Growth conditions effects on morphology and transport properties of an oligothiophene semiconductor M.E. Hajlaoui a,b,* , Francis Garnier b , Lotﬁ Hassine a , Fayc ¸al Kouki a,b , Habib Bouchriha a a LPMC, Faculte ´ des Sciences de Tunis Campus Universitaire, Tunis 1002, Tunisia b Laboratoire des Mate ´riaux Mole ´culaires, CNRS, 2 Rue Henri Dunant, Thiais 94320, France Received 3 January 2002; accepted 12 February 2002 Abstract In this paper we report X-ray diffraction measurements and UV–visible spectra of thin ﬁlm of 8T oligothiophene grown at two substrate temperatures 25 and 175 8C. We also report the scanning electron microscopy pictures of 8T ﬁlms deposited at different temperatures (25, 120, 150 and 175 8C). These experimental results account for a different structural organization of organic molecules in the thin ﬁlms. We also show that the increase of substrate temperature enhances the charge transport in 8T-based ﬁeld-effect transistors. UV–visible spectroscopy shows that heating the substrate during 8T ﬁlm deposition enhances the molecular order and leads to large crystallites. The last result is conﬁrmed by scanning electron microscopy. Carrier mobility in 8T ﬁeld-effect transistor increases rapidly when the substrate temperature exceeds 120 8C, up to a value of 0.33 cm 2 V 1 s 1 . # 2002 Published by Elsevier Science B.V. Keywords: Oligothiophene semiconductor; Scanning electron microscopy; UV–visible spectroscopy 1. Introduction Organic materials have been widely studied over the two last decades for their potential applications as optoelectronic components and ﬁeld-effect transistors [1,2]. Linear oli- gothiophene built of a few molecules of thiophene (<10) present a particular interest for theses applications due to their suitable charge transport properties to their high lumines- cence efﬁciency [3] and to their high chemical and thermal stability [4]. Hence ﬁeld-effect transistors made from oli- gothiophene thin ﬁlms lead to a charge carrier mobility of about 0.33 cm 2 V 1 s 1 [5], which is comparable to that obtained with amorphous silicon. The broad absorption spec- trum of oligothiophenes in the visible range opens the use of these materials for photovoltaic applications like solar cells. The performance enhancement of these devices requires not only a perfect control of their processing but also a better understanding of orientation and structural organization of organic molecules in grown thin ﬁlms [6]. In this work, we report on the role of the substrate tem- perature and the growth rate on structure and organization of 8T vacuum evaporated thin ﬁlms using three techniques: UV–visible spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). 2. Experimental Octithiophene (8T) was synthesized and puriﬁed accord- ing to procedures described elsewhere [7]. A polycrystalline octithiophene layer was deposited on quartz substrates by vacuum evaporation at a low pressure of 6  10 6 Pa. The XRD analysis was carried out using the Bragg–Brentano method (W–2W scanning) in symmetrical reﬂection mode, with Cu Ka and a graphite monochromator. UV–visible spectroscopy was performed on ﬁlms, with a Varian Cary 500 spectrometer equipped with a Glan–Taylor polarizer. The scanning electron microscope observations were made on a LEO 1530 ﬁeld-effect gun facility. Thin ﬁlm transistors were built on thermally oxidized highly doped silicon wafers. The highly doped substrate acts as the gate electrode. The silicon oxide layer has a capacitance of 8 nF/cm 2 . Gold source and drain electrodes (channel length: 50 mm, channel width: 1 mm) were vacuum evaporated on top of the organic ﬁlm through a shadow mask. Current–voltage characteristics were measured with a Hewlett-Packard 4140B picoamper- meter/double voltage source. Synthetic Metals 129 (2002) 215–220 * Corresponding author. Present address: LPMC, Faculte ´ des Sciences de Tunis Campus Universitaire, Tunis 1002, Tunisia. 0379-6779/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII:S0379-6779(02)00040-1