Synthesis, Characterization and Properties of Porphyrin Derivatives for Organic Photovoltaic Devices as Near-Infrared Dyes

 

Ashis Kumar Sarkar*, Pijush Kanti Roy

Department of Chemistry, Faculty of Science, Mawlana Bhashani Science and Technology University, Santosh, Tangail-1902, Bangladesh

*Corresponding Author E-mail: ashischemru@gmail.com

We designed and synthesized porphyrin derivatives, in which a porphyrin ring was linked to two diketopyrrolopyrrole-4-fluorobenzene units named Pzn-DPP-P-F by ethynylene bridges. This new donor material is characterized by using ¹H NMR, MALDI-TOF-Mass, UV-Visible spectrophotometer and cyclic voltammetry. The resulted material exhibited a very low optical band gap of 1.50 eV and a broad light absorption to 825 nm. The photovoltaic performances of these donor materials were successfully evaluated in organic solar cells. PCBM was used as acceptor moiety in this work. Consequently, the organic photovoltaic device (OPV) based on Pzn-DPP-P-F showed J_SC of 12.67 mA/cm², V_OC of 0.69 V, a FF of 0.52, the overall power conversion efficiency of (η) 4.55%. The facile synthetic route and purification technique provide the durability of this donor material as future OPV development.

 

 

INTRODUCTION:

Solution-processed small molecule (SM) bulk heterojunction organic solar cells (BHJ OSCs) have adopted acute study very recently on account of the defined molecular structure and weight, high purity, and good batch-to-batch reproducibility in cell performance.^1-4 These amenities make it easier to understand the relationships between the molecular structures and device performance and realize the crucial performance limit of these materials and devices. Not surprisingly, dramatically onward PCEs up to 10% have been indicated for single layer SM BHJ solar cells^5-10, compared to the supreme of polymer solar cells^11-15.

 

Stimulated by the instinctive photosynthetic systems which utilize chlorophylls to absorb light and carry out photochemical charge separation to stowage light energy, porphyrins and their derivatives have been pursued for a long time as the effective materials in dye-sensitized solar cells with the maximum efficiency of 13%¹⁶, and in BHJ OSCs but with very low PCEs^17-28.

 

Nowadays, A-π-D-π-A structural design technique to synthesize porphyrin-based donor materials has been practised, and practically achieved high PCEs more than 4% applying PCBM as the acceptor units^3,29-33, exhibiting the potential of porphyrins for OSCs by their virtues of high molar absorption coefficients, simple chemical structure modification, and identical photophysical properties. To date, however, a very limited number of porphyrin-based materials have been exhibited with efficiencies higher than 4% in BHJ OSCs, since it is very challenging to gain a balance between solubility and intermolecular interactions for porphyrin molecules, synchronously targeting optically positioned energy levels (to ensure a high V_OC) and an increased solar flux coverage and charge carrier mobility.

To enhance the backbone conjugation and extend intermolecular π-π interaction, we designed and prepared porphyrin small molecule Pzn-DPP-P-F (Scheme 2) in which DPP was symmetrically conjugated to porphyrin core, then finished with alkyl chain terminal units. Elaborate studies show that the adequate alkyl chains on meso-position and DPP conjugated backbone can not only enhance the directional intermolecular π-π stacking in films but also form the blend films with PCBM with characteristics of bicontinuous, interpenetrating networks requisite for efficient charge separation and transportation. The processing engineering for their blend films with PCBM afforded power conversion efficiencies of 4.55% in BHJ OSC based on Pzn-DPP-P-F.

 

EXPERIMENTAL:

Materials:

All air and water-sensitive reaction were performed under nitrogen atmosphere. Diketopyrrolopyrrole (DPP), N-bromosuccinimide (NBS), Pd(PPh₃)₄, phenylboronic acid, 4- fluorophenylboronic acid were purchased from Sigma-Aldrich. Chloroform, Tetrahydrofuran (THF), Magnesium sulfate (MgSO₄), Potassium carbonate (K₂CO₃) were purchased from Samchun chemicals. All of other chemicals were purchased from Sigma-Aldrich and Samchun South Korea. Organic solvents used in this work were purified using standard process. The other materials were of the common commercial level and used as received.

