Carbon nanotube electrodes in organic transistors

Irina Valitova,a Michele Amato,b Farzaneh Mahvash,c Giovanni Cantele,d

Antonio Ma!ucci,e Clara Santato,c Richard Martelf and Fabio Cicoira*a

The scope of this Minireview is to provide an overview of the recent progress on carbon nanotubeelectrodes applied to organic thin "lm transistors. After an introduction on the general aspects of thecharge injection processes at various electrode–semiconductor interfaces, we discuss the great potentialof carbon nanotube electrodes for organic thin "lm transistors and the recent achievements in the "eld.

Introduction

Injection of electrons and holes from the drain and sourcemetal electrodes into the organic channel plays a key role in theperformance of organic thin !lm transistors (OTFTs).1 Contactsbetween metals and organic semiconductors are generally non-

ohmic because charge carriers have to overcome an energybarrier (Schottky barrier) at the interface, which signi!cantlydeteriorates the device performance thus limiting the capacityto scale-down the devices.2,3 A possible solution to this problemis to replace the metallic electrodes of OTFTs with carbon-basedelectrodes, such as carbon nanotubes (CNTs), i.e. rolled sheetsof graphene in a seamless cylinder of diameter on the nano-meter scale.4,5 CNT electrodes are gaining much momentum inthe organic electronic community. Besides their high electricaland thermal conductivity, they o!er properties that are highlydesirable for applications in organic electronics, such as facileprocessing and printing, mechanical "exibility and chemicalstability.6,7 The main motivations for the use of CNT electrodesin organic electronics are: (i) improving the injection of chargecarriers and (ii) providing a replacement to metal electrodes in"exible carbon-based electronics.8–17 CNTs present uniqueelectrostatic behaviour due to their low dimensionality and

Richard Martel, Professor ofChemistry at the Universite deMontreal, holds a CanadaResearch Chair on electroactivenanomaterials and interfaces.He is known internationally forhis leading role in forefrontresearch on carbon nanotubeelectronics. Martel received aPhD in 1995 from the UniversiteLaval in Quebec city and workedfrom 1995 until 2003 at the IBMT.J. Watson Research Center

(NY, USA), !rst as a Postdoctoral Fellow and then as a ResearchSta! Member of Physical Sciences. Martel is a fellow of theAmerican Physical Society and received IBM OutstandingAchievement Awards for breakthroughs in nanotube electronics.

Fabio Cicoira is a professor ofChemical Engineering at EcolePolytechnique de Montreal. Heis known for his studies oforganic !eld-e!ect transistors,growth of organic semi-conductors and organic electro-chemical transistors. Hisactivities focus on organic bio-electronics and carbon basedelectrodes in organic electronicdevices. Fabio received his PhDfrom the Ecole Polytechnique

Federale de Lausanne (Switzerland) in 2002 and worked as aresearcher at the National Research Council of Italy (2002–2005and 2007–2011) and as a postdoctoral fellow at INRS-EMT (2005–2007). He was a visiting scientist at Cornell University from 2007to 2009. From 2007 to 2010 he was a recipient of the Marie CurieInternational Outgoing Fellowship of the European Commission.

aDepartment of Chemical Engineering, Ecole Polytechnique de Montreal, Montreal,Quebec, H3C 3A7, Canada. E-mail: fabio.cicoira@polymtl.cabLaboratoire des Solides Irradies (LSI), Ecole Polytechnique, 91128 Palaiseau, FrancecDepartment of Engineering Physics, Ecole Polytechnique de Montreal, Montreal,Quebec, H3C 3A7, CanadadCNR-SPIN Complesso Universitario Monte S. Angelo, Dipartimento di Scienze Fisiche,Ed. 6, Via Cintia, I-80126, Napoli, ItalyeDepartment of Electrical and Information Engineering, University of Cassino andSouthern Lazio, Via Di Biasio 43, I-03043, Cassino, ItalyfDepartment of Chemistry, Universite de Montreal, Montreal, Quebec, H3C-3J7,Canada

Cite this: Nanoscale, 2013, 5, 4638

Received 20th November 2012Accepted 27th March 2013

DOI: 10.1039/c3nr33727h

www.rsc.org/nanoscale

4638 | Nanoscale, 2013, 5, 4638–4646 This journal is ª The Royal Society of Chemistry 2013

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high aspect ratio (typically they are several mm long and have adiameter of about 1 nm). This low-dimensional electrostatics isparticularly suitable for OTFT electrodes and it is expected tofacilitate tunnelling processes for charge carrier injection intoorganic semiconductors.

