Dye-Sensitized Solar Cells Application of TiO₂ Using Spirulina and Chlorella Algae Extract

Abstract

The present study investigates dye-sensitized solar cells (DSSCs) incorporating natural extracts from the microalgae Spirulina and Chlorella as photosensitizers. TiO₂-based electrodes were prepared and immersed in methanolic algae extracts for 24 and 48 h. UV–Vis spectroscopy revealed absorption peaks near 400 nm and 650 nm, characteristic of chlorophyll. Electrochemical analyses, including photochronoamperometry and open-circuit potential, confirmed the photosensitivity and charge transfer capabilities of all systems. The cell sensitized with Chlorella after 48 h of immersion exhibited the highest energy conversion efficiency (0.0184% ± 0.0015), while Spirulina achieved 0.0105% ± 0.0349 after 24 h. Chlorella’s superior performance is attributed to its higher chlorophyll content and enhanced light absorption, facilitating more efficient electron injection and interaction with the TiO₂ surface. Although the efficiency remains lower than that of conventional silicon-based solar cells, the results highlight the potential of natural colorants as sustainable and low-cost alternatives for photovoltaic applications. Nonetheless, further, improvements are required, particularly in dye stability and anchorage, to improve device performance. This research reinforces the viability of natural photosensitizers in DSSC technology and supports continued efforts to optimize their application.

1. Introduction

Energy consumption has become one of the biggest issues of our time, as the rising demand for energy and continued dependence on non-renewable sources are having a detrimental impact on the environment, raising concerns about the greenhouse effect and the environmental legacy we will leave for future generations [1,2].

With the increase in population and economic progress, it is essential to invest in sustainable energy alternatives. In this context, solar energy has emerged as a promising option, as it is a clean and inexhaustible source, whose generation does not produce greenhouse gases, contributing significantly to the environment [1,3]. Furthermore, technological advances in photovoltaic cells have promoted significant improvement in terms of efficiency and cost reduction, increasing the accessibility and competitiveness of this technology in the energy sector [4].

According to projections, the capacity of photovoltaic solar plants could increase nearly sixfold over the next decade, and projections indicate that this source of renewable energy continues to expand. Furthermore, between the years 2000 and 2018, photovoltaic energy worldwide increased from 1 GW to 480 GW. The Brazilian Solar Energy Association points out that, on the national scene, Brazil is currently in 16th place in the ranking of solar energy generation, even though it has shown significant growth in this sector in the last years [5].

The development of photovoltaics began in 1839 with the discovery of the photovoltaic effect by Edmond Becquerel, who observed that two brass plates immersed in an electrolyte generated electricity when exposed to sunlight, which became known as the photovoltaic effect. However, one of the most important landmarks in the progress of this technology was achieved in 1954, when scientists D.M. Chapin, C.S. Fuller and G.L. Pearson making silicon solar cells, enabled the first modern photovoltaic cell to be developed [5,6].

The operating principle of solar cells is based on the photovoltaic effect, which occurs in semiconductor materials. These materials are characterized by electrons distributed between two energy bands—the valence band (VB) and the conduction band (CB)—which are separated by a forbidden energy region known as the bandgap. When this region is exposed to light and an electric field is applied, the electrons can pass through the bandgap, generating an electric current [7].

One method of converting solar energy into electrical energy involves the use of solar cells that incorporate nanoparticles into their structure, known as Grätzel cells, or even photochemical solar cells or dye-sensitized nanocrystalline solar cells (DSSC). In their manufacture, these devices use titanium dioxide (TiO₂), a material that is particularly attractive due to its low cost compared to silicon, as well as the fact that it can be obtained from mineral reserves located in Brazil [7]. These systems show promise by incorporating a semiconductor oxide and a photosensitizing material, such as a dye, which enables charge generation and separation through distinct mechanisms. This represents an advantage over conventional silicon-based photovoltaic systems.

The dyes most used in photochemical solar cells are ruthenium-based metal complexes, which contain coordinating groups that enable them to adsorb on the surface of the semiconductor, resulting in efficient absorption in the solar spectrum, including the infrared region. However, these dyes are specifically designed for photovoltaic applications and involve complex synthetic pathways, which significantly increase their cost. Given their high cost, it is essential to study and develop new and more accessible dyes for solar energy generation [7,8].

