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Synthesis, Spectral Properties of Zinc Hexadecafluorophthalocyanine (ZnPcF 16 ) and Its Application in Organic Thin Film Transistors Yingli Sun 1 , Xianggao Li 2 , Shirong Wang 2 , Lijun Zhang 1 and Feng Ma 1,* 1 Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, Peoples Republic of China 2 School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, Peoples Republic of China A stable n-type semiconductor material, ZnPcF 16 , was synthesized and characterized by infrared (IR), UV-vis and fluorescence spectra. ZnPcF 16 showed a monomer characteristic in 1, 2-dichlorobenzene (DCB) while exhibited an aggregation property in tetrahydrofuran (THF) and dimethylformamide (DMF). The ZnPcF 16 /p-6p (ZnPcF 16 on p-6p) organic thin film transistors (OTFTs) using ZnPcF 16 as an active layer and p-6p as an inducing layer was fabricated by the physical vapor deposition technique. The ZnPcF 16 semiconductor film was characterized by XRD and SEM. And the results showed that ZnPcF 16 molecules can be oriented after the employment of the p-6p inducing layer. Charge carri- er field-effect mobility (μ) and threshold voltage (V T ) of the ZnPcF 16 /p-6p OTFTs were 1.3 × 10 2 cm 2 /V s and 22 V, respectively. [doi:10.2320/matertrans.M2016168] (Received May 12, 2016; Accepted October 11, 2016; Published December 25, 2016) Keywords: phthalocyanines, semiconductors, thin film transistors, aggregation effects, electron mobility 1. Introduction Organic thin film transistors (OTFTs) employing organic semiconductors as the active layer have received widely stud- ies because of their potential application in displays, logic circuits and sensors. 1–3) To date, the performance of p-type pentacene-based OTFTs has reached the level of a-Si devic- es. 3,4) Compared with p-type materials, the mobility of the n-type materials with high air stability is relatively low. The search for high performance and stable n-type organic semi- conductors has been a challenging task. 5) Phthalocyanines (Pc) show a long standing record of inter- est in both basic research and applications referring to their photoelectrical properties. 6) Copper hexadecafluorophthalo- cyanine (CuPcF 16 ) is one of the few promising n-type candi- dates as air stable and high mobility semiconductor materi- als. 7) Synthesis and investigations of CuPcF 16 were described in reference literature. 8) To improve the performance of CuP- cF 16 -based OTFTs, Yan et al. 3) employed an organic hetero- junction buffer layer to decrease the contact resistance of or- ganic/metal. The electron field-effect mobility is exponen- tially enhanced from 4.2 × 10 2 to 7.6 × 10 2 cm 2 /V·s. Fan et al. 9) have reported CuPcF 16 -based transparent OTFTs based on Ag/LiF bilayer transparent S/D electrodes with good elec- tron mobility of 1.31 × 10 2 cm 2 /V·s. The fluorination of p-type metal phthalocyanines may produce n-type semicon- ductors, allowing many n-type semiconductors designed in accordance with this approach. 5,9) Although the synthesis of fluorinated metal phthalocya- nines has been reported, 8,10,11) the fundamental properties of hexadecafluorinated phthalocyanines have not been suffi- ciently studied. 8) In this paper, zinc hexadecafluorophthalocy- anine (ZnPcF 16 , shown in Fig. 1) was synthesized in a slight- ly modified procedure and its spectral properties were charac- terized. Especially we employed p-6p (shown in Fig. 1) as an inducing layer to fabricate ZnPcF 16 -based thin film transis- tors using the vapor deposition techniques and investigated their electrical characteristics. * Corresponding author, E-mail: [email protected] Fig. 1 Molecular structures of (a) ZnPcF 16 and (b) p-6p. Materials Transactions, Vol. 58, No. 1 (2017) pp. 103 to 106 ©2016 The Japan Institute of Metals and Materials

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Page 1: Synthesis, Spectral Properties of Zinc ...Synthesis, Spectral Properties of Zinc Hexadeca˜uorophthalocyanine (ZnPcF 16) and Its Application in Organic Thin Film Transistors Yingli

Synthesis, Spectral Properties of Zinc Hexadeca�uorophthalocyanine (ZnPcF16) and Its Application in Organic Thin Film Transistors

Yingli Sun1, Xianggao Li2, Shirong Wang2, Lijun Zhang1 and Feng Ma1,*

1Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, People’s Republic of China2School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China

