Synthesis of Peripherally Tetrasubstituted Phthalocyanines and Their Applications in Schottky Barrier Diodes

1Faculty of Arts and Science, Department of Chemistry, Yildiz Technical University, Esenler, 34210 Istanbul, Turkey 2Faculty of Chemical and Metallurgical Engineering, Department of Metallurgical and Materials Engineering, Yildiz Technical University, Esenler, 34210 Istanbul, Turkey 3Faculty of Arts and Science, Department of Physics, Yildiz Technical University, Esenler, 34210 Istanbul, Turkey 4Department of Occupational Health and Safety, Esenyurt University, Esenyurt, 34510 Istanbul, Turkey


Introduction
Phthalocyanines (Pcs) have drawn special interest due to their particular unique physicochemical properties and exhibit a variety of superb properties, such as architectural flexibility, diverse coordination properties, increased stability, improved spectroscopic characteristics, and both semi-and photoconductive characteristics [1,2].These properties make them of considerable interest in diverse technological and scientific areas such as gas sensors [3], optoelectronic devices [4], static induction transistors [5], Langmuir-Blodgett films [6], electrophotographic applications [7], optical data storage [8], solar cells [9], organic field-effect transistors (OFETs) [10], and nonlinear optics [11].In the fields of electronics and optoelectronics, Pcs are very promising candidates for future technology.The application of Pc compounds for electronics and optoelectronics devices is related to the dielectric relaxation process and electrical conductivity in the Pc compounds and these are frequently the deciding factors about the compatibility of the material for a specific device application [12,13].
As main building blocks of various electronic and optoelectronic devices such as microwave diodes, solar cells, and photodetectors [14,15], Schottky barrier diodes (SBDs) play a crucial role in the further development of the electronic devices.High barrier height and low leakage current are the most important performance parameters for a SBD.Although important advances have been made toward high performance SBD, a lot of improvement is still needed for practical applications.There are currently a vast number of experimental studies on the realization and the manipulation of the Schottky barrier height and the leakage current using an organic interlayer.In this respect, Pc compounds offer several advantages over inorganic counterparts such as strontium titanate (SrTiO 3 ) [16] and hafnium dioxide (HfO 2 ) [17] because of their low cost, suitability for synthetic modification, and ease of device fabrication.Therefore, the understanding of the physical behavior of these compounds under dc condition is essential in order to decide suitability and improving the quality and performance of SBD based electronic devices.

Experimental Section
2.1.Materials and Equipment.,  -Dimethylformamide (DMF), dimethyl sulfoxide (DMSO), chloroform (CHCl 3 ), diethyl ether, methanol (MeOH), tetrahydrofuran (THF), ethanol (EtOH), dichloromethane (DCM), n-hexane, npentanol, and acetone were purchased from MERCK.4-Nitrophthalonitrile, 2-hydroxymethyl-1,4-benzodioxan, 1,8diazabicyclo [5.4.0]undec-7-ene (DBU), potassium carbonate (K 2 CO 3 ), zinc(II) acetate, cobalt(II) acetate, nickel(II) acetate, copper(II) acetate were purchased from Aldrich.All chemicals were of reagent grade.All solvents were dried and stored over molecular sieves.The progress of the reactions was monitored by thin layer chromatography (TLC).Melting points of the substances were determined using an Electrothermal Gallenkamp device.The IR spectra were recorded using a Perkin Elmer spectrophotometer with ATR sampling accessory.UV-Vis spectra were recorded on a Agilent 8453 UV/Vis spectrophotometer.Elemental analyses were carried out by a LECO CHNS 932 instrument.A Varian Unity Inova 500 MHz spectrophotometer was used for 1 H NMR and 13 C NMR spectra.Mass spectra of the synthesized substances were acquired by using a Bruker Microflex LT MALDI-TOF MS.GC-MS spectrum was obtained on an Agilent Technologies 6890N GC-System-5973 IMSD.[1,4]dioxin-2-yl)methoxy)phthalonitrile ( ). 2-Hydroxymethyl-1,4-benzodioxan (0.52 g, 2.89 mmol) was dissolved in DMF (30 ml) under nitrogen and 4-nitrophthalonitrile (0.5 g, 2.89 mmol) was added.After stirring for 20 min at room temperature, finely ground anhydrous K 2 CO 3 (1.96g, 14.45 mmol) was added to this mixture in portions over 2 h with efficient stirring.The reaction mixture was stirred under a nitrogen atmosphere at 50 ∘ C for a total of 24 h.The reaction mixture was cooled to room temperature and was then poured into 200 ml ice-water, and the precipitate that formed was filtered off, washed with water, n-hexane, and diethyl ether, and then dried.The creamy crude product was recrystallized from MeOH.Finally the pure powder was dried in a vacuum.This compound was soluble in MeOH, EtOH, acetone, DCM, and CHCI 3 .Yield: 0.76 g (90%