ThisMinireview focuses on recent studies and applications ofCNT electrodes for OTFTs. It is worth mentioning that CNTelectrodes are also being extensively investigated in organiclight-emitting diodes and organic photovoltaic devices.10–15,18–21

Along with graphene,22–26 CNTs are expected to play a primaryrole as electrode materials in future organic electronics. Tobetter emphasize the relevance of CNT electrodes for organicelectronic applications, we brie"y discuss the physics of chargeinjection for low dimensional electrode–semiconductor junc-tions found in electronic devices.

Interfaces at metal–CNT and CNT–organic semiconductorjunctions

The planar metal–semiconductor junction has been extensivelystudied because of its high relevance in electronic devices.Charge carrier injection at this junction is dependent on thenature of the contact.26 Moreover, scaling down the devicedimensions leads to unexpected electrostatic e!ects, for whichthe device physics is still poorly understood.

The junction between a metal and a semiconductor isgenerally described using the concept of band bending. Thismodel sketches the band displacement resulting from the "owof charges across the metal–semiconductor junctions thatoccurs in order to reach the thermodynamic equilibrium, i.e.when the Fermi levels (chemical potentials) are equalized.27 Inmost cases however, the Fermi level is pinned right at themetal–semiconductor junction because of the presence of a!nite Density of States (DoS) within the band gap of the semi-conductor (an example of such gap states is Metal Induced GapStates, MIGS).27,28 This modi!ed but simple contact model wassuccessfully applied to describe planar junctions down to thesub-micrometer scale.27,29–32

For nanoscale contacts, this model needs to be adjusted inorder to take into account a drastic reduction of the MIGS andchanges of the e!ective work function of the metal caused bythe formation of chemical bonds at the interface.33,34 Thecontrol of these nanoscale processes has been indicated as ane!ective tool for tuning fundamental properties such as theelectrode work function35,36 or the suppression of the pinning ofthe Fermi level due to the !nite size of the system (as alreadydemonstrated for nanowires, nanodots, organic molecules andnanotubes).37–40

The structure of a metal–CNT junction is a signi!cantdeparture from that of the most common planar metal–semi-conductor bulk junction. The low dimensional CNT electro-statics has been extensively discussed in the context of contactbarriers in CNT !eld-e!ect transistors (CNT FETs).41 Theperformance of CNT FETs is mainly determined by the nature ofthe contact between the CNT and the metal electrodes. That is,experimental and theoretical studies demonstrated that thetransistor action mainly occurs because of a varying contact

resistance rather than channel conductance. For this reason,CNT FETs are indicated to act as unconventional Schottkybarrier transistors.42,43

The simplest CNT FET architecture consists of a single CNTplaced on top of two metal electrodes, pre-deposited on a SiO2

gate dielectric grown on a heavily doped Si back gate (the so-called side-on contact, Fig. 1).44 For metal–CNT junctions, themost important di!erence with respect to metal–semi-conductor junctions is that a geometrical alternative to the side-on is possible: the end-bonded contact geometry (Fig. 2). Acontact with side-on geometry can be described by a modi!edMIGS model with a signi!cantly lower density of states withrespect to planar bulk junctions, leading to a weaker pinning ofthe Fermi level.28,42

For end-bonded contacts, calculations have shown that theinduced charge forms a dipole ring rather than a dipole sheet(Fig. 2, le! panel), resulting in a quickly decaying electrostaticpotential.5,45 For such contacts, it is possible to neglect thepinning of the Fermi level. A general model should take intoaccount the CNT bandgap, the work function of both CNT andmetal, the geometry and orientation of the contact, and also thedetails of the local chemical bonds at the CNT–organic inter-face.43,46 In addition, due to quantum e!ects associated with thenanoscale dimension of the contacts, the transport across thebarrier is dominated by quantum tunnelling rather thanconventional thermionic e!ects.47–49

Before starting a discussion on the coupling of CNTs,employed as electrode materials, to organic semiconductors, itis important to recognize the richness of the physicochemicalinterfacial phenomena, which take place in organic electronicdevices.2,26,50–52

The description of electrode–organic semiconductorcontacts as given by the energy-level alignment rule has to takeinto account the peculiar characteristics of electrode–organicsemiconductor interfaces, i.e. the absence of long-range order,

Fig. 1 Side-on con!guration for a CNT–metal junction. The dipole sheet at theinterface is shown in the left panel.5

