Making Contacts to n-Type Organic Transistors Using Carbon Nanotube Arrays Fabio Cicoira, †,§ Carla M. Aguirre, ‡ and Richard Martel §, * † Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche, Via alla Cascata 56/c, Trento 38123, Italy, ‡ De ´partement de Genie Physique, E ´ cole Polytechnique de Montre ´al, Montre ´al, Que ´bec H3C 3A7, Canada, and § De ´partement de Chimie, Universite ´ de Montre ´al, Montre ´al, Que ´bec H3C 3J7, Canada T he performance of organic thin film transistors (OTFTs) has improved considerably over the last decades owing to important progress in organic semiconductor synthesis and device engineering. 1-4 The electrical characteris- tics of OTFTs are, however, limited by the contacts, whose resistance depends criti- cally on the electrostatics and on the local details of the electrodes/organic semicon- ductor interface. 5-8 For instance, charge carrier injection efficiency can be limited by the presence of Schottky barriers that originate from an offset between the metal workfunction and the highest occupied mo- lecular orbital (HOMO, for hole injection) or the lowest unoccupied molecular orbital (LUMO, for electron injection) of the organic semiconductor, 9,10 or to additional offsets induced by local charge transfer to accep- tor/donor states. The presence of impuri- ties, morphological discontinuities, interfa- cial traps, and structural disorder in the organic film at the electrode interface are other factors affecting the energy level alignment and the charge carrier injection properties. 11-13 Low injection efficiency has been so far a severe performance hurdle for bottom contact OTFTs (with bottom gate), that is, the most interesting geometry for organic microelectronics. Although progress has been achieved using thiol self- assembled monolayers, 14-17 a universal ap- proach to optimize the charge injection ef- ficiency in OTFTs remains to be developed. The use of carbon nanotubes (CNTs) to optimize charge carrier injection in OTFTs is attractive because their one-dimensional (1D) structure can induce strong electro- static effects at the electrode/semiconduc- tor interface. One-dimensional electrostatic effects around CNTs were clearly identified in carbon nanotube field-effect transistors and shown to promote charge carrier tun- neling across the contact Schottky barriers. 18-22 Other advantages for the use of CNTs are high electrical conductivity, ease of processing, compatibility with flexible elec- tronics, and chemical inertness. 23 CNT elec- trodes have already been used in organic electronic devices. Hole injection has been demonstrated in organic light emitting di- odes (OLEDs) and in p-type OTFTs. 24-28 Re- cently, OLEDs having CNT layers as anode and cathode have been reported. 29 In this work, we investigated the perfor- mance of CNT arrays as source/drain elec- trodes for n-type phenyl-C61-butyric acid methyl ester (PCBM) OTFTs on a SiO 2 /Si sub- strate. As shown in Figure 1a, the CNT elec- trodes consist of a disordered array of indi- vidual (or small bundle of) nanotubes having one end connected to a large metal (Ti) pad and the other end embedded in the organic semiconductor layer. This elec- trode geometry preserves the 1D structure of the nanotubes and aims to provide opti- mum conditions enabling 1D electrostatics. Compared to conventional Au source/drain *Address correspondence to r.martel@umontreal.ca. Received for review October 9, 2010 and accepted November 23, 2010. Published online December 9, 2010. 10.1021/nn1027032 © 2011 American Chemical Society ABSTRACT We investigated the performance of carbon nanotube (CNT) array electrodes applied to n-type and ambipolar phenyl-C61-butyric acid methyl ester (PCBM) thin film transistors on a SiO 2 dielectric substrate. Compared to conventional Au electrodes, CNT arrays provide better injection efficiency, improved switching behavior, higher electron mobility, and lower contact resistance. Experiments on ambipolar PCBM transistors indicate that the injection performance is enhanced by the electrostatics of the CNT contacts, which promotes electron and hole tunneling across Schottky barriers at the PCBM/nanotube interface. The use of CNT arrays is a valid replacement to low workfunction metals, which are often reactive in air and difficult to process. Our work paves the way for a widespread use of carbon nanotube array electrodes in high-performance n-type and p-type organic thin film transistors. KEYWORDS: organic transistors · charge carrier injection · ambipolar transport · carbon nanotubes · Schottky barriers · contacts ARTICLE www.acsnano.org VOL. 5 ▪ NO. 1 ▪ 283–290 ▪ 2011 283