Synthesis of High Functionalized Fluorinated alkoxy based Porphyrin Materials for efficient Organic Solar Cells

 

Ashis K. Sarker*

Department of Chemistry, Mawlana Bhashani Science and Technology University,

Santosh, Tangail-1902, Bangladesh

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

 

ABSTRACT:

We have planned and synthesized a bunch of three new fluorinated alkoxyporphyrin constructed donor materials named as 1, 2 and 3. These new donor materials are characterized by using NMR, MALDI-TOF Mass, UV/Visible spectrometer and cyclic voltammetry. 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 OPV based on 1, 2 and 3 showed JSC of 4.07 mA/cm2, 7.25 mA/cm2 and 10.15 mA/cm2, VOC of 0.78 V, 0.67 V and 0.80 V, a FF of 0.31, 0.48 and 0.52, the overall power conversion efficiency of (η) 1.28%, 2.07% and 4.26% respectively. The easy synthetic route and purification technique provide the viability of these donor materials as future OPV development.

 

KEYWORDS: Porphyrin, Fluorination, Alkoxy-thiophene, Sonogashira reaction, Energy conversion.

 

 


INTRODUCTION:

Small molecule (SM) in bulk heterojunction organic solar cells (BHJ OSCs) have received deep interest because the molecular structure, molecular weight, purity, cell performance reproducibility are highly well-disciplined [1-11]. The relationships between the molecular structures and device performance are easier to understand by the reproducibility of these small molecules and the performance limit of these materials and devices can be realized. The PCEs of single layer SM BHJ solar cells have been increased up to 10% which are comparable to the polymer solar cells [12-26]. Chlorophylls can absorb light and carry out photochemical reaction to produce energy. Scientists are inspired by this process and synthesized porphyrins and their derivatives to explore the active materials in dye-sensitized solar cells with the efficiency of 13% however, BHJ OSCs showed very low PCEs [27-38].

 

Porphyrin-based donor materials has been considered to study and achieved a high efficiency more than 4% using PCBM as acceptor moieties. The potential of porphyrins for OSCs are their virtues of high molar absorption coefficients, easy chemical structural modification and distinctive photophysical properties [14]. Higher than 4% efficiency of porphyrin-based materials are very limited in BHJ OSCs because to achieve a balance between solubility and required functionality of porphyrin molecules are very difficult as well as optically positioned energy levels and solar flux coverage with charge carrier mobility.

 

We designed and synthesized three porphyrin-based molecules 1, 2 and 3 (Scheme-2) in which DPP was symmetrically conjugated to porphyrin core, then ended with alkyl chain terminal units. To enhance the directional intermolecular π-π stacking in films and form the blend with PCBM, the appropriate alkyl chains on meso position and DPP conjugated backbone are necessary. This will help to interpenetrating networks for efficient charge separation and transportation. The blend films with PCBM gave power conversion efficiencies of 1.28%, 2.07% and 4.26% in BHJ OSC based on 1, 2 and 3, respectively.

EXPERIMENTAL:

Materials

Nitrogen atmosphere has been used for air/water sensitive reaction. We purchased all chemicals from Sigma-Aldrich and Samchun, South Korea. All organic solvents were purified by standard process used in this work. The other materials were used as received.

 

Methods. Column chromatography was performed using silica gel 230-300 mesh (purchased from Aldrich) as the solid support. A Bruker Advance DPX 400 MHz spectrometer was used for all NMR data at 25 C in CDCl3 and 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. A BAS 100B/W electrochemical analyzer with a three-electrode cell was used for cyclic voltammetry in a 0.1 N Bu4NBF4 solution in acetonitrile at a scan rate of 50 mV/s.

 

Scheme 1. Synthesis of a, b and c.

 

Synthesis of 2,3,4,5,6-pentafluorobenzaldehyde (a). 2,3,4,5,6-pentafluorobenzaldehyde was purchased from Sigma Aldrich. 1H NMR (400 MHz, CHCl3-d, 25 C), δ (ppm)=10.28 (s, 1H). (GC-mass): m/z 196.18 [M]+; C7HF5O(196.51).