Fig. 2 End-bonded con!guration for a CNT–metal junction. The dipole ring atthe interface is shown in the left panel.5,45

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the presence of contaminants, and morphological discontinu-ities of the interface.53 These characteristics can cause strongvariations of the electronic properties of the contacts, inparticular of the ionization potential of the organic semi-conductor and the work function of the metal electrode.33,36

More complex models to describe the interaction betweenorganic semiconductors andmetals take into account the pillowe!ect: due to Pauli repulsion, the metal wavefunction tail getscompressed at the interface, thus reducing the metal workfunction.54 Chemical reactions, formation of interface states(also referred to as induced density of interface states, IDIS) andorientation of molecular dipoles might also occur. Interfacialdipole formation determines a variation of the electrostaticenergy at the interface, herea!er indicated with D, that leads toa shi! of the vacuum level.3,55–58

At the organic semiconductor–electrode interface, a Schottkybarrier is present because of the o!set between the lowestunoccupied molecular orbital (LUMO) or the highest occupiedmolecular orbital (HOMO) levels of the organic semiconductorand the Fermi level of the metal.59–61 It is possible to estimatethe electron (hole) injection barrier from the di!erence betweenthe LUMO (HOMO) levels of the organic semiconductor and theFermi level of the metal. Therefore, the hole injection barrier(FB,h) is de!ned as:

FB,h ! IP " Fm " D, (1)

and the electron injection barrier (FB,e):

FB,e ! Fm " EA + D, (2)

where the IP and EA are the ionization potential and electrona"nity of the organic semiconductor.30 In the absence of aninterfacial dipole layer, the height of the barriers can be foundfrom eqn (1) and (2) with D ! 0.

Examples of energy band diagrams for electrode–organicsemiconductor interfaces are shown in Fig. 3 for copper-phthalocyanine (CuPc, p-type) and phenyl-C61-butyric acidmethyl ester (PCBM, n-type) organic semiconductors and ametal with a work function of 4.8 eV (close to that of CNTs).62–64

1D electrostatic e!ects were clearly identi!ed in CNT FETsand shown to promote charge carrier tunnelling across Schottkybarriers.41–43,65,66 In OTFTs however, the charge carrier injectionprocess takes place at the CNT–organic interface, which is awayfrom the metal electrode. Because of the absence of screeningby the metallic electrode, di!erences in the injection mecha-nisms are expected depending on the temperature and appliedelectrical bias (Fig. 4).67 As pointed out by Rinzler et al., animportant di!erence is that not only the barrier width can bemodulated by the electric !eld, but also the barrier height.68

Indeed, the Fermi level of the low-DoS CNTs can be shi!ed bythe gate !eld, in contrast to high-DoS of metals. Consequently, acombination of two di!erent e!ects alters the contact barrier:thinning and lowering. This combination implies then thatdirect and Fowler–Nordheim tunneling processes are favouredat low temperature for the CNT–organic junction, whichcontrast with the bulk junction where only Fowler–Nordheimtunneling is observed.67,69 At relatively high temperature,Richardson–Schottky injection (or thermionic emission) canalso take place and the injection is then dominated by thermalemission because the thermal energy of the charge carriers issigni!cant compared to the injection barrier. At low tempera-ture, the carrier thermal energy is insu"cient and chargeinjection is mainly dominated by direct tunnelling.65,66 At strong

Fig. 3 Energy band diagrams for CuPc (normally a p-type semiconductor) andPCBM (normally a n-type semiconductor), with respect to a metal electrode workfunction of 4.8 eV (close to that of CNTs).62–64

Fig. 4 (a) Schematic diagram of interfacial barriers at di"erent temperaturesand voltages. The circles represent the charge carriers and the arrow indicates thecharge carrier injection processes: (I) thermionic emission, (II) direct tunneling,and (III) Fowler–Nordheim (F–N) tunneling. (b) Typical diagram of injectionregimes at various temperatures and voltages.67 (c) A !nite element calculation ofthe !eld ampli!cation at the tip of a carbon nanotube as a function of the aspectratio. The inset illustrates the amplitude of the !eld for a carbon nanotube 20 nmin length and 1 nm in diameter.70

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electric !eld, the trapezoidal shape of the barrier changes into atriangular one and the electrons can tunnel by a Fowler–Nord-heim mechanism.

