Fermi 1, 80055 Portici, Naples, Italy. Tel.: +39 081 7723386; fax: +39 081 7723344. E-mail address: angelica.grimaldi@enea.it (I.A. Grimaldi). mental stability [11], good solubility in common organic solvents such as toluene (TOL), chloroform (CF) and o-dichlorobenzene (DCB). In particular, this last feature opens the way to the fabrica- tion of OTFTs by direct printing methods, like the inkjet technology, which are able to guarantee many advantages including low costs, low material wastage, selective deposition of materials and non- contact patterning [12]. In electrically active organic devices, the semiconductor morphology plays a crucial role in determining the final performances given the widely demonstrated correla- tion between the crystalline microstructure of the organic layer and the corresponding charge transport properties [13]. This issue is even more critical if the semiconductor is deposited by inkjet printing, since the drop drying process drastically modifies the morphology of the deposited material and can strongly affect also its overall uniformity [14]. The poor uniformity of inkjet printed films is mainly caused by the so-called ‘coffee-stain’ effect. This phenomenon is ruled by the convective flow occurring inside the sessile drop from its centre towards the edges, where the evapora- tion rate is higher, for replenishing the evaporation losses [15]. As a consequence, at the end of the drying process, the organic semicon- ductor results largely localized at the rim of the printed droplet. A possible approach to reduce the ‘coffee-stain’ effect is based on the use of mixtures of solvents with different boiling points and surface tensions. By mixing suitable high- and low-boiling point solvents, indeed, temperature and surface tension gradients can be gener- ated producing a Marangoni flow from droplet rim to the centre (Fig. 1) [16–18]. This capillary flow balances the ‘coffee-stain’ effect 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.08.004