Abstract
In order to obtain higher power conversion performance in dye-sensitized solar cells, phthalocyanine compounds (ZnPc-1 and ZnPc-2) containing electron donor methoxy groups and aldehyde groups as anchors were synthesized in this study. The photovoltaic and electrochemical properties of these compounds were studied and their applicability as photosensitizers in DSSCs was investigated. The photovoltaic cell efficiencies (PCE) of the devices were in the range of 0.43–0.76 % under simulated AM 1.5 solar irradiation of 100 mW/cm2. Considering the photovoltaic performance of the produced DSSC devices, the anchor group and the chelate effect, it was observed that the efficiency increased, respectively, ZnPc-1 < ZnPc-2. The highest PCE value of 0.76 % was obtained with asymmetric ZnPc-2 based DSSC under. It has been explained that methoxy groups are electron donors and contribute to intramolecular electron mobility and that better electron transfer with single aldehyde anchor increases cell efficiency. In addition, the increase in the number of methoxy groups with known donor properties on the molecule also contributed to the increase in cell efficiency by increasing electron transfer. All compounds synthesized were characterized using FTIR, UV-vis and MS spectroscopic data.
Introduction
Dye-sensitized solar cells (DSSC), first developed by O’Regan and Grätzel [1], are still developing rapidly today. One of the most important steps to obtain high efficiency while creating these cells is to choose an effective dye [2]. These dyes can be purchased commercially or synthesized. Studies show that higher efficiencies are obtained from donor-π-acceptor (D-π-A) dyes [3–6]. Phthalocyanines are also suitable compounds for use as dyes in dye-sensitive solar cells. Some of the reasons why phthalocyanines are preferred are that they have a conjugated 18-pi electron system, a colored molecule, a stable structure, and the ability to bind different substituents to different positions [7, 8].
When the studies were examined, it was shown that the transmission of electrons from the dyes to the TiO2 layer with a single anchor group provides higher cell efficiency. This showed that asymmetric phthalocyanines would provide higher efficiency than symmetrical structures [7, 9]. In the presence of symmetrical phthalocyanine-bound anchor groups, electron movement did not occur sufficiently, but instead, the presence of electron donating/repelling groups and one electron transferring anchor group increased cell efficiency as it provided electron mobility in the molecule [10–12].
In this study, we attached aldehyde, known as a good anchor group, to the phthalocyanine molecule [13]. We also chose methoxy as the donor group. When the literature is examined, it is seen that the DSSC efficiency increases significantly with the increase in the number of methoxy groups in a molecule [14]. We noticed that the effect of methoxy groups on cell efficiency in phthalocyanines was not comparable. Based on this data, two symmetric and asymmetric phthalocyanine molecules with different numbers of substituted methoxy groups were synthesized. The synthesized zinc metal symmetric and asymmetric phthalocyanine molecules are shown in Fig. 1. The structure of the synthesized molecules was characterized by MALDI-MS, FTIR, UV-vis and fluorescent spectroscopic methods.
Structure of compounds of symmetric ZnPc-1 and asymmetric ZnPc-2.
Reagents
All solvents, 4-hydroxy-3,5-dimethoxybenzaldehyde, 3,4,5-trimethoxybenzaldehyde, and 4-nitrophthalonitrile were purchased from Merck. The FTIR spectra were recorded on a spectrum one Perkin Elmer 1600 FTIR spectrophotometer. Absorption spectra were recorded with an Agilent 8453 UV-visible spectrophotometer. Fluorescence spectra were recorded on a Varian Eclipse spectrofluorometer using 1 cm path length cuvettes at room temperature. Mass spectra were determined with QTOF Agilent 6530 at Yildiz Technical University Central Laboratory and Bruker microflex LT MALDI-TOF MS at the Gebze Technical University MALDI-TOF Mass Laboratory. The J–V measurements were carried with an AM 1.5G sun simulator.
Synthesis
The starting compounds 4-(4-formyl-2,6-dimethoxyphenoxy)phthalonitrile (F1) [8], 4-(3,4,5-trimethoxyphenoxy)phthalonitrile (F2) [16] were synthesized and purified according to well-known literature. Before synthesizing the F2 molecule, 3,4,5-trimethoxybenzaldehyde was reduced to 3,4,5-trimethoxyphenol [17].
Synthesis of ZnPc-1
ZnPc-1 molecule was synthesized using compound F1 (0.15 g, 0.82 mmol) and anhydrous zinc acetate (0.09 g, 0.48 mmol) as reported in the literature [8]. FTIR (cm–1): 2941 and 2838 (C-H) m, 1698 (C = O) s, 1595 (C = N) s, 1230 (Ar-O-Ar) s, 1126 (Aliphatic ether) s. MALDI-TOF m/z: 1298.428 [M]+.
Synthesis of ZnPc-2
F1 (0.02 g, 0.05 mmol), F2 (0.05 g, 0.16 mmol), anhydrous zinc acetate (0.03 g, 0.16 mmol) were dissolved in 2 mL n-pentanol in sealed tube. The reaction mixtures were stirred and heated at 150°C for 24 h under Argon atmosphere. The color started to turn green gradually after 10 minutes. The cooled reaction mixture was added drop by drop to 50 mL n-hexane until the precipitation was complete. The precipitate was filtered through a sintered glass filter and then zinc(II) phthalocyanines on silica gel using THF as eluent and final products were dried in vacuum. FTIR (cm–1): 2929 and 2854 (C-H) m, 1719 (C = O)s, 1596 (C = N) s, 1232 (Ar-O-Ar) s, 1123 (Aliphatic ether) s. MALDI-TOF m/z: 1361.646 [M]+ + Na + 2H2O.