This simple picture of the CNT–organic interface is consis-tent with recent experiments by Khondaker and Sarker.67 Usingtemperature-dependent charge carrier transport measurementson aligned CNT array–pentacene interfaces, they measured lowinjection barriers. In the high temperature range (200–300 K),charge carrier injection was dominated by thermal emission,which is well described by the Richardson–Schottky model. Thecalculated barrier height at the aligned CNT array–pentaceneinterface was 0.16 eV, which is lower than that of Au–pentaceneand Pd–pentacene (0.35 eV) interfaces. At temperatures below200 K, a transition from thermal emission to direct tunnellingwas observed for electrical bias lower than the threshold voltage(Vt ! 2.8 V). For electrical bias larger than 2.8 V, the shape ofthe tunnel barrier changed from trapezoidal to triangular, andthe current density vs. voltage relationship was successfullydescribed by Fowler–Nordheim tunnelling.

The overall advantage of the CNT electrodes described aboveis in fact related to the length/diameter ratio of the nanotubes.This ratio is extremely large for individual nanotubes, which iskey for enhancing the electric !eld at the nanotube–organicinterface. In a vacuum, the electric !eld can be hundred timeshigher compared to planar structures. In Fig. 4c, an example of!eld enhancement at the tip of a metallic CNT in a vacuum, as afunction of the aspect ratio, is shown. For a carbon nanotubehaving 1 mm length and 1 nm diameter, the electric !eldcalculated by !nite element calculations is ampli!ed more than350 times. This e!ect is similar to !eld enhancement exploitedin !eld-e!ect emitters, which emit electrons into the vacuum.By analogy similar enhancement e!ects are expected to takeplace at the carbon nanotube organic semiconductor interface,thus helping injection by narrowing down the contact barrierand, in turn, promoting Fowler–Nordheim tunneling.

Recent advances on the use of CNT electrodes in planarorganic transistors

CNTs can be used as electrodes for organic transistor electrodeseither individually or in arrays of isolated (individual) CNTs thatare randomly dispersed or aligned. On one hand, CNT arrays arebest suited as electrodes in thin !lm transistors, as they cancover much larger areas.71 On the other hand, the properties ofisolated CNTs depend on their chirality and cause device-to-device inhomogeneity in performance.

One of the most interesting applications of CNT electrodes isin the fabrication of short channel OTFTs, which are required inorder to achieve higher switching speed and on-currents.Attempts to fabricate short channel OTFTs making use of metalelectrodes have been unsuccessful so far, due to the high chargeinjection barriers. Typically, downscaling the channel lengthincreases the contact resistance at the metal–organic semi-conductor junction with respect to the channel resistance andleads to a signi!cant decrease in the apparent charge carriermobility, m, and short channel e!ects (note here that the termapparent mobility is used because the true mobility of the

channel cannot be extracted from the I–V characteristics of atransistor with contact barriers). To tackle this issue, severalgroups have fabricated short channel OTFTs using CNT elec-trodes.72–78 A review article by Cao et al.79 has extensively coveredthe use of individual CNT electrodes in short channel OTFTs.Therefore, here we will focus on recent results on CNT arrayelectrodes.

CNT arrays have been successfully used as electrodes for p-type, n-type and ambipolar OTFTs produced by solution pro-cessing as well as vacuum-sublimation of semiconductors. The!rst application of CNT array electrodes in OTFTs was demon-strated by Aguirre and co-workers, who investigated the injec-tion e"ciency of electrodes made of individual CNTs and CNTarrays in pentacene TFTs.80 A schematic illustration of thesetransistor con!gurations, along with a composite of the scan-ning electron microscopy (SEM) and the atomic force micros-copy (AFM) images, is shown in Fig. 5.

To fabricate OTFTs with CNT array electrodes, a densenetwork (z10 nanotubes mm"2) of single walled CNTs(SWCNTs) was transferred by vacuum !ltration onto a highlydoped Si wafer covered SiO2. Metal (Ti) contact pads werepatterned by optical lithography and li!-o!, and pentacene wassubsequently vacuum-sublimed on the patterned substrate.OTFTs using CNT array electrodes exhibited barrier-free-likeinjection and overall better mobility. OTFTs contacted witheither Au or Ti electrodes displayed nonlinear behavior due tothe presence of large contact barriers at the metal–organicsemiconductor interface. In the linear regime, the mobility ofOTFTs making use of the CNT array, Au, and Ti contacts was1.4 # 10"1, 9 # 10"2, and $1 # 10"3 cm2 V"1 s"1, respectively.

