Organic metal–organic semiconductor blended contacts in single crystal field-effect transistors

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Organic metal–organic semiconductor blended contacts in single crystalfield-effect transistors†

Raphael Pfattner,ab Marta Mas-Torrent,*ab C�esar Moreno,d Joaquim Puigdollers,d Ram�on Alcubilla,d

Ivano Bilotti,e Elisabetta Venuti,e Aldo Brillante,e Vladimir Laukhin,abc Jaume Vecianaab

and Concepci�o Roviraab

Received 9th May 2012, Accepted 19th June 2012

DOI: 10.1039/c2jm32925e

A novel approach to blend organic source and drain electrodes with semiconducting organic single

crystals in field-effect transistors is described. The devices fabricated show a very high performance

which is ascribed to a notable reduction of the contact resistance as measured by Kelvin probe

microscopy. The average mobility is found to be four-fold that obtained from devices where no

interpenetration of the two materials takes place. This work highlights therefore the importance of the

contacts in organic field-effect transistors not only in terms of the alignment of the energy levels but also

with respect to the interface morphology.

Introduction

During the last decade, there have been dramatic advances in the

area of organic field-effect transistors (OFETs) mainly motivated

by the low-cost production and large-area coverage that organic

materials offer.1,2 However, to study the intrinsic charge trans-

port properties of organic semiconductors (OSC), single crystal

devices are required due to their high molecular ordering and the

absence of grain boundaries.3–5 In this sense, single crystals of

dithiophene-tetrathiafulvalene (DT-TTF)6 have been shown to

be very promising due to their high field-effect mobility (mFE) and

solution-processability.6–8 It should be considered though that

the OFET performance is not only determined by the active

material but also depends greatly on the device architecture and

composition. A critical issue thus, in obtaining high performing

devices, is to reduce the parasitic contact resistances at the source

and drain electrodes,9–11 which can have a strong impact on the

overall device charge carrier mobility.12,13 Recently, the organic

charge-transfer salt tetrathiafulvalene-tetracyanoquinodi-

methane (TTF-TCNQ) has been used as source and drain

contacts in OFETs by evaporating the organic metal on

aInstitut de Ci�encia de Materials de Barcelona (ICMAB-CSIC), CampusUAB, 08193 Bellaterra, Spain. E-mail: [email protected] Research Center on Bioengineering, Biomaterials andNanomedicine (CIBER-BBN), Bellaterra, SpaincInstituci�o Catalana de Recerca i Estudis Avancats (ICREA), ICMAB-CSIC, 08193-Bellaterra, SpaindDept. Eng. Electronica and Center for Research in NanoEngineering(CrNE), UPC, Barcelona-08034, SpaineDipartimento di Chimica Fisica e Inorganica and INSTM-UdR Bologna,Universit�a di Bologna, 40136 Bologna, Italy

† Electronic supplementary information (ESI) available: AdditionalOFET device characterization, Raman, AFM, and KPM analyses. SeeDOI: 10.1039/c2jm32925e

This journal is ª The Royal Society of Chemistry 2012

previously deposited OSCs (top-contact configuration, TC) or by

evaporating the OSC on the TTF-TCNQ contacts (bottom-

contact configuration, BC).14–21 The improved performance

observed in all these devices was ascribed to a better matching of

the energy levels, a more favourable organic–organic interface, a

smaller change of the electrical potential at the OSC–organic

metal interface, or to the lower temperatures applied in the TC

configuration for the evaporation of the source and drain

contacts compared to when gold contacts are used, which do not

damage the OSC.

Here we report a novel approach to grow the source and drain

TTF-TCNQ contacts in DT-TTF (Fig. 1A) single crystal OFETs

using a solution-based technique to deposit the OSC. We

demonstrate not only that the organic metal contacts are

compatible with wet techniques but also that the method

employed here results in a blending of the contacts with the OSC

greatly reducing the contact resistance and significantly

improving the device performance.