 

3-Fluoro-4-((2-hexyldecyl) oxy) benzaldehyde (b).3-Fluoro-4-hydroxybenzaldehyde (4.0 g, 0.02 mol), 7-(bromomethyl) pentadecane (6.80 g, 0.04 mol), and anhydrous K2CO3 (5.68 g, 0.04 mol) were heated and refluxed in acetone (100 mL) under nitrogen for 20 h. After cooling, the residue was filtered, and the filtrate was intense by vacuum. We dissolved the crude product in chloroform (40mL) and then added trifluoroacetic acid (4 mL) and water (4 mL). The mixture was stirred under reflux for 1 h. After extraction, the solvent was dried by vacuum. The product was purified by column chromatography to give the title compound (2.40 g, 42%) as colorless oil. 1H NMR (400 MHz, CHCl3-d, 25 C), δ (ppm) = 9.78 (s, 1H), 7.54 (d, 2H), 6.98 (s, 1H), 3.90 (d, 2H), 1.77 (m, 1H), 1.36-1.19 (m, 24H), 0.80 (m, 6H).MALDI-TOF-MS: m/z: calcd. for C23H37FO2:364.54 [M]+; found 365.14.

 

4-((2-Ethylhexyl)oxy)-3-fluorobenzaldehyde (c).3-Fluoro-4-hydroxybenzaldehyde (4.0 g, 0.02 mol), 3-(bromomethyl) heptane (5.98 g, 0.04 mol), and anhydrous K2CO3 (5.68 g, 0.04 mol) were heated and refluxed in acetone (100 mL) under nitrogen for 20 h. After cooling, the residue was filtered, and the filtrate was intense by vacuum. We dissolved the crude product in chloroform (40 mL) and then added trifluoroaceic acid (4 mL) and water (4 mL). The mixture was stirred under reflux for 1 h. After extraction, the solvent was dried by vacuum. The product was purified by column chromatography to give the title compound (2.90 g, 43%) as colorless oil. 1H NMR (400 MHz, CHCl3-d, 25 C), δ (ppm) = 9.78 (s, 1H), 7.54 (d, 2H), 6.98 (s, 1H), 3.90 (d, 2H), 1.77 (m, 1H), 1.43-1.17 (m, 8H), 0.80 (m, 6H). MALDI-TOF-MS: m/z: calcd. for C15H21FO2: 252.32 [M]+; found 253.87.

 

Scheme 2. Synthetic route to molecule 1, 2 and 3.

Synthesis of Dipyrromethane. At first, AcOH (300 mL) and MeOH (100 mL) were mixed together and then paraformaldehyde (1.2 g, 40 mmol) and pyrrole (30 mL, 1 mol) were dissolved in that mixture. The mixture was stirred at 25C for 20 h. Water (200mL 2) and aqueous KOH (0.2 M; 200 mL 2) solution were used to wash the reaction mixture. Flash column chromatography was used to purify the product as dark green crystal in 40% yield (2.40 g). 1H NMR (CDCl3, 400 MHz) δH 7.9 (b, 2H), 6.62 (d, 2H), 6.01 (t, 2H), 5,95 (d, 2H), 3.90 (s, 2H).

 

Synthesis of porphyrin derivatives 7, 8, 9. Dipyrromethane (1.0 g, 6.86 mmol) and compound 4, 5, 6 (2.48 g, 6.86 mmol) were mixed in DCM (1 L) and degassed by nitrogen flow then trifluoroacetic acid (0.46 mL, 74.6 mmol) was added to the mixture. Under nitrogen atmosphere, the solution was stirred at room temperature for 5 h and DDQ (2.34 g, 10.28 mmol) was added and stirred for 1 h. Et3N (1.16 mL) was mixed to the reaction mixture to basify the solution and silica gel was used to filter it. The solvent was removed by reduced pressure and the product was purified by column chromatography. The products were recrystallized from MeOH/CHCl3 to get the pure product (2.04 g,35%) as a colored solid. 1H NMR (CDCl3, 400 MHz) δH 10.20 (s, 2H), 9.32 (d, 4H), 8.96 (d, 4H), 7.71 (t, 2H), 7.02 (d, 4H), -3.12 (s, 2H). (MALDI-TOF-MS): m/z 643.89 [M+]; C32H12F10N4 (642.45).

