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1. Introduction
Over the last years, various types of solar cells have been developed to convert sunlight to electricity. Crystalline, polycrystalline, and amorphous silicon solar cells have been widely used for different domestic and industrial application [1–3]. Multijunction semiconductor solar cells have shown the world record efficiency of 46% [4]. However, their applications are mostly limited to space industry. There are other types of less efficient and low-cost cells, such as dye-sensitized solar cells (DSSCs) [5] and organic solar cells [6]. These cells have been around for years and stimulated useful studies; however, their implementation for large-scale applications is still limited.
DSSC was firstly reported by O’Regan and Grätzel in 1991 [5]. The highest power conversion efficiency (PCE) reported for DSSCs using ruthenium complex dyes (N719) was 11-12% [7, 8]. One of the main challenges of DSSCs is the long-term stability. Electrolyte leakage, dye desorption, and degradation of the dye itself are considered the most important parameters affecting the cell stability [9, 10].
Researchers have been focusing on the modification of each component of the DSSC with the aim to improve the PCE. For instance, in order to obtain more effective nanostructured semiconductor photoanodes, different shapes have been utilized such as nanoparticles, nanorods, nanotubes, nanosheets, and mesoporous structure [11–15]. Ruthenium and osmium metal-organic complexes have been the most stable and effective dyes used for DSSCs [16, 17]. Due to the fact that these dyes are toxic, expensive, and difficult to synthesize, growing activities for using natural dyes have been reported [18–20]. Natural-based DSSCs have not shown high efficiency compared to the artificial ones, mainly due to the weak binding with TiO2 film as well as the low charge-transfer absorption in the whole visible range [21]. However, many reports have been recently published on using extracted natural dyes from natural products and tested for DSSCs [22–31]. Karakuş et al. employed Pelargonium hortorum and Pelargonium grandiflorum as sensitizers in their DSSCs and achieved a PCE of 0.065% and 0.067%, respectively [32]. Ramanarayanan et al. extracted the dye from the leaves of red amaranth and studied the effect of using different solvents, such as water and ethanol, and achieved PCE of 0.230% and 0.530%, respectively [33]. Hosseinnezhad et al. extracted the dye from Sambucus ebulus and PCE of 1.15% was reported [34]. Despite the fact that all these studies showed low PCE compared to other conventional cells, still the mechanism of operation and performance are of great interest, mainly to explore new insights and understanding for these sophisticated cells.
In this work, different dyes were extracted from red cabbage, onion peels, and spinach and used as sensitizers for the DSSCs. The optical and structural properties of the dyes and the fabricated cells were studied. Furthermore, the interface between the dye and TiO2 was investigated by impedance spectroscopy. The degradation in the PCE of N719 and natural-based DSSCs was monitored.
2. Experimental
2.1. Materials
Onion peels, red cabbage, and spinach leaves used in this study were collected from Fayoum City, Egypt. HCl and acetic acid were purchased from Loba Chemie. Isopropanol was purchased from Fisher Scientific. FTO conductive glass (sheet resistance: 7 Ω/sq), P25 TiO2 nanopowder, titanium isopropoxide, α-terpineol, ethyl cellulose, and di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2
2.2. Extraction of the Natural Dyes
Water was used as the extraction solvent for onion peels and red cabbage. 6 gm of onion peel and 147 gm of chopped red cabbage were dispersed into 250 ml and 400 ml of distilled water, respectively. The dispersions were heated up at 90°C for 24 hours. After cooling down to room temperature, the dispersions were filtered through filter papers to extract the anthocyanin (dye) for use as sensitizers. A third dye was extracted from spinach using acetone as the extraction solvent. 11 gm of spinach was crushed into fine powder using a mortar and dispersed in acetone. The solution was then filtered, and the resulting filtrate was used as natural sensitizer. All dye solutions were stored in the dark.