To provide an alternative to synthetic dyes, several studies have focused on the development of dye-sensitized solar cells (DSSCs) using low-cost dyes extracted from natural products as algae extracts. Although these dyes have a lower energy efficiency, their lower cost makes it possible to use them for photovoltaic systems [8].

Natural dyes typically serve important biological functions and participate in various metabolic processes in living organisms. Several studies have investigated the efficiency of natural dyes in the development of dye-sensitized solar cells (DSSCs) as a sustainable alternative to conventional sensitizers. Ammar et al. [9] evaluated the performance of DSSCs using dyes extracted from spinach (chlorophyll), onion and red cabbage as sensitizers, the latter due to their anthocyanin content. Anthocyanins were extracted using distilled water at 90 °C for 24 h, while chlorophyll was extracted using acetone as the solvent. Among the tested dyes, chlorophyll exhibited the best performance, with a short-circuit current density of 0.41 mA/cm², an open-circuit potential of 0.59 V, and a power conversion efficiency of 0.17%. However, when compared to the commercial dye N719, all natural dyes tested demonstrated inferior performance, primarily due to weaker anchoring to the TiO₂ surface. Additionally, the efficiency of the natural dye-based cells declined by approximately 50% after seven days, attributed to electrolyte evaporation and dye desorption.

Pathak et al. [10] investigated the use of natural dyes extracted from pomegranate, beet root, lemon leaves and spinach as sensitizers in DSSCs. Various proportions of these extracts were tested, with the best results observed for samples DS2 (composed of lemon and beet root) and DS6 (composed of pomegranate, beet root and spinach). The dyes from pomegranate and beet root were obtained by slicing and grinding the fruits/vegetables using a mixer, followed by the blending with ethanol and boiling for 30 min. The final solution was adjusted to pH 3.5 using HCl 0.1 M. In contrast, the dyes extract from lemon and spinach were prepared by drying the leaves in an oven at 60 °C for 24 h, after, which 2 g of dried leaves were immersed in ethanol and kept in the dark at room temperature for 24 h. Among the test samples, DS6 exhibited the best performance, with a efficiency of 0.03% and a short-circuit current density of 0.055 mA/cm². These results indicate that natural dyes extracted from fruits and vegetables are viable alternatives as sensitizers in DSSCs. However, further research is needed to optimize preparation conditions and achieve efficiencies comparable to or exceeding those obtained with the synthetic dye N719.

Chlorophyll is an important example of a natural dye, present in microalgae such as Chlorella, and in cyanobacteria such as Spirulina [11,12]. According to previous studies, Chlorella contains chlorophyll a and chlorophyll b in the order of 0.31 and 0.12 pg.cel-1, respectively [11], while Spirulina has an approximate concentration of 1.2% chlorophyll [13].

Chlorophylls are macrocyclic compounds derived from porphyrin, with magnesium (Mg) acting as the central atom (Figure 1). Among the different forms of chlorophyll, chlorophyll A is the most predominant, performing a crucial role in the first stage of the photosynthesis process. The other variants, such as chlorophylls B, C and D, function as accessory pigments that contribute to capturing light during photosynthesis [14].

Photosynthetic pigments, especially chlorophylls, are present in all organisms which carry out photosynthesis. These pigments include chlorophylls A and B (carotenoids and phycobilins), which act to absorb solar radiation and transfer this energy to electrolytic sites. Each chlorophyll molecule is capable of absorbing 1 quantum of energy at a time and causing the excitation of an electron, which moves to a higher energy level as it distances itself from the atomic nucleus. The energy from this process is then used by the photosynthesis process [14].

Therefore, these pigments have maximum absorption efficiency in the blue and red light bands of the electromagnetic spectrum, optimizing the photosynthetic process through electronic excitation and subsequent energy transfer. Based on this information, the present study aims to evaluate the electrochemical behavior of titanium dioxide (TiO₂) solar cells sensitized with natural extracts of Spirulina and Chlorella under different immersion durations.

2. Materials and Methods

2.1. Assembly for the Cell

The TiO₂ paste for the working electrode was prepared according to the methodology described by Parussulo [15], using 3 g of anatase TiO₂ (Sigma Aldrich, São Paulo, SP, Brazil), 0.1 mL of acetyl acetone (Catalão, Goiás, Brazil), 0.1 mL of Triton X (Catalão, Goiás, Brazil), 1 mL of polyethylene glycol 200 (Synth) and 4 mL of deionized water. The deposition was carried out on a conductive FTO substrate (tin oxide doped with fluorine ~7 Ω sq⁻¹), via an aerography process, which was then calcined at 450 °C for 30 min.