 

Methods:

Column chromatography was performed using silica gel 230-300 mesh (purchased from Aldrich) as the solid support. All NMR spectra were recorded on a Bruker Advance DPX 400 MHz spectrometer at 25 °C in CDCl₃and THF. ¹H NMR chemical shifts are reported in δ units, part per million (ppm) relative to the chemical shift of residual solvent. Deuterated solvents were used as received from Aldrich. Reference peaks for chloroform in ¹H NMR spectra were set at 7.18 ppm. Molecular masses were measured by Matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (MALDI-TOF-MS). MALDI-TOF-MS was performed on Bruker Daltonics LRF20 with dithranol (1,8,9-trihydroxyanthracene) and HCCA (α-Cyano-4-hydroxycinnamic acid) as the matrix. The absorption spectra were measured on a SHIMADZU/UV-2550 model UV-visible spectrophotometer. Recycling SEC was performed on JAI model LC9021 equipped with JAIGEL-1H, JAIGEL-2H and JAIGEL-3H columns using THF (DUKSAN) as the eluent. Cyclic voltammetry was performed on a BAS 100B/W electrochemical analyzer with a three-electrode cell in a 0.1 N Bu₄NBF₄ solution in acetonitrile at a scan rate of 50 mV/s.

 

Scheme 1: Synthetic route to Br-DPP-P-F.

 

Synthesis of DPP-Br 2:

To a stirred solution of 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (5.00 g, 9.52 mmol) in dry chloroform (300 mL), N-bromosuccinimide (NBS, 1.69 g, 9.52 mmol) was slowly added at 0°C. The mixture was allowed to warm up to room temperature and stirred 2 h. The reaction mixture was poured into water and then extracted with chloroform. The combined organic layers were washed with water and dried over anhydrous MgSO₄. The solvent was removed under reduced pressure and the mixture of products was separated using column chromatography (silica gel, eluent gradient 8:2 > 5:5 hexane: DCM). The target compound (2.59 g) was obtained as a red flaky powder (yield = 2.59 g, 45%). ¹H NMR (400 MHz, CDCl₃) δ: 8.90 (dd, 1H), 8.63 (d, 1H), 7.64 (dd, 1H), 7.28-7.26 (m, 1H), 7.22 (d, 1H), 4.03-3.99 (m, 2H), 3.98-3.92 (m, 2H), 1.88-1.80 (m, 2H), 1.38-1.23 (m, 16H), 0.90-0.84 (t, 12H).

 

Synthesis of Compound 3:

To a mixture of compound 2 (3.00 g, 4.97 mmol) and 4-fluorophenylboronic acid (1.04 g, 7.45 mmol) in dry THF (40 mL), Pd(PPh₃)₄ (0.24 g, 0.21 mmol) and aqueous K₂CO₃ (2.0 M, 20 mL; N₂ bubbled before use) were added and the mixture was stirred for 24 h at 60 °C. After cooling to room temperature, the reaction mixture was poured into water and then extracted with chloroform. The combined organic layers were washed with water and dried over anhydrous MgSO₄. After filtration and evaporation, the product was purified by silica gelusing column chromatography (eluent: CHCl₃), recrystallized from chloroform/methanol, and dried under vacuum to afford compound 3 as a dark violet solid (yield = 3.12 g, 92%). ¹H NMR (400 MHz, CDCl₃) δ: 8.93 (d, 1H), 8.90 (d, 1H), 7.67-7.60 (m, 4H), 7.40 (d, 1H), 7.15-7.10 (m, 2H), 4.07-4.02 (m, 4H), 1.90-1.87 (m, 2H), 1.38- 1.25 (m, 16H), 0.92-0.85 (t, 12H).

 

Synthesis of Br-DPP-P-F 4:

To a stirred solution of 3 (5.00 g, 9.52 mmol) in dry chloroform (300 mL), N-bromosuccinimide (NBS, 1.69 g, 9.52 mmol) was slowly added at 0 °C. The mixture was allowed to warm up to room temperature and stirred 2 h. The reaction mixture was poured into water and then extracted with chloroform. The combined organic layers were washed with water and dried over anhydrous MgSO₄. The solvent was removed under reduced pressure and the mixture of products was separated using column chromatography (silica gel, eluent gradient 8:2 > 5:5 hexane: DCM). The target compound (3.00 g) was obtained as a red flaky powder (yield = 3.00 g, 59%). ¹H NMR (400 MHz, CDCl₃) δ: 8.30 (d, 1H), 8.13 (d, 1H), 8.04 (d, 2H), 7.78 (d, 2H), 7.32 (d, 2H), 3.09-3.04 (m, 4H), 2.18-2.14 (m, 2H), 1.56-1.50 (m, 8H), 1.23-1.18 (m, 8H), 0.90-0.84 (t, 12H).