Fig. 1 (A) Molecular structure of DT-TTF. Optical microscopy images

of DT-TTF single crystals grown from toluene (B) and PhCl (C); other

crystals were broken to measure one single crystal.

J. Mater. Chem., 2012, 22, 16011–16016 | 16011

Fig. 2 Schematic representation (left) and output characteristics (right)

of a typical BC device prepared from PhCl.Measured (A) the same day of

fabrication, (B) one day after fabrication and storage under ambient

conditions and darkness, and (C) measured after five days of storage

under ambient conditions and darkness.

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Results and discussion

Conventional TC single crystal OFETs of DT-TTF (ref. 22) were

prepared from drop casting a solution of DT-TTF either in

chlorobenzene (PhCl) or toluene (c¼ 1 mg ml�1) on top of the Si/

SiO2 substrates and allowing the solvent to evaporate slowly. It is

well-known that different polymorphs can be formed when

modifying the experimental conditions, which can have a crucial

impact on the device performance.23 However, in this case, it was

previously shown that the crystallisation of DT-TTF in both

these solvents (i.e. toluene and PhCl) leads to the same poly-

morph.8 Then, TTF-TCNQ contacts were thermally evaporated

on the top through a shadow mask. The extracted average

mobility value in the saturation regime was found to be mFE ¼1.5 cm2 V�1 s�1 with a low threshold voltage of about VTH ¼0.12 V (see ESI†). No difference in performance was observed

between the crystals prepared from toluene and PhCl solutions,

confirming that both solvents result in crystals of similar quality.

The slightly enhanced performance achieved in comparison with

previously reported TC devices with DT-TTF,8 using Au and

graphite source and drain electrodes, is in agreement with the

results published for other TTF-TCNQ based OFETs.14–21

BC devices were also fabricated by drop-casting the DT-TTF

solution on a substrate containing a previously thermally evap-

orated TTF-TCNQ electrode array. Also here toluene and PhCl

were employed to solubilise the OSC. In this crystallisation

process, the crystals are formed in the solution and then fall

down on the substrate.8 After a few hours, the solvent evaporates

and single crystals can be observed by optical microscopy on top

of the organic electrode array. Differences in the OSC crystal–

organic metal interface between the devices prepared using the

two solvents were already noticeable in the optical microscopy

images. While the crystals grown from toluene are not disturbed

on the TTF-TCNQ electrode (Fig. 1B), when using PhCl the DT-

TTF crystals seem to ramify once they come in contact with the

organic metal (Fig. 1C). Further, atomic force microscopy

(AFM) analysis of the single crystals grown on top of the TTF-

TCNQ electrodes also elucidates the different connection of the

OSC with the charge transfer salt, showing that in the case of

PhCl the crystals and electrodes are interconnected (see ESI†).

This is caused by the fact that the TTF-TCNQ salt shows certain

solubility in the most polar solvent PhCl.

Following, all the prepared devices (i.e. more than 20 for each

fabrication process) were measured in air. For toluene based DT-

TTF single crystal OFETs with TTF-TCNQ BC electrodes an

average field-effect mobility of 0.6 cm2 V�1 s�1 and a low

threshold voltage were extracted in the saturation regime,

showing a very similar performance compared to already pub-

lished DT-TTF single crystal OFETs with Au BC electrodes

(average mFE ¼ 0.57 cm2 V�1 s�1).8 The OFET performance of

these devices stored in air remained constant for over a month.

Surprisingly, the electrical characterization of the devices

prepared from PhCl just after fabrication exhibited a very poor

performance with hardly any source–drain current (ISD).

However, after some days of storing the devices under ambient

conditions and darkness a clear field-effect could be observed.

