Investigate the utilization of novel natural photosensitizers for the performance of dye-sensitized solar cells (DSSCs)

2.1 Materials

All solvents, including Isopropyl alcohol (IPA, 99 %), alcohol (99 %), methanol (99 %), and aquadest (99 %) were sourced locally from West Sumatra, Indonesia except the Zamzam solvent was purchased from Saudi Arabia. The raw natural dyes, such as gambier leaves, gambier branch, cinnamon, and petiole of tectona leaves were obtained and extracted from Indonesian natural resources, while the date fruits were procured from Saudi Arabia. Titanium dioxide (TiO₂, 99 %, 5–10 nm) and Indium thin oxide (ITO, 7 – 10 $\mathrm{\Omega }$ /sq, 20 × 20 × 7 mm) were supplied by Titanos, China and Latech Scientific, Singapore, respectively. Potassium iodide (KI) and Iodine (I₂) (99 %, solid) were obtained from ROFA Laboratorium Center, Indonesia.

2.2 Photosensitizer extraction processes

The extraction procedures begin with the raw natural materials (leaves of gambier, stem wood of gambier, cinnamon, date fruits, and petioles of tectona leaves). These materials were initially cleaned with water approximately 10 min, followed by washing with aquadest for ∼25 min. The cleaned materials were then left at room temperature for around 180 min. Each cleaned material was ground in a blender for approximately 25 min. Weighed samples, up to 250 g each, were then mixed with 300 mL of solvent (Isopropyl alcohol, alcohol, methanol, and aquadest) and macerated at room temperature for 24 h.

Following maceration, the dyes were evaporated for about 60 min and cooled at room temperature for approximately 130 min. The pure dyes were then filtered for 45 min and stored at room temperature. The filtered dyes were divided into two portions: one was kept at room temperature for future use, while the other was subjected to characterization. The optical and structural characterization was performed using an UV–Visible spectroscopy (wavelength range: 200–1200 nm and absorption range: 0–1 a.u) and Shimadzu IR Prestige-21 FTIR spectroscopy (wavenumber range: 500–4500 cm⁻¹, with transmittance range: 0–50 %).

2.3 Fabrication of the photoanode and cathode of DSSCs

The fabrication of the primary components of DSSCs proceeded as follows: first, we prepared the photoanode, which comprises ITO glass, TiO₂, and the photosensitizer extract. The ITO glasses were cleaned using the aquadest for approximately 5 min and IPA for about 15 min in an ultrasonic cleaner. After cleaning, the glasses were dried at room temperature for 15 min and their resistivity was measured. A 1 cm² area on the positive side of the ITO glass was masked with tape as shown in Fig. 2a. Concurrently, 0.4 g of TiO₂ was mixed with 0.6 mL of aquadest using ultrasonic cleaner until a paste was formed. This TiO₂ paste was applied to the 1 cm² ITO glass using the doctor blade techniques, resulting in a layer approximately 11 μm thick, as illustrated in Fig. 2a. The semi photoanode was then sintered at approximately 250 ^oC for 60 min and cooled at room temperature for 45 min. Following the step, semi-photoanode was soaked in photosensitizer for 12 h, resulting the final photoanode as depicted in Fig. 2b. Next, we fabricated the photocathode or counter electrode, which consists of ITO and carbon derived from candle soot (Toor and Sayyad, 2021). We used the cleaned ITO glass from previous process. The active area of the ITO glass was coated with carbon from candle soot, and the design was cleaned with cotton wool to define a 1 cm² area and with a thickness of 11 μm (the same with photoanode) as shown in Fig. 2c. Finally, the electrolyte was prepared by dissolving 5 g of potassium iodide (KI) and 0.5 g of iodine (I₂) in 10 mL of distilled water (aquades) and mixing in an ultrasonic cleaner for 15 min.

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Fig. 2

Illustration of DSSC fabrication (a) photoanode deposition process from left to rigth (b) deposite a photosensitizer on photoanode (c) cathode and (d) DSSC divice (Reproduced from ref. (Chebrolu and Kim, 2019) with permission from the Royal Society of Chemistry).

