Comparative Performances of Natural Dyes Extracted from Mentha Leaves, Helianthus Annuus Leaves, and Fragaria Fruit for Dye-Sensitized Solar Cells

Abstract

During the last four centuries, there have been extensive research activities looking for green and clean sources of energy instead of traditional (fossil) energy in order to reduce the accumulation of gases and environmental pollution. Natural dye-sensitized solar cells (DSSCs) are one of the most promising types of photovoltaic cells for generating clean energy at a low cost. In this study, DSSCs were collected and experimentally tested using four different dyes extracted from Mentha leaves, Helianthus annuus leaves, Fragaria, and a mixture of the above extracts in equal proportions as natural stimuli for TiO₂ films. The result show that solar energy was successfully turned into electricity. Additionally, DSSCs based on mixtures of dyes showed better results than those based on single dyes. Efficiency (η) was 0.714%, and the fill factor (FF) was 83.3% for the cell area.

1. Introduction

Nowadays, the used energy in the industry sector is transitioning towards a more environmentally-friendly future. Fossil fuels are an environmental menace that will be depleted sooner. To triumph over this situation, alternatives to these fossil fuel reserves must be found [1]. Photovoltaic cells are one of the promising technologies for harvesting solar energy [2,3]. Unnatural solar cells are widely used because they have good efficiency, but they are high in cost [1,4]. Therefore, the increasing need for sustainable energy prompted researchers to focus their efforts on the development of photovoltaic technologies to meet the need at a lower cost [5,6,7]. Dye-based solar cells are used as a viable alternative to conventional (inorganic) solar cells as they are an environmentally and economically beneficial technology, as well as being easy to manufacture and providing a method for tuning optical properties through molecular design; they were first invented by O’Regan and Gratzel in 1991 [8,9,10]. DSSCs are photovoltaic cells based on semiconductors and light sensors that convert sunlight (photons) or artificial radiation into electrical energy [11]. DSSCs consists of a photodiode, a counter electrode (conductor glass covered with graphite, pt, or carbon), and electrolytes (including redox pairs as iodide or tri-iodide) [12]. At the moment, the maximum efficiency (η) observed for these cells (DSSCs) is 13% [13,14,15,16]. The operating concept of DSSCs is based on the natural photosynthesis of plants, and pigments play a vital role in widening the cell spectrum’s sensitivity. After photoexcitation of the sensitizer dye, the cascade transmission of excited electrons results in charge separation and rapid regeneration of oxidizing dyes [17,18,19].

A phototype is an inorganic substance that is sensitized by a donor substance called a sensitizer (dye) [20]. Titanium dioxide (TiO₂) is a broadband-gap semiconductor widely used in DSSCs due to the physical properties that make it suitable for use in DSSCs. They meet one of the criteria for effective electron injection by having a conduction band edge that is just below the excited state energy level of many dyes. When the DSSC is highlighted, photons are absorbed by the dye, electrons are transferred and excited, and the dye is oxidized. The excited electrons are sent into the range where TiO₂ can conduct electricity, and then they spread out through the porous film of the TiO₂ conductive glass fluorine-doped tin oxide (FTO) [21].

To obtain cells that are less costly and environmentally friendly, studies in the literature have suggested using DSSCs dyed with natural dyes or synthetic dyes [22,23]. On the other hand, synthetic dyes cause serious environmental difficulties because they are toxic, carcinogenic, refractory, and difficult to dissolve using water treatment technology [24]. To fight the health risks and environmental problems caused by synthetic dyes around the world, preventive steps must be taken to cut down on how much of these dyes are used in the environment [25]. Different research institutions and government groups have come up with different ideas for how to solve the problems of making water treatment technologies. These ideas include enhanced oxidation processes, reducing the amount of dye used, and using natural dyes instead [26,27]. Replacement with natural dyes has been suggested as a possible green solution for dealing with wastewater problems and reducing the environmental impact of these industrial operations. Natural dyes made from renewable bioresources are safe for the environment, biodegradable, and cheaper than synthetic dyes. They can be used as a good replacement for synthetic dyes [28,29]. Natural dyes derived from plants have been employed in recent years to improve the performance of DSSCs and photo-stimulation therapies [30,31]. To be classified as a sensitizer, a natural dye must meet the following criteria: possessing a broad and robust capacity to absorb light in the visible and near-infrared ranges; ensuring the stability and effective charge injection of the system by firmly adhering to the semiconductor surface, and containing optimal lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels for effective charge injection into the semiconductor conduction band (CB) [31,32]. Sensitizer development has been a major focus of recent years, since it plays an important role in DSSCs [26,27]. Chlorophylls seem to be in charge of collecting and delivering light energy to photosynthesis reaction centers, according to the research [33].