Fig. 2 shows the output characteristics of one typical device

measured at different days after preparation. The same day of

preparation, grown crystals were found well allocated on the

16012 | J. Mater. Chem., 2012, 22, 16011–16016

electrodes under the optical microscope, but when characterized

electrically, no ISD could be measured (Fig. 2A). We suggest here

that electrically, the crystals in these cases were not connected

properly. After one day a distinct field-effect was measured;

however, high hysteresis effects appeared between forward and

reverse sweeps of the source–drain voltage (VSD). This could be

attributed to some solvent resting at the interface between the

DT-TTF single crystal and the SiO2 as also shown schematically

in Fig. 2B. After further storage under ambient conditions and

darkness for five days, the same device was measured again and

the corresponding characteristics are shown in Fig. 2C. Hardly

any hysteresis between forward and reverse swept VSD was

observed. The device showed textbook like behavior which can

be explained with the well formed connection of the DT-TTF

crystal both with the dielectric and the source–drain electrodes.

Fig. 3 shows the evolution of the mFE and the VTH of the device

during days. As clearly observed, mFE increases in the first five

days and then completely stabilizes at about mFE ¼ 1.9 cm2 V�1

s�1. Similarly, the VTH also varies notably during the first three

days and then stabilizes at about 5 V. After the fifth day, the

performance of the device remains completely stable for weeks.

All the devices prepared from PhCl showed the same trend in

performance, varying only slightly in the time delays until the

device reached the optimum performance.

This effect regarding the improvement of the device perfor-

mance with time was not observed when toluene was used as

solvent, due to its lower boiling point as well as the fact that in

this case no strong interconnections are created between the

This journal is ª The Royal Society of Chemistry 2012

Fig. 3 Evolution of mFE, and VTH for a PhCl solution grown DT-TTF

single crystal OFET with TTF-TCNQ source–drain electrodes. The open

symbol represents day one of measurement, where the device was still not

working properly, i.e. no ISD; assumption: mFE ¼ 0 cm2 V�1 s�1.

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organic semiconductor and the organic metal. Additionally,

other effects related to the different solvent polarities which can

influence charge trapping can also take place. All this implies that

the device fabrication process takes a longer time to reach the

optimum performance when PhCl is employed.

Remarkably, the performance achieved when using PhCl was

always higher than with toluene. The characteristics of a typical

device, with a channel width (W) and length (L) of 9 mm and

304 mm, respectively, are shown in Fig. 4, exhibiting an ideal

OFET behaviour and a very high performance. As clearly

observed in the output characteristics shown in Fig. 4A, hardly

any hysteresis between forward and reverse sweeps of source–

drain voltages was found. The device saturated at low opera-

tional voltages and showed a very high hole mobility, extracted

Fig. 4 Electrical properties of a typical PhCl solution grown DT-TTF

single crystal OFET: (A) output characteristics measured sweeping the

source–drain voltage forward and reverse with channel width and length

of 9 mm and 304 mm, respectively. (B) Corresponding transfer charac-

teristics and extraction of the field-effect mobility in the saturation

regime.

This journal is ª The Royal Society of Chemistry 2012

in the saturation regime, of 2.7 cm2 V�1 s�1. The low threshold

voltage (2.2 V) as well as the switch-on voltage (VSO) reflects the

low level of unintentional doping.

A summary of the mobility values including the standard error

obtained for BC devices either employing Au as source and drain

with DT-TTF crystals grown from PhCl (ref. 8) as well as

employing TTF-TCNQ as source and drain electrodes with DT-

TTFcrystals grownboth from toluene andPhCl is shown inFig. 5.