OTFTs with individual CNT electrodes were fabricated usingdouble-walled CNTs dispersed in dichloroethane and spin-coated onto a heavily doped n+ Si wafer covered with 200 or100 nm thick SiO2. Metal contact pads (0.5 nm thick Ti and20 nm thick Pd) were patterned on top of the CNTs by photo-lithography and li!-o!. Nanoscale electrodes were obtained byelectrical breakdown of metallic CNTs contacted by metallic

Fig. 5 (a) Schematic diagram (top) of an individual pentacene island contactedby two metallic CNTs that act as source and drain electrodes. The 50 nm gap iscreated by the electrical breakdown of a double walled metallic CNT. A compositeSEM/AFM image of a nanoscale pentacene transistor (bottom). (b) Schematicdiagram of a CNT array electrode in the con!guration used to fabricate thepentacene TFT devices (top). A SEM image of an array-contacted OTFT deviceshowing the CNTs sticking out from the titanium electrodes (bottom).80

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pads. The resulting gap length was in the range of 20–200 nm.A!er electrode fabrication, the pentacene adduct 13,6-N-sul!-nylacetamidopentacene was spin-coated on the CNT electrodes.When the adduct was converted to pentacene by a thermaltreatment, islands preferentially nucleated in the gaps betweentwo CNT ends. The mobility of the pentacene transistors wasm ! 8 # 10"1 cm2 V"1 s"1. Individual CNT electrodes showedbetter injection e"ciency than Au electrodes having compa-rable channel lengths.

Two successive studies demonstrated the good performanceof CNT array electrodes for the injection of both types of chargecarriers in thin !lms of PCBM and CuPc TFTs.81,82 In both cases,thermal annealing in a vacuum was used to convert unipolardevices to ambipolar. For PCBM TFTs, the output characteris-tics taken before thermal annealing in a vacuum (10"7 Torr), at300 K, using CNT (see Fig. 6a) and Au (see Fig. 6c) electrodesshowed unipolar n-type transistor behaviour. Speci!cally, thebehaviour in the Id–Vd characteristics of PCBM TFTs based onCNT array electrodes at low Vd indicated good injection e"-ciency (Fig. 6b and d). Indeed, upon application of the sameelectrical bias, a sublinear Id–Vd behaviour was obtained inanalogous PCBM OTFTs based on Au electrodes, indicating acharge injection hindered by the presence of an injectionbarrier.

A higher mobility for PCBM TFTs based on CNT electrodesthan that using Au electrodes was also observed at Vg ! 20 V (m$ 2 # 10"2 cm2 V"1 s"1 for CNT OTFTs and $1 # 10"3 cm2 V"1

s"1 for Au OTFTs). The mobility di!erence (CNT vs. Au devices)decreased with increasing Vg (at Vg! 40 V, m$ 1# 10"2 cm2 V"1

s"1 for CNT OTFTs and m$ 6# 10"3 cm2 V"1 s"1 for Au OTFTs).In the linear regime the mobility of CNT OTFTs was about twoorders of magnitude larger than that of Au OTFTs (at Vd ! 2 V, m$ 1 # 10"2 cm2 V"1 s"1 for CNT OTFTs and m $ 1 # 10"4 cm2

V"1 s"1 for Au OTFTs). Also, OTFTs based on CNT electrodesshowed lower Vth and higher ION/IOFF (at Vg ! 20 V, Vth $ 6 V,ION/OFF ! 106 for CNT OTFTs; Vth $ 10 V, ION/OFF ! 105 for AuOTFTs). The unipolar n-type PCBM TFTs were converted to

ambipolar by vacuum thermal annealing. This conversionmight be attributed to desorption of O2 and H2O moleculesadsorbed at the surface of the SiO2 dielectric, in analogy withresults obtained on CNT and graphene transistors.83,84 Forambipolar devices, CNT electrodes provided ohmic injection forboth types of carriers. The better performance of the CNTOTFTs compared with Au OTFTs was attributed to the one-dimensional form factor of CNT electrodes, which promotestunnelling injection across the Schottky barrier as well as on thefavorable morphology of the CNT–organic interfaces.

