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Organic Electronics 8 (2007) 759–766

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Air stability of n-channel organic transistors basedon nickel coordination compounds

Hiroshi Wada, Tomohiro Taguchi, Bunpei Noda, Takuya Kambayashi,Takehiko Mori *, Ken Ishikawa, Hideo Takezoe

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan

Received 10 May 2007; received in revised form 25 May 2007; accepted 20 June 2007Available online 3 July 2007

Abstract

A series of tetraphenyl nickel dithiolene complexes, 1, are prepared, and the air stability of the n-channel organic field-effect transistors (OFET) is investigated. The fluoro and trifluoromethyl derivatives with reduction potentials higher than0 V afford reasonably air-stable n-channel OFET. Metallic organic charge-transfer complex (TTF)(TCNQ) (TTF: tetra-thiafulvalene and TCNQ: tetracyanoquinodimethane) is used as source and drain electrodes, and realizes n-channel tran-sistor properties even when Al electrodes do not work.� 2007 Elsevier B.V. All rights reserved.

PACS: 73.61.Ph

Keywords: Organic transistor; Metal complex; n-Channel; Charge-transfer complex

1. Introduction

Organic field-effect transistors have attracted agreat deal of attention due to the potential applica-tions to large area and flexible devices [1]. Althoughsatisfactory p-channel FET characteristics havebeen achieved by various compounds such as poly-acenes and oligothiophenes, good n-channel organicmaterials for OFETs are relatively limited [2]. C60

and naphthalene tetracarboxy dianhydride deriva-tives are known as representative n-channel OFETsemiconductors as well as a variety of fluoro and tri-

1566-1199/$ - see front matter � 2007 Elsevier B.V. All rights reserved

doi:10.1016/j.orgel.2007.06.007

* Corresponding author. Tel.: +81 3 5734 2427; fax: +81 3 57342876.

E-mail address: mori.t.ae@m.titech.ac.jp (T. Mori).

fluoromethyl compounds, and some of them showvery high performance [2]. These n-channel OFETsare, however, usually not very stable in air, thoughseveral air-stable compounds have been known[2d,2f]. It has been reported that n-type organicsemiconductors are unstable with respect to oxida-tions by H2O and O2, which take place at redoxpotentials of �0.66 V and 0.57 V (vs. SCE), respec-tively [3]. Compounds with reduction potentials tothe monoanionic (1-) states higher than the former(�0.66 V) is reported to be stable in air [4], as repre-sented by F16CuPc (copper hexadecafluoro phthalo-cyanine) [2d]. However, even these n-typesemiconductors are not always stable dependingon the conditions. Reduction potentials higher thanthe latter (0.57 V) are realized only in an extremely

.

760 H. Wada et al. / Organic Electronics 8 (2007) 759–766

strong electron acceptor. In this connection, metalcoordination compounds are promising becausemetal complexes show a wide variety of redoxpotentials in comparison with pure organic com-pounds. There are several metal coordination com-pounds that have been used for OFET (Scheme 1).In addition to copper phthalocyanine, which is awell-known p-channel material [5], bis(1,2-phenyl-enediamino)Ni has been reported to show p-channelcharacteristics [6]. In addition to F16CuPc [2d], n-channel characteristics have been observed in N-octadecylpyridinium Ni(dmit)2 (dmit: 4,5-dimerca-pto-1,3-dithiole-2-thione) and bis(benzoquinonedi-oximato)Pt [7]. Ambipolar OFET has been alsoreported in bis(4-dimetylaminodithiobenzyl)Ni [8].We have reported that a tetraphenyl nickel dithio-lene complex exhibits n-channel characteristics (1a

in Scheme 2) [9]. Interestingly, a nickel diaminocompound, coordinated by four nitrogen atoms(Scheme 1), is a strong electron donor and worksas a p-channel semiconductor, while a nickel dithio-lene compound with four-sulfur coordination is astrong electron acceptor and achieves an n-channelOFET. Since OFET of 1a is still not very stable in

NH

Ni

HN

NH

HN

bis(1,2-phenylenediamino)Ni N

bis(benzoquinonedioximato)Pt

NPt

N

N

NOH

O

O

OH

bis

Scheme 1. Metal complexes sho

Me Et

R CHO R

OR

OHKCN

EtOH / H2O

P2

R =

a

2

b

3

c

Scheme 2. Preparation of bis(dith

air, we have prepared a series of tetraphenyl nickeldithiolene complexes substituted at the phenylgroups (Scheme 2). These compounds have exhib-ited n-channel FET properties, and particularlythe air stability is improved in the compounds withfluoro substituents. Here we report correlationbetween the redox potentials and the air stabilityof the OFETs in these materials.