Worth noting is the fact that the average OFETmobility in BC

devices prepared from PhCl solution reaches a value of 2.5 cm2

V�1 s�1, which is four-fold that obtained from devices prepared

from solutions of DT-TTF in toluene. The relatively large spread

of mobilities found in the case of PhCl with TTF-TCNQ contacts

can be ascribed to the different degrees of interconnection

between the organic material and both source and drain contacts

(see below). Additionally, it should be noted that similar spreads

in high mobility solution-processed single crystals OFETs are

commonly found.5

The chemical nature of the OSC–organic electrode interface

was studied by confocal Raman microscopy. The spectra of the

intramolecular modes in the range 1350–1650 cm�1 for DT-TTF

crystallised from PhCl lying on the metal contact could be fitted

to the sum of the bands corresponding to TTF-TCNQ and DT-

TTF (see ESI†). The absence of new bands confirms that the two

materials maintain their chemical identity and have not deteri-

orated during the device fabrication. In addition, Raman

phonon spectra (Fig. 6), which detect the lattice modes, were

recorded by a linear scan (arrow direction) on a thin DT-TTF

single crystal grown from toluene or PhCl, starting from the

channel region and moving to the TTF-TCNQ source electrode

(see schematic illustration, Fig. 6C and D). The phonon spectra

of DT-TTF devices prepared from toluene were always the same

in the channel region, far from the TTF-TCNQ electrode, and

when reaching the source electrode (Fig. 6A). This confirms that

the crystalline nature of DT-TTF is also preserved on top of the

organic electrode. On the contrary, in the case of devices

prepared from PhCl, in the contact area where the DT-TTF

crystal reaches the TTF-TCNQ electrode, a fading of the modes

occurs, which can be explained by a disruption of the lattice and,

Fig. 5 Summary of the average mFE extracted for solution grown

DT-TTF single crystal OFETs in a BC architecture with either Au or

TTF-TCNQ as source and drain electrodes and PhCl or toluene as

solvent to grow the single crystals. Data for Au as source and drain

electrodes were taken from ref. 8.

J. Mater. Chem., 2012, 22, 16011–16016 | 16013

Fig. 6 Lattice phonon Raman spectra from the linear scan (arrow

direction) of a toluene (A) and PhCl (B) solution grown DT-TTF single

crystal far (bottom trace) and across the TTF-TCNQ electrode contact

area. Schematic illustration of the Raman scan (C) and (D).

Fig. 8 Contact resistance (R � W) extracted with the effective potential

measured by Kelvin probe microscopy at both the drain (RD, top) and

source (RS, bottom) electrodes for a DT-TTF single crystal grown from

PhCl and toluene at VSD ¼ �5 V.

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therefore, supports the idea of blending of the two materials

(Fig. 6B).

To explore the OSC–organic metal interface in detail, focused

ion beam-scanning electron microscopy (FIB-SEM) studies were

performed on the cross-section of the single crystal OFETs. In

devices in which toluene was employed to grow the DT-TTF

crystals, a clear separation between the TTF-TCNQ and the DT-

TTF was observed (Fig. 7). In contrast, in devices with PhCl

grown crystals there is no clear interface between the TTF-

TCNQ and the DT-TTF but, instead, an interpenetration can be

observed, resulting in a larger effective contact area (compare

Fig. 7A and B).

The significant improvement of the device performance in

OFETs with the same electrodes, and the same active materials,

but slightly different fabrication method, was attributed to

differences in the contact resistance caused by the blending of the

OSC and the organic metal. A suitable technique to extract the

effective electrical potential drop close to the electrodes of a

working field-effect transistor is Kelvin probe microscopy

(KPM).11 In fact, by combining the measured electrical potential

close to both the source and drain electrodes whilst also

measuring the source–drain current ISD, the contact resistance

can be calculated. Fig. 8 shows the contact resistance extracted at

the source (bottom) and drain (top) electrode, for solution

prepared BC OFETs, with the DT-TTF single crystal on top of

the TTF-TCNQ electrodes, crystallised from toluene (closed

symbols) and PhCl solutions (open symbols) at VSD ¼ �5 V.

Fig. 7 Cross-section of single crystal OFETs by an FIB-SEM image

with a DT-TTF single crystal (bright regions) grown from toluene (A)

and from PhCl (B), melting into the TTF-TCNQ electrode (dark regions)

on top of a Si/SiO2 substrate.

16014 | J. Mater. Chem., 2012, 22, 16011–16016

Further information about the effective electrical potential drop

at the electrodes and the extraction of the contact resistance is

included in the ESI.† Clearly a lower contact resistance (a factor

of about 5 for drain and source electrodes), can be observed in

the OFETs prepared from PhCl, which stays in accordance

with the highest performance found in these devices. Higher

blending between the materials at the OSC–source and OSC–

drain interfaces led to a lower contact resistance. Typically, in

our experiments the contact resistance improvement caused by

the blending of the materials resulted in mobility values four

times larger than when the DT-TTF crystal was lying on the

organic metals (Fig. 5).

