Solid-State Electronics 54 (2010) 710–714

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Solid-State Electronics

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Effects of thermal annealing on structure, morphology and electrical propertiesof F16CuPc/a6T heterojunction thin films

Rongbin Ye a,*, Mamoru Baba a, Koji Ohta a, Kazunori Suzuki b

a Faculty of Engineering, Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japanb Iwate Industrial Research Institute, 3-35-2 Iioka-shinden, Morioka 020-0852, Japan

a r t i c l e i n f o

Article history:Received 12 August 2009Received in revised form 2 February 2010Accepted 23 March 2010Available online 10 April 2010

The review of this paper was arranged byeditor Y. Kuk

Keywords:Organic thin film transistorAmbipolar characteristicsField-effect mobilityThermal annealing

0038-1101/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.sse.2010.03.014

* Corresponding author. Tel.: +81 19 621 6364; faxE-mail address: ye@iwate-u.ac.jp (R. Ye).

a b s t r a c t

We report the effects of thermal annealing on structure, morphology and electrical properties of fluori-nated copper phthalocyanine (F16CuPc)/a-sexithiophene (a6T) heterojunction thin films. The morphol-ogy and structure of the thin films were examined by atomic force microscopy and X-ray diffractiontechniques. The thermal annealing significantly improved a6T molecular ordering although F16CuPcmolecular was slightly disordered in the thermal annealing process. The smallest full width of halfmaximum of the 200 diffraction lines was gained at the thermal annealing temperature of 150 �C.Similarly, ambipolar performance of the F16CuPc/a6T heterojunction transistor could be improved bythe thermal annealing. At the thermal annealed temperature of 150 �C, the ambipolar device achievedhigh-performance with field-effect hole and electron mobilities of 8.84 � 10�3 cm2 V�1 s�1 and1.00 � 10�2 cm2 V�1 s�1, respectively.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Organic thin films have been widely investigated as active lay-ers in electronic and optoelectronic devices, such as light-emittingdiodes (LEDs), thin film transistors (TFTs), and photovoltaic (PV)cells [1]. The use of more complex structures than just an organicthin film is needed in many organic devices. Several organic layersof different materials are used in organic LEDs, organic ambipolarTFTs or organic PV cells, where basically a pn junction allows theeffective transport of electrons and holes towards or away fromthe organic-organic interface [2–10]. Similarly, additional organiclayers as electron or hole blocking layers are often used to improvethe performance of these devices. Especially, the microscopicstructure of organic pn heterojunction plays a key role in currentand future electronic devices built from organic semiconductingmolecules.

It has been known that fluorinated copper phthalocyanine(F16CuPc)/copper phthalocyanine (CuPc) and pentacence/fluorinated pentacence have a similar molecular shape and a sim-ilar crystal structure with field-effect mobility of the same order[8,11–13]. These highly ordered polycrystalline thin films couldbe deposited on amorphous SiO2/Si substrates under similar opti-mized growth conditions. High-performance ambipolar transistorsbased on these quasi-homoheterostructure devices have been

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reported [7,8,14–16]. However, although high-performance ambi-polar transistors could gained from organic heterostructure be-tween dissimilar organic semiconductors such as pentacene/C60

and F16CuPc/2, 5-bis (4-biphenylyl) bithiophene, little is knownabout how such organic interfaces evolve during growth andhow the emerging morphology and structure affects device perfor-mance of organic ambipolar TFTs [9,10]. On the other hand, it hasbeen known that the thermal annealing process can improvemolecular ordering or phase separation and the performance ofthe corresponding organic LEDs, organic TFTs and organic PVs withrespect to the as-grown devices [17–20].

In this study, we used two dissimilar organic materials F16CuPcand a-sexithiophene (a6T). The effects of the thermal annealing onthe morphology and structure of F16CuPc/a6T thin films were ana-lyzed by X-ray diffraction (XRD) and atomic force microscopy(AFM). Furthermore, we also investigated the effect of thermalannealing on the electrical properties of ambipolar TFTs based onthe two organic materials.

