Asymmetric phthalocyanine compounds in the structure D- π -A containing cyano groups: Design,synthesis and dye-sensitized solar cell applications

Abstract

In this study, asymmetric zinc phthalocyanine compounds with Donor-π-Anchor (D-π-A) property that enable the movement of electrons in molecular structure in one direction were synthesized. Phthalocyanines were designed to ensure electron mobility within the molecule and to facilitate the transfer of electrons to the TiO₂ layer. The synthesized asymmetric zinc phthalocyanines (ZnPc-1 and ZnPc-2) are molecules with three donor biphenyls and one anchor aldehyde group and three acceptor/anchor cyano and one anchor aldehyde group, respectively. The effect of biphenyl and cyano groups on cell efficiency with aldehyde anchor group was investigated. The structure of the synthesized phthalocyanines was characterized by Fourier Transform Infrared Spectrometry (FTIR), Mass Spectrometry (MS), UV-vis, Fluorescence spectroscopy. The experimentally calculated optical band gap values were supported by the values found by Density Functional Theory (DFT) calculations. dye sensitive solar cells were measured and the efficiencies were evaluated with reference to the N719 standard dye. In the solar cell measurements of the designed phthalocyanines, the structure containing the cyano group has been given a higher photovoltaic cell thanks to the higher short circuit photo-current (Jsc). In this way, the highest power conversion efficiency value was achieved among the cyano group molecules.

Keywords

Dye-sensitized solar cell Zn(II)cyano anchor asymmetric pc.

1 Introduction

Dye-Sensitized Solar Cells (DSSC), also known as Grätzel cells, which appeared as a result of the studies led by M. Grätzel and B. O’Regan in 1991 [1], have attracted high attention from the first day to this day. The advantages of DSSCs in cost and production stages compared to standard silicon (Si) based solar cells are the main reasons of this rising concern. Dye-Sensitized Solar Cells consist of four basic parts: conductive glass, semiconductor layer (TiO₂), sensitizer (dye), and electrolyte solution. Optimization of these key parts is the most important factor of determining cell efficiency [2].

Continuous improvement since the discovery of dye-sensitized solar cells in 1991 by Brian O’Regan and Michael Grätzel [1]. Phthalocyanines (Pcs), which have a conjugated 18π electron system, are very interesting for dye-sensitized solar cells because these properties facilitate electron transfer [3, 4]. In addition, it is preferred in many industrial applications because of its different colors, having different metal cations in its center, having peripheral and nonperipheral substituents, and being stable molecules [5, 6]. Donor and anchor groups that bind to the phthalocyanine molecule play an important role in increasing cell productivity [7 –9]. When dye-sensitized solar cell applications of asymmetric metal phthalocyanine compounds were examined, the highest efficiency was reported as 6.49% in the literature [10].

In the previous study where we examined the effects of cyano groups on dye-sensitized solar cells, the highest efficiency obtained with molecules containing cyano groups was 0.97. In this study, it was aimed to investigate the synthesis of asymmetric phthalocyanine compounds containing one aldehyde as an anchor group and three cyano groups as anchor / acceptor in the donor group, and the effects of cyano groups on cell productivity in comparison with our previous study [11]. In the literature, the contribution of the CN group, whose both anchor [12 –14] and acceptor properties are known, to DSSC efficiency has been examined [15, 16]. The aldehyde group, which has high efficiency and long-lasting use in DSSC applications, is used in compounds as anchors. It was desired to obtain devices with high cell potential with the designed asymmetric phthalocyanine compounds. The structure of synthesized asymmetric phthalocyanine compounds was characterized by FTIR, Uv-vis, fluorescence, MALDI-MS, H-NMR and C-NMR analyzes. The solar cells of the compounds were prepared, their power conversion efficiencies were measured and optimization studies of the devices were carried out [17].