2.3. DSSC Fabrication
FTO conductive glass substrates were firstly cleaned in labosol solution for 30 min followed by rinsing in water-ethanol solution of NaOH for another 30 min. TiO2 blocking layer was prepared by adding 2.4 ml of titanium isopropoxide to 34 ml of isopropanol into a plastic bottle with stirring. Then, 0.8 ml of 2 M HCl was added dropwise and the solution left under stirring for 24 hours. The coating solution was spread on the surface by using spin coater (1000 rpm for 10 s followed by 3000 rpm for 60 s). The formed layer was sintered at 120°C for 120 min. TiO2 mesoporous layer was prepared by adding 1.5 gm of titania with 6 ml of terpineol-ethyl cellulose mixture. 0.25 ml of acetic acid was slowly added to the mixture with continuous grinding for 15 min. Then, 6 ml of isopropanol was added with grinding for 15 min until it gets homogenous. The paste was deposited on the FTO conductive glass by doctor-blading technique to obtain a TiO2 mesoporous with a thickness of 15 μm and an area of 1 cm2. The layer was preheated at 120°C for 150 min then sintered at 460°C for 15 min. After cooling to 80°C, the TiO2 electrode was immersed in dye solutions for 24 h. The iodide/triiodide (I-/I3-) was used as the electrolyte solution. DSSC was assembled by filling the electrolyte between a TiO2 electrode (anode) and a conductive glass substrate plated with Pt (cathode).
2.4. Characterizations
The UV-vis absorption was recorded using Agilent Cary 60 spectrometer. The Fourier transmission infrared (FTIR) spectra were recorded using Mattson Satellite IR to analyze the functional groups of the natural dyes. The steady-state photoluminescence spectroscopy was carried out using AVANTUS Ava-florescence setup featured with AvaSpec-ULS2048L-USB2 detector with the following specifications: back-thinned CCD (charged coupled device) image sensor array of 2048 pixels, symmetrical Czerny-Turner monochromator (600 line/mm), 200–1160 nm of wavelength scanning range, 25 μm slit, DCL-UV/VIS-200 detector collection lens, AvaLight-LED355, 450 nm light sources, and two FCR-UV200/600-2-IND fiber optics. Time-resolved PL measurements were performed using a time correlated single photon counting device (PicoQuant “PicoHarp-300”). A pulsed diode laser head at different repetition rates was used to excite the sample at 440 nm controlled by (PicoQuant PDL 800-D pulsed driver controller). The pulse duration of the laser was about 200 ps. The PL from the sample was filtered using long pass filters from 520 nm to pick up only band edge emission. The emitted photons were focused onto a fast avalanche photodiode (MPD-100-CTB, SPAD, Micro Photon Device). The response time of the photodiode was <50 ps. The excitation photon flux was controlled using neutral filters with different optical density.
The surface morphology of the TiO2 films was characterized by scanning electron microscope (Carl ZEISS Gemini, Sigma 500 VP). The photocurrent-voltage (
3. Results and Discussion
Figure 1 displays the optical images of the extracted natural dyes from onion peels, spinach, and red cabbage and their representative UV-vis absorption. The UV-vis absorption of N719 was also shown for the sake of comparison. It can be noticed that N719 have two wide absorption peaks at 387 nm and 530 nm. These peaks have been previously reported [35]. For the anthocyanin extracted from red cabbage and onion peels, absorption peaks at 544 nm and 486 nm have been shown, respectively. The absorption of the chlorophyll extracted from spinach shows two different peaks at 662 nm and 431 nm. The shifts in the absorption peaks are mainly due to the different chemical structure of these dyes.
[figures omitted; refer to PDF]
Figure 2(a) shows the schematic diagram of the main structure of DSSC prepared in this work. The detailed preparation conditions were described in Section 2. It should be emphasized here that the PCE is very sensitive to every single preparation step. Our reported results were repeated several times to make sure about the effect of the varied parameters on the PCE. Figure 2(b) presents SEM micrograph of TiO2 mesoporous layer formed on the FTO-coated glass. It can be seen that the TiO2 particles were aggregated to form homogenous and crack-free nanoclusters. Similar morphology was reported in [36]. The morphology of the photoanode strongly affects the photoelectrochemical activity of the DSSCs. The effect of TiO2 concentration in the mesoporous layer on the absorption is shown in Figure 2(c). Layers with different TiO2 concentrations (4%, 6%, 8%, and 10%) were formed and the representative UV-vis absorption was recorded. 10% TiO2 mesoporous layer resulted the highest possible absorption.