This paper presents an experimental investigation on the performance of four natural dyes extracted from mint, Helianthus annuus, Fragaria, and a mixture of the three dyes in equal proportions. The dye extracted from Helianthus annuus has been shown to contain Helianthus annuus auxin, which is the main reason for the sensitivity of the Helianthus annuus flower and its rotation with the sun, whereas mint leaves contain chlorophyll, which is one of the most important elements that help plants in absorbing sunlight. Fragaria extract was chosen because it contained anthocyanins, and DSSCs were made from these dyes. We found that these dyes are good for the environment and are a good alternative to the typical dyes. The inhibition of TiO₂ particle crystallization was investigated by energy dispersion X-ray analysis of the efficacy of natural photosensitizers, and the DSSCs were manufactured by adding the extracted dyes as the photosensitizers. Maximum power point (P_max), fill factor (FF), energy conversion efficiency (η), and short circuit current (I_SC) are the primary characteristics of a solar cell.

2. Materials and Methods

2.1. Materials

The materials used in this research included Mentha leaves, Helianthus annuus, Fragaria, and acetone to extract the natural colors from plants. A liquid platinum catalyst and sealing tape were used in this process. An electrolyte (iodide and triiodide) (I⁻/I⁺³), FTO (fluorine-doped tin oxide) glass as a transparent conducting oxide was used; it has a surface resistance of 10–20 Ω and also needs nitric acid (HNO₃) and TiO₂ powder.

2.2. Preparation of Natural Dye Sensitizers

The dyes extracted from the leaves of mint and the leaves of the annual plant Helianthus were prepared in the same way. The leaves of both plants were first washed with distilled water, and then the leaves were dried at room temperature in the dark. Then the leaves were crushed separately using an electric grinder, and 1 g of mint leaves was dissolved in 60 mL of acetone, and also 1 g of Helianthus annuus was dissolved in 60 mL of acetone in an airtight package and left for 24 h, after which it was filtered. Using filter paper to get rid of large portions, the extracts were kept in a container away from light until use. These steps are illustrated in Figure 1.

As an alternative, the Fragaria fruit extract was made by crushing the fruit in a pestle and extracting the fruit’s juices from 1 mg of fruit extract in 6 mL of acetone. After 24 h, they were filtered using filter paper to get rid of large pieces and kept in a tightly closed container in the dark until used. The fourth dye was prepared by mixing equal amounts of all of the aforementioned dyes, and the same method of preserving the other dyes was followed.

2.3. The TiO₂ Paste and Photoelectrode Preparation

The amount of 2 g of TiO₂ powder was dissolved in 6 mL of dilute nitric acid (HNO₃) (PH = 3) to make a TiO₂ paste. The components were mixed with a mortar and pestle for 30 min, followed by magnetic stirring for another 30 min, to create a homogeneous mixture. Ethylene glucose and glucose X-100 were added, which enhance the paste’s adhesion to FTO and the doughs’ adhesion to each other. Magnetic stirring was then carried out for an hour and a half to fasten the dissolution of the TiO₂ paste. Figure 2 shows the texture of the TiO₂ paste.

2.4. Fabrication of DSSCs

The FTO glass was thoroughly cleaned, and then the FTO piece was wrapped with tape on all four sides so that the thickness of the TiO₂ layer was controlled and the active area of the substrate was determined, which was equivalent to 1 cm². Drops of TiO₂ paste were applied and spread on the FTO substrate. Then, the FTO-coated TiO₂ was annealed at 500 °C for 80 min. After that, it was left to cool at room temperature. Then, four FTO substrates were soaked in each natural dye for 24 h. The excess dye was then removed by rinsing the substrate with ethanol. Using another FTO glass, the counter electrode was prepared with a Pt distribution and then heated at 450 °C, and these steps are illustrated in Figure 3, Figure 4 and Figure 5.