2 Materials and methods

2.1 Instruments and chemicals

4-Phenylphenol, 4′-hydroxy-4-biphenylcarbonitrile, and 4-Hydroxybenzaldehyde were purchased from Merck. The FTIR spectra were recorded with Perkin Elmer 1600 FTIR spectrophotometer. Absorption spectra were recorded with an Agilent 8453 UV–visible spectrophotometer. Fluorescence excitation and emission spectra were recorded on a Varian Eclipse spectra fluorometer using 1-cm-path-length cuvettes at room temperature. Mass spectra were determined with Bruker microflex LT MALDI-TOF MS at the Gebze Technical University MALDI-TOF Mass Laboratory.

2.2 Synthesis

The starting compounds 4-[([1,1′-biphenyl]-4-yl)oxy] phthalonitrile (BP), 4-[(4′-isocyano[1,1′-biphenyl]-4-yl) oxy]phthalonitrile (CBP) were synthesized in the same way as in our previous publication [11]. and 4-(4-formylphenoxy)phthalonitrile (FP) [18] were synthesized according to the procedure reported in the literature.

2.2.1 2(3), 9(10), 16(17)- Tris [4-(1,1′-biphenyl) -4-yl) oxy]-23(24)- [4-(4′- isoaldehyde (1,1′-biphenyl) -4-yl) oxy] phthalocyaninato zinc(II) (ZnPc-1)

4-[([1,1′-biphenyl]-4-yl)oxy] phthalonitrile (BP) (0.020 g, 0.0676 mmol), 4-(4-formylphenoxy)phthalonitrile (FP) (0.006 g, 0.0225 mmol) and anhydrous zinc acetate (0.04 g, 0.0225 mmol) were dissolved in 2 mL pentanol in sealed tube. The reaction mixtures were stirred and heated at 150 °C under argon atmosphere for 14 hours. The color was yellow, and then it started to turn green gradually after 15 min. The cooled reaction mixture was added drop by drop to 50 mL n-hexane. Then, it was left in the refrigerator for 2 h to complete the precipitation. The precipitate was filtered through a sintered glass filter and the final products were dried in vacuum. FTIR (cm^–1): 2860, 2930 (C-H, s), 1650 (C = O, s), 1230 (Ar–O–Ar, s). MALDI-TOF m/z: 1203.08 [M]⁺.

2.2.2 2(3), 9(10), 16(17)- Tris [4-(1,1′-isocyano) -4-yl) oxy]-23(24)- [4-(4′- isoaldehyde (1,1′-biphenyl) -4-yl) oxy] phthalocyaninato zinc(II) (ZnPc-2)

4-[(4′-isocyano[1,1′-biphenyl]-4-yl)oxy] phthalonitrile (CBP) (0.015 g, 0.0467 mmol), 4- (4-formylphenoxy) phthalonitrile (FP) (0.004 g, 0.0156 mmol) and anhydrous zinc acetate (0.003 g, 0.0156mmol) were dissolved in 2 mL pentanol in sealed tube. The reaction mixtures were stirred and heated at 150 °C under argon atmosphere for 14 hours. The color was light brown, and then it started to turn green gradually after 10 min. The cooled reaction mixture was added drop by drop to 50 mL n-hexane. Then, it was left in the refrigerator for 2 h to complete the precipitation. The precipitate was filtered through a sintered glass filter and final products were dried in vacuum. FTIR (cm^–1): 2860, 2930 (C-H, s), 2230 (C≡N), 1730 (C = O, s) 1600 (HC = N, s), 1230 (Ar–O–Ar, s). MALDI-TOF m/z: 1276.83 [M]⁺.

2.3 Preparation of dye-sensitized solar cell (DSSC)

J/V graphs of cells prepared with synthesized ZnPc molecules were made under 100 mW cm^–2 illumination with AM1.5G solar simulator. Nanostructured TiO₂ paste [19] used in solar cells was prepared and using the dr blade coating technique, TiO₂ paste is placed in front of the coating blade and the blade is moved to flush with the surface to form a wet film [20]. It was coated on indium tin oxide (ITO, the sheet resistance of 8–10 Ω/cm²) in an area of 0.2 cm². It was covered with blade technique. After drying for half an hour at room temperature, it was sintered at 500°C for 3 hours. The dye molecules were adsorbed onto the TiO₂ surface by immersing them in the prepared dye solutions and waiting for 4 hours at room temperature. The films removed from the dye solution were washed with ethanol and allowed to dry at room temperature. Then, solar cell measurements were taken, a J/V curve was drawn, cell efficiencies were calculated and the gaph showed in Fig. 5