2. Results and discussion

2.1. Syntheses and electrochemistry

The substituted tetraphenyl nickel dithiolenecomplexes have been investigated as dyes [10].Except for commercially available ‘‘bis(dithioben-zyl)nickel’’ 1a, the nickel dithiolene compoundsare prepared according to the two-step sequence(Scheme 2) similarly to [10]. Compounds 3 are pre-pared from aromatic aldehydes by the benzoin con-densation [11]. Although the yield of the benzoincondensation is excellent for the non-substitutedbenzaldehyde, the fluorine substituted compoundsresult in relatively low yields. Accordingly the ben-

NNi

S

S

S

SS

S

S

S

SS

C16H37

-octadecylpyridinium-Ni(dimt)2

(4-dimethylaminodithiobenzyl)Ni

SNi

S

S

S

N

N

wing FET characteristics.

SNi

S

S

SR

R

R

R

F F3C

S5

dioxane1

d e

NiCl2.6H2O

iobenzyl)nickel derivatives.

H. Wada et al. / Organic Electronics 8 (2007) 759–766 761

zoins 3 are purified by vacuum distillation. The sec-ond step proceeds smoothly for non-fluoro deriva-tives 1b and 1c, but precipitates are not easilyobtained for the fluoro derivatives 1d and 1e. Inthese cases, stirring under air for a few days affordsthe desired precipitation. This may be related to therelatively high oxidation potentials of these com-pounds. The ethyl derivative 1c is a previouslyunknown compound. All obtained compounds arepurified by sublimation, giving black microcrystals.

The redox potentials are measured by cyclic vol-tammogram as summarized in Fig. 1. These com-pounds are reduced to the monoanionic (1-) statesat around 0 V; these compounds are moderatelystrong acceptors. The introduction of fluoro or triflu-oromethyl groups enhances the acceptor ability. Theshift of the redox potential in the trifluoromethylcompound is larger than that in the fluoro compound.In contrast, the methyl substitution slightly decreasesthe acceptor ability. These redox potentials corre-spond to the LUMO levels at about 4.4–4.6 eV [12];these values are a little larger than the work functionof Al (4.3 eV), so that good electron injection from Alelectrodes is expected.

2.2. Thin-film morphology and crystal structure

Thin films of the nickel complexes are made byvacuum evaporation on SiO2 substrates. TheAFM images are shown in Fig. 2. The non-fluori-nated compounds, 1a, 1b and 1c, show no large con-tinuous domains or terraces, but have fibril-like

Fig. 1. Redox potentials E1/2 of the n

structures. For 1a, relatively large microcrystalsare observed, and there are few flat parts. On thecontrary, 1c enlarges the continuous area, andobscures the boundaries between microcrystals andflat parts. Accordingly the ethyl substitutionimproves the thin-film property. The thin film of1d does not show fibril-like structure like the alkylderivatives, but shows a smooth but inhomogeneouspattern. Compound 1e shows continuous surfacewith no clear grain boundary. In general, fluorocompounds (1d and 1e) afford much improved thinfilms.

In the X-ray diffraction (XRD) measurement,clear peaks are observed only for 1c and 1e.Observed diffraction peaks of 1c and 1e correspondto the d-spacings of 15.9 A and 16.2 A, respectively.The molecular lengths are 20.1 A and 16.3 A fromthe geometry-optimized molecular orbital calcula-tion. Accordingly the tilt angle is 38� for 1c, whilethe molecules are approximately perpendicular tothe substrate for 1e. The alkyl and CF3 groups seemto improve the molecular packing and the film mor-phology. The difference of the tilt angles, however,suggests that the molecular arrangement varies asa sensitive function of the end groups.

Single crystal X-ray crystal structure analysis hasbeen carried out for 1d (Fig. 3) [13]. The centralnickel dithiolene part has a bent structure owingto the strong dimerization, similarly to the previ-ously reported crystal structure of 1a [14]. The spacegroup (P�1) is, however, different from 1a (P21/n).This is because all dimers in 1d are arranged parallel

ickel complexes (vs. Ag/AgCl).

Fig. 2. AFM images of nickel complexes. (a)–(e) correspond to 1a–e.

Fig. 3. Crystal structure of 1d. (a) The dimer, and (b) theprojection along the a axis.

762 H. Wada et al. / Organic Electronics 8 (2007) 759–766

to each other, whereas the molecules in 1a are tiltedalternately. The phenyl groups are out of the centraldithiolene part, making dihedral angles of 12.7�,19.5�, 30.4�, and 58.7� for Molecule A and 24.5�,52.0�, 52.3� and 83.4� for Molecule B.