2.5. Measurement of the DSSC’s Photoelectric Conversion Rate

Analysis of the absorption spectra of natural dye solution and the combined solution of TiO₂ and natural dye was carried out using a UV-VIS spectrophotometer (Jasco, V650). The DSSCs’ photoelectric conversion efficiency was also examined in a lab environment using a source of artificial sunlight (AM1.5). Starting with the current–voltage (I–V) curve, the fill factor (FF) was specified at the time of the foundation as follows:

$$\mathrm{FF}=\frac{P_{max} }{I_{sc}\times V_{OC}}$$

(1)

The maximum current and voltage values are denoted as $I_{max}$ and $V_{max}$, respectively, while the short circuit current and open voltage are denoted as $I_{sc}$ and $V_{OC}$, respectively. Here is how to calculate the total energy conversion efficiency:

$$\eta =\frac{I_{sc}\times V_{OC}\times FF}{P_{in}}$$

(2)

where $P_{in}$ specifies the energy of the incident photon.

The parameters of DSSCs were measured using a device (Keithley 2400), where a digital ammeter and voltage meter are connected at both ends of the DSSC, as shown in Figure 6.

3. Results and Discussion

3.1. X-ray Diffraction Analysis

X-ray investigations were carried out on the films prepared on FTO glass, since the nature of TiO₂ plays a very important role in dye adsorption, and controls the efficiency and photocatalytic processes due to the details of different binding patterns. The results of the X-ray show that all of the films are in the anatase-phase through peaks (20.144, 25.005, 37.795, 47.776, 54.212, 53.658, and 62.370) with the respect to TiO₂. This phase is considered an active phase due to its surface chemistry and its high conduction band. This phase also shows better performance for DSSCs than the Rutile phase [34,35,36,37]. From the X-rays, a noticeable increase in the height of the peaks was noted, as shown in Figure 7 and Figure 8. These results are consistent with the tables of the American Society of Mechanical Engineers (A.S.T.M). It is also observed from Figure 7 and Figure 8 that there were no phases of impurities or other oxides in the crystal structure. The crystal size, which is the primary determinant in influencing the electron transport properties of materials, can be determined through the following equation [38]:

$$d=\frac{0.9\lambda }{\beta \cos \left(\theta \right)}$$

(3)

In this equation, θ is the diffraction angle, β is the full width at half maximum for each peak, and λ is the employed X-ray wavelength.

3.2. Field Emission Scanning Electron Microscopy (FESEM)

To confirm the creation of TiO₂ nanostructured films, FESEM measurements are often performed [39]. As TiO₂ films were heated to less than 450 °C, they were found to produce an imperceptible current even in the A range, making them ideal for solar cells [40]. According to the FESEM images in Figure 9, the TiO₂ grains were formed as spherical particles covering the FTO substrate with an average size of 27 ± 3 nm. Good agglomeration is responsible for this change, and it increases the ability of the FTO glass cover to stick together, which increases the quality of the conversion of light photons into electrons and the improved flow of electricity through the particle grid.

3.3. Absorbance Behavior of the Prepared TiO₂ Film

Figure 10 shows the absorption behavior of the prepared TiO₂. It shows that the annealed TiO₂ film at 500 °C has an absorption peak of 318 nm, while the maximum absorption wavelength of 304 nm occurs with a power band gap of 4.079 eV. The energy band gap (E_g) of the dyes absorbed by the TiO₂ surface was calculated as follows [41]:

$$Eg=\frac{1240}{\lambda }$$

(4)

The results of this research are in good agreement with other investigations on the optical energy band gap [42,43].

Figure 11 shows that the TiO₂ film that was poured on the FTO glass allows the transmission of visible light and absorbs ultraviolet rays. The average visible light transmission of TiO₂ after casting on the FTO substrate reduced to about 74.34%. However, coated FTO allows maximum transmission of visible light to the dye (sensitive material) for optimal absorption and conversion to electricity in DSSCs, while blocking harmful UV rays.

4. UV-Vis Analysis

The optical measurements were made using UV-VIS spectroscopy for the prepared dyes (Mentha leaves, Helianthus annuus leaves, Fragaria, and a dye consisting of a mixture of each of the extracted pigments in equal proportions). The absorption spectra showed that each type of dye had its absorption peak in the visible range.

In the Mentha leaves case, the absorption spectrum of mint leaves showed a peak absorption rate in the visible region at wavelengths of 450–500 and 580–700 nm, showing three peaks at 655, 430, and 485 nm, as shown in Figure 12.

In the Helianthus annuus case, the absorption spectrum of Helianthus annuus showed a maximum absorption plausible region from the 350–450 nm to the 500–600 nm regions, where its absorption peaks appeared at 415 and 515 nm, as shown in Figure 12.

In the Fragaria case, the absorbance spectrum for Fragaria showed a maximum absorption peak in the visible regions at 512 and 650 nm wavelengths. It had a wide absorption range in the region of 450–690 nm, as shown in Figure 12.