3 Result and discussions

Asymmetric zinc phthalocyanines, whose molecular shape is shown in Figure 1, were synthesized. When designing these compounds, 4-[([1,1′-biphenyl]-4-yl)oxy] phthalonitrile (BP) groups were chosen as the donor group. The 4-[(4′-isocyano [1,1′-biphenyl] -4-yl) oxy] phthalonitrile (CBP) group is known for both anchor [13 –15] and acceptor [16] properties. and in our previous study, DSSC efficiencies were investigated by synthesizing phthalocyanines with these groups. Differently in this study, it is known that aldehyde groups have a good anchor feature. Therefore, 4- (4-formylphenoxy) phthalonitrile (FP) was selected as the anchor group and phthalocyanines were synthesized and DSSC measurements were made. As a result, the highest DSSC efficiency [21] was achieved among the molecules with cyano groups.The flow chart of the synthesis of phthalonitrile and phthalocyanine compounds is shown in Fig. 2 and Fig. 3. The structure of the synthesized zinc phthalocyanine compounds is determined by FTIR spectra. In the spectrum of the phthalonitrile compound, the NO₂ band is seen at 1310 cm^–1. In addition, a strong cyano (C≡N) band was seen at 2230 cm^–1. In the spectra of ZnPc-1 and ZnPc-2 phthalocyanines, it is seen that the peak of the NO₂ band disappears and a new Ar-O-Ar absorption peak appears at 1230 cm^–1. Imine (C = N) vibrations were observed at 1600 cm^–1 in ZnPc-1 and ZnPc-2. The cyano (C≡N) peak on biphenyl groups in the synthesized ZnPc-2 molecule is seen at 2220 cm^–1 in their infrared spectra (Supplementary Information S2 and S3). FTIR peak values for ZnPc-1, 2860, 2930 (C-H, s), 1650 (C = O s), 1230 (Ar–O–Ar, s). FTIR peak values for ZnPc-2, 2860, 2930 (C-H, s), 2230 (C≡N), 1730 (C = O, s), 1650 (C = O s), 1600 (HC = N, s), 1230 (Ar–O–Ar, s).

Fig. 1

Molecular structures of the asymmetrical zinc phthalocyanines.

Fig. 2

Synthesis of phthalonitrile ligands.

Fig. 3

Synthesis of Zinc(II) phthalocyanine complexes.

In the MALDI-TOF mass spectra, the molecular ion peaks [M]⁺ of ZnPc-1 and ZnPc-2 were found at 1203.08 and 1276.83 respectively. MALDI-TOF mass spectrum consists of a molecular ion peak at (Supplementary Information S4, S5).

UV-vis spectra of the solutions of ZnPc-1 and ZnPc-2 compounds prepared in chloroform (1×10^–3 M) were measured. In the UV-vis spectrum of ZnPc-1 and ZnPc-2, the Q band absorption of zinc phthalocyanines is observed as a single band of high intensity in the visible region at 683 and 680 nm, respectively. B band absorption peaks were also seen at 344 and 346 nm, respectively. The absorption spectrums of ZnPc’s are shown in (Supplementary Information S6, S7). The fluorescence excitation and emission spectra of the complexes are typical zinc phthalocyanine complexes with Stokes shifts (λemis -λexc) ranging 18 and 16 nm. Fluorescence quantum efficiencies (Φ_Φ) of ZnPc-1 and ZnPc-2 were investigated in CHCl₃. The Φ_Φ value of the ZnPc complexes is shown in Table 1.

Table 1

UV–Vis spectral and photophysical parameters of asymmetrical ZnPcs in CHCl₃

Sample	λ_B (Abs) (nm)	λ_B (Ems) (nm)	λB (Exc) (nm)	ΔλStokes (nm)	Φ _Φ
ZnPc-1	613, 683	699	681	18	0.192
ZnPc-2	612, 680	699	683	16	0.191

The optical band gap of ZnPc’s was calculated from UV-vis spectrum [22]. It was calculated by using the absorbance and wavelength values according to equations 1 and 2. The band gap values which were obtained from the UV-vis spectrum of the ZnPcs are shown in Fig. 4. In order to support the results, the band gap values obtained by DFT calculation are also shown in Table 2.