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Upon fabricating all the DSSC core components, the next step was assembling the DSSC devices in a sandwich structure, as shown in Fig. 2d. Then, we measured the current and voltage (I-V) characteristics of the DSSC devices using a 30 W lamp set to an intensity of 10 mW/cm². The setup included two multimeters to monitor the I-V flow, a lux meter to measure the input power, and a potentiometer to control the I-V flow as depicted in Fig. 1a. In this study, we fabricated twelve DSSC devices using photosensitizer extracted from gambier leaves, gambier branches, cinnamon, date fruits, and petioles of tectona leaves. We also considered to extract those photosensitizers using Isopropyl alcohol, alcohol, methanol, zamzam, and aquadest.

3.1 Optical and structural characterizations of natural photosensitizer extracts

UV–Vis spectroscopy results: The performance of DSSC is critically influenced by the light absorption capacity of the dye sensitizer and the efficient transfer of excited electrons through the mesoporous semiconductor layer. In this study, we collected gambier leaves, gambier branches, petioles of tectona leaves, and date fruits, and immersed them in various solvents including alcohol, distilled water, methanol, isopropyl alcohol, and Zamzam water (99 %). Altogether, we characterized twelve photosensitizer samples by examining their optical properties using UV–Visible spectroscopy across wavelengths ranging from 200–1200 nm.

Fig. 3a displays the absorbance results of gambier leaves dissolved by five solvents. The initial absorption peak, appearing between 350 – 400 nm, align with previous reports on extracted flavonoid/tannin compounds (Megala and Rajkumar, 2016; Nasyori and Noor, 2021; Rather et al., 2020). Additionally, secondary peaks at 655 and 666 nm were observed for samples immersed in alcohol and isopropyl alcohol, respectively. This indicates a transition from flavonoid to chlorophyll compounds (Rajaramanan et al., 2023). These results emphasize the dominance of flavonoid/tannin compounds in gambier leaves. Moving on to Fig. 3b, the optical properties of cinnamon, gambier branches, and petioles of tectona leaves were immersed in isopropyl alcohol and methanol. The Zamzam water was used for soaking date fruits and gambier branches. The prominent peaks between 300 – 375 nm across all solvents indicate the presence of flavonoid or tannin compounds (Rather et al., 2020). Furthermore, in Fig. 3b, the peaks at 394 nm and 666 nm for gambier branches immersed in isopropyl alcohol indicate a transition from tannin to chlorophyll compounds. Fig. 3c highlights the use of Zamzam water for the first time to extract compounds from date fruits and gambier branches. Interestingly, while immersion in isopropyl alcohol, alcohol, and methanol resulted in solidification of dates after 24 h, extraction using Zamzam solvent successfully yielded the active compounds from ajwa and sukkari dates. The absorbance peaks at 369 and 343 nm correspond to flavonoid/tannin compounds (Hammouda et al., 2014; Megala and Rajkumar, 2016; Nasyori and Noor, 2021; Rather et al., 2020).

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Fig. 3

UV–Vis spectra of the natural photosensitiser extract with (a) gambier leaves using five solvents, (b) cinnamon, gambier branches, petioles of tectona leaves, using I=isopropyl alcohol, M=methanol and (c) sukkari date, ajwa date, and gambier branches using Z=Zamzam water.

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FTIR spectra analysis: Each natural photosensitizer extract contains distinct pigment compounds, leading to varied electrical properties when exposed to sunlight. This pigment compounds can be categorized based on their characteristic absorbance spectra. As highlighted in the UV–Vis spectra results, we further explore the chemical composition of these natural photosensitizers, often classified within anthocyanin, flavonoid, curcumin, and chlorophyll groups. Fig. 4 presents the fundamental molecular structures of these pigment categories.

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Fig. 4

Chemical structures of key organic compound found in natural photosensitiser extracts, including flavonoid, curcumin, chlorophyll, and anthocyanin.

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Flavonoids are characterized by a C6-C3-C6 carbon structure, facilitating extensive physiological activity. This structure includes a flavone or G-ring, that connects two benzena rings, as shown in Fig. 4, allowing efficient charge transfer from the highest-occupied molecule orbital (HOMO) to the lowest-unoccupied molecular orbital (LUMO), requiring minimal energy (Khammee et al., 2023; Mejica et al., 2022; Seithtanabutara et al., 2023). The electron injections from these dyes into the conduction band of a semiconductor enhances photoelectric conversion efficiency. Chlorophyll, one of the most commonly used photosensitizer in DSSCs, absorbs ultraviolet, red, and blue light, with absorbance peaks typically between 400 – 660 nm (Rajaramanan et al., 2023; Seithtanabutara et al., 2023). This pigment is abundant in plants, algae, and cyanobacteria. Another widely studied pigment is anthocyanin, responsible for blue, red, and yellow coloration in fruits and plants. Structurally similar to flavonoids, anthocyanins possess an oxygen atom and a C-ring, with absorbance peaks between 500–555 nm (Mejica et al., 2022; Seithtanabutara et al., 2023), whereas flavonoid peaks occur at 350–400 nm (Bella et al., 2015; Grätzel, 2003; Yadav et al., 2023). Curcumin, a deep yellow pigment derived from ginger, has a structure consisting of carbon double bonds, methoxy, and hydroxyl groups, with absorbance peaks in the 420–580 nm range (Seithtanabutara et al., 2023; Suyitno et al., 2018).

The chemical groups present in the natural photosensitizer extracts were characterized using FTIR spectroscopy in the range of 4000–500 cm⁻¹. Fig. 5a shows the FTIR analysis of gambier leaves extracted with five different solvents. The spectra for gambier leaves extracted in alcohol, methanol, and isopropyl alcohol exhibit consistent peaks, including C=C stretching at 2152 cm⁻¹, 1629 cm⁻¹, and 1521 cm⁻¹ (Gómez-Ortíz et al., 2010), C–H bending of methyl groups at 1466 cm⁻¹ and 1379 cm⁻¹(Gómez-Ortíz et al., 2010; Seithtanabutara et al., 2023), and C-O vibration at 1266 cm⁻¹(Gómez-Ortíz et al., 2010). Additional peaks at 1200 cm⁻¹, 1146 cm⁻¹, 1106 cm⁻¹, 1063 cm⁻¹, 1050 cm⁻¹, and 1035 cm⁻¹ correspond to C–O–C stretching, while peaks at 992 cm⁻¹ indicate =C–H, and those at 820 cm⁻¹, 764 cm⁻¹, 666 cm⁻¹, and 630 cm⁻¹ represent C–H bonds of alkene groups (Gómez-Ortíz et al., 2010; Seithtanabutara et al., 2023). For gambier leaves extracted in aquadest and Zamzam water, C=C stretching is observed at 2122 cm⁻¹ and 1737 cm⁻¹, with additional peaks for −C=C−stretching at 1629 cm⁻¹, −C−C aromatic stretch at 1520 cm⁻¹, C–H at 1466 cm⁻¹ and 1394 cm⁻¹, and −C−C−and −C−O stretching between 1285 cm⁻¹ and 1107 cm⁻¹. Peaks at 1052 cm⁻¹, 945 cm⁻¹, 821 cm⁻¹, and 630 cm⁻¹ represent C−O−C stretching and −COOH deformation (Rather et al., 2020). The region from 4000–2500 cm⁻¹ indicates the presence of benzene rings, characteristic of flavonoid/tannin compounds forming intermolecular H-bonded frameworks and methyl groups, attributed to strong −OH stretching (Sampaio et al., 2019). Isopropyl alcohol shows a peak at 2930 cm⁻¹ corresponding to alkane C–H stretching, consistent with weak absorption bands (Çakar et al., 2016).

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Fig. 5

FTIR spectra of the natural photosensitiser extracts showing (a) gambier leaves using five different solvents (alcohol, methanol, isopropyl alcohol, aquadest, and Zamzam water), (b) cinnamon, gambier branches, petioles of tectona leaves, using I=isopropyl alcohol, M=methanol, and (c) sukkari date, ajwa date, and gambier branches using Z=Zamzam water.

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Fig. 5b shows the petiole of tectona leaves, gambier branch, and cinnamon extracted in isopropyl alcohol and methanol. For tectona petioles in isopropyl alcohol, peaks at 1637 cm⁻¹, 1346 cm⁻¹, 1077 cm⁻¹, and 576 cm⁻¹ correspond to −C=C−stretching, −OH, C−O−C, and −COOH groups, respectively. Methanol extraction exhibits peaks at 1636 cm⁻¹, 1456–1385 cm⁻¹, 1154–1077 cm⁻¹, and 613 cm⁻¹, indicating − C=C−, −C−C−aromatic stretches, C−O−C stretching, and C–H bonds (Seithtanabutara et al., 2023). The gambier branch shows alkane C–H stretching at 2927 cm⁻¹, C=O or carbonyl stretching at 1629 cm⁻¹, C−O elongation at 1461 cm⁻¹ and 1380 cm⁻¹, and C–O vibrations at 1265 cm⁻¹. Peaks at 1145 cm⁻¹, 1072 cm⁻¹, 919 cm⁻¹, and 621 cm⁻¹ indicate C−O−C stretching and C–H bonds (Gómez-Ortíz et al., 2010; Rather et al., 2020; Seithtanabutara et al., 2023). For cinnamon extracted with isopropyl alcohol, peaks at 1621 cm⁻¹, 1446–1056 cm⁻¹, and 977–628 cm⁻¹ represent C=O, C−O, and C−H groups (Hirose et al., 2019).

The use of Zamzam water for extraction was tested on gambier branches, ajwa dates, and sukkari dates. Peaks for gambier branches include 2099 cm⁻¹, corresponding to carbonyl stretching at 1636 cm⁻¹, C−O elongation at 1461 cm⁻¹, C−O−C stretching at 1145 cm⁻¹, and C–H at 565 cm⁻¹. For sukkari dates, strong − OH stretching peaks at 3904–3841 cm⁻¹ indicate anthocyanin compounds (Sampaio et al., 2019). Ajwa and sukkari dates show peaks at 2938 cm⁻¹, 2106 cm⁻¹, 1636 cm⁻¹, 1411–1059 cm⁻¹, and 919–591 cm⁻¹, corresponding to alkane C–H stretching, −C=C−stretching, C=O, C−O, and C−H groups (Gómez-Ortíz et al., 2010; Rather et al., 2020; Sampaio et al., 2019; Seithtanabutara et al., 2023).

It is worth noting that each natural dye exhibits a distinct compound profile, with transmittance values serving as indicative markers of predominant functional groups. The presence of carbonyl and hydroxyl groups significantly impacts the adherence of these dyes to semiconducting TiO₂ photoanodes, making them promising candidates for photosensitizers (Seithtanabutara et al., 2023). Strong anchoring between functionalized dye groups and TiO₂ is essential for efficient electron injection from the dye’s lowest energy levels into the TiO₂ conduction band, ultimately contributing to high-efficiency DSSCs (Rajaramanan et al., 2023).

3.2 Dye-Sensitizer solar cell performances

The next step of this study involved evaluating the performance of DSSC extracted from various natural photosensitizers. We fabricated twelve DSSC devices, each with an active area of 1 cm² and immersed them in different natural dye extracts using a range of solvents. The devices were initially tested under the direct sunlight as illustrated in Fig. 1a. However, due to unpredictable weather conditions, we switched the measurement setup by using 30 W lamp, calibrated to an intensity of 10 mW/cm² as illustrated in Fig. 1a. An I-V circuit was connected to enable accurate measurements.

Fig. 2d displays the key parameter such as Jmax, Vmax, Jsc, Voc, and FF, which are obtained directly. The measurements were recorded and calculated manually using two multimeters and a potentiometer. This manual approach facilitated straightforward calculation of the open-circuit voltage (Voc) and short-circuit current density (Jsc). Voc is defined as the voltage across the device when no current is flowing, while Jsc represents the current density when no external voltage is applied. The maximum power output (Pmax) and fill factor (FF) were subsequently calculated using Eq. (1).

Gambier leaves extracted from five different solvents: Our initial study focused on extracting Gambier leaves using five different solvents and evaluating their effectiveness as photosensitizers in DSSC devices. Notably, the highest efficiency of 4.71 %, with a Jsc of 2.95 mA/cm² and a Voc of 0.64 V, was achieved with the photosensitizer extracted using aquadest, as shown in Fig. 6a and Table 1. This result aligns with our FTIR transmittance analysis, which revealed that aquadest extraction led to a strong presence of C=C stretching, −C=C−, and −C−C aromatic stretching, indicating a higher extraction yield of flavonoid/tannin compounds (Rather et al., 2020). Interestingly, the consistency of Voc values across solvents such as alcohol, aquadest, and zamzam suggests that solvent choice has a minimal impact on Voc, likely due to their similar absorbance peak ranges. This observation is in line with previous literature findings (Grätzel, 2003). It is important to note that the choice of solvent can significantly influence other performance parameters and should be carefully considered during optimization.

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Fig. 6

Performance curves of DSSCs with photoanode sensitized using extract from various natural materials and different solvents. (a) Gambier leaves, (b) Gambier branches, (c) Petiole of tectona leaves, (d) Cinnamon, and (e-f) Date fruits.

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The second-highest efficiency was obtained with the photosensitizer extracted in isopropyl alcohol, yielding an efficiency of 3.15 %, with a Jsc of 1.69 mA/cm² and a Voc of 0.48 V. Our analysis, supported by UV–Vis and FTIR data, indicates that the combination of flavonoid and chlorophyll compounds in this extract may contribute to the enhanced performance of the DSSCs. This transition between flavonoid and chlorophyll components highlights the complex interplay between different photosensitizer compounds and their effects on device performance (Rajaramanan et al., 2023; Seithtanabutara et al., 2023).

Isopropyl alcohol and methanol solvents: The second study investigated the performance of DSSCs sensitized with natural extracts from cinnamon, gambier branch, and petioles of tectona leaves, extracted in isopropyl alcohol (IPA) and methanol solvents. The devices exhibited promising efficiencies, with recorded values above 1 %, demonstrating the potential of these natural extracts as viable photosensitizers for DSSCs, as shown in Fig. 6b-e and Table 2. Particularly noteworthy were the efficiencies achieved with the gambier branch extracted in IPA and petioles of tectona leaves in methanol, reaching 3.46 % (with Jsc = 1.70 mAcm⁻² and Voc = 0.58 V) and 3.31 % (with Jsc = 1.40 mAcm⁻² and Voc = 0.63 V), respectively. These results underscore the efficacy of natural extracts as viable alternatives to traditional synthetic dyes in DSSCs. The superior performance of DSSCs sensitized with gambier branch extracted in IPA can be attributed to several factors. Spectral analysis revealed a high peak in absorbance spectra at 666 nm, signifying robust light absorption. Additionally, the presence of alkane C–H stretching at 2927 cm⁻¹ suggests a synergistic interaction between flavonoid and chlorophyll compounds. These findings are consistent with prior studies (Grätzel, 2003; Rajaramanan et al., 2023; Rather et al., 2020; Seithtanabutara et al., 2023). Similarly, DSSCs sensitized with petioles of tectona leaves in methanol also demonstrated notable efficiency, highlighting the potential of this natural extract as a photosensitizer. The observed performance may be attributed to the unique chemical composition of tectona leaves, which may enhance efficient light absorption and charge transfer processes.

Zamzam water solvent: Our next study investigated the utilization of Zamzam water as a solvent for