The working electrode was sensitized by immersing the oxide films in Spirulina and Chlorella extracts for either 24 h or 48 h. The immersion times of 24 and 48 h were selected based on preliminary tests. Longer immersion periods, above 48 h, led to a decrease in the charge transport capacity of the device, probably due to excessive dye adsorption on the TiO₂ surface. This excess can block the porous structure of the semiconductor, hindering electron injection. The extracts were prepared in a proportion of 0.6 g of in natura dye (Spirulina and Chlorella) per 100 mL of methanol.

The counter electrode consisted of a platinum layer deposited onto FTO via cyclic voltammetry. The tests were carried out using a cell of 3 electrodes: a working electrode, FTO, an Ag/AgCl reference electrode and a platinum plate as a counter electrode. The electrolyte solution was prepared with K₂PtCl₆ 1 × 10⁻⁴ mol L⁻¹ in 0.1 mol L⁻¹ HCl. Four cycles were performed at a scan rate of 10 mV s⁻¹ between ± 0.5 V (vs. Ag/AgCl).

The cell was assembled in a sandwich configuration, as illustrated in Figure 2, with an active area of 0.2 cm². An iodine-based electrolyte was used, consisting of 0.5 mol L⁻¹ of tert-butyl pyridine, 0.6 mol L⁻¹ of tetrabutylammonium iodide, 0.1 mol L⁻¹ of lithium iodide and 0.1 mol L⁻¹ of resublimated iodine, all dissolved in methoxypropionitrile [16,17]. The tests were conducted in triplicate.

2.2. Characterization of Dyes and DSSC

To determine the absorption spectra of Spirulina and Chlorella extracts, absorbance was measured in the ultraviolet and visible regions using a Shimadzu^® (Guarapuava, Paraná, Brazil) UV–Vis spectrophotometer (model UV-1800) at the CCMN laboratory at UNICENTRO.

Electrochemical measurements of the DSSCs were performed using a Zahner potentiostat (model Zennium Electrochemical Workstation), coupled with an Xpot and a LOT Oriel-Quantum Design GmbH solar simulator equipped with a xenon lamp and a beam diameter of 25 nm (Quantum Design, Guarapuava, Paraná, Brazil). The electrochemical measurements consisted of open-circuit potential (OCP) tests.

Photochronoamperometry was conducted over 660 s, with the light source intermittently turned off and on every 60 s, to evaluate the charge injection capacity and the charge–discharge behavior of the solar cell.

Photochronoamperometric efficiency was calculated based on the corresponding current–time curves. Current density–voltage (j-V) curves were used to determine the photovoltaic parameters required to calculate the energy conversion efficiency of the system, as shown in Equation (1). According to Equation (1), η represents the energy efficiency, J_sc the short circuit current, V_oc the open-circuit potential, FF the fill factor and P_in the incident power [8].

$$\mathsf{\eta }={\displaystyle \frac{{\mathrm{J}}_{\mathrm{s}\mathrm{c}}·{\mathrm{V}}_{\mathrm{o}\mathrm{c}}·\mathrm{F}\mathrm{F}·100\%}{{\mathrm{P}}_{\mathrm{i}\mathrm{n}}}}$$

(1)

3. Results and Discussion

3.1. UV–VIS

The UV–VIS spectra obtained for the Spirulina and Chlorella samples are shown in Figure 3.

As shown in Figure 3, Spirulina and Chlorella exhibited absorbance peaks near 400 nm and 650 nm, which are characteristic of chlorophyll absorption [18,19].

Strong absorption peaks at 420 nm and 660 nm in the visible region suggest the potential of these extracts as natural sensitizers for as a natural sensitizer in the visible light-driven applications, as shown by Al-Alwani et al. [20]. The literature shows that chlorophyll molecules, especially Chlorophylls A and B, contain alternate single and double bonds, as well as orbitals that can be delocalized, thus providing a stable and effective photoreceptor or as an electron donor [21,22]. Chlorophylls A and B have two principal absorption peaks, a short wavelength absorption of 400–500 nm and a long wavelength absorption between 600–700 nm [23].