Device preparation and performance
Fluorine doped tin oxide (FTO) substrates purchased from Spi Co were first cleaned. Cleaning was done in an ultrasonic bath for 5 minutes, first with acetone, then with isopropanol, and finally with distilled water. The cleaned FTO surfaces were dried with nitrogen gas. Paste was prepared using 20–35μm thick TiO2. It was coated on the FTO with the dr blade technique, which is the paste coating technique that was prepared. Then sintering was done, under weather conditions, at 400°C for 4 hours. After this process was completed, the TiO2 film-coated FTOs, which came to room temperature, were immersed in ZnPc-1 and ZnPc-2 dye solutions and waited for 24 hours for adsorption to take place. A thin-film platinum-coated counter electrode was sandwiched. H2PtCl4 solution was used to coat the FTO glass Pt. The coating was completed by heating at 450 °C for 10 minutes. The electrolyte was prepared as reported by Smestad in the literature. It was prepared by mixing 0.05 M iodine and 0.5 M potassium iodide in pure ethylene glycol [15]. Photovoltaic performance was measured under simulated sunlight, with ABET 1.5G solar simulation at a power density of 100 mW/cm2. Keithley 2400 Digital Source Meter was used. The current density-voltage (J-V) characteristics of the cells were recorded at room temperature.
The cell efficiency and fill factor (FF), which determine the electrical performance of a solar cell, are calculated using the equations Equation 1 and 2, respectively. The symbols in the equations mean: η cell efficiency, FF fill factor, Pmax maximum power, Vmp maximum power point voltage, Imp maximum power point current, Voc open circuit voltage, Isc short circuit current.
The starting materials 4-(4-formyl-2,6-dimethoxyphenoxy)phthalonitrile (F1) and 4-(3,4,5-trimethoxyphenoxy)phthalonitrile (F2) phthalonitrile compounds were first synthesized. Then, symmetrical zinc metal phthalocyanine was synthesized with the obtained F1 phthalonitrile. Asymmetrical zinc metal phthalocyanine was synthesized from F1 and F2 phthalonitriles in a ratio of 1:3 moles. In the synthesis procedure, zinc(II)acetate 2H2O was refluxed with phthalonitrile and DBU in absolute n-pentanol at 150°C for 24 hours.
In the FTIR spectra of the F1 and F2 compounds, it was observed that the stretch vibration band of the -NO2 group in 4-nitrophthalonitrile disappeared around 1350 cm-1 and the stretch vibration bands of the Ar-O-Ar groups were formed at 1249 and 1250 cm–1, respectively. In addition, strong -C≡N vibrational bands were also observed in phthalonitrile compounds at 2242 and 2230 cm–1, respectively. The stretch vibration peaks of the aldehyde group (H-C = O) were observed at 1698 and 1719 cm–1, respectively. Vibration bands of aliphatic ether peaks were seen at 1126 and 1123 cm–1, respectively.
In the mass spectra, the molecular ion peaks of the ZnPc-1 and ZnPc-2 compounds appeared at m/z: 1298.42. [H]+ and 1361.64 [2H2O + Na]+ in the MALDI-TOF spectra, respectively, as shown in Figs. 2, 3., Mass spectral data confirmed the proposed structure of ZnPc-1 and ZnPc-2.
The MALDI-TOF spectrum of ZnPc-1.
The MALDI-TOF spectrum of ZnPc-2.
Zinc phthalocyanines gave the same results as metalophthalocyanines found in the literature, and Q band absorption was observed as a single high-intensity band in the visible region. The UV-vis spectra of the compounds are shown in Fig. 4. The fluorescent excitation and emission spectra of zinc phthalocyanines appear to have Stokes shifts (λemis-λexc) ranging from 10 to 17 nm. Fluorescent quantum yields (Φ Φ ) were investigated in CHCl3 as solvent and the results are presented in Table 1.
UV-vis absorption spectra of ZnPc-1 and ZnPc-2 compounds in CHCl3.
Photo physical parameters for ZnPc-1 and ZnPc-2 compounds in CHCl3
Solar cell measurements were carried out under room conditions with an AM1.5G solar simulator under 100 mW cm–2 illuminations. The paste prepared with TiO2 (20 nm) in the anatase phase was coated on FTO glass with the dr blade technique. The films were immersed in the dye solution for 24 hours to adsorb the synthesized phthalocyanines (dyes) on TiO2. Measurements were made using I–/I3 as the electrolyte and J/V graphs were obtained. J/V graphic is seen in Fig. 5.
Current density-voltage curves of illuminated DSSCs using dyes ZnPc-1 and ZnPc-2.
Photovoltaic measurements obtained from the prepared DSSC devices were evaluated and the effects of the methoxy group on the PCE of the cell were observed. It is thought that there are two important reasons why ZnPc-2 has a higher PCE value. It was observed that the increase in the molecular structure of the methoxy group, which is the donor group, caused an increase in the PCE value. Another important reason is that the molecular structure is asymmetrical, which caused the existing electrons to be directed towards a single anchor group and increased the PCE value. The results of all these devices are summarized in Table 2.
Photovoltaic values obtained from DSSC measurements of ZnPc-1 and ZnPc-2
The power conversion values of the solar cell of symmetrical and asymmetrical phthalocyanine compounds designed by adding different numbers of methoxy groups selected as donor groups to the molecule were compared. In the asymmetrically synthesized Pc compound, the use of the aldehyde group as an anchor and the increase in the number of methoxy groups significantly increased the power conversion efficiency value of the produced DSSC device.
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).