G. ZANG et al.: SYNTHESIS, CHARACTERIZATION OF COPPER PERFLUOROPHTHALOCYANINE (F16CuPc) ... 827–831 SYNTHESIS, CHARACTERIZATION OF COPPER PERFLUOROPHTHALOCYANINE (F 16 CuPc) AND ITS APPLICATION IN ORGANIC THIN-FILM TRANSISTORS SINTEZA IN KARAKTERIZACIJA BAKROVEGA PERFLUOROFTALOCIANINA (F 16 CuPc) IN NJEGOVA UPORABA V ORGANSKIH TANKOPLASTNIH TRANZISTORJIH Guangjing Zhang 1,2 , Feng Ma 1* , Luzhen Wang 1 ,BoSun 1 , Jiongpeng Zhao 1 , Fude Liu 1 1 School of Chemistry and Chemical Engineering, Tianjin University of Technology, 300384 Tianjin, China 2 Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering, Tianjin University of Technology, 300384 Tianjin, China Prejem rokopisa – received: 2018-12-06; sprejem za objavo – accepted for publication: 2019-06-27 doi:10.17222/mit.2018.260 A stable n-type semiconductor material, F16CuPc, was synthesized with liquid-phase synthesis and its optical, thermal and electrical properties were characterized. The electron energy levels were studied with UV–vis absorption and cyclic voltammetry. The F16CuPc compound embodied a suitable lowest unoccupied molecular orbital (LUMO) level for electron injection. The thermal analysis showed that the compound had an excellent thermal stability with a decomposition temperature above 498 °C. F16CuPc-based organic thin-film transistors (OTFTs) were fabricated with the physical-vapor-deposition technique. The charge-carrier field-effect mobility (μ), on-off current ratio (Ion/Ioff) and threshold voltage (VT) were 0.02cm 2 /V s, 10 5 and 11 V, respectively. Keywords: organic semiconductors, thin-film transistors, thermal properties, phthalocyanines Avtorji ~lanka so v teko~i fazi sintetizirali stabilni n-tip polprevodni{kega materiala na osnovi F16CuPc in tako dolo~ili njegove opti~ne, termi~ne in elektri~ne lastnosti. Energijske nivoje elektronov so {tudirali z UV–vis absorpcijo in cikli~no voltametrijo. F16CuPc je spojina z najni`jim nezasedenim molekularno-orbitalnim (LUMO) nivojem za zasedbo elektrona. Termi~na analiza je pokazala da ima spojina odli~no termi~no stabilnost s temperaturo razpada nad 498 °C. Organski tankoplastni tranzistor na osnovi F16CuPc (OTFTs) so izdelali s tehniko depozicije (odlaganja) iz parne faze. Ugotovili so, da je u~inek poljske mobilnosti nosilcev naboja (μ) 0,02 cm 2 /Vs, razmerje vklop-izklop toka (Ion/Ioff)10 5 in vr{na napetost (VT)11V . Klju~ne besede: organski polprevodniki, tankoplastni tranzistorji, termi~ne lastnosti, ftalocianini 1 INTRODUCTION Organic thin-film transistors (OTFTs) employing organic semiconductors as the active layer have been widely studied because of their potential application in displays, logic circuits and sensors. 1–3 The performance of p-type pentacene-based thin-film transistors has reached the level of a-Si devices. 3,4 Compared with the p-type semiconductor materials, the mobility of the n-type materials with a high thermal stability is relatively low. However, the n-type semiconductor materials are essential for the fabrications of organic complementary circuits, p-n junction diodes and bipolar transistors. Therefore, the synthesis of high-performance and stable n-type organic semiconductors has been one of the re- search hotspots in the organic-optoelectronics field. Phthalocyanines (Pc) show a long-standing record of interest in both the basic research and applications regarding their electrical and photoelectrical properties. 5 Phthalocyanines have a great application potential in the areas related to semiconductors, chemical sensors, nonlinear optics, display devices, information-storage systems and others. 6 While unsubstituted phthalocyan- ines exhibit the p-type behavior due to the doping with electron-accepting molecules, thin films of some metal hexadecafluorophthalocyanines exhibit the n-type behav- ior. A metal hexadecafluorophthalocyanine is a kind of an n-type semiconductor material with stable air and high mobility. 