Zhou et al. [24] reported that the two intense bands present in the region of 400 and near 670 nm represent an electronic transition π → π* and n → π* with auxochrome groups present; these bands are characteristic of Chlorophyll A and Chlorophyll B, respectively. In addition, these absorption regions are characteristic of chromophore and auxochrome groups (C=O, C-OH) in the molecular structure of natural sensitizer, which could coordinate with the Ti⁺⁴ sites and provide an efficient charge transfer process. These differences in composition affect how dyes absorb and distribute across the porous oxide films; therefore, the impregnation time must be carefully analyzed to achieve a balance between sufficient coordination and adequate dye loading, while avoiding excessive coverage that can block electron transport [25].

These results demonstrate the potential of using Chlorella and Spirulina as natural dyes in DSSCs.

3.2. Photochronoamperometry

The photochronoamperometry curves for Chlorella and Spirulina after 24 and 48 h of immersion are shown in Figure 4.

Samples A and B (Figure 4) showed a current flow when the light source was re-established, demonstrating that all the systems tested are photosensitive. Moreover, the devices exhibited excellent charge–discharge responsiveness, as the current changed promptly upon illumination cycling. It can also be seen that there was no significant degradation of the dye under the incidence of light, as the current remained stable at close to the same levels during the analysis time.

As indicated by the results in Figure 3, the dye derived from Spirulina extract produced the highest photocurrent, with 24 h of immersion being the most efficient (with currents close to 1 × 10⁻⁴ A cm⁻²). The dye with Chlorella extract, on the other hand, showed lower photocurrent values and was more effective after 48 h of immersion.

3.3. Open-Circuit Potential

Figure 5 shows the open-circuit potential (OCP) curves as a function of time, obtained under constant temperature and lighting conditions, after 24 and 48 h.

As can be seen in Figure 5A,B, all the systems showed small potential variations around 300 s, which was expected due to the increase in temperature caused by the solar simulator’s OCP. After 48 h of immersion, the potential values for Spirulina and Chlorella were approximately 325 mV and 260 mV, respectively. While immersion in the dyes for 24 h presented a lower potential, showing that longer immersions cause greater amounts of active substances to be adsorbed on the surface of the solar cell, increasing the electron flow [8].

The potentials for Spirulina at both times were lower than for Chlorella. This indicates that Spirulina is less efficient. This suggests that Spirulina contains multiple accessory pigments in addition to chlorophyll. In accordance with the literature, the pigment known as phycocyanin is responsible for the blue–green color of Spirulina, and is conjugated to proteins, phycobiliproteins, that are responsible for the mechanism of light absorption and can be found in the absorption range of 610–620 nm [26,27,28,29]. Phycobiliproteins are known to be able to absorb incident light directly at wavelengths which are not covered by chlorophyll, and thus are considered accessory pigments [30].

3.4. Current Density Curve as a Function of Potential (j-V)

The j-V curves are presented in Figure 6 and the photovoltaic parameters are given in

Table 1. Photovoltaic parameters of cells sensitized with Chlorella and Spirulina.

	Jsc mA/cm2	E (V)	FF	η (%)
Chlorella 24 h	0.0910 ± 0.0119	0.313 ± 0.0170	0.327 ± 0.013	0.0093 ± 0.0021
Chlorella 48 h	0.1385 ± 0.0098	0.3637 ± 0.0059	0.365 ± 0.004	0.0184 ± 0.0015
Spirulina 24 h	0.1095 ± 0.0300	0.2970 ± 0.0735	0.320 ± 0.029	0.0104 ± 0.0300
Spirulina 48 h	0.1034 ± 0.0349	0.2990 ± 0.0858	0.338 ± 0.050	0.0105 ± 0.0349

, considering: J_sc, as the short circuit current, V_oc the open-circuit potential, FF the fill factor and P_in the incident power (100 mW cm⁻²).

Based on the photochronoamperometric curves (Figure 5), the solar cell sensitized with Chlorella after 48 h of immersion exhibited the highest performance. Spirulina, on the other hand, showed a more efficient response for the sample after 24 h of immersion.

As shown in Table 1, the energy conversion efficiencies (η) obtained for the cells sensitized with Chlorella and Spirulina after 24 and 48 h of immersion ranged from 0.0093 to 0.0184%, but a higher energy efficiency (η) of 0.0184% was obtained for the solar cell sensitized with Chlorella after 48 h of immersion, which can be attributed to the higher absorbance peak (Figure 3).