In the dye mixture case, the optical absorption spectrum of the dye mixture showed absorption in the visible region at wavelengths of 380–445, 575–700 and 510–450 nm, where its absorption peaks appeared at 433, 484, and 652 nm, as shown in Figure 12.

It has been observed that the three pigments can absorb the light in the visible area when melting, which makes them suitable as catalysts in DSSCs. Thus, it can be concluded that the nature of the solvent used to extract the dyes can affect the concentration of the dye due to the presence of different chemical compounds in plants. Accordingly, they have different solubility in different solvents. Additionally, it can be concluded that the optimal combination of dyes showed better cumulative absorption properties, as its absorption spectrum was broader and higher. Taking light in the visible range makes it more likely that higher levels of solar energy can be turned into an electrochemical form.

Photovoltaics Performance of DSSCs

Under white light irradiation (100 mW/cm²) from a high-pressure mercury arc lamp, photovoltaic studies of DSSCs manufactured using natural dyes as catalysts were performed by measuring the J–V curve of each cell. The short circuit current (J_sc), fill factor (FF), open circuit voltage (V_oc), and power conversion efficiency (ɳ) were used to evaluate the performance of natural dyes as catalysts in DSSCs. Figure 13 shows J–V curves of DSSCs utilizing sensitizers taken from Mentha leaves, Helianthus annuus, Fragaria, and a mixture of these dyes. The outcomes reveal that the sensitizing dye has a significant impact on the performance of the DSSC, as dyes absorb sunlight and convert it into electrical energy.

The short-circuit current had the highest value in the DSSC based on the mixture of dyes, while it showed the highest open circuit voltage with the DSSC based on Mentha leaves. The power output power of the DSSC was computed using IV data as P = IV. Figure 14 shows the power estimated as a function of V for DSSC sensitized by a dye combination as an example. The photoelectron chemical properties of the DSSCs sensitized with natural dyes are listed in

Table 1. Performance of DSSCs.

Dye	${\mathit{V}}_{\mathit{o}\mathit{c}} \left(\mathbf{v}\right)$	${\mathit{J}}_{\mathit{s}\mathit{c}} \left(\mathbf{mA}\right)$	ɳ (%)	F (%)
Fragaria	0.38	0.51	0.09	46.44
Mentha leaves	0.41	0.64	0.15	57.16
Helianthus annuus leaves	0.48	1.19	0.29	50.77
Mix	0.59	1.59	0.69	73.55

. As displayed in Table 1, the fill factor of the fabricated DSSCs ranged between 46.44% and 73.55%. The $V_{oc}$ changed from 0.38 to 0.59 V and the $J_{sc}$ varied from 0.51 to 1.59 mA/${\mathrm{cm}}^2$. The DSSC sensitization based on the mixture of dyes yielded the best results, with a cell efficiency of 0.69%. This dye showed the highest and most absorbance peaks in the visible region, due to carbonyl and hydroxyl groups in its chemistry, which allow it to attach to the TiO₂’s surface, enhancing the energy conversion efficiency.

5. Conclusions

In this study, four different natural dyes extracted from mint, helianthus annuus, Fragaria, and a mixture of the three dyes were investigated, and their performances were calculated. The dye sensitizer is a key factor in DSSCs, as it acts as an electronic pump to transfer light energy from the sun to an electrical power generation device. These dyes were selected, as Fragaria extract and mint leaves contain anthocyanin and chlorophyll pigments, and Helianthus annuus leaves contain auxin. The adopted methodology in this study has extracted the dyes successfully, and then the absorption conditions were examined and the photocurrent activity was tested. The outcomes have shown that a mixture of dyes (anthocyanins, auxin, and chlorophyll) has a better efficiency among the other dyes, reaching approximately 0.69%. This is due to the carbonyl and hydroxyl groups in its chemistry, which allow it to attach to the TiO₂’s surface, enhancing energy conversion efficiency. By contrast, the efficiency magnitudes for Mentha leaves, Fragaria, and Helianthus annuus leaves were 0.15%, 0.09%, and 0.29%, respectively. The results obtained lack the detailed research needed to increase the efficiency and stability of DSSCs based on natural dyes. Accordingly, the study concludes that natural dyes have good potential to form photosensitizers in DSSCs, and they are cheap, safe, environmentally friendly, and easy to extract. Therefore, it can be concluded that DSSCs that are based on natural dyes will shortly become one of the photovoltaic sources that overcome traditional electrical energy sources.