 

Scheme 2: Synthetic route to Pzn-DPP-P-F.

Synthesis of Dipyrromethane 5:

Paraformaldehyde (0.6 g, 20 mmol) and pyrrole (15 mL, 500 mmol) were dissolved in a mixture of AcOH (150 mL) and MeOH (50 mL). The solution was stirred for 20 h at 25°C. Then the reaction mixture was washed with water (100 mL × 2) and aqueous KOH solution (0.1 M; 100 mL × 2). The product was purified by flash column chromatography as eluent of DCM to obtain the product 5 (the chemical structure in Scheme 2) as dark green crystal in 40% yield (1.40 g). ¹H NMR (CDCl₃, 400 MHz) δ: 7.9 (br, 2H), 6.62 (d, 2H), 6.01 (t, 2H), 5.95 (d, 2H), 3.90 (s, 2H).

 

Synthesis of porphyrin derivatives 6:

To a degassed solution of dipyrromethane (0.5 g, 3.43mmol) and corresponding aldehyde (1.24 g, 3.43 mmol) in DCM (500mL) was added trifluoroacetic acid (0.23 mL, 37.3 mmol). The solution was stirred under nitrogen atmosphere at 23°C for 4 h, and charged with DDQ (1.17 g, 5.14 mmol). Then, the mixture was stirred for an additional 1 h. The mixture was basified with Et₃N (0.58 mL) and filtered through silica. after the solvent was removed under reduced pressure, the residue was purified by column chromatography (silica gel) using DCM/hexane = 1/2 as eluent. The products were recrystallized from MeOH/DCM to give the product (1.02 g,30%) as a purple powder. ¹H NMR (CDCl₃, 400 MHz) δ: 10.13 (s, 2H), 9.26 (d, 4H), 8.96 (d, 4H), 7.51 (t, 2H), 6.92 (d, 2H), 2.65 (d, 4H), 1.76 (m, 2H), 1.33 (m, 12H), 1.25 (m, 20H), 0.89 (t, 12H), -3.12 (s, 2H). MS (MALDI-TOF): m/z 811.24 [M⁺]; C₅₂H₆₆S₂N₄ (812.20).

 

Synthesis of 5,15-Dibromo-porphyrin derivatives7:

To a stirred solution of compound 6 (0.35g, 0.36 mmol) in DCM (150 mL) was slowly added a solution of NBS (0.13 g, 0.75 mmol) in DCM (50 mL) in a period of 6 h at 0°C under inert condition. The reaction was quenched with adding acetone (30 mL). After the solvent was removed under reduced pressure, the residue was purified by column chromatography (silica gel) using DCM/hexane = 1/2 as eluent. Recrystallization from MeOH/DCM gave the product (0.32 g, 81%) as a purple powder. ¹H NMR (CDCl₃, 400 MHz) δ: 9.26 (d, 4H), 8.96 (d, 4H), 7.51 (t, 2H), 6.92 (d, 2H), 2.65 (d, 4H), 1.76 (m, 2H), 1.33 (m, 12H), 1.25 (m, 20H),0.89 (t, 12H), -3.12 (s, 2H). MS (MALDI-TOF): m/z 969.03 [M⁺]; C₅₂H₆₄S₂Br₂N₄ (969.57).

 

Synthesis of [5,15-Dibromo-porphinato] zinc (II) derivatives 8:

A suspension of the compound 7 (0.291 g, 0.243mmol) and Zn(OAc)₂.2H₂O (0.135 g, 0.615mmol) in a mixture of DCM (110 mL) and MeOH (25mL) was stirred at 23 °C for 3 h. The reaction was quenched with adding water (10 mL), and the mixture was extracted with DCM (3 × 10 mL). The combined extracts were washed with water and dried over anhydrous MgSO₄. The solvent was removed under reduce pressure to give the product (0.25g, 96%). ¹H NMR (CDCl₃, 400 MHz) δ: 9.26 (d, 4H), 8.96 (d, 4H), 7.51 (t, 2H), 6.92 (d, 2H), 2.65 (d, 4H), 1.76 (m, 2H), 1.33 (m, 12H), 1.25 (m, 20H), 0.89 (t, 12H). MS (MALDI-TOF): m/z 1032.39 [M⁺]; C₅₂H₆₂S₂Br₂N₄Zn (1033.15).