Hidayah et al. [19] conducted a preliminary study on the fabrication and characterization of DSSCs using Spirulina in the solid state under varying conditions. Among the tested configurations, the best-performing device exhibited a short-circuit current density of 0.44 µA/cm², an open-circuit potential of 0.17 V and a fill factor of 0.250 under 1 mW/cm² incandescent illumination. These results were attributed to the presence of PSI/PSII pigment–protein complexes in the active materials, as higher PSI/PSII content tends to enhance the photocurrent response of the devices. In comparison, the results obtained in the present study demonstrate superior performance, with a short-circuit current density (J_sc) of 0.1095 mA/cm², an open-circuit potential (E) of 0.2970 V and a fill factor of 0.320, indicating the promising potential of Spirulina-based DSSCs for solar energy conversion.

As observed by Ganta et al. [31], Chlorella is known to produce a greater amount of chlorophyll than any other algae and is therefore appropriate for the construction of DSSCs.

Current density intensity depends on the strength of interaction between sunlight, the dye and TiO₂, as well as the absorption coefficient of the dye. Higher current densities generally result in higher efficiencies for converting light to energy, corroborating the results presented in this study [31].

The short circuit current (J_sc) presents the photocurrent incident on the sample per unit area in mA/cm² and the j-V curve (Figure 5) shows that the samples tested ranged between 0.1385 mA/cm² and 0.1095 mA/cm² for Chlorella after 48 h of immersion and Spirulina after 24 h of immersion, respectively. The parameters presented in Table 1 show slight variation when comparing Spirulina at different immersion times, although the sample immersed for 24 h demonstrated higher efficiency.

The literature shows that Spirulina contains first-generation degradation products such as Pheophytin A and Pheophorbide A [32], corroborating the results of this study, showing better results for Spirulina after 24 h, decreasing after 48 h of immersion.

Commercial dyes, such as N719, based on ruthenium-based complex, typically exhibit higher efficiencies (η ~ 5%) due to their ability to absorb across the electromagnetic spectrum. Additionally, these dyes contain highly effective coordinating groups within their molecular structure, enabling strong anchoring to the semiconductor oxide surface, which enhances charge transfer and overall device performance [33]. However, despite their excellent efficiency, ruthenium-based dyes are associated with high production costs and complex synthesis routes, limiting their large-scale application in low-cost photovoltaic devices. Furthermore, it is noteworthy that in this study, the cells were produced without the application of certain optimizations commonly used in DSSC construction, such as the introduction of a blocking layer (e.g., TiCl₄ treatment) to reduce electron recombination, or the use of effective cell-sealing techniques [34]. These factors could potentially improve the photovoltaic performance of the devices developed in this work.

The experimental results demonstrate clear performance trends for DSSCs sensitized with natural dyes. However, the application of accurate physical models could further improve understanding and prediction of device behavior. Studies have shown that modeling contributes to optimizing photovoltaic performance by accounting for parameters such as light absorption, reflection and device geometry [35].

In addition, comparing the efficiency of dye-sensitized solar cells with traditional silicon cells, the performance is lower. This is because of a number of conditions, such as (1) these dyes can suffer photodegradation over time, (2) the long-term stability of the electrolytes and (3) the adhesion of the dye to the TiO₂ nanoparticles [31,36]. Therefore, studies are essential into the best conditions for obtaining plant-based dyes in solar cells.

4. Conclusions

The application of natural dyes derived from Spirulina and Chlorella extracts in TiO₂-based cells has shown promising potential for use in DSSCs, although their efficiency remains lower than that of conventional dyes. The samples analyzed were responsive to the light source, demonstrating their potential as photosensitizers and effectively converting sunlight into electrical energy.

The solar cell sensitized with Chlorella showed the best results after 48 h of immersion, with J_sc equal to 0.1385 mA/cm² and efficiency equal to 0.0184%. The solar cell sensitized with Spirulina showed J_sc equal to 0.1034 mA/cm² and efficiency equal to 0.0105% after 48 h of immersion.

The development of solar cells utilizing natural dyes may offer a sustainable and cost-effective alternative for renewable energy generation. However, further research is necessary to optimize the system and enhance the overall efficiency of these devices.