7 Although the synthesis of fluorinated metal phthalocyanines was reported a long time ago, an interest in the study of the properties of these compounds is rekindled. F 16 CuPc was prepared with solid-phase synthesis with a maximum yield of 44 %. 7 X. Yan et al. 3 employed an organic heterojunction buffer layer to decrease the contact resistance of the organic/metal interface and the electron field-effect mobility of OTFTs w a s7 . 6×1 0 –2 cm 2 /V·s. N. Zhang et al. 8 reported on F 16 CuPc-based transparent OTFTs based on Ag/LiF bilayer transparent S/D electrodes with a good electron mobility of 1.31 × 10 –2 cm 2 /V·s. Materiali in tehnologije / Materials and technology 53 (2019) 6, 827–831 827 UDK 621.315.5:621.382.3:620.1 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(6)827(2019) *Corresponding author's e-mail: mafontune@sina.com.cn (Feng Ma) In this paper, a copper hexadecafluorophthalocyanine (F 16 CuPc, shown in Figure 1) was synthesized with liquid-phase synthesis and its optical, thermal and electrical properties were characterized. The F 16 CuPc- based thin-film transistors were fabricated with vapor- deposition techniques and their electrical characteristics were investigated. 2 EXPERIMENTAL PART For the synthesis of F 16 CuPc, tetrafluorophthalo nitrile and copper (I1) acetate with a molar ratio of 5:1 were mixed in an N-methyl pyrrolidone solvent under nitrogen. The mixture was refluxed for 24 h, cooled to room temperature and suction filtered. Then petroleum ether was added to the blue filtrate that was then sub- merged into an ice bath. A dark blue precipitate formed and was suction filtered through a fine Teflon filter. The F 16 CuPc compound was recrystallized from sulfuric acid and isolated in a 52.51 % yield. The IR (KBr) spectrum of F 16 CuPc is shown in Figure 2. The main absorption peaks are (1615, 1527, 1490, 1459, 1318, 1275, 1151, 964, 840 and 754) cm –1 . MS (TOF, Methanol) m/e 862.86. The F 16 CuPc-based OTFT configuration is given in Figure 3. A 30-nm layer of F 16 CuPc was deposited on t o po ft h eS i O 2 substrate with vacuum deposition. The organic film was deposited in vacuum (10 –4 –10 –5 Pa)ata rate of 0.50 nm min –1 . An Au source and drain electrodes with a thickness of 30 nm were prepared using thermal deposition with a shadow-mask-defining 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-measure- ment units under ambient conditions at room tem- perature. 3 RESULTS AND DISCUSSION 3.1 UV–vis and fluorescence spectra Figure 4 shows the UV–vis absorption spectra of 1×1 0 –6 mol·L –1 F 16 CuPc solutions in tetrahydrofuran (THF), pyridine and dimethylformamide (DMF), res- pectively. The absorption spectra were measured with an EVOLUTION300 spectrometer. The absorption maxima for the Q-band are seen at 683 nm with a shoulder peak at 651 nm for the F 16 CuPc dissolved in ptridine and at 680 and 686 nm for the F 16 CuPc solution in THF and DMF. And with the increase of the polarity of the solvents, the ground state is more stable than the excited G. ZANG et al.: SYNTHESIS, CHARACTERIZATION OF COPPER PERFLUOROPHTHALOCYANINE (F16CuPc) ... 828 Materiali in tehnologije / Materials and technology 53 (2019) 6, 827–831 Figure 2: IR spectrum of the F 16 CuPc Figure 1: Molecular structure of F 16 CuPc Figure 4: UV–vis spectra of F 16 CuPc in different solvents Figure 3: Configuration of F 16 CuPc-based thin-film transistors state in the – * transition system, so the transition energy gap increases, inducing a Q-band shift to a shorter wave length to some extent. In Figure 4, the optical-gap energy can be obtained from the edge of the absorption band. 9 The absorption edge of the F 16 CuPc in DMF is found at around 770 nm, from which the optical-band-gap energy of F 16 CuPc, Eg, is estimated to be 1.61 eV. The fluorescence spectrum of 1 × 10 –6 mol·L –1 F 16 CuPc solution in 1, 2-dichlorobenzene (DCB) was measured on a CARY Eclipse fluorescence spectro- photometer, as shown in Figure 5. The emission maxima are observed at 712 nm corresponding to the red-light emission. 