Fabrication and Characterization of SBDs.
The p-type Si wafers with (100) orientation and resistivity in the 8-10 Ω cm range were used as substrate.After cleaning the Si wafers using standard RCA cleaning procedure, Al metal was thermally evaporated under 4 × 10 −5 mbar pressure onto the Si wafers and followed by a heat treatment at 500 ∘ C for 3 min in N 2 atmosphere in order to form the Ohmic contacts.After the removing of native oxide layer on the polished surface of the Si wafers by HF/H 2 O (1 : 8) solution, thin films of the Pc compounds were formed onto the polished surface of Si substrate by spin coating technique.The thickness of the films was determined by ellipsometric technique.After the spin coating of the interface layer, 1 mm in diameter and 2000 Å thick Ag Schottky contacts were deposited on the the Pc layer by thermal evaporation technique.In this way, Ag/Pc film/p-Si Schottky barrier diodes were fabricated.Currentvoltage (-) characteristics of the devices were measured under vacuum (<10 −3 mbar) using an electrometer (Keithley, Model 617).[1,4]dioxin-2-yl)methoxy)phthalonitrile ( ) and its metal-free ( ), zinc(II) ( ), cobalt(II) ( ), nickel(II) ( ), and copper(II) ( ) phthalocyanine complexes was given in Scheme 1.Initial compound was synthesized by the nucleophilic aromatic substitution reaction of compound 2hydroxymethyl-1,4-benzodioxan with 4-nitrophthalonitrile, in DMF at 50 ∘ C for 24 h.New phthalocyanines ( -) were synthesized using corresponding anhydrous metal salts and/or without metal salt in 3 mL n-pentanol for 24 h at 160 ∘ C under N 2 atmosphere in the presence of DBU.Structures of synthesized compounds were verified by elemental analysis, UV-vis, FT-IR, 1 H NMR, 13 C NMR, and mass spectroscopy techniques.According to the IR spectrum of , a new vibration that appeared at 2228 cm −1 proved the formation of .The FT-IR spectrum of showed aromatic bond -CH peaks at around 3013-3048 cm −1 and aromatic -C=C peaks at around 1604-1592 cm −1 .The stretching peaks at 2935-2880 cm −1 confirmed the presence of -CH and -CH 2 groups. 1 H NMR spectrum of (Figure S1) exhibited aromatic protons, integrating for a total of 7 protons, at 7.88-7.86,7.60-7.59,7.43-7.41,and 6.75-6.71ppm.In addition, CH and CH 2 protons of were observed at 4.56-4.52 and 4.42-4.09ppm which integrated for 5 protons. 13C NMR of (Figure S2) is another evidence for proposed structure; the new bands observed at 115.49 and 115.08 ppm could be defined as carbons of nitrile groups.The molecular ion peak for was observed at 292 / in its GC-MS spectrum which verified the formation of this phthalonitrile.

Results and Discussion
FT-IR spectra of the phthalocyanine compounds are very similar to each other.The proposed target structures of all new phthalocyanines were affirmed in the FT-IR spectra by the disappearance of the -C≡N vibration at 2228 cm −1 for phthalonitrile ( ).For metal-free phthalocyanine ( ), inner core -NH vibration was observed at 3289 cm −1 , and this peak was the essential difference between the IR spectral data of metallophthalocyanines and metal-free ones.In the FT-IR spectra of phthalocyanines ( -), stretching vibrations of aromatic CH groups around 3064-3044 cm −1 , aliphatic-CH, CH 2 groups around 2954-2870 cm −1 , and aromatic -C=C groups around 1610-1489 cm −1 appeared at expected frequencies.In the 1 H NMR spectra of , , and phthalocyanines (Figures S3, S4, and S5), the bands belonging to the phthalocyanine and substitute 2-hydroxymethyl-1,4-benzodioxan aromatic protons were observed between 7.67 and 6.82 ppm for compound , between 7.74 and 6.44 ppm for compound , and between 7.74 and 6.44 ppm for compound as broad multiplet peaks.In addition, the aliphatic-CH protons for , , and were assigned at around 4.95, 4.94, and 4.95 ppm, respectively, as broad single peaks.The aliphatic CH 2 protons for , , and were also assigned at around 4.60-4.20,4.54-4.17,and 4.57-4.21ppm, respectively, as broad multiplet peaks.For , the typical shielding of inner core protons could not be observed due to the strong aggregation between Pc molecules [18].The 1 H NMR spectrum of compounds and could not be determined because of the presence of paramagnetic copper and cobalt ions [19]  UV-Vis spectroscopy is thought to be the basic method to verify the formation of phthalocyanines.The phthalocyanine compounds show two main electronic transitions named as Q band (600-700 nm in the visible region), assigned to the - * transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the Pc ring, and B band (300-350 nm in the UV region), resulting from the deeper - * transitions [20][21][22].The UV spectra of the metallophthalocyanines ( -) display intense Q band at 675 nm for , 663 nm for , 671 nm for , and 675 nm for in THF.The shoulders of phthalocyanines were observed at 609, 603, 617, and 611 nm, respectively.For the Q band of phthalocyanines, the longer wavelength absorptions are owing to the monomeric species and shorter wavelengths (shoulders) are due to the aggregated species [2].The B bands were observed at 351 nm for , 329 nm for , 327 nm for , and 341 nm for .For the metal-free phthalocyanine ( ), a split Q band was observed at 701 and 664 nm, while B band was observed at 332 nm (Figure 2).