 

Synthesis of [5,15-Di(triisopropylsilyl)ethynyl-porphinato] zinc (II)9:

The compound 8 (50 mg, 42 μmol), anhydrous toluene (10 mL), (triisopropylsilyl) acetylene (37µL, 168 μmol), and copper (I) iodide (1.2 mg, 6.3 μmol) in a round-bottom flask were stirred under nitrogen. Triethylamine (1 mL) was then added into the mixture, which was then degassed by N₂bubbling. After the addition of bis (triphenylphosphine) palladium (II) dichloride (4.4 mg, 6.3μmol), the solution was heated at 85°C overnight. The solvent was removed under vacuum, and the residue was purified by column chromatography (silica gel) using methylene chloride/hexane (1/3) as eluent. Solvent were removed to afford compound 9(0.26 mg, 87%) as dark purple solid.¹H NMR (CDCl₃, 400 MHz) δ: 9.26 (d, 4H), 8.96 (d, 4H), 7.51 (t, 2H), 6.92 (d, 2H), 2.65 (d, 4H), 1.76 (m, 2H), 1.33 (m, 36H), 1.25 (m, 20H), 1.02 (t, 12H), 0.89 (t, 18H). MS (MALDI-TOF): m/z 1235.33 [M⁺]; C₇₄H₁₀₄S₂Si₂N₄Zn(1236.89).

 

Synthesis of porphyrin derivatives10:

To a solution of the compound 9 (100mg, 0.071 mmol) in THF (5 mL) was added tetra-n-butylammonium fluoride (0.715 mL, 0.71 mmol) at RT. Prior to the addition of water, the mixture was stirred for 1 h. Then, the solution was extracted with methylene chloride. The green organic layer was collected, and the solvent was evaporated under reduced pressure to get the deprotected products.

 

Synthesis of compound 1:

To a solution of the deprotected intermediate in a degassed mixture of THF (20 mL) and NEt₃ (3 mL) was added DPP-Br (21.8 mg, 85 μmol), Pd₂(dba)₃ (3.25 mg, 3.55 μmol), and AsPh₃ (43 mg, 142 μmol). The mixture was stirred at 85 °C for 5 h. The solvent was removed under vacuum and the residue was purified by column chromatography (silica gel) using CH₂Cl₂/CH₃OH (19/1) as eluent. Recrystallization from CH₂Cl₂/CH₃OH gave products 1 (29.7 mg, 30%). ¹H NMR (THF-d₈, 400 MHz)δ: 9.26 (d, 4H), 8.96 (d, 4H), 8.24 (d, 2H),8.13 (d, 2H), 8.04 (d, 4H),7.78 (d, 4H),  7.51 (t, 2H), 7.32 (d, 4H), 6.92 (d, 2H), 3.09 (m, 8H), 2.65 (d, 4H),2.18 (m, 4H),1.76 (m, 2H),1.56 (m, 16H),1.33 (m, 12H), 1.28 (m, 20H),1.25 (m, 16H),1.02 (t, 12H),0.89 (t, 24H).MS (MALDI-TOF): m/z 2156.35 [M⁺]; C₁₂₈H₁₄₆F₂N₈O₄S₆Zn (2156.98).

RESULTS AND DISCUSSION:

Synthesis:

Pzn-DPP-P-F containing meso-substituent was synthesized as small molecule donor. The synthetic procedures of the porphyrin donor is outlined in Scheme 1 and summarized in the experimental part in detail. Briefly, monobromo-DPP was synthesized by controlled NBS bromination reaction of DPP at room temperature. DPP-P-F has been synthesized by Suzuki coupling reaction with DPP-Br and (4-fluorophenyl) boronic acid in basic condition. Br-DPP-P-F has been synthesized by NBS bromination reaction with DPP-P-F.