3.2 Thermal properties The thermal properties of F 16 CuPc were characterized with a thermogravimetric analysis (TGA) at a heating rate of 10 °C min –1 under a nitrogen atmosphere. The TGA curve was obtained with a TG 209 F3 thermogravi- metric analyzer. F 16 CuPc is relatively stable and the mass loss is less than 10 % below 100 °C. The TGA measurements indicate that the F 16 CuPc compound has a high decomposition temperature of 498 °C (T d , corres- ponding to a 10-% mass loss, Figure 6). F 16 CuPc exhibits an excellent thermal stability and so its semicon- ductor thin film can be prepared with the thermal- deposition technique. 3.3 C-V curve The electron-transport ability and electrochemical properties of F 16 CuPc were examined with solution cyclic voltammetry (CV). The cyclic voltammogram (Figure 7) was obtained on a CHI760E electrochemistry workstation at room temperature in DMF, measured against a saturated calomel electrode (SCE) with tetrabutylammonium perchlorate (Bu 4 NClO 4 , 0.10 M) as the supporting electrolyte. In Figure 7, the chemical- oxidation and reduction peaks of E OX = –0.78 V and E RE = –0.64 V are observed for F 16 CuPc. The reductive process started at –0.29 V. The energy level of the lowest unoccupied molecular orbital, E LUMO , is estimated from the reductive onset potential to be –4.45 eV. 10,11 The low LUMO energy level is favorable for electron injection and transport; in other words, F 16 CuPc should be a good electron-transport material. The energy level of the highest occupied molecular orbital, E HOMO , can be calculated by adding E g from E LUMO as determined by the electrochemistry curve. This leads to an estimation of E HOMO to be –6.06 eV for F 16 CuPc. 3.4 Current-voltage characteristics Typical output characteristic curves of the F 16 CuPc-based OTFTs are shown in Figure 8 at different gate-source voltages (V GS ) from 0 to 50 V. Positive voltage signals imply an electron-accumulated process in these OTFTs. With an increase in V DS , the linear region and the saturation region can be observed. For a lower V DS , ranging from0Vt o2 0V ,I DS is almost linearly increased with the increasing V DS . In contrast, for a higher V DS , I DS tends to saturate. G. ZANG et al.: SYNTHESIS, CHARACTERIZATION OF COPPER PERFLUOROPHTHALOCYANINE (F16CuPc) ... Materiali in tehnologije / Materials and technology 53 (2019) 6, 827–831 829 Figure 5: Fluorescence-emission spectrum of F 16 CuPc in DCB Figure 7: Cyclic voltammogram of F 16 CuPc in DMF Figure 6: TGA curve of F 16 CuPc Figure 9 shows typical transfer characteristics of the F 16 CuPc-based OTFTs with different gate voltages at a fixed V DS of 50 V. The field-effect mobility was extracted from the saturation region (V (V GS – V T )) based on the following formula: 3 I W L CV V i DS GS T =− 2 2 () (1) Here, I DS is the drain-source current, W and L are the width and length of the channel, respectively, μ is the field-effect mobility, V GS is the gate voltage and V T is the threshold voltage. The capacitance per unit area of the insulator (Ci) is 8 nF/cm 2 . When a positive I DS is observed upon the application of positive V GS and V DS , the semiconductor is of the n-type since the electrons are mobile. According to the electrical properties, the n-type conductivity of the F 16 CuPc semiconductor material was confirmed. A field-effect mobility (μ) of 0.02 cm 2 /V s, on-off current ratio (I on /I off )of10 5 and threshold voltage (V T ) of 11 V were extracted from the saturation region in Figure 9. 