 

Synthesis of 5, 15-Dibromo-porphyrin derivatives 10, 11, 12. Compound 7, 8, 9 (0.70 g, 0.72 mmol) in DCM (300 mL) was stirred and NBS (0.26 g, 1.5 mmol) in DCM (100 mL) was added slowly at 0C under nitrogen atmosphere. To quench the reaction, acetone (60 mL) was added. After completion of reaction, the solvent was removed by vacuum and column chromatography was used to purify the product. The pure product was recrystallized by MeOH/CHCl3 to get the product (0.65 g, 83%) as colored solid.1H NMR (CDCl3, 400 MHz) δH 9.26 (d, 4H), 8.96 (d, 4H), 7.71 (t, 2H), 7.02 (d, 4H), -3.12 (s, 2H). (MALDI-TOF-MS): m/z 801.89 [M+]; C32H10Br2F10N4 (800.24).

 

Synthesis of [5,15-Dibromo-porphinato] zinc (II) derivatives 13, 14, 15. A suspension of the compound10,11,12(0.291 g, 0.243mmol) and Zn(OAc)22H2O (0.135 g, 0.615mmol) in a DCM (110 mL) and MeOH (25 mL) mixture was stirred at 25C for 4 h. By adding water (15 mL), the reaction was quenched and DCM (215 mL) was used to extract the mixture. Water was used to wash the combined extracts and anhydrous MgSO4 was used to dry the mixture. To get the product, the solvent was removed by vacuum (0.25g, 96%). 1H NMR (CDCl3, 400 MHz) δH 9.26 (d, 4H), 8.96 (d, 4H), 7.71 (t, 2H), 7.02 (d, 4H). (MALDI-TOF-MS): m/z 863.97 [M+]; C32H8Br2F10N4Zn (863.60).

Synthesis of [5,15-Di (triisopropylsilyl) ethynyl-porphinato] zinc(II) 16, 17, 18. The compound 13, 14, 15 (50mg, 42μmol), toluene (15mL), (triisopropylsilyl) acetylene (37L, 168μmol), and copper (I) iodide (1.2 mg, 6.3μmol) were taken in a flask and the mixture was stirred under inert condition. After the addition of trimethylamine, the mixture was degassed using nitrogen bubbling. Bis (triphenylphosphine) palladium (II)dichloride (4.4 mg, 6.3μmol) was added to the reaction mixture and the solution was heated overnight at 90C. High vacuum was used to remove the solvent and column chromatography was used to purify the final product. Rota-evaporator was used to dry the solvent and the final product was dried in a vacuum condition (0.26 mg, 87%).1H NMR (CDCl3, 400 MHz) δH 9.51 (d, 4H), 8.80 (d, 4H), 7.71 (t, 2H), 7.02 (d, 4H), 1.43-1.39 (m, 42H). (MALDI-TOF-MS): m/z 1067.98 [M+]; C54H50F10N4Si2Zn (1066.54).

 

Synthesis of porphyrin derivatives 19, 20, 21. To a solution of the compound 16, 17, 18 (110 mg, 0.081 mmol) in THF (5 mL), tetra-n-butylammoniumfluoride (0.815 mL, 0.81 mmol) was added at room temperature. The mixture was stirred for 1 h and then water was added to the reaction mixture. Methylene chloride was used to extract the crude. The green layer was separated and to get the deprotected product solvent was evaporated under vacuum.

 

Synthesis of compound 1, 2 and 3. The deprotected intermediate was added in a degassed THF (25 mL) and NEt3 (4 mL). 4 (25.8 mg, 90μmol), Pd2(dba)3 (3.45 mg, 3.65μmol), and AsPh3 (44 mg, 147μmol) were added to the reaction mixture, subsequently. After stirring 6 h at 90 C, to remove the solvent we used vacuum pump. And column chromatography was used to purify the product. For more purification, the product was recrystallized from CH2Cl2/CH3OH (29.7 mg, 30%). 1H NMR (400 MHz, THF) δ (ppm) = 9.49 (d, 4 H, J = 4.6 Hz), 9.11(d, 2 H, J = 4.4 Hz), 9.13 (m, 2 H), 8.87 (d, 4 H, J = 4.3 Hz), 8.18 (d, 4 H, J = 8.1 Hz), 7.83 (d, 2 H, J = 4.2 Hz), 7.74 (d, 2 H, J = 4.6 Hz), 7.39 (d, 4 H, J = 7.8 Hz), 7.31 (t, 2 H, J = 4.2 Hz), 4.34 (t, 4 H, J = 6.5 Hz), 4.00 (m, 8 H), 2.02 (m, 4 H), 1.96 (m, 4 H), 1.32 (m, 32H), 0.93(m, 18 H). MS(MALDI-TOF): m/z: calcd. for C96H86F10N8O4S4Zn: 1799.39 [M] +; found 1798.78.