2. Experimental

The molecular structures of F16CuPc and a6T are shown inFig. 1, along with their relative energy levels and the OTFT deviceschematic structure [11,21]. A heavily n-doped Si substrate actsas the gate electrode with a 300 nm thermally grown SiO2 layer(Ci � 10 nF/cm2) as the gate dielectric. F16CuPc thin films of 5 nmand a6T thin films of 20 nm were continually vacuum-deposited

Fig. 1. Molecular structures of a6T and F16CuPc, schematic structure of OTFTs based on F16CuPc/a6T heterojunctions and energy levels (HOMO and LUMO) of F16CuPc, a6Tand work function of Au. The device dimensions are not scaled.

R. Ye et al. / Solid-State Electronics 54 (2010) 710–714 711

from two deposition sources. During deposition, the substrate tem-peratures were set at 120 �C for F16CuPc films and room tempera-ture (RT) for a6T films under a base pressure of less than4 � 10�4 Pa. Film thicknesses and growth rates were monitoredby a thickness and rate monitor (CRTM-6000, ULVAC). Sampleswere annealed in Ar-ambient and dark conditions at temperaturesranging from 130 �C, 150 �C, and 170 �C, respectively. The XRDanalysis was performed on a diffractometer (Rint 2200 V, RIGAKUCo., Ltd.) with graphite monochromatized Cu Ka radiation(k = 1.54056 Å), operating in the H-2H mode. The morphology ofthe films was examined using AFM (Nanocute, Seiko InstrumentsCo., Ltd.), the cantilevers were used in the tipping mode with alength of 90 lm and a force constant of 15 N/m. The grain sizeand the root-mean-square roughness (RMS) were given by theAFM instrument for the individual scans (5 lm � 5 lm). Further-more, electrical characteristics of F16CuPc/a6T heterojunction TFTswith a channel width of 5 mm and a length of 70 lm were mea-sured using a two-channel voltage current source/monitor system(R6245, ADVANTEST) under ambient laboratory air conditions.

Fig. 2. XRD spectra of (a) a reference a6T single layer of 50 nm and a referenceF16CuPc single layer of 20 nm, and F16CuPc/a6T heterojunction thin films, (b) beforeand (c–e) after thermal annealing (130 �C, 150 �C, and 170 �C, respectively). For thetwo single-layers, the growth conditions were not optimized.

3. Results and discussion

In Fig. 2a, the typical XRD patterns of F16CuPc thin films and a6Tthin films with d200 spacings of 1.44 nm and 2.18 nm were ob-served, and these molecules are standing up with respect tosubstrate surface [11,12,22]. Fig. 2b shows XRD pattern of as-deposited F16CuPc/a6T thin films. Two new reflections at 2H of4.31� and 4.92� were observed except the F16CuPc reflection of6.12�. The peak at 2H of 4.92� could be an intermixing layer, whichoriginates from the interactions or relaxations between F16CuPcand a6T at the interface [7,16]. The peak at 2H of 4.31� is supposedas a disordered a6T layer with smaller d-spacing of 2.05 nm grownon the above-mentioned intermixing layer. Due to effects of thesurface roughness and the lower surface energy of the F16CuPcfilms, the a6T films are much disordered and slant than the filmsdirectly deposited on the SiO2/Si substrates (with d200 spacing of2.18 nm) [22]. Fig. 2c–e show XRD patterns of F16CuPc/a6T thinfilms after thermal annealing at 130 �C, 150 �C and 170 �C, respec-tively. After thermal annealing, all of the XRD spectra are com-posed of a series of (k 0 0) peaks (observed in Fig. 2c–e) andcorresponding to the crystalline structure of a6T is present. In con-

trast, the intermixing layer is significantly reduced. On going to thehigher annealing temperatures of 170 �C, the (k 0 0) peaks weresignificantly reduced indicating that the crystalline structureof a6T was damaged at higher annealing temperatures. Fig. 3

Fig. 3. FWHM of the (2 0 0) diffraction lines as a function of TA. The inset shows d200

spacings of the (2 0 0) diffraction lines of a6T as a function of TA.

712 R. Ye et al. / Solid-State Electronics 54 (2010) 710–714

shows the effect of the thermal annealing temperature (TA) on fullwidth of half maximum (FWHM) along with d200 spacings of the(2 0 0) diffraction lines of a6T. The thermal annealing significantlyimproves a6T molecular ordering and the smallest FWHM and the

Fig. 4. AFM images of F16CuPc/a6T heterojunction thin films (a) before and (b–d) aft

same d200 spacing were gained at the TA = 150 �C although F16CuPcmolecular was slightly disordered.

