Applied Surface Science 254 (2008) 7885–7888

Structure and morphology of CuPc and F16CuPc pn heterojunction

Rongbin Ye a,*, Mamoru Baba a, Kazunori Suzuki b, Kunio Mori a

a Faculty of Engineering, Iwate University, 3-18-8 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:

Available online 19 March 2008

Keywords:

Organic thin films

pn junction

Morphology

Ambipolar transport

Field-effect mobility

A B S T R A C T

This article reports structure and morphology of copper phthalocyanine (CuPc) and fluorinated copper

phthalocyanine (F16CuPc) pn heterojunction. Highly ordered CuPc and F16CuPc polycrystalline thin films

with the 2 0 0 plane spacing s of 1.30 and 1.56 nm, respectively, could be continuously grown via an

intermediate-phase layer. Compared with CuPc, the intermediate-phase layer is much thinner when

F16CuPc is used as the first layer. The rougher the first layer is, the thicker the intermediate-phase layer is.

Similarly, the 2 0 0 plane spacings of the intermediate-phase layer are dependent on morphology of the

first layer. Furthermore, morphology of the heterostructure is mainly dominated by that of CuPc films.

Due to the thicker intermediate-phase layer in the CuPc/F16CuPc heterostructure, the thin film transistors

(TFT) performance is obviously inferior to that of the F16CuPc/CuPc device.

� 2008 Elsevier B.V. All rights reserved.

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Fig. 1. XRD spectra of (a) a reference CuPc single layer of 50 nm and a reference

1. Introduction

Organic thin films have been widely investigated as activelayers in electronic and optoelectronic devices, such as light-emitting diodes (LEDs), thin film transistors (TFTs), and photo-voltaic (PV) cells [1]. The use of more complex structures than justan organic thin film is needed in many organic devices. Severalorganic layers of different materials are used in organic LEDs orsolar cells, where basically a pn junction allows the effectivetransport of electrons and holes towards or away from the organic–organic interface. Similarly, additional organic layers as electron orhole blocking layers are often used to improve the performance ofthese devices. The use of more complex structures than just anorganic thin film on a substrate is needed in many organic devices.Especially, the microscopic structure of organic pn heterojunctionsplays a key role in current and future electronic devices built fromorganic semiconducting molecules. Very little is known about howsuch organic interfaces evolve during growth and how theemerging morphology and structure affects the function, perfor-mance and lifetime of organic devices, such as OLEDs, organicambipolar TFTs or organic photovoltic cells [2–4].

It has been known that fluorinated copper phthalocyanine(F16CuPc), one of air-stable n-type organic semiconductors, as wellas copper phthalocyanine (CuPc) has a similar molecular shape anda similar crystal structure with field-effect hole mobility of thesame order [5–8]. The highly ordered polycrystalline thin films of

* Corresponding author. Tel.: +81 19 6216364; fax: +81 19 6216978.

E-mail address: ye@iwate-u.ac.jp (R. Ye).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.03.048

both F16CuPc and CuPc could be deposited on amorphous SiO2/Sisubstrates under similar optimized growth conditions [6–8].Previously, we have reported on high-performance air-stableambipolar transistors based on an F16CuPc/CuPc (F16CuPc, the firstdeposited layer) pn junction [4,9]. In this study, we discuss theinterface between the two organic materials. Structure andmorphology of CuPc and F16CuPC heterostructure thin films were

F16CuPc single layer of 20 nm, (b) a 5 nm/15 nm of F16CuPc/CuPc heterostructure

layer, (c) a 10 nm/10 nm of F16CuPc/CuPc heterostructure layer, (d) a 5 nm/15 nm of

CuPc/F16CuPc heterostructure layer, (e) a 10 nm/10 nm of CuPc/F16CuPc

heterostructure layer. For the two single-layers, the growth conditions were not

optimized.

Fig. 2. (2 mm � 2 mm) AFM images of F16CuPc single layers of (a) 5 nm, (b) 10 nm, CuPc single layers of (c) 5 nm, (d) 20 nm, (e) an F16CuPc(5 nm)/CuPc(10 nm) heterostructure

layer, (f) a CuPc (5 nm)/F16CuPc (15 nm) heterostructure layer, respectively.

R. Ye et al. / Applied Surface Science 254 (2008) 7885–78887886

analyzed by X-ray diffraction (XRD) and atomic force microscopy(AFM). Furthermore, we give the effect of the interface on theelectrical properties of ambipolar OTFTs based on the two organicmaterials.