Considering all, we can affirm that the methodology employed

for growing the DT-TTF OSC from the more polar PhCl solu-

tion on the top of the organic metal causes a certain re-dissolu-

tion of the TTF-TCNQ contact and subsequent precipitation of

the two materials. This process does not damage the organic

materials but, instead, results in a lower contact resistance in the

devices due to the larger effective contact area, which, in turn,

has a crucial influence on the final device performance.24

Experimental section

TTF-TCNQ electrodes were thermally evaporated at a base

pressure p ¼ 10�6 mbar and a temperature of T ¼ 110 �C with a

typical deposition rate of about 10 �A s�1. The pattern for the

source and drain electrodes was defined using a shadow mask.

EDX-microanalysis was performed to prove the stoichiometry of

the organic charge transfer salt, which was found to be about

TTF1-TCNQ1.

OFETs: DT-TTF was synthesized as previously described.22

For the preparation of single crystal OFETs, Si/SiO2 substrates,

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with typically di ¼ 200 nm of dielectric thickness and the corre-

sponding gate capacitance Ci ¼ 17.25 nF cm�2 were used. The

small molecule DT-TTF was dissolved in chlorobenzene (PhCl)

or toluene (c ¼ 1 mg ml�1) and after being drop cast (drops of

about 50 ml), the solvent was allowed to evaporate slowly (2–3 h),

either on prefabricated TTF-TCNQ source and drain electrodes

(BC-architecture) or directly onto the dielectric for the TC

geometry. The standard current–voltage characteristics of the

OFETs were performed using a two channel Keithley 2612

SourceMeter connected via GPIB to a measurement computer,

equipped with MATLAB and instrument control toolbox 2.5.

Homemade MATLAB routines were used to characterize the

single crystal OFETs in the different regimes. All measurements

were carried out under ambient conditions and darkness (T ¼28 �C and RH ¼ 50%). The field-effect mobility as well as the

threshold voltage was calculated in the saturation regime

applying the widely used relation:25

mSAT ¼ 2L

WCi

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiISD;SAT

pvVSG

!2

where Ci is the insulator capacitance per unit area, and W and L

are the width and length of the crystal between the electrodes,

respectively.

Focused ion beam-scanning electron microscopy (FIB-SEM):

Experiments were performed on a Zeiss Neon 40 with cross-

beam (FIB/SEM) for observation, selective milling and deposi-

tion, tomography and elemental analysis including a focused ion

beam of gallium with 7 nm resolution. The focused beam of Ga+

ions was used to cut OFETs at the very dielectric–OSC interface

in the vertical direction and to study the cross-section by means

of SEM.

Raman spectra of DT-TTF and especially of TTF-TCNQ are

strongly dependent on the excitation energy, so that different

spectra are observed depending on the excitation. Single crystals

and OFETs were placed on the stage of an optical microscope

(Olympus BX40) interfaced to a Jobin Yvon T64000 Raman

spectrometer (647.1 nm and 752.5 nm excitations) or to a

Renishaw System 1000 (514.5 nm excitation). By using a 100�microscope objective, a spatial resolution ranging from 0.7 to

1 mm, depending on the excitation wavelength was reached. The

theoretical field depth was about 7 mm. In these conditions the

full thickness of the crystals was sampled as seen from the Si

bands of the substrate always present in the reported spectra and

subtracted when needed. The spectra were recorded spanning the

region 10–2000 cm�1, with particular attention to the low

frequency region of the lattice phonons (10–150 cm�1). The

incoming power was reduced with a neutral filter whose optical

density was selected in each experiment to prevent thermal effects

on the sample. To explore in detail the organic metal–organic

semiconductor interface, the point to point variation of the

Raman spectra of the DT-TTF single crystals in the OFET was

obtained by moving the sample with a motorized stage of the

microscope. This allowed us to scan each crystal starting away

from the TTF-TCNQ electrode up to the area lying on top of it.