A stable n-type semiconductor material, ZnPcF16, was synthesized and characterized by infrared (IR), UV-vis and �uorescence spectra. ZnPcF16 showed a monomer characteristic in 1, 2-dichlorobenzene (DCB) while exhibited an aggregation property in tetrahydrofuran (THF) and dimethylformamide (DMF). The ZnPcF16/p-6p (ZnPcF16 on p-6p) organic thin �lm transistors (OTFTs) using ZnPcF16 as an active layer and p-6p as an inducing layer was fabricated by the physical vapor deposition technique. The ZnPcF16 semiconductor �lm was characterized by XRD and SEM. And the results showed that ZnPcF16 molecules can be oriented after the employment of the p-6p inducing layer. Charge carri-er �eld-effect mobility (µ) and threshold voltage (VT) of the ZnPcF16/p-6p OTFTs were 1.3 ×  10−2 cm2/V s and 22 V, respectively. [doi:10.2320/matertrans.M2016168]

(Received May 12, 2016; Accepted October 11, 2016; Published December 25, 2016)

Keywords:  phthalocyanines, semiconductors, thin �lm transistors, aggregation effects, electron mobility

1.  Introduction

Organic thin �lm transistors (OTFTs) employing organic semiconductors as the active layer have received widely stud-ies because of their potential application in displays, logic circuits and sensors.1–3) To date, the performance of p-type pentacene-based OTFTs has reached the level of a-Si devic-es.3,4) Compared with p-type materials, the mobility of the n-type materials with high air stability is relatively low. The search for high performance and stable n-type organic semi-conductors has been a challenging task.5)

Phthalocyanines (Pc) show a long standing record of inter-est in both basic research and applications referring to their photoelectrical properties.6) Copper hexadeca�uorophthalo-cyanine (CuPcF16) is one of the few promising n-type candi-dates as air stable and high mobility semiconductor materi-als.7) Synthesis and investigations of CuPcF16 were described in reference literature.8) To improve the performance of CuP-cF16-based OTFTs, Yan et al.3) employed an organic hetero-junction buffer layer to decrease the contact resistance of or-ganic/metal. The electron �eld-effect mobility is exponen-tially enhanced from 4.2 ×  10−2 to 7.6 ×  10−2 cm2/V·s. Fan et al.9) have reported CuPcF16-based transparent OTFTs based on Ag/LiF bilayer transparent S/D electrodes with good elec-tron mobility of 1.31 ×  10−2 cm2/V·s. The �uorination of p-type metal phthalocyanines may produce n-type semicon-ductors, allowing many n-type semiconductors designed in accordance with this approach.5,9)

Although the synthesis of �uorinated metal phthalocya-nines has been reported,8,10,11) the fundamental properties of hexadeca�uorinated phthalocyanines have not been suf�-ciently studied.8) In this paper, zinc hexadeca�uorophthalocy-anine (ZnPcF16, shown in Fig. 1) was synthesized in a slight-ly modi�ed procedure and its spectral properties were charac-terized. Especially we employed p-6p (shown in Fig. 1) as an inducing layer to fabricate ZnPcF16-based thin �lm transis-

tors using the vapor deposition techniques and investigated their electrical characteristics.

* Corresponding author, E-mail: [email protected]

Fig. 1 Molecular structures of (a) ZnPcF16 and (b) p-6p.

Materials Transactions, Vol. 58, No. 1 (2017) pp. 103 to 106 ©2016 The Japan Institute of Metals and Materials

Page 2: Synthesis, Spectral Properties of Zinc ...Synthesis, Spectral Properties of Zinc Hexadeca˜uorophthalocyanine (ZnPcF 16) and Its Application in Organic Thin Film Transistors Yingli

2.  Experimental Procedures

For the synthesis of ZnPcF16, sublimed tetra�uorophthalo-nitrie and zinc acetate in an equimolar 4:1 ratio were inten-sively mixed in a mortar. The mixture was �lled in a glass vessel, three times �ushed with nitrogen and vacuum and �-nally the glass ampoule was sealed under vacuum (1.33 ×  10−3 Pa). After heating for 8 h at 220°C the blue product was isolated and washed with ethanol and acetone to remove the soluble organic admixture. The resulting blue deposit with yield of 48.2% after puri�cation by concentrated sulfuric acid was identi�ed as zinc hexadeca�uorophthalocyanine. IR (KBr) 1610, 1523, 1450, 1148, 832 and 745 cm−1. MS (TOF, Methanol) m/e 865.76. The ZnPcF16/p-6p OTFTs con�gura-tion is given in Fig. 2. The device was prepared with a semi-conductor ZnPcF16 and a rod-like conjugated oligomer p-6p molecule. ZnPcF16 and p-6p were puri�ed twice by thermal gradient sublimation prior to processing. A 6 nm thick �lm of p-6p was �rst deposited on a SiO2 substrate at 180°C, and then a 30 nm thick layer of ZnPcF16 was deposited on top of the p-6p surface by vacuum deposition.