2. Experimental

CuPc (98%, TCI) and F16CuPc (80%, Aldrich) were economicallypurchased and used with further purification. CuPc and F16CuPc

Table 1Ratios of the integrated intensity (RII) (%) for these XRD peaks in various

heterostructure layers

Heterostructure The ratios of integrated intensity (RII) (%)

F16CuPc The intermediate-

phase layer (d02 0 0)

CuPc

F16CuPc (5 nm)/CuPc (15 nm) 24.5 11.3 (1.33 nm) 64.2

CuPc (5 nm)/F16CuPc (15 nm) 28.8 59.5 (1.43 nm) 11.7

F16CuPc (10 nm)/CuPc (10 nm) 40.7 26.5 (1.38 nm) 32.7

CuPc (10 nm)/F16CuPc (10 nm) 23.7 61.4 (1.39 nm) 14.9

Fig. 3. Dependence of the PV and RMS on the film thickness.

Fig. 4. Dependence of RII (%) of the intermediate-phase layer on PV of the first layer.

R. Ye et al. / Applied Surface Science 254 (2008) 7885–7888 7887

thin films could be continually vacuum-deposited on heavily n-doped Si substrates with a 300 nm thermally grown SiO2 layer (Ci

�10 nF/cm2). During deposition, the substrate temperature wasset at 120 8C and under a base pressure of less than 5 � 10�4 Pa.Film thickness and growth rates (ca. 0.01 and ca. 0.04 nm/s forF16CuPc films and CuPc films, respectively) were monitored by athickness and rate monitor (CRTM-6000, ULVAC). The X-raydiffraction analysis was performed on a diffractometer (RINT-250V/PC, RIGAKU), operating in the 2Q mode with a fixed incidentangle of 28. The morphology of films was examined by AFM (SPA500, Seiko Instruments Co., Ltd.); the cantilevers were used in thetapping mode and had a length of 90 mm and a force constant of0.12 N/m. The peak-to-valley height (PV) and the root mean squareroughness (RMS) were obtained using the AFM instrument for eachindividual scan (2 mm � 2 mm). Furthermore, the top-contactdevices were constructed. Au source and drain electrodes of ca.100 nm were vacuum-deposited through a shadow mask with achannel width of 5 mm and a length of 70 mm. The characteristicsof OTFTs were measured with a two-channel voltage currentsource/monitor system (R6245, ADVANTEST) under ambientlaboratory air conditions.

3. Results and discussion

In Fig. 1(a), the typical XRD patterns of CuPc and F16CuPc thinfilms with d2 0 0 spacings of 1.30 nm and 1.56 nm were observed,respectively. These values of d2 0 0 are slightly larger than thosemeasured by XRD operating in the Q – 2Q mode [4]. Fig. 1(b)–(e)shows XRD patterns for various heterostructure layers of CuPc andF16CuPc. In all cases, except the above-mentioned two reflections,new reflections at 2Q of 6.648 (d02 0 0 ¼ 1:33 nm) for the films of5 nm/15 nm and 6.388 (d02 0 0 ¼ 1:38 nm) for the films of 10 nm/10 nm in the F16CuPc/CuPc heterostructures (F16CuPc, the firstdeposited layer), and at 2Q of 6.188 (d02 0 0 ¼ 1:43 nm) for the filmsof 5 nm/15 nm and 6.348 (d02 0 0 ¼ 1:39 nm) for the films of 10 nm/10 nm in the CuPc/F16CuPc heterostructures (CuPc, the firstdeposited layer) were observed, respectively. Unlike other organicheterostructures, such as PTCDA (perylene-3,4,9,10-tetracar-boxylic dianhydride) and H2Pc with completely different d-spacingand crystal structure, in which the structure of the second layer iscompletely disrupted by the first layer, highly ordered F16CuPc andCuPc polycrystalline thin films could be continuously grown via anintermediate-phase layer in the heterostructure [2,10]. Theseintermediate-phase layers originate from the interactions orrelaxations between F16CuPc and CuPc at the interface. From thedetailed XRD analysis, the ratios of the integrated intensity (RII) (%)for these XRD peaks in the various heterostructures are shown inTable 1. Compared with CuPc, the intermediate-phase layer ismuch thinner when F16CuPc is used as the first layer.