2.3. FET characteristics

Bottom-contact type FETs have been fabricatedon SiO2 substrates. An organic charge-transfer com-plex (TTF)(TCNQ) as well as Al is used as sourceand drain electrodes. (TTF)(TCNQ) is a metalliccomplex, and has been used as source and drainelectrodes in single-crystal OFETs [15]. We canexpect a good contact between the electrodes andthe active layer, because both are composed oforganic materials [16]. In addition, the Fermi levelof (TTF)(TCNQ) is higher than Au, and electroninjection is possible [15].

The FET performance is summarized in Table 1.OFETs of all compounds have shown n-channelcharacteristics, though the mobilities are in theorder of 10�4–10�6 cm2/V s. Since in our study ofTTF derivatives, dimerized compounds have notshown transistor properties [17], it is rather surpris-ing that these strongly dimerized compounds showtransistor properties. The presence of the out-of-plane phenyl groups is disadvantageous to themolecular packing and the FET properties, result-ing in the observed moderate mobility values. This,however, facilitates the vacuum evaporation, andconsequently all these compounds show FET prop-erties. In comparison with our previous report on 1a

[9], the mobility values are improved. For 1b,(TTF)(TCNQ) electrodes have given much betterresults than Al electrodes. For 1c–1e, OFET proper-ties have been observed only for (TTF)(TCNQ)electrodes.

Table 1FET performance of bis(dithiobenzyl)nickel and their derivatives

Compound Electrodes Electron mobilitycm2/V s (air)

Electron mobilitycm2/V s (vacuum)

On/off ratio(air/vacuum)

Vt (air/vacuum) (V)

1a Al 3.0 · 10�6 2.0 · 10�5 102/102 68/101b Al – 1.3 · 10�5 –/2 · 102 –/4.0

(TTF)(TCNQ) – 1.3 · 10�4 –/2 · 102 –/ 451c (TTF)(TCNQ) – 6.0 · 10�5 –/104 –/ 271d (TTF)(TCNQ) 9.0 · 10�5 5.0 · 10�5 20/30 7/�101e (TTF)(TCNQ) 6.6 · 10�6 6.4 · 10�6 10/<10 �69/�110

H. Wada et al. / Organic Electronics 8 (2007) 759–766 763

Output characteristics of these nickel complexesin vacuum are shown in Fig. 4, where characteristicsafter 5 min air exposure are also depicted for (d) 1a,(f) 1d, and (h) 1e. These figures are arranged in theorder of the acceptor strength. In vacuum, good sat-uration properties are observed for all compounds.Remarkable difference is observed in the measure-ments in air. Output characteristics of 1d and 1eshow good saturation properties which are basicallythe same as the vacuum measurements (Fig. 4f andh). Compound 1e exhibits a decrease of the off-cur-rent in air, showing better characteristics in air thanin vacuum. Output characteristics become moreunstable with decreasing the acceptor ability. Themeasurement for 1a is depicted in Fig. 4d, but for1c and 1d, the FET properties are similarly lost inair. The air stability seems to change abruptlybetween 1a and 1d. For all compounds, p-channelcharacteristics are not observed even in air.

As depicted in Fig. 5, there is a good correlationbetween the threshold voltage and the redox poten-tials. When the redox potential shifts to positive,making the molecule stronger acceptors, the thresh-old voltage becomes more negative. The change ofthe former of 0.25 V induces a more than 100 Vchange of the latter. The threshold voltage is notnaively determined by the potential difference, butby the barriers at the junctions.

Since 1d and 1e are considerably air-stable, wehave investigated time dependence of the transistorcharacteristics after exposure to air. As shown inFig. 6, the threshold voltage gradually increases inair. Compound 1d loses FET properties within1 h. The FET of 1e degrades during several hours,but this drop stops at about 10 h. We have observedalmost the same n-channel FET properties evenafter more than 37 days as those after 10 h.

Air-stable n-channel OFETs have been reportedfor F16CuPc, naphthalene tetracarboxy diimide,and a hybrid dmit complex [2c,2e,4]. The LUMOlevel of F16CuPc is reported to be 4.61 V [18], which

corresponds to the redox potential of +0.17 V.Therefore, F16CuPc is an acceptor as strong as thepresent compound 1e. The reduction potentials oftetracarboxy diimides are about �0.5 V [19].Although the fluoro compounds are expected tohave a little higher redox potentials, these com-pounds are not very strong acceptors. In addition,FET of TCNQ (redox potential: +0.18 V) isreported to maintain its activity for days [2b]. Theresults obtained in the present work, together withthe previous observations, indicate that organicacceptors with positive redox potentials have a goodchance of showing reasonably air-stable FET prop-erties, and the threshold to the stability is about 0 V.These observations suggest the possibility to muchimproved air stability in n-channel OFET.

3. Experimental

Bis(dithiobenzyl)nickel 1a was purchased fromTokyo Kasei, and used after sublimation.