Fig. 4 shows the 5 lm � 5 lm AFM topography images of theF16CuPc/a6T heterojunction thin films before and after the thermalannealing. In Fig. 4a, many small projections are observed, and inFig. 4b, some adjacent grains are joined together, which suggeststhat the local recrystallization has occurred due to the thermalannealing. As can be seen in Fig. 4c, the grain size and densityare similar to those of Fig. 4b, but the grains have coalesced to formclusters approximately 1 lm in size. On the basis of the above re-sults, the annealing temperature for recrystallization is thought tobe close to 150 �C. Fig. 5 shows the effect of the thermal annealingon grain size and the RMS. By the thermal annealing process, thegain size was increased up to about three times although the var-iation of RMS was very slight. Similar to pentacene, the grain sizeand RMS were decreased due to reevaporation at TA over 170 �C[17]. On the other hand, the thermal annealing only caused a localaggregation of a6T molecules which led to a slight variation in theRMS roughness, unlike pure thermal diffusion growth on elevatedsubstrates [21–23].

Fig. 6 shows the drain current–drain voltage (ID � VD) character-istics of F16CuPc/a6T heterojunction TFTs before and after thethermal annealing. These devices exhibit ambipolar operatingcharacteristics of both the hole-accumulation and electron-accu-

er thermal annealing (at 130 �C, 150 �C, and 170 �C, respectively) (5 lm � 5 lm).

Fig. 5. Effect of thermal annealing on grain size and RMS roughness.

R. Ye et al. / Solid-State Electronics 54 (2010) 710–714 713

mulation modes. Increasing TA, the p-channel operating character-istics were enhanced, whereas the bulk currents were suppressed.This implies that ambipolar performance of the F16CuPc/a6T het-erojunction transistors could be improved by the thermal anneal-ing. However, when TA is over 170 �C, the p-channel operatingcharacteristics (shown in Fig. 6d) were sharply decreased, whichis due to the reevaporation of a6T. These results imply that it isimportant to control TA for obtaining a balanced ambipolartransport.

Generally, the electric parameters of these devices wereestimated using the standard analytic theory of metal oxide semi-

Fig. 6. Drain current–voltage (ID � VD) characteristics of ambipolar OTFTs based on the F1

130 �C, 150 �C, and 170 �C, respectively).

conductor field-effect transistors (MOSFETs) [24]. Due to bulk cur-rents in the devices (shown in Fig. 6a and b), field-effect hole andelectron mobilities were only accurately derived in the linear re-gion as followed:

ID ¼WL

Cil ðVG � VTÞVD �12

V2D

� �; ð1Þ

where W, L, and Ci are the channel width, channel length, and gatedielectric capacitance per unit area, respectively. From Fig. 7a, thedependence of ambipolar mobilities on TA is shown in Fig. 7b. Byincreasing TA, field-effect hole mobility is increased, whereasfield-effect electron mobility is almost unchanged, which well de-scribes the dependence of the device performance predicated fromthe ID � VD characteristics (Fig. 6) on TA. The increase of field-effecthole mobility originates from improving a6T molecular ordering. AtTA = 150 �C, the ambipolar devices achieved high-performance withfield-effect hole and electron mobilities of 8.98 � 10�3 cm2 V�1 s�1

and 1.01 � 10�2 cm2 V�1 s�1, respectively.Furthermore, a bulk current is harmful to device performance

such as current on/off ratio. As shown in Fig. 8, bulk current isdependent on TA, which results from the interface dipolar, andthe electrons and holes are free charges, which are significantlydistinguished from ionized impurities, localized in the spacecharge region for conventional inorganic pn junctions. Due to thelow charge carrier density of F16CuPc, the quantities of the chargesare equivalent and determined by the first layer (F16CuPc). Asshown in the inset of Fig. 8, channel resistance (RC) is proportionalto TA. It has been known that the conductivity (d ¼ nel) of a6Tfilms (deposited on glass substrate at RT) in the parallel is aboutone-tenth of that in the perpendicular (referred to the substrate

6CuPc/a6T heterojunction thin films: (a) before and (b–d) after thermal annealing (at

Fig. 7. (a) Drain current–gate voltage (ID � VG) characteristics of the ambipolar OTFTs based on F16CuPc/a6T heterojunction thin films at |VD| = 5 V and (b) Dependence ofambipolar nobilities of the F16CuPc/a6T heterojunction OTFTs on TA.