All organic �lms were deposited by vapor deposition in vacuum (10−4–10−5 Pa) at a rate of 0.50 nm min−1. Finally, Au source and drain electrodes with 30 nm thickness were prepared by thermal deposition with a shadow mask de�ning channel width (W) and length (L) of 6000 µm and 200 µm, respectively. The output and transfer characteristics of the transistors were measured with two Keithley 2400 source-measurement units under ambient conditions at room temperature.

3.  Results and Discussion

3.1  UV-vis and �uorescence spectraFigure 3 shows the UV-vis absorption spectra of 5  × 

10−5 mol L−1 ZnPcF16 solutions in 1, 2-dichlorobenzene (DCB), tetrahydrofuran (THF) and dimethylformamide (DMF), respectively. The absorption spectra were measured by an EVOLUTION300 spectrometer. Absorption maxima for Q band are seen at 680 nm with shoulder peak at 650 nm for ZnPcF16 dissolved in DCB and at 644 and 640 nm for ZnPcF16 solution in THF and DMF. The absence of aggrega-tion effects indicates monomer behavior of ZnPcF16 in DCB.8,11) And with the increase of the polarity of solvents, the ground state is more stable than the excited state in the π-π* transition system, so the transition energy gap increases, which will induce the Q band shift to a shorter wave length to

some extent. The spectrum of ZnPcF16 dissolved in THF dif-fers from the spectrums of this compound in DCB solvents since it shows two strong absorption peaks at 644 and 674 nm. It demonstrates that ZnPcF16 aggregates in THF solvent ac-cording to reference literature.8,11)

Figure 4 shows the UV-vis absorption curves of different concentrations of ZnPcF16 in DMF. In the low concentration region, there are two strong absorption peaks at 640 and 674 nm for Q band. With the concentration of ZnPcF16 in-creasing, the absorption peak at about 640 nm grows while the intensity of absorption peak at 674 nm declines and even disappears. It is a typical UV-vis absorption pattern for aggre-gation behavior. The higher the concentration, the more easi-ly it aggregates in DMF.

The �uorescence spectrum of ZnPcF16 in DCB was mea-sured on a CARY Eclipse �uorescence spectrophotometer, as shown in Fig. 5. The emission maxima are observed at 728 nm corresponding to the red light emission.

3.2  Thermal propertiesThe thermal properties of ZnPcF16 were characterized by

thermogravimetric analysis (TGA) at a heating rate of 10°C min−1 under nitrogen atmosphere. The TGA curve was

Fig. 3 The UV-vis spectra of ZnPcF16 in different solvents.

Fig. 4 UV-vis absorption spectra of different concentration ZnPcF16 solu-tion in DMF.Fig. 2 The device con�gurations of ZnPcF16/p-6p thin �lm transistors.

104 Y. Sun, X. Li, S. Wang, L. Zhang and F. Ma

Page 3: Synthesis, Spectral Properties of Zinc ...Synthesis, Spectral Properties of Zinc Hexadeca˜uorophthalocyanine (ZnPcF 16) and Its Application in Organic Thin Film Transistors Yingli

measured by a TG 209 F3 thermo gravimetric analyzer. ZnPcF16 is relative stable and the mass loss is less than 10% below 100°C. TGA measurements indicate that ZnPcF16 compound has high decomposition temperature of 497°C (Td, correspond to a 10% mass loss, Fig. 6). ZnPcF16 exhibits ex-cellent thermal stability and so its semiconductor thin �lm can be prepared by thermal deposition technique.

3.3  XRD of ZnPcF16/p-6p �lmFigure 7 shows the XRD pattern of the ZnPcF16/p-6p thin

�lm. The X-ray diffraction was performed in a Rigaku D/max 2500 PC X-ray diffractometer with a Cu Kα radiation (λ =  1.54056 Å). The diffraction peaks of the ZnPcF16/p-6p �lm at diffraction angle 2θ =  6.24, corresponding to d200 spac-ing of 14.15 Å, are observed. According to the reference liter-atures and the process for preparing semiconductor thin �lms by vapor deposition, the XRD pattern indicates that the ZnP-cF16 molecules are approximately standing-up on the p-6p layer.12–15)

3.4  SEM of ZnPcF16/p-6p �lmThe SEM image was obtained by a FEI Nanosem 430 elec-

tron microscopy instrument. The surface image of ZnPcF16/ p-6p thin �lm is shown in Fig. 8. Dish-like ZnPcF16 mole-cules can be oriented to form high quality semiconductor thin �lms. The image shows a strip grain pattern of ZnPcF16/p-6p �lm. It implies that the π-π stack direction of the ZnPcF16

molecules is parallel to the p-6p layer, which will facilitate the carrier transportation.