4 CONCLUSIONS In summary, an n-type semiconductor material, F 16 CuPc, was synthesized and characterized. The F 16 CuPc compound has a high T d , above 498 o C, showing that it has a good thermal stability. The LUMO level of F 16 CuPc is –4.45 eV, which is beneficial to the electron transportation. F 16 CuPc-based OTFTs were fabricated using the physical-vapor-deposition technique and their electronic properties were demonstrated. The field-effect mobility, on-off current ratio and threshold voltage of the OTFTs were 0.02 cm 2 /V s, 10 5 and 11 V, respectively. Therefore, F 16 CuPc is a good n-type semiconductor material and can be used in organic electronic devices such as organic field-effect transistors. Acknowledgements The authors are grateful to the Donghang Yan Research Group at the Changchun Institute of Applied Chemistry of the Chinese Academy of Sciences for the help with the fabrication of devices. The work was partially supported by the National Natural Science Foundation of China (no. 21401138) and the Tianjin Project of the Innovation Team of Colleges and Univer- sities in Tianjin (TD13–502). 5 REFERENCES 1 H. E. A. Huitema, G. H. Gelinck, J. B. P. H. Van Der Putten, K. E. Kuijk, C. M. Hart, E. Cantatore, D. M. De Leeuw, Active-Matrix Displays Driven by Solution-Processed Polymeric Transistors, Adv. Mater., 14 (2002) 17, 1201–1204 2 B. Crone, A. Dodabalapur, Y.-Y. Lin, R. W. Filas, Z. Bao, A. Laduca, R. Sarpeshkar, H. E. Katz, W. Li, Large-scale complementary integrated circuits based on organic transistors, Nature, 403 (2000), 521–523, doi:10.1038/35000530 3 X. Yan, J. Wang, H. Wang, H. Wang, D. Yan, Improved n-type organic transistors by introducing organic heterojunction buffer layer under source/drain electrodes, App. Phys. Lett., 89 (2006) 5, 053510, doi:10.1063/1.2227714 4 T. N. Jackson, Y. Y. Lin, D. J. Gundlach, H. Klauk, Organic thin-film transistors for organic light-emitting flat-panel display backplanes, IEEE J. Sel. Top. Quantum Electron., 4 (1998) 1, 100–104, doi:10.1109/2944.669475 5 H. Brinkmann, C. Kelting, S. Makarov, O. Tsaryova, G. Schnurpfeil, D. Wöhrle, D. Schlettwein, Fluorinated phthalocyanines as molecular semiconductor thin films, Phys. Stat. Sol. (a), 205 (2008) 3, 409–420, doi:10.1002/pssa.200723391 6 E. Kol’tsov, T. Basova, P. Semyannikov, I. Igumenov, Synthesis and investigations of copper hexadecafluorophthalocyanine CuPcF16, Mater. Chem. Phys., 86 (2004) 1, 222–227, doi:10.1016/ j.matchemphys.2004.03.007 7 T. Wang, D. Ebeling, J. Yang, C. Du, L. Chi, H. Fuchs, D. Yan, Weak Epitaxy Growth of Copper Hexadecafluorophthalocyanine (F16CuPc) on p-Sexiphenyl Monolayer Film, J. Phys. Chem. B, 113 (2009)8, 2333–2337, doi:10.1021/jp8080639 8 N. Zhang, J. Lin, J. S. Luo, Y. T. Li, Z. H. Gan, Y. Fan, X. Y. Liu, N-channel transparent organic thin-film transistors with Ag/LiF G. ZANG et al.: SYNTHESIS, CHARACTERIZATION OF COPPER PERFLUOROPHTHALOCYANINE (F16CuPc) ... 830 Materiali in tehnologije / Materials and technology 53 (2019) 6, 827–831 Figure 9: Transfer characteristics of F 16 CuPc-based OTFTs at a fixed V DS (50 V) Figure 8: Output characteristics of F 16 CuPc-based OTFTs bilayer transparent source-drain electrodes fabricated by thermal evaporation, Appl. Phys. Express, 7 (2014) 2, 021610, doi:10.7567/ apex.7.021601 9 H. Jin, X. Li, T. Tan, S. Wang, Y. Xiao, J. Tian, Electrochromic pro- perties of novel chalcones containing triphenylamine moiety, Dyes and Pigments, 106 (2014), 154–160, doi:10.1016/j.dyepig.2014. 02.018 10 J. Yang, T. Wang, H. Wang, F. Zhu, G. Li, D. Yan, Ultrathin-Film Growth of para-Sexiphenyl (I): Submonolayer Thin-Film Growth as a Function of the Substrate Temperature, J. Phys. Chem. B, 112 (2008) 26, 7816–7820, doi:10.1021/jp711455u 11 J. Yang, T. Wang, H. Wang, F. Zhu, G. Li, D. Yan, Ultrathin-Film Growth of para-Sexiphenyl (II): Formation of Large-Size Domain and Continuous Thin Film, J. Phys. Chem. B, 112 (2008) 26, 7821–7825, doi:10.1021/jp711457p G. ZANG et al.: SYNTHESIS, CHARACTERIZATION OF COPPER PERFLUOROPHTHALOCYANINE (F16CuPc) ... Materiali in tehnologije / Materials and technology 53 (2019) 6, 827–831 831