TFTs with CNT array electrodes based on CuPc thin !lms,deposited by supersonic molecular beam epitaxy (SuMBE),showed ohmic hole injection, high hole mobility and high ON/OFF ratio.82 In the saturation mode, at Vgs ! "20 V, m ! 1.2 #10"2 cm2 V"1 s"1 for CNT OTFTs and 5 # 10"3 cm2 V"1 s"1 forAu OTFTs. In the linear regime at Vds ! "1 V, Ids was about twoorders of magnitude higher for CNT OTFTs, which also had ahigher ON/OFF ratio (z106 for CNT OTFTs and z103 for AuOTFTs, for 0 # Vg # 20 V). As in the case of n-type PCBM TFTs,the unipolar p-type CuPc transistors were converted to ambi-polar by vacuum annealing for 24 h at 400 K. The n-type Ids/Vdscurves at low Vds were linear (ohmic) for CNT OTFTs andnonlinear for Au OTFTs (Fig. 7a and b). Extending the annealingtime of CNT OTFTs to 96 h further increased the electroncurrent by about a factor of 5 (Fig. 7c).

Fig. 6 Output characteristics (Id versus Vd at various Vg) at 300 K of PCBM TFTswith CNT (a) and Au (c) S/D electrodes for Vg ! 0, 5, 10, 15, and 20 V (the highestcurrents being measured at Vg ! 20 V). Output characteristics for 0 < Vd < 5 of thesame devices with CNT (b) and Au (d) electrodes. Vd has +50 mV steps andW/L !2000/10 mm/mm.81

Fig. 7 n-Type output characteristics of CNT OTFTs after vacuum annealing of (a)24 h and (b) 96 h. Vgs ! 0, 10, 20, 30, 40, and 50 V. (c) Ambipolar transfercharacteristics at Vds ! "20 V of CNT OTFTs before annealing (black dashed line),after 24 h (blue solid line) and after 96 h (red pointed line) annealing at 400 K. Vds

steps are +50 mV in (a) and (b). Vgs steps are +100 mV for the annealed samplesand +500 mV for the unannealed sample.W/L ! 2000/5 mm/mm. The inset of (c):HOMO–LUMO levels of CuPc with respect to a work function of 4.8 eV (close tothat of CNTs).82

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The contributions described above demonstrated that CNTarray electrodes provide ohmic charge injection in p-type, n-typeas well as ambipolar OTFTs.

By making use of transparent !lms of SWCNTs produced byairbrushing from aqueous solutions as the source, drain andgate electrodes, Southard et al. demonstrated high-performancepentacene and poly-3-hexylthiophene (P3HT) TFTs.85 Trans-parent CNT electrodes were patterned onto a "exible andtransparent polyethylene terephthalate (PET) substrate byphotolithography and O2 reactive ion etching. A transparentPMMA/Al2O3 bilayer was used as the gate dielectric. The holemobility was 9.3 # 10"2 cm2 V"1 s"1 for pentacene devices(ION/IOFF > 106) and 1.4 # 10"2 cm2 V"1 s"1 for P3HT devices(ION/IOFF > 105). For pentacene devices, the contact resistivity (rc)was gate voltage-dependent and was <30 kU cm for Vg < 40 V. ForP3HT TFTs rc was lower than 50 kU cm at all gate voltages.

Compared tomultilayer arrays, where the tube self-screeninglimits charge transport, perfectly two-dimensional (2D) arrays ofaligned SWCNTs are expected to exhibit electronic propertiesthat approach the intrinsic properties of an individual nano-tube.71 Khondaker and co-workers have advanced further thistopic using CNT arrays containing hundreds of alignedSWCNTs. They fabricated TFTs by directly growing P3HT crys-talline nanowires on CNTs previously deposited on asubstrate.72 Ultrahigh density SWCNTs were aligned from theiraqueous solutions via dielectrophoresis, then electron beamlithography (EBL) and oxygen plasma etching were used tode!ne the SWCNT electrode pattern.70 A!erwards, P3HT crys-talline nanowires were grown to connect the SWCNT interdigi-tated electrodes. The so-fabricated P3HT TFTs showed a Vth of"20 V and an ION/IOFF of 3.1 # 105. The mobility in the satu-ration and linear regimes of this device was 6.5 # 10"2 and4.0# 10"2 cm2 V"1 s"1, respectively. The highmobility and highON/OFF ratio together with the absence of short channel e!ectsand the good charge carrier injection were attributed to theimproved contact via strong p–p interaction between SWCNTelectrodes and crystalline P3HT nanowires.