The growth behavior of the p-6p thin �lm has been studied systematically.16,17) A highly-ordered smooth p-6p ultrathin �lm can supply a high quality substrate for the growth of phthalocyanine molecules. The phthalocyanine molecules can be oriented on the inducing layer of rod-like p-6p mole-cules, which likely results from the geometrical channels of the p-6p layer surface and the dominant direction of semicon-ductor molecules on the p-6p thin �lm surface. And through the method of organic vapor deposition at high substrate tem-perature, organic phthalocyanine molecules can grow upright on the substrate. Thus the π-π conjugated direction is parallel to the p-6p layer.

3.5  Current-voltage characteristicsTypical output characteristic curves of the ZnPcF16/p-6p

OTFTs are shown in Fig. 9 at different gate-source voltages (VGS) from 0 to 100 V. The positive voltage signals imply an electron-accumulated process in these OTFTs. With the in-crease of VDS, the linear region and the saturation region can be observed. At VGS =  0 V, IDS tends to 0 A for VDS <  60 V and IDS is increased sharply with the increasing VDS for the VDS >  60 V. At VGS =  20 and 40 V, IDS only shows weak current val-ue for VDS from 0 to 100 V. At VGS =  60, 80 and 100 V, IDS is almost linearly increased with increasing VDS for lower VDS from 0 to 30 V while IDS tends to saturate for higher VDS above 30 V.

Fig. 5 Fluorescence emission spectra of ZnPcF16 in DCB.

Fig. 6 TGA curves of ZnPcF16.

Fig. 7 X-ray diffraction spectra of ZnPcF16/p-6p thin �lm.

Fig. 8 SEM of ZnPcF16/p-6p thin �lm.

105Synthesis, Spectral Properties of Zinc Hexadeca�uorophthalocyanine (ZnPcF16) and Its Application in Organic Thin Film Transistors

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In the electron accumulating mode, when the drain voltage is low, the saturated region continues to be compressed as VGS decreases, but for a high drain voltage, IDS increases dramati-cally as VDS increases. This is a typical ambipolar transport character, and both electrons and holes can be transported in the semiconductor. The p-6p is a p-type semiconductor, while ZnPcF16 is an n-type semiconductor. Due to the heterojunc-tion effect, the transistor presents ambipolar behavior.

Figure 10 shows the typical transfer characteristics of the ZnPcF16/p-6p OTFTs with different gate voltages at a �xed VDS of 100 V. The �eld effect mobility was extracted from Fig. 10 in the saturation region (V ≥  (VGS −  VT)) based on

IDS =W2LµCi(VGS − VT)2 (1)

Where IDS is the drain-source current, W and L are the width and length of the channel, respectively, µ is the �eld-effect mobility, VGS is the gate voltage and VT is the threshold Volt-age. The capacitance per unit area of the insulator (Ci) is 8 nF/cm2. When a positive IDS is observed upon the applica-tion of positive VGS and VDS, the semiconductor is n-type since the electrons are mobile. According to the electrical properties, n-type conductivity of the ZnPcF16 semiconductor

material was con�rmed. The �eld effect mobility of 1.3 ×  10−2 cm2/V s and the threshold Voltage of 22 V were extract-ed from the saturation region in Fig. 10.

4.  Conclusions

In summary, an n-type semiconductor material, ZnPcF16, was synthesized and characterized by infrared (IR), MS, UV-vis and �uorescence spectra. The ZnPcF16-based OTFTs us-ing p-6p as the inducing layer was fabricated by the physical vapor deposition technique. The electrical properties of the OTFTs were investigated and n-type of conductivity was con-�rmed. And the charge carrier �eld-effect mobility of the ZnPcF16/p-6p OTFTs was 1.3 ×  10−2 cm2/V s. The stable ZnPcF16 compound with high mobility will be a promising material for optical and electronic devices application.

Acknowledgements

The authors are grateful to Donghang Yan research group in Changchun Institute of Applied Chemistry of Chinese Academy of Sciences for support of devices’ fabrication. The work has been partially supported by National Natural Sci-ence Foundation of China (no. 21401138), High School Sci-ence and Technology Fund Planning Project of Tianjin (no. 20130508).

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Fig. 9 Output characteristics of ZnPcF16/p-6p OTFTs.

Fig. 10 Transfer characteristics of ZnPcF16/p-6p OTFTs at a �xed VDS (100 V).

106 Y. Sun, X. Li, S. Wang, L. Zhang and F. Ma