Fig. 8. ID � VD characteristics of the ambipolar OTFTs (shown in Fig. 6) for VG = 0 Vand at a low VD. The inset shows dependences of the channel resistances RC on TA.

714 R. Ye et al. / Solid-State Electronics 54 (2010) 710–714

plane) [22]. Thus the thermal treatment is yearned for suppressingbulk current by improving a6T molecular ordering. These resultsimply that the optimal annealing temperature for F16CuPc/a6T het-erojunction thin film ordering for use in ambipolar organic TFTs isabout 150 �C.

4. Conclusions

We investigate effects of the thermal annealing on structure,morphology and electrical properties of F16CuPc/a6T heterojunc-tion thin films. The morphology and structure of the thin films wereexamined by AFM and XRD techniques. The thermal annealing sig-nificantly improves a6T molecular ordering although F16CuPcmolecular was slightly disordered in the thermal annealing process.The smallest FWHM of the 200 diffraction lines was gained at TA of

150 �C. Similarly, ambipolar performance of the F16CuPc/a6T het-erojunction TFTs could be improved by the thermal annealing. AtTA of 150 �C, the ambipolar device achieved high-performance withfield-effect hole and electron mobilities of 8.98 � 10�3 cm2 V�1 s�1

and 1.01 � 10�2 cm2 V�1 s�1, respectively.

Acknowledgements

The authors would like to thank Y. Ishi for his experimentalassistance and Iwate Industrial Research Institute for its contribu-tion of the XRD and AFM apparatus.

References

[1] Forrest SR. Chem Rev 1997;97:1793.[2] Tang CW, Van Slyke SA. Appl Phys Lett 1987;51:913.[3] Peumans P, Forrest SR. Appl Phys Lett 2001;79:126.[4] Granström M, Petritsch K, Arias AC, Lux A, Andersson MR, Friend RH. Nature

1998;395:257.[5] Peumans P, Uchida S, Forrest SR. Nature 2003;425:158.[6] Dodabalapur A, Katz HE, Torsi L, Haddon RC. Appl Phys Lett 1996;68:1108.[7] Ye R, Baba M, Oishi Y, Mori K, Suzuki K. Appl Phys Lett 2005;86:253505.[8] Inoue Y, Sakamoto Y, Suzuki T, Kobahashi M, Gao Y, Tokito S. Jpn J Appl Phys

2005;44:3663.[9] Wang J, Wang HB, Yan XJ, Huang HC, Jin D, Shi JW, et al. Adv Funct Mater

2006;16:824.[10] Kuwahara E, Kusai H, Nagano K, Takayanagi T, Kubozono Y. Chem Phys Lett

2005;413:379.[11] Bao Z, Lovinger AJ, Brown J. J Am Chem Soc 1998;120:207.[12] Ye R, Baba M, Ohishi Y, Mori K, Suzuki K. Mol Cryst Liq Cryst 2006;444:203.[13] Xiao K, Liu Y, Yu G, Zhu D. Appl Phys A 2003;77:367.[14] Ye R, Baba M, Mori K. Jpn J Appl Phys 2005;44:L581.[15] Ye R, Baba M, Suzuki K, Mori K. Jpn J Appl Phys 2007;46:2878.[16] Ye R, Baba M, Suzuki K, Mori K. Appl Surf Sci 2008;254:7885.[17] Ye R, Baba M, Suzuki K, Ohishi Y, Mori K. Jpn J Appl Phys 2003;42:4473.[18] Ahn T, Jung H, Suk HJ, Yi MH. Synth Metal 2009;159:1277.[19] Sakai J, Taima T, Yamanari T, Saito K. Sol Energy Mater Sol Cells 2009;93:1149.[20] Ahn JH, Wang C, Widdowson NE, Pearson C, Bryce MR, Petty MC. J Appl Phys

2005;98:054508.[21] Watkins NJ. PhD Thesis, University of Rochester, 2002.[22] Sevvet S, Horowitz G, Rise S, Logorsse O, Alnot P, Yassar A, et al. Chem Mater

1994;6:1809.[23] Tsamouras D, Palasantzas G. Appl Phys Lett 2002;80:4528.[24] Sze SM. Physics of semiconductor devices. New York: Wiley; 1981.