Kelvin probe microscopy measurements were performed with a

Veeco Dimension 3100 microscope operated under ambient

conditions. We used commercial conductive B-doped diamond

coated Si tips with k ¼ 2.8 N m�1 (nanosensors) and a resonance

This journal is ª The Royal Society of Chemistry 2012

frequency of 75 kHz. Lift mode at a distance of 15 nm from the

surface was used to measure the surface contact potential

difference (CPD) in a second scan after recording the topography

in first step. To detect the electrostatic force, an ac bias (0.5 kHz,

1 V) is applied between the tip and sample, in addition to a dc

bias. By using a lock-in amplifier the electrostatic force can be

nullified by adjusting dc bias to match the CPD. The statistical

error in the KPM signal was determined to be about 20 mV. The

absolute values for the CPD measured signal depend on the

capacitive coupling between the cantilever and the electrodes.

For this reason a scaling is accomplished to correct the difference

between the CPD measured and the voltage applied to drain and

source contacts.26 To apply the voltages to the source, drain and

gate contacts as well as to record the current through the tran-

sistor device in the drain electrode a Keithley 2636 electrometer

was used. As reported,27 contact resistance RS,D can be deter-

mined as RS,D ¼W(DVS,D)/ID whereW is the channel width and

DVS and DVD are the potential drops at the source and drain (see

ESI†) electrode edges, respectively.

Conclusion

Typically in OFETs, special emphasis has been placed on the

matching of the energy levels of the organic semiconductor and

the metal workfunction to ensure efficient charge injection.15,28

Some efforts have been devoted to improving the organic semi-

conductor–metal interface by functionalising the metal with

self-assembled molecular monolayers to modify the metal work-

function or/and achieve higher molecular ordering at the inter-

face.29 In our work, we would like to underline that, besides the

very important matching of the Fermi level of the source and

drain electrodes with the HOMO level of the active material for p-

channelOFETs, the quality of the contact in terms ofmorphology

and interface also plays a crucial role and opens the possibility to

design new strategies to improve the device performance.

Acknowledgements

The authors thank the EU Large Project One-P (FP7-NMP-

2007-212311), Marie Curie Est FuMaSSEC, the Networking

Research Center on Bioengineering, Biomaterials and Nano-

medicine (CIBER-BBN), the DGI (Spain) with project POMAS

CTQ2010-19501/BQU and TEC2011-27859, and Consolider

HOPE project CSD2007-07 and Generalitat de Catalunya

2009SGR516.

Notes and references

1 M. Mas-Torrent and C. Rovira, Chem. Soc. Rev., 2008, 37, 827.2 J. Zaumseil and H. Sirringhaus, Chem. Rev., 2007, 107, 1296.3 C. Reese and Z. Bao, Mater. Today, 2007, 10, 20.4 W. Warta and N. Karl, Phys. Rev. B: Condens. Matter Mater. Phys.,1985, 32, 1172.

5 H. Minemawari, T. Yamada, H. Matsui, J. Tsutsumi, S. Haas,R. Chiba, R. Kumai and T. Hasegawa, Nature, 2011, 475, 364.

6 M. Mas-Torrent, M. Durkut, P. Hadley, X. Ribas and C. Rovira,J. Am. Chem. Soc., 2004, 126, 984.

7 M. Leufgen, O. Rost, C. Gould, G. Schmidt, J. Geurts,L. Molenkamp, N. Oxtoby, M. Mas-Torrent, N. Crivillers,J. Veciana and C. Rovira, Org. Electron., 2008, 9, 1101.