[figures omitted; refer to PDF]
Different DSSCs were prepared using TiO2 mesoporous layers with different concentrations and N719 as a sensitizer. The
[figures omitted; refer to PDF]
Table 1
Photoelectrochemical parameters of DSSCs using different TiO2 concentrations.
TiO2 wt (%) | FF (%) | |||||
---|---|---|---|---|---|---|
4% | 0.89 | 0.583 | 0.34 | 0.24 | 46.23926 | 0.168578 |
6% | 2.91 | 1.77 | 0.39 | 0.26 | 40.54983 | 0.554458 |
8% | 3.96 | 2.16 | 0.73 | 0.46 | 34.37111 | 1.197108 |
10% | 5.1 | 4.04 | 0.73 | 0.46 | 49.91673 | 2.239036 |
Figure 3(b) shows the efficiency of the 10% TiO2 DSSC over a week. The power conversion efficiencies were 2.2%, 1.88%, 1.68%, and 1.15% for days 1, 2, 3, and 7, respectively (Table 2).
Table 2
Photoelectrochemical parameters of DSSCs over a week.
Days | FF (%) | |||||
---|---|---|---|---|---|---|
1 | 5.1 | 4.04 | 0.73 | 0.46 | 49.91673 | 2.239036 |
2 | 3.95 | 3.25 | 0.73 | 0.48 | 54.10092 | 1.879518 |
3 | 3.67 | 2.9 | 0.75 | 0.48 | 50.57221 | 1.677108 |
7 | 2.63 | 2.07 | 0.67 | 0.46 | 54.0378 | 1.147229 |
FTIR studies were done to confirm the chemical structure of the extracted dyes. The natural dyes need to own specific functional groups in order to effectively adsorb on the TiO2 layers [41]. As shown in Figure 4, the chlorophyll dye extracted from spinach shows a peak at 3435 cm-1 due to the presence of the hydroxyl group. The peaks at 2923 cm-1 and 2854 cm-1 correspond to C–H stretching vibrations confirming the presence of aromatic C–H group. C=O stretching vibrations shows a peak at 1643 cm-1. The peak at 1056 cm-1 is attributed to the C–O–C stretching vibrations of acid and carbohydrate groups. C–N–C bending vibrations demonstrate a peak at 1385 cm-1. As observed from the functional groups of anthocyanin dye extracted from onion and red cabbage in Figure 3, the OH group among molecules indicates peaks at 3444 cm-1 and 3467 cm-1, respectively.
[figure omitted; refer to PDF]C=O stretching vibration shows a peak at 1639 cm-1. Stretching vibrations of C–O–C esters demonstrate peaks at 1037 cm-1 and 1033 cm-1, respectively. These functional groups confirm the presence of chlorophyll and anthocyanin [42].
Figure 5(a) represents the steady-state photoluminescence of the extracted dyes measured in parallel configuration of Aventus setup. The data was collected after 10 ms of acquisition time (in case of onion and red cabbage) and 1 ms for spinach. All data was averaged over 10 times of measurements. The spectra were fitted and normalized according to Gaussian distribution.
[figures omitted; refer to PDF]
All extracted dyes exhibited a spectral shift from UV to visible region. For anthocyanin dye, it showed an emission peak at 565 nm, which is red-shifted by 15 nm compared to the extracted dye from red cabbage. On the other hand, the spinach dye emits at 485 nm which in prominent for an efficient photoexcitation process between absorption and emission.
The behavior of photoexcitation process in the three dyes was investigated with time-resolved photoluminescence (TRPL) as shown in Figure 5(b). It demonstrates a long lifetime for chlorophyll dye relative to anthocyanin dyes. Consequently, higher efficiency for chlorophyll-based solar cells compared to anthocyanin ones was observed.
[figures omitted; refer to PDF]
Table 3
Photoelectrochemical parameters of the DSSCs with natural extracts.