 

Dipyrromethane has been synthesized with pyrrole and aldehyde in an aqueous medium in the presence of an acid catalyst. 5,15-disubstituted cross-condensation and successive oxidation reactions of dipyrromethane and substituted aldehyde were performed. The Zn-incorporated porphyrin ring 8 was obtained via the metalation using zinc acetate and the meso-bromination reaction using bromosuccinimide (NBS). Trimethylsilyl acetylene was introduced to the porphyrin derivative through the Sologashira coupling reaction, and this reaction was followed by the removal of the trimethylsilyl group to give ethynyl-bearing porphyrin 10. Porphyrin derivatives was synthesized according to the literature procedure. Finally, porphyrin donor 1 was synthesized using Br-DPP-P-F and ethynyl-bearing porphyrin 10 by Sonogashira coupling. The molecular structures of the synthesized porphyrin small molecule was identified by ¹H NMR and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), which are recorded in the SI.

 

Optical properties:

The UV-vis absorption spectra of Pzn-DPP-P-F in CHCl₃ solution and film state are shown in Figure 1a and b, and corresponding spectroscopic data are summarized in Table 1. The absorption spectra of the porphyrin dyes exhibit a typical intense Soret band with 400-500 nm and less intense Q bands in a range of 550-700 nm. The Q bands of this molecule is higher intense than Soret band and red-shifted to 650-820 nm. For porphyrin moiety there is no absorption in the range of 500-600 nm. Highly conjugated DPP moiety connected with Acetylene bridge covered the absorption in the range of 500-620 nm which is perfectly complementary absorption to the porphyrin ring. These DPP porphyrin can cover full visible range from 400 to 700 nm and extended to the NIR region up to 820 nm. DPP moiety attached with acetylene to the porphyrin will be the best choice for OPV application. Compared to the Pzn donor in a solution state, Q band absorption of the porphyrin derivative in a film state showed strong bathochromic shifts, indicating the formation of J-type aggregates. The full-width half-maximum (fwhm) of the Q-band absorption by the Pzn donor in film was ~100 nm higher than those in a solution state, implying that intermolecular interactions among the Pzn donors were very strong in a film state. Because DPP units and focal porphyrin are connected via ethyne bridges, Pzn donors possibly have planar backbone structures. Thus, high planarity would have contributed to strong molecular π-π interactions. We also choose the 2-(2-butyloctyl) thiophene molecule connected to the β-position of porphyrin to compare the optical properties of the molecule. Meso-substituted 2-(2-butyloctyl) thiophene shown Q band at 750 nm. The main difference from porphyrin core is that this molecule contains two additional butyloctyl groups on the thiophene group which results in a 150 nm red shift of the Q-band, indicating that the butyloctyl groups are not only favorable for suppressing dye aggregation but also can extend the absorption to longer wavelengths and achieving a striking onset absorption wavelength of 820 nm.

 

Figure 1: Absorption spectra (a) in solution (chloroform) and (b) solid film of Pzn-DPP-P-F.

 

Table 1: Optoelectronic properties of Pzn-DPP-P-F in solution and film state.

Compound	Band λab(nm) (log ɛ)×105	Onset λab (nm)	Egopt (eV)
Pzn-DPP-P-F (soln)	330 (0.14), 465 (0.28), 580 (0.32), 750 (0.40)	825	1.50
Pzn-DPP-P-F (film)	350, 475, 600, 750, 810	1000	1.24

 

Electrochemical properties:

To evaluate the matching of energy levels between the porphyrin donor and PCBM, the highest occupied molecular orbital (HOMO) energy levels and the lowest unoccupied molecular orbital (LUMO) energy levels of porphyrin donor was determined using cyclic voltammetry (CV, Figure 2) and optical bandgap values. These results are summarized in Table 2. Electrochemical cyclic voltammetry of Pzn-DPP-P-F was carried out in acetonitrile solution (Figure 2) to measure the energy levels of Pzn-DPP-P-F. The onset oxidation potential (E_ox^onset) of Pzn-DPP-P-F was estimated to be 0.06 V vs. Ag/AgCl. The highest occupied molecular orbital (HOMO) energy level was determined to be -5.22 eV on the basis of the equation: E_HOMO = -e(E_ox^onset) + 4.78 (eV). The LUMO energy levels of the three are determined by the reduction potential from cyclic voltammetry. And the lowest unoccupied molecular orbital (LUMO) energy level was calculated to be -3.67 eV from the equation E_HOMO = -e(-E_red^onset) + 4.78 (eV). Figure 3 shows the energy level diagram and the energy level of Pzn-DPP-P-F is suitable of its application as a donor in bulk heterojunction solar cells. The electron donating meso-butylhexylthiophene groups raised the HOMO energy levels of the porphyrin donors. The electron donating meso-butylhexylthiophene groups decreased the HOMO energy levels of the porphyrin donors. Thus, the electrochemical bandgaps of porphyrin donor Pzn-DPP-P-F was determined as 1.55 eV, by the oxidation reduction values of the films. The energy levels of PCBM was measured using identical methods. The HOMO and LUMO energy levels of the porphyrin donor and PCMB are summarized in Table 2. The energy level of porphyrin donor was appropriate to collect holes from PCBM and to transfer electron to PCBM.

 

Figure 2: Cyclic voltammetry of Pzn-DPP-P-F (a) Oxidation and (b) Reduction.

 

Table 2: Electrochemical properties of molecule Pzn-DPP-P-F.

Donor	Egelec (eV)	EHOMO (eV)	ELUMO (eV)
Pzn-DPP-P-F	1.55	-5.22	-3.67
PCBM	2.05	-6.00	-3.95

 

Figure 3: Energy level diagram of Pzn-DPP-P-F and PCBM.

 

Photovoltaic properties:

Solution-processed BHJ OSCs were fabricated using porphyrin Pzn-DPP-P-F as the electron donor material and PCBM as the electron acceptor materials with an inverted device structure of ITO/ZnO/BHJ/MoOx/Ag. ZnO (30 nm) acts as an electron transport layer. Bulk heterojunctions as active layers were achieved by mixing porphyrin Pzn-DPP-P-F as electron donor and PCBM (1:1.2 w/w) with DIO (0.8 v/v %) and pyridine (0.5 v/v %) as additives. The active materials were dissolved in chloroform and stirred for 2 h in an ambient environment. The donor concentration was 7.5 mg/mL. The thickness of the active layer was 100 nm. MoOx (8 nm) as the hole transport layer and Ag (100 nm) as the electrode were deposited using thermal evaporation at low pressure (10^-6 bar). The active area was measured to be 0.707 cm². The current density-voltage (J-V) characteristics of the porphyrin donor: PCBM-based SM-OPV devices are shown in Figure 4, and the results are summarized in Table 3. The Pzn-DPP-P-F-OPV devices fabricated from meso-substituted 2-(2-butylhexyl) thiophene donor Pzn-DPP-P-F showed higher PCEs with improved Jsc and FF values. Thus, the porphyrin donor Pzn-DPP-P-F: PCBM device attained the highest PCE of 4.55%, Jsc of 12.67 mA/cm² and FF of 0.52.

 

Figure 4: Current density−voltage (J−V) curves.

 

Table 3. Photovoltaic properties of Pzn-DPP-P-F.

Donor: PCBM	Voc (V)	Jsc (mA/cm2)	FF (%)	PCE (%)
Pzn-DPP-P-F	0.69	12.67	0.52	4.55

 

CONCLUSION:

In summary, the new porphyrin small molecule of Pzn-DPP-P-F has been developed for BHJ OSCs. The structural engineering with the horizontal conjugation of DPP to porphyrin-core with the vertical Pzn-DPP-P-F peripheral substitutions can not only successfully increase the solar flux coverage in the visible and near-infrared region, but also optimize molecular packing through polymorphism linked with the long alkyl chain appended. Simultaneously their blend film with PCBM demonstrated efficient photogenerated exciton dissociation and charge collection when processed by DIO additive. As a result, the excellent device performances with PCEs of 4.55% was achieved for Pzn-DPP-P-F based OSC.

 

ACKNOWLEDGEMENT:

This work was supported by Kookmin University, Seoul, South Korea and Mawlana Bhashani Science and Technology University, Santosh, Tangail-1902, Bangladesh.

 

CONFLICT OF INTEREST:

The author declares that there are no conflict of interest related of this article.

 

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Received on 09.07.2019                    Modified on 31.07.2019

Accepted on 16.08.2019                   ©AJRC All right reserved