8 R. Pfattner, M. Mas-Torrent, I. Bilotti, A. Brillante, S. Milita,F. Liscio, F. Biscarini, T. Marszalek, J. Ulanski, A. Nosal,

J. Mater. Chem., 2012, 22, 16011–16016 | 16015

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June

201

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://pu

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View Online

M. Gazicki-Lipman, M. Leufgen, G. Schmidt, L. Molenkamp,V. Laukhin, J. Veciana and C. Rovira, Adv. Mater., 2010, 22, 4198.

9 N. Tessler and Y. Roichman, Appl. Phys. Lett., 2001, 79, 2987.10 T. J. Richards and H. Sirringhaus, J. Appl. Phys., 2007, 102, 094510.11 C. Moreno, R. Pfattner, M. Mas-Torrent, J. Puigdollers,

S. T. Bromley, C. Rovira, J. Veciana and R. Alcubilla, J. Mater.Chem., 2012, 22, 345.

12 D. Braga and G. Horowitz, Adv. Mater., 2009, 21, 1473.13 D. Natali and M. Caironi, Adv. Mater., 2012, 24, 1357.14 B. Mukherjee and M. Mukherjee, Langmuir, 2011, 27, 11246.15 K. Shibata, K. Ishikawa, H. Takezoe, H. Wada and T. Mori, Appl.

Phys. Lett., 2008, 92, 023305.16 K. Shibata, H. Wada, K. Ishikawa, H. Takezoe and T. Mori, Appl.

Phys. Lett., 2007, 90, 193509.17 Y. Takahashi, T. Hasegawa, Y. Abe, Y. Tokura, K. Nishimura and

G. Saito, Appl. Phys. Lett., 2005, 86, 063504.18 Y. Takahashi, T. Hasegawa, Y. Abe, Y. Tokura and G. Saito, Appl.

Phys. Lett., 2006, 88, 073504.19 Y. Takahashi, T. Hasegawa, S. Horiuchi, R. Kumai, Y. Tokura and

G. Saito, Chem. Mater., 2007, 19, 6382.20 H. Wada, K. Shibata, Y. Bando and T. Mori, J. Mater. Chem., 2008,

18, 4165.21 S. Haas, Y. Takahashi, K. Takimiya and T. Hasegawa, Appl. Phys.

Lett., 2009, 95, 022111.

16016 | J. Mater. Chem., 2012, 22, 16011–16016

22 N. Crivillers, N. S. Oxtoby, M. Mas-Torrent, J. Veciana andC. Rovira, Synthesis, 2007, 1621–1623.

23 M. Mas-Torrent and C. Rovira, Chem. Rev., 2011, 111, 4833.24 A device with mobility as high as 6.2 cm2 V�1 s�1 has already been

achieved, indicating that the OSC–organic metal blending can befurther optimised. However, this high mobility value has not beenconsidered in all the data analysis.

25 (a) G. Horowitz, M. Hajlaoui, H. Bouchriha, R. Bourguiga andM. Hajlaoui, Adv. Mater., 1998, 10, 923; (b) E. J. Meijer,C. Tanase, P. W. M. Blom, E. van Veenendaal, B.-H. Huisman,D. M. de Leeuw and T. M. Klapwijk, Appl. Phys. Lett., 2002, 80,3838.

26 S. G. J. Mathijssen, M. Colle, A. J. G. Mank, M. Kemerink,P. A. Bobbert and D. M. de Leeuw, Appl. Phys. Lett., 2007, 90,192104.

27 L. B€urgi, H. Sirringhaus and R. H. Friend,Appl. Phys. Lett., 2002, 80,2913.

28 S. Braun, W. R. Salaneck and M. Fahlman, Adv. Mater., 2009, 21,1450.

29 (a) X. Y. Cheng, Y. Y. Noh, J. P. Wang, M. Tello, J. Frisch,R. P. Blum, A. Vollmer, J. P. Rabe, N. Koch and H. Sirringhaus,Adv. Funct. Mater., 2009, 19, 2407; (b) K. Asadi, Y. Wu,F. Gholamrezaie, P. Rudolf and P. W. M. Blom, Adv. Funct.Mater., 2009, 21, 4109.

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