Fig. 2(a) and (d) shows 2 mm � 2 mm topographic AFM imagesof F16CuPc and CuPc films deposited on SiO2 with various filmsthicknesses at the substrate temperature of 120 8C. For these films,the growth of elongated bent strips that are lying parallel to the

substrate surface could be observed. As shown in Fig. 3, PV andRMS increase with increasing the film thickness, and morphologyof CuPc films is always rougher than that of F16CuPc films with thefilm thickness of less than 20 nm. Dependence of RII (%) of theintermediate-phase layer on the PV of the first layer is shown inFig. 4. This result suggests that the rougher the first layer is, thethicker the intermediate-phase layer is. On the other hand, d02 0 0 ofthe intermediate-phase layer is between d2 0 0 of F16CuPc and CuPc,and dependent on morphology of the first layer. As shown in Fig. 4,the smoother the first layer is, d02 0 0 of the intermediate-phase layeris much near to d2 0 0 of the second layer. On the contrary, d02 0 0 ismuch near to that of the first layer. Furthermore, Fig. 2(e) and (f)shows 2 mm � 2 mm topographic AFM images of F16CuPc (5 nm)/CuPc (15 nm) and CuPc (5 nm)/F16CuPc (15 nm) films, respectively.Similar to these single layer films, the two heterostructure filmsalso consisted of elongated bent strips that are lying parallel to thesubstrate surface. The CuPc (5 nm)/F16CuPc (15 nm) films areslightly rougher than the F16CuPc (5 nm)/CuPc (15 nm) films andsimilar to the CuPc single layer films of 20 nm, which suggests thatmorphology of the heterostructure is mainly dominated by that ofCuPc films.

For the F16CuPc/CuPc heterostructure devices, ambipolartransport is dependent on the first active layer thickness andambipolar transport is observed at film thickness of the first layer(F16CuPc) between 4 and 12 nm [11]. Due to the thickerintermediate-phase layer in the CuPc/F16CuPc heterostructure,

Fig. 5. Drain current–voltage (ID–VD) characteristics of a CuPc/F16CuPc TFT working

in hole-enhancement and electron-enhancement modes.

Fig. 6. Drain current–gate voltage (ID–VG) characteristics of (a) CuPc/F16CuPc TFT

and (b) F16CuPc/CuPc TFT at VD = 5 V and �5 V.

R. Ye et al. / Applied Surface Science 254 (2008) 7885–78887888

the TFT performance of the second layer (F16CuPc) is poorer, andeven not possible to be obtained. When film thickness of CuPc isdecreased to 4 nm, ambipolar characteristics (shown in Fig. 5) canbe obtained. The p-channel operational characteristics of theambipolar TFT are almost identical to those of TFTs with only aCuPc layer. Only for VG = 0 V and VD < �40 V, the increase of ID canalso seen, which originates from drain-induced electrons. The n-channel operational characteristics of the ambipolar TFT aredifferent from those of TFTs with only an F16CuPc layer. Above acertain gate voltages VG > 30 V, the characteristics resemble anormal behavior for an n-channel device for small VD withVD �VG < �15 V. For VG � 30 V, ID does not saturate and rapidlyincreases for VD � VG < �15 V, which can be explained by thecontribution of drain-induced holes.

The electric parameters of the ambipolar TFT were estimatedusing the standard analytic theory of metal oxide semiconductorfield-effect transistors (MOSFETs) [12]. Due to bigger currents ofthe drain-induced holes in the saturation region for n-channel,field-effect hole and electron mobilities of 1.48 � 10�3 cm2 V�1 s�1

and 7.35 � 10�4 cm2 V�1 s�1, respectively, were only accuratelyderived in the linear region (shown in Fig. 6(a)). The ambipolarmobilities are obviously inferior to those of the F16CuPc/CuPcdevice shown in Fig. 6(b) (field-effect hole and electron mobilitiesof 2.96 � 10�3 cm2V�1 s�1 and 9.49 � 10�3 cm 2V�1 s�1, respec-tively). Therefore, it is very important to control morphology aswell as thickness of the first layer in the heterostructure devices inorder to obtain the high TFT performance of the second layer.

4. Conclusions

We have studied structure and morphology of CuPc andF16CuPc pn heterojunction in details by XRD and AFM analysis.Highly ordered CuPc and F16CuPc polycrystalline thin films withthe 2 0 0 plane spacings of 1.30 and 1.56 nm, respectively, could be

continuously grown via an intermediate-phase layer. Comparedwith CuPc, the intermediate-phase layer is much thinner whenF16CuPc is used as the first layer. The rougher the first layer is, thethicker the intermediate-phase layer is. Similarly, spacings (d02 0 0)of the intermediate-phase layer are dependent on morphology ofthe first layer. Furthermore, morphology of the heterostructure ismainly dominated by that of CuPc films. Due to the thickerintermediate-phase layer in the CuPc/F16CuPc heterostructure, theTFT performance is obviously inferior to that of the F16CuPc/CuPcdevice.

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