The same group fabricated high performance pentaceneTFTs with a channel length L ! 0.7 mm using densely alignedSWCNT array source and drain electrodes86 (Fig. 8). For thesedevices, the mobility was as high as 6.5 # 10"1 cm2 V"1 s"1 withan average of (2.5 % 1.6) # 10"1 cm2 V"1 s"1 and the ON/OFFratios were greater than 104, with a maximum of about 106. Thebehaviour of the output curves at low bias indicated a goodinterfacial contact between the aligned CNT electrode andpentacene. For analogue OTFTs using gold electrodes, themobility was in the range of 5 # 10"3 to 5 # 10"2 cm2 V"1 s"1

with an average of (2.0 % 1.0) # 10"2 cm2 V"1 s"1, and the ON/OFF was in the range of 102 to 104. Therefore, the maximummobility and ON/OFF ratios of the devices using CNT electrodeswere about 10 times and about 100 times higher than those ofdevices using gold electrodes, respectively. All the CNT tips wereopen-ended through oxidative etching, which may explain whythe nanotube injection e"ciency was enhanced.68 Therefore, alarge number of parallel nanotubes with open-ended tipscontributed to the !eld-induced tunneling simultaneouslyresulting in higher charge injection in the devices.

Alternative architectures (vertical CNT electrodes)

The application of CNT array electrodes for vertical organictransistors was proposed by the group of Rinzler.87–91 Based onthe speci!c properties of CNTs, the vertical architecture allowstheuseof low-mobility organic semiconductors and increases therange of potential materials for organic transistors.

Liu et al. proposed vertical organic !eld e!ect transistors(VOFETs) and vertical organic light-emitting transistors(VOLETs) using a network of isolated SWCNTs as the sourceelectrode (Fig. 9).87 For the VOFET architecture, the source elec-trode has to be perforated, since a continuous metal sourceelectrode would completely screen the gate !eld from the activelayer. Networks of SWCNTs are good candidates as source elec-trodes due to their ‘open’ structure. Another advantage of thevertical structure is that the VOFET channel length correspondsto the thickness of the active layer, which can be controlledeasily. The SWCNT networks were transferred on a 200 nm thickthermal SiO2 on heavily doped Si substrates, which served as the

Fig. 8 (a) SEM image of a part of the nanotube electrodes. (b) High magni!-cation AFM image of the SWNT tips of the electrodes. AFM images of a depositedpentacene thin !lm on (c) nanotube electrodes, and (d) gold electrodes. The scalebar in !gures (a)–(d) is 1 mm.86

Fig. 9 The standard TFT (a) and the VFET (b and c) architectures. An AFM imageof the percolating nanotube network source electrode (scale$0.5 mm# 1mm) isshown in (c). Also shown is the wiring diagram for the device. A short channellength, CL, in the standard TFT requires tight patterning of the source and drainelectrodes, an issue circumvented in VFETs. The current in the standard TFT (redarrows and lines indicate paths) scales with the channel width, CW, while in theVFET it scales with the overlap area between source and drain electrodes.87

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bottom gate electrodes. A thin !lm of poly[(9,9-dioctyl-"uorenyl-2,7-diyl)-alt-co-(9-hexyl-3,6-carbazole)] (PF-9HK) or N,N0-di(1-naphthyl)-N,N0-diphenyl-1,10-diphenyl-1,40-diamine (NPD) wasdeposited over the entire substrate on top of the SWCNTnetworks. Finally, a thin Au !lm (20 nm thick) thermally evapo-rated onto the organic !lm served as the drain electrode. Asimilar approach was used to fabricate VOLETs, where indiumtin oxide was employed as the gate electrode.86 SWCNTs (sourceelectrode) were deposited on a 160 nm thick Al–Ti oxide layeracting as the gate dielectric. A PF-9HK hole-injecting layer wasspun onto the CNTs. NPD (hole transport layer) and tris(8-hydroxyquinoline) aluminum (emitting layer) were subsequentlyevaporated. Device fabrication was completed by evaporation ofLiF (electron injecting layer) and Al (drain electrode). The lumi-nance of the device was 540 cd m"2 at a current density of17.3 mA cm"2, for an e"ciency of 3.1 cd A"1.