3.1. Preparation of benzoin derivatives 3b–e

In a flask were placed benzaldehyde derivative 2,EtOH, and 0.16 equivalent of potassium cyanidedissolved in water. The mixture was refluxed for3 h. The resulting mixture was washed with waterand the solvent was removed by evaporation. Theobtained pale yellow oil was distilled under vacuumcondition, giving a pale yellow solid.

3.2. Preparation of bis(dithiobenzyl)nickel derivative

1b–e

Benzoin derivative 3 was refluxed with 1.4 equiv-alent of P2S5 in dioxane for 1 h. To the cooled andfiltered reaction solution 0.5 equivalent ofNiCl2 Æ 6H2O dissolved in water were added, thenthe mixture was refluxed for 1 h. After cooling,black crystals of the complex were formed and

a b

c d

e f

g h

Fig. 4. Output characteristics of OFETs with Al electrodes for 1a and (TTF)(TCNQ) electrodes for 1b–e.

764 H. Wada et al. / Organic Electronics 8 (2007) 759–766

Fig. 5. Correlation between the threshold voltages for(TTF)(TCNQ) electrodes, and the redox potentials to themonoanionic state.

H. Wada et al. / Organic Electronics 8 (2007) 759–766 765

collected by filtering the solution, giving a blacksolid. The product was purified by sublimation.

The redox potentials were measured by a YanacoVMA-010 cyclic voltammetric analyzer. All electro-chemical measurements were conducted on a glassycarbon working electrode vs. Ag/AgCl in benzonit-rile containing tetrabutylammonium hexafluoro-phosphate as a supporting electrolyte.

AFM images of the evaporated thin films on theSi/SiO2 substrates were taken with a SII scanningprobe microscope system SPI3800N and SPA 300by using a Si3N4 cantilever. Scanning range of thethin films was 5 · 5 lm. X-ray diffraction profilesof organic thin films deposited on the Si/SiO2 sub-strates were obtained with a Phillips X’Pert-MPD-OEC PW3050 with a monochromated Cu Ka radi-ation (k = 1.541 A) at room temperature using theh–2h technique for 2� 6 2h 6 30�. The single-crystalX-ray data of the nickel complex 1d were collectedon a Rigaku AFC-7R diffractometer with a rotating

30

25

20

15

10

5

0100806040200-20-40

5 min

30 min

1 h

Dra

in C

urre

nt [n

A]

Gate Voltage [V]

1d

Dra

in C

urre

nt [n

A]

a b

Fig. 6. Time dependence of the transfer cha

anode X-ray generator with graphite monochro-mated Mo Ka radiation at room temperature andusing the 2h–x scan technique to a maximum 2hof 60�. The structure was solved by the directmethod (SIR 92) and refined by full-matrix least-squares analysis.

Bottom-contact type FETs were fabricated onheavily doped n+-Si substrates with 300 nm ther-mally grown SiO2. Powder of (TTF)(TCNQ) in a cru-cible was thermally deposited (<10�5 Torr) through ashadow mask on the substrates. Al electrodes weresimilarly deposited. Subsequently, the substrateswere loaded into an evaporator and the nickel com-plexes were evaporated under high-vacuum condi-tions (<10�6 Torr) to thickness of approximately50–100 nm. The substrate temperature was kept atroom temperature during the deposition and the mea-surement. The channel dimensions were W/L (lm/lm) = 2000/100 for 1b–1e except for 6000/50 (vac-uum) and 1000/20 (air) for 1a. The FET propertieswere measured by a Keithley 4200-SCS semiconduc-tor parameter analyzer. The measurements in airwere carried out by using an OYAMA manual probersystem model TB-2-H-200, and the in situ measure-ments were done by connecting the device by usingcopper wires and silver paste. In the measurementunder vacuum, the substrates were once taken outfrom the evaporation chamber and transferred to ameasurement chamber, and kept under vacuum over-night. Mobilities (l) were estimated from the transfercharacteristics in the saturation regime (Vds = 60 V)

Ids ¼WCl2LðV g � V tÞ2; ð1Þ

where C (13.3 nF/cm2) was the oxide capacitance,and Vg was the gate voltage. The threshold voltage

1e16

14

12

10

8

6

4

2

0-100 -50 0 50 100

Gate Voltage [V]

1 h2.5 h

4 h5 h

6.5 h

8 h10 h12 h

racteristics in air in (a) 1d and (b) 1e.

766 H. Wada et al. / Organic Electronics 8 (2007) 759–766

Vt was estimated from the intercept of the linear sec-tion of the plot of Vg vs. (Id)1/2.

Acknowledgement

This work was partly supported by Grant-in Aidfor Scientific Research (No. 15073211 and No.18350095) from the Ministry of Education, Culture,Sports, Science and Technology.

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