Synthetic Metals 160 (2010) 2061–2064 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Improved device performance based on crosslinking of poly (3-hexylthiophene) Manoj Gaur a , Jaya Lohani b , R. Raman b , V.R. Balakrishnan b , P. Raghunathan c , S.V. Eswaran a,∗ a St. Stephen’s College, University of Delhi, Delhi 110007, India b Solid State Physics Laboratory, Lucknow Road, Delhi 110054, India c National Brain Research Centre, Manesar 122050, India article info Article history: Received 6 April 2010 Received in revised form 14 July 2010 Accepted 20 July 2010 Available online 21 August 2010 Keywords: Poly (3-hexylthiophene) Crosslinkers Bisazides Diode devices abstract Diode devices (glass/ITO/polymer/Al) have been fabricated using poly (3-hexylthiophene) (P3HT) crosslinked with two different biaryl crosslinkers. Crosslinking was performed by exposing the thin films with different wt% of crosslinker to UV irradiation and progress of crosslinking was monitored by IR spec- troscopy. An increase in hole mobility of two orders of magnitude has been observed after crosslinking. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Poly (3-hexylthiophene) (P3HT) has emerged as one of the most widely studied conjugated materials for electronic device applica- tions due to its good film forming properties, strong absorption and high hole mobilities. P3HT has been used for various device applica- tions such as fabrication of memory devices [1], thin film transistors (TFTs) [2,3], photovoltaic devices [4–8], optoelectronic devices [9] and for the preparation of biosensors [10,11]. Hole mobility is an important parameter especially for photovoltaic applications. Hole mobilities of regioregular P3HT have been reported to be in order of 10 -5 cm 2 V -1 s -1 as calculated from space charge limited cur- rent (SCLC) method [12–15]. However, in highly ordered films high field-effect mobilities in the range of 0.1–1 cm 2 V -1 s -1 have been achieved [16]. The structural, optical and electrical properties of conjugated materials are governed by the extent of inter- as well as intra- chain delocalization of -orbitals. In solid state, dense packing of the polymer chains can allow overlap to occur in three-dimension. This 3D-electronic connectivity creates low resistivity pathways for the charge carrier to travel and hence dense chain packing with better overlap of -orbitals can lead to high electrical conductiv- ity. Since past few years, instead of synthesising new materials, the focus has shifted on the development of newer strategies to increase hole mobilities. Some of the approaches employed in this respect involves increasing molecular weight and decreasing poly- ∗ Corresponding author. Tel.: +91 11 27667462. E-mail address: sv.eswaran@gmail.com (S.V. Eswaran). dispersity [17,18], introduction of conjugated bridges in polymer backbone [19,20], doping [21], blending with different materials [22,23] and thermal crosslinking by modifying the polymer side chains and heating at high temperatures [24]. In continuation of our earlier work on crosslinking of aryl azides [25–27] and use of biaryls [28] for electronic device applications, we report here crosslinking of P3HT with two new biaryl based bisazides. This simple and novel approach can be used as an alter- native strategy for better device performance. 2. Experimental 2.1. Materials and methods Chemicals and solvents were purchased from Merck or S.D. Fine Chemicals. Regioregular P3HT (∼87 kDa) was purchased from Sigma–Aldrich Chemical Co. and used without any additional purification. Purification of compounds was done by column chro- matography over silica gel (60–120 mesh) with a gradient elution using hexane with increasing proportions of ethyl acetate. Melting points were measured on an electrothermal melting point appa- ratus and are uncorrected. Optical measurements were done on a CARY-5E UV-Vis-NIR spectrophotometer on a quartz substrate. IR spectra were recorded on Spectrum BX series spectrophotometer using KBr or Nujol (for compounds) and on GaAs substrate for film state. 1 H and 13 C NMR spectra were recorded on Bruker AVANCE 300 MHz instrument. Mass spectra were recorded on Waters Micro- analysis LCT Mass spectrometer (Model No. KC455). Film thickness was measured by SENTECH SE500 ellipsometer and AFM. AFM measurements were carried out on a Veeco CP II instrument in non- 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.07.023