Fig. 4

Optical band gap values calculated from the Uv-vis spectrum.

Table 2

HOMO-LUMO energy levels and band gap values by DFT calculation of ZnPc-1 and ZnPc-2

Sample	HOMO (eV)	LUMO (eV)	Eg (eV)
ZnPc-1	–5.12	–3.04	2.08
ZnPc-2	–5.38	–3.29	2.09

In Equation 1. Equation is the optical band gap and λ is the wavelength symbol. $Eg (opt) (eV) = \frac{1239.95}{λ (nm)}$ (1)

In equation 2, α is the absorbance, hv is the energy of the photon, A is the proportionality constant. $ahv = A {(hv - Eg (opt))}^{1 / 2}$ (2)

For DSSCs fabrication, two transparent TiO₂ pastes were deposited using Doctor Blade’s method on the conductive side of ITO substrate. In order to prepare the dye-sensitized TiO₂ films (photoanodes), the coated ITO substrates were immersed into a 0.3 M ZnPc-1 and ZnPc-2 solution in CHCl. The Pt counter electrodes were prepared on the ITO substrates by casting platinum paste solution. Both the photoanode and counter electrode were sealed with Surlyn film. The I^- / I₃ redox electrolyte was injected between the electrodes through a drilled hole at the counter electrode. The current density-voltage (J-V) characteristics were investigated by using the potentiostat/galvanostat under AM 1.5 global one sun illumination (100 mW cm^–2). Detailed photovoltaic data are listed in Table 3. Accordingly, the maximum product of current and voltage is divided by incoming solar energy to obtain the efficiency of conversion of sunlight to electrical energy.

Table 3

Photovoltaic performance parameters of the DSSCs based on ZnPc-1 and ZnPc-2

Sample	V_oc (V)	J_sc (mA/cm²)	FF	η (%)
N-719	0.41	10.38	0.31	1.321
ZnPc-1	0.39	1.02	0.26	0.10
ZnPc-2	0.29	1,89	0.25	0.14

The results show that the aldehyde anchor group [23, 24] on the asymmetric ZnPc-2 provides electron transfer to the TiO₂ layer and the cyano group improves photovoltaic performance by providing electron mobility. The short circuit current density (J_sc) value of ZnPc-2, which has a cyano group in its molecular structure, is 1.89 mA/cm². This value is higher than ZnPc-1 and it caused higher cell efficiency. The open-circuit voltages (V_oc) value was measured as 0.29 V and the cell efficiency calculated as 0.14. The measured J-V character curves of the DSSC with the Pcs sensitized onto TiO₂ flms sintered at 500 °C are shown in Fig. 5. When the photovoltaic performances of paints containing cyano groups are examined, it is thought that our results are better than those in the literature and can be improved.

Fig. 5

J-V characteristics of the DSSCs based on ZnPc-1 and ZnPc-2.

4 Conclusions

This work showed high cell potentials were achieved with DSSC applications using newly synthesized Donor-acceptor/anchor structured ZnPcs dyes. While the cyano and biphenyl groups in the dye molecules increase the electron mobility in the molecule, the transfer of electrons to the external circuit with the aldehyde anchor group through the TiO₂ layer has increased the efficiency. The efficiency of cells was increased approximately two fold by binding the aldehyde group, which is known as one of the best anchor groups according to the literature. It was observed that the power conversion efficiency of the cyano group as an acceptor/donor group could be increased in the applications of asymmetric ZnPc phthalocyanines in DSSCs. In phthalocyanine compounds, improvements in solar cell performance can be achieved by using different electron donor and π-conjugation systems in the structure besides the cyano group.

Footnotes

Acknowledgments

This work was supported by The Scientific and Technological Research Council of Turkey (TUBITAK) Ph.D. Scholarship Program in Priority Areas (2211/C) and Council of Higher Education (YOK) Ph.D. Scholarship Program (100/2000).

The supplementary material is available in the electronic version of this article: DOI: .

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