Dye | FF (%) | |||||
---|---|---|---|---|---|---|
Spinach | 0.41 | 0.309 | 0.59 | 0.46 | 58.75982 | 0.171253 |
Onion | 0.24 | 0.158 | 0.48 | 0.34 | 46.63194 | 0.064723 |
Red cabbage | 0.21 | 0.156 | 0.51 | 0.32 | 46.61064 | 0.060145 |
Table 4
Photoelectrochemical parameters of the DSSCs with spinach extract over a week.
Days | FF (%) | |||||
---|---|---|---|---|---|---|
1 | 0.41 | 0.309 | 0.59 | 0.46 | 58.75982 | 0.171253 |
2 | 0.37 | 0.286 | 0.56 | 0.44 | 60.73359 | 0.151614 |
3 | 0.29 | 0.198 | 0.57 | 0.44 | 52.70417 | 0.104964 |
7 | 0.26 | 0.181 | 0.55 | 0.38 | 48.0979 | 0.082867 |
The interfacial kinetics and reactions of DSSC were investigated under dark conditions at 0 V by measuring the electrochemical impedance spectroscopy (EIS). The results of EIS are shown in Figure 7. Normally, the Nyquist plot of DSSC exhibits three frequency regions. The high-frequency region can be attributed to the charge transfer resistance at Pt/electrolyte interface. The middle-frequency region corresponds to the charge transfer recombination resistance at TiO2/dye/electrolyte interface. The low-frequency region is assigned to Warburg resistance and the diffusion properties of the redox couple (I3-/I-) in the electrolyte. The Nyquist plots of different dyes were fitted using a suitable circuit as shown in Figure 7. The big semicircles in the middle-frequency region indicated low charge recombination at the TiO2/dye/electrolyte interface. Natural dyes anchored to TiO2 showed a larger impedance compared to N719, which explains the higher performance of DSSC based on N719 compared to the extracted dyes. The DSSCs based on onion peel dye showed higher resistance to recombination than the other dyes. Larger radius indicated slower charge recombination rate [46].
[figure omitted; refer to PDF]Table 5 summarizes the fitting results.
Table 5
Electrochemical impedance parameters of DSSCs based on different dye sensitizers.
Dye | |||||
---|---|---|---|---|---|
N719 | 22.17 | 4.343 | 79.76 | 17.498 | 9.095 |
Spinach | 27.53 | 48.452 | 126.3 | 7.586 | 20.98 |
Onion | 25.57 | 60.445 | 371.3 | 30.98 | 5.137 |
Red cabbage | 25.41 | 16.021 | 430.6 | 35.48 | 4.486 |
4. Conclusions
The extraction, preparation, and photovoltaic performance of DSSCs based on natural sensitizers and N719 were optimized. Natural dyes can be easily and safely extracted by simple techniques. The UV-visible absorption and photoluminescence properties of the extracted dyes were studied. Among the dyes extracted, chlorophyll gave the longest lifetime and the highest possible efficiency. The DSSCs prepared with a photoelectrode thin film of 10% TiO2 showed the highest photoelectric conversion efficiency of 2.239%. The DSSC based on chlorophyll dye showed the highest performance among the natural extracted dyes with power conversion efficiency of 0.17%.
Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.
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Abstract
Here, three natural dyes were extracted from different fruits and leaves and used as sensitizers for dye-sensitized solar cells (DSSCs). Chlorophyll was extracted from spinach leaves using acetone as a solvent. Anthocyanin was extracted from red cabbage and onion peels using water. Different characterizations for the prepared natural dyes were conducted including UV-vis absorption, FTIR, and steady-state/time-resolved photoluminescence spectroscopy. Various DSSCs based on the extracted dyes were fabricated. The degradation in the power conversion efficiencies was monitored over a week. The effect of the TiO2 mesoporous layers on the efficiency was also studied. The interfaces between the natural dyes and the TiO2 layers were investigated using electrochemical impedance spectroscopy.
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1 Physics Department, Environmental and Smart Technology Group (ESTG), Faculty of Science, Fayoum University, Fayoum 63514, Egypt
2 Physics Department, Environmental and Smart Technology Group (ESTG), Faculty of Science, Fayoum University, Fayoum 63514, Egypt; State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, 430070 Wuhan, Hubei, China
3 Chemistry Department, Faculty of Science, Fayoum University, Fayoum 63514, Egypt