Characterization of SBDs.
A device without Pc interface layer was also fabricated in order to verify the efficacy of the insertion of Pc layer on the charge transfer characteristic of the devices.For comparison, the room temperature current density () voltage characteristics of the devices are presented in Figure 3.As is clear from Figure 3, all the devices exhibit rectification behavior with different rectification ratio between 7 for Ag/p-Si and 310 for Ag/ /P-Si structure at ±5.This results prove the good rectification performance for the Pc layer inserted devices.Leakage current is an important factor influencing the performance of an SBD.It was found that the value of the leakage current density for the Pc layer inserted devices varies between 2.29 × 10 −7 A/cm 2 and 6.56 × 10 −7 A/cm 2 , while the observed leakage current density was 1.38 × 10 −4 A/cm 2 for Ag/p-Si structure.It should be mentioned here that the observed level of leakage current density is significantly low when compared with the reported values with similar structure such as Au/PDI/p-Si [23] and Al/DNA/p-Si [24] Schottky devices.This findings clearly show that the main performance parameters of a SBD such as rectification ratio and leakage current can be improved by modifying p-type silicon surface with 2(3),9 (10),16 (17) Various models, such as Fowler-Nordheim tunneling, Poole-Frenkel emission, and Ohmic and space charge limited (SCL) current, have widely been used to understand the nature of the charge transport through an organic thin film [25][26][27].To investigate the dc conduction mechanism in fabricated devices, the obtained - data were replotted on double logarithmic scale for forward and reverse bias conditions in Figure 4.
It will be clear from the analysis of the data presented in Figures 4(a) and 4(b) that there are two regions with different slopes in double logarithmic - plots for both forward and reverse bias conditions, which is consistent with earlier reports on Metal/Phthalocyanine/Metal (MIM) structures [28,29].A relatively small slope in - curves at low voltage regions is clear for both directions.Special attention must be given to interpreting the presented data in Figure 4, because different conduction mechanism can give rise to this type of - characteristics.A close analysis of the slopes of the curves shown in Figure 4 indicates that the slopes of the curves vary between 0.92 and 1.16 for reverse bias conditions, which is indicative of the Ohmic conduction in this voltage region.On the other hand, it was observed that the slopes of the - curves for forward bias vary between 2.15 and 2.68, which cannot be considered in the framework of Ohmic conduction.Therefore, in order to identify the conduction mechanism taking place in Ag/Pc/p-Si structures, the - curves of the fabricated devices were analyzed separately for forward and reverse bias directions.As mentioned before, the reverse bias - characteristic of the devices at lower voltage range can be explained by the Ohmic behavior of the device.However, a power law dependence in the form of   with  > 2 reveals the presence of space charge limited conductivity (SCLC) controlled by exponentially distributed trapping levels.This type of - characteristic has been observed in some other Pc films such as copper phthalocyanine thin film [30].It should be mentioned here that the reverse bias voltage at which the exponent  deviates from unity is nearly the same for all Pc modified devices.In order to investigate whether our experimental data can be explained using Poole-Frenkel emission, we have plotted, in Figure 5, ln(/) versus  1/2 curves for all samples.
In the case of Poole-Frenkel emission, the relationship between the current density and the applied voltage can be expressed as given in [15] where  is a fitting parameter,  is the thickness of the film,  PF is the required energy for an electron to escape from the trap,  is the slope parameter, and  PF √  is barrier lowering.According to (1), the plot of ln(/) versus √  should be linear if the charge transport takes place through Poole-Frenkel emission.Good correlation coefficients ( 2 ) obtained from ln(/) versus √  plots reveal that the mechanism responsible for conduction in Ag/Pc/p-Si structures under forward bias conditions can be described by the Poole-Frenkel emission.

Conclusion
In this work, we have defined the synthesis and characterization of peripherally tetrasubstituted metal-free, zinc, cobalt, nickel, and copper phthalocyanine derivatives.These new five compounds were characterized by various techniques.
In order to get more quantitative information about charge transport mechanism and the usability of these compounds to modify the main SBD parameters such as rectification ratio and the leakage current, SBDs with the structure of Ag/Pc/p-Si were fabricated and characterized.Our experimental results showed that charge transport in these compounds take place via different mechanism in reverse and forward bias conditions.Results from this preliminary analysis indicated that the compounds can be considered, among other candidates, as a potential passivation layer for the new SBD diodes with high rectification ratio and low leakage current.