The same group applied the vertical architecture usingSWCNT networks as the source electrode to build high-perfor-mance non-volatile memory elements.88 The SWCNT randomnetwork source electrode facilitated charge injection into thecharge storage layer, which was a 12 nm thick benzocyclobutene(BCB) polymer layer, deposited on top of a SiO2 gate dielectric.A!erwards, poly(9,9-dioctyl-"uorene-co-N-(4-butylphenyl)-di-phenylamine) (TFB) was deposited as the charge transport layer(organic active layer). The Au top drain electrode was evaporatedonto the active layer. The ON/OFF ratio of the memory elementwas >104. At Vds ! "5 V, the ON state current was about 0.8 mAfor the 0.035 mm2 pixel size, corresponding to a current densityof 2.3 mA cm"2. Large hysteresis is essential for non-volatilememory applications. A hysteresis of 157 V was shown for the"100 V to 100 V gate bias scan. The hysteresis in the device wasfully programmable, depending on the gate bias.

Rinzler et al. also fabricated a vertical OFET using a networkof SWCNTs as the source electrode and dinaphtho-[2,3-b:20,30-f]-thieno[3,2-b]thiophene (DNTT) as the organic semiconductor.89

DNTT-based CNT-VOFETs on the Al2O3/BCB dielectric showedON/OFF ratios as high as 105 for a Vg range of 4 V (for Vds !"1.0V), with an ON-current density of 110 mA cm"2. If these CNT-contacted VOFETs were to drive an OLED of comparable size,with a luminance e"ciency of 4 cd A"1, at a Jd of 25 mA cm"2,the luminance of the OLED would be 1000 cd m"2.

Successively, Rinzler et al. applied the vertical architectureusing pentacene as the organic semiconductor.90 An ION/IOFF ofabout 105 was obtained while the Jd was about 50 mA cm"2. TheION/IOFF remained above 104 when Jd was 225 mA cm"2 at Vds !"3 V. It was proven by X-ray di!raction and atomic forcemicroscopy that the CNTs induced a reorientation of the pen-tacene a–b plane toward the near vertical direction, whichresulted in increasing the !lm roughness (Fig. 10). CNTs reor-iented the a–b plane of pentacene in the direction perpendic-ular to the substrate, which resulted in higher mobility (i.e. inthe direction of current "ow).

Recently the same group employed the vertical approach todemonstrate organic light-emitting transistors integratingred, green and blue (RGB) emitters.91 The device was namedCNT-enabled vertical organic light-emitting transistors (CNT-VOLETs). As a source electrode they exploited a CNT network

possessing good electrical percolation and high optical trans-mittance (greater than 98% with the visible spectrum). Eachcolor of the CNT-VOLET was obtained operating the devicewithin a %3 V Vgs range. The luminance was 500 cd m"2 at a Vdsof "6.8 V (red), "4.9 V (green), "5.7 V (blue), with contrastratios (the ratio of the luminance at a given Vds between the onand o! states) approaching 104.

Conclusions

CNT array electrodes have been employed in OTFTs based ondi!erent classes of organic semiconductors (e.g., polymers andsmall molecules), processed by solution-based and vacuum-based techniques. The results reported so far indicate that CNTarray electrodes for OTFTs lead to improved charge carrierinjection properties compared to more conventional metalelectrodes, such as Au, thus suggesting the general validity ofthe CNT array approach. The underlying hypotheses for thisapproach are: (i) the 1D structure of CNTs facilitates chargecarrier injection by !eld emission, (ii) the reduced DoS providesa lever to further reduce the Schottky barrier height and width,and (iii) the conjugated structure of both CNTs and organicsemiconductors provides a favorable interface for chargeinjection, with a low density of charge carrier traps. Given theimproved performance for the injection of both holes andelectrons, CNT array electrodes are attractive for applications ine"cient organic light-emitting transistors, where light genera-tion requires ambipolar injection and transport.92,93

Acknowledgements

F. C., C. S. and R. M. acknowledge !nancial support from NSERC(Discovery grant) and from the Fonds de Recherche du Quebec –Nature et Technologies (FRQNT). F. C. acknowledges support fromthe Department of Chemical Engineering of Ecole Polytechniquede Montreal. M. A. acknowledges support from the EuropeanUnion through the FP7MarieCurie Intra-EuropeanFellowshipno.326794 (EXPRESS project). G.C. acknowledges !nancial support

Fig. 10 AFM micrographs of (A) a dilute CNT network on BCB/glass, (B) pen-tacene on BCB/glass, (C) pentacene on a dilute CNTnetwork on BCB/glass and (D)pentacene on a 45 nm thick CNT !lm on glass. Pentacene was thermally evapo-rated to a thickness of 560 nm at 1 A s"1, with the substrate at room temperature,at a pressure of 3 # 10"7 Torr.90

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from the European Union Seventh Framework Program (FP7/2007–2013) under grant agreement no. 264098 –MAMA.

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