 

RESULTS AND DISCUSSION:

Synthesis. As small molecule donors, three porphyrin derivatives comprising different meso-substituents were synthesized. The synthetic procedures of porphyrin donors are outlined in Scheme 1 and 2, and summarized in the experimental part in detail. Acid-catalyzed cross-condensation and successive oxidation reactions have been performed to synthesize 5,15-disubstituted porphyrin 7-9 by using dipyrromethane and corresponding substituted aldehydes. The metalation with zinc acetate and the bromination using N-bromosuccinimide were done to incorporate Zn in the porphyrin ring.

 

To the porphyrin derivatives, trimethylsilyl acetylene were brought together through the Sonogashira reaction. The trimethylsilyl groups were removed by TBAF reagent to produce ethynyl bearing porphyrin 19-21. DPP-Br and ethynyl-bearing porphyrin 19, 20 and 21gave the final products 1, 2 and 3 by Sonogashira coupling reaction. The synthesized porphyrin molecular structures were confirmed by proton NMR and Mass spectrometry.

 

UV-Visible Absorption Spectroscopy/Optical properties. The UV/visible absorption spectra of 1, 2 and 3 in chloroform solutions are shown in figure 1, and Table 1 showed the corresponding spectroscopic data. 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 nm6. The Q bands of these 3 molecules are similar intense of Soret band and red-shifted to 600-800 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 period of 500-600 nm which is perfectly corresponding 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 800 nm. DPP moiety attached with acetylene to the porphyrin will be the best choice for OPV application. We also choose the fluorine atom connected to the β-position of porphyrin to compare the optical properties of the molecules. Meso substituted pentafluorobenzene shown Q band at 710 nm. Octyl-hexyl alkoxy long chain associated with fluorobenzene (2) shown red shifted for ca. 15 nm relative to those pentafluorobenzene (1). Ethyl-butyl methoxy short chain associated with fluorobenzene shown red shifted for ca. another 15 nm relative to those octyl-hexyl methoxy long chain fluorobenzene (2). Compared with compound 1, the introduction of octyl-hexyl methoxy or ethyl-butyl methoxy groups to the fluorobenzene in 2 and 3 were observed to induce red shift of the Q bands for 15 and 30 nm, respectively. The molecular structure of 2 is quite similar to the porphyrin 3. The main difference is that 2 contains two additional octyl hexyl groups on the phenyl group which results in a 15 nm red shift of the Q-band, indicating that the octyl-hexyl methoxy groups are not only favorable for destroying dye aggregation but also can prolong the absorption to longer wavelengths. However, the attachment of the ethyl-butyl methoxy group into porphyrin (compound 3) dramatically red-shifted the λmax of the Q-band to 740 nm, achieving a striking onset absorption wavelength of 800 nm. Ethyne bridges have been connected to the DPP units and porphyrin ring. Strong molecular π-π interactions have been contributed because of the high planarity of these small molecule donors.

 

Figure 1. UV/Visible absorption spectra of molecule 1, 2 and 3.

 

Table 1. Optoelectronic properties of 1, 2 and 3.

Compound

Band λab(nm) (log ɛ)105

Onset λab (nm)

Egopt (eV)

1

467 (1.19), 568 (1.15), 710 (0.97)

780

1.58

2

465 (1.04), 570 (0.76), 725 (0.91)

808

1.53

3

465 (0.90), 570 (0.83), 740 (0.99)

810

1.53

 

Electrochemical Properties:

The highest occupied molecular orbital (HOMO) energy levels and the lowest unoccupied molecular orbital (LUMO) energy levels of porphyrin donors were determined by cyclic voltammetry to evaluate the matching of energy levels between the porphyrin donors and PCBM. The optical bandgap was also determined by onset absorption of UV/Visible spectra. Table 1 shows the summary of these results. The oxidation potential of porphyrin donors 1, 2 and 3 were -0.42, -0.60 and -0.58, respectively (Figure 2) and HOMO energy levels were found to be -5.11, -5.28 and -5.23, respectively. The HOMO energy levels of porphyrin donors 1 and 2 were higher and slightly lower, respectively than that of porphyrin donor 3. The HOMO energy levels of the porphyrin donors were raised up by the electron donating meso-pentafluorophenyl groups. The electron donating meso-octyl-hexyl methoxymonofluorophenyl groups (2) decreased the HOMO energy levels of the porphyrin donors compared to porphyrin donor 3. The LUMO energy levels of the three are recorded by the reduction potential from cyclic voltammetry. The LUMO energy levels were calculated to be -3.68, -3.08 and -3.29, respectively (Table 2). Thus, the electrochemical bandgaps of porphyrin donors 1, 2 and 3 were determined as 1.43, 2.20 and 1.94 eV, respectively, by the oxidation reduction values of the films (Figure 3).The energy levels of PCBM was measured using similar procedure. Table 2 shows the HOMO and LUMO energy levels of the porphyrin donors and PCMB. The energy level of porphyrin donor 3 was suitable to gather holes from PCBM and to transfer electron to PCBM.

 

Figure 2. Cyclic voltammetry of molecule 1, 2 and 3.

 

Table 2. Electrochemical properties of molecule 1, 2 and 3.

Donor

Egelec (eV)

EHOMO(Ev)

ELUMO (eV)

1

1.43

-5.11

-3.68

2

2.20

-5.28

-3.08

3

1.94

-5.23

-3.29

PCBM

2.05

-6.00

-3.95

 

Figure 3.Estimated energy level of donor 1, 2, 3 and PCBM.

 

Photovoltaic properties. Porphyrin donors 1, 2 and 3 were used in BHJ OSCs as electron donor materials and PCBM as the electron acceptor materials. The inverted device structure was ITO/ZnO/BHJ/MoOx/Ag. ZnO (30 nm) was used as electron transport layer (Figure 4). The active layers were prepared by different donor materials and PCBM (1:1.1 w/w) with DIO (0.4 v/v %) and pyridine (0.6 v/v %) as additives. Chloroform was used as solvent to dissolve active materials and stirred for 3 h in air. The concentration of donor material was 7.3 mg/mL, 100 nm thick layer was prepared with the active materials. As hole transport layer MoOx (10 nm) was used and thermal evaporator has been applied to deposit Ag (90 nm) as the electrode and 0.69cm2active area was measured. Figure 5 shown the current density-voltage characteristics of the porphyrin donor: PCBM-structure and the results summary are shown in Table 3. The SM-OPV devices fabricated from meso-ethylhexylmethoxythiophene-substituted donors (3) showed higher efficiency with enhanced Jsc and FF values than that of pentafluorophenylthiophene and octyl-hexyl-methoxythiophene-substituted donor 1 and 2, respectively. In addition, meso-ethylhexylmethoxythiophene-substituted porphyrin donor 3 showed better performance than the pentafluorophenylthiophene and octyl-hexyl-methoxythiophene substituted porphyrin donor 1 and 2. Thus, the porphyrin donor 3: PCBM device achieved the maximum PCE of 4.26%, Jsc of 10.15 mA/cm2 and FF of 0.52.

 

Figure 4. Schematic structure of device used in this study.

 

Figure 5. Current densityvoltage (JV) curves.

 

Table 3. Photovoltaic properties of 1, 2 and 3.

Donor: PCBM

Voc (V)

Jsc (mA/cm2

FF

PCE (%)

1

0.78

4.07

0.31

1.28

2

0.67

7.25

0.48

2.07

3

0.80

10.15

0.52

4.26

 

CONCLUSION:

Three new porphyrin small molecules of 1, 2 and 3 were synthesized for bulk heterojunction organic solar cells. To increase the solar flux coverage in the visible and NIR region, the horizontal conjugation of DPP was incorporated to porphyrin-core with the vertical pentafluoro, monofluro-octyl-hexyl and monofluro-ethyl-butyl methoxyphenyl peripheral substitutions. To optimize molecular packing through polymorphism connected with the long alkyl chain was added. DIO additive was added to the blend film with PCBM to demonstrate efficient photogenerated exciton dissociation and charge collection. As a result, the excellent device performances with PCEs of 1.28%, 2.07% and 4.5% were achieved for 1, 2 and 3-based OSCs, respectively.

 

ACKNOWLEDGEMENT:

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

 

CONFLICTS OF INTEREST:

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

 

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Received on 08.07.2019 Modified on 21.07.2019

Accepted on 06.08.2019 AJRC All right reserved

Asian J. Research Chem. 2019; 12(4):193-198.

DOI: 10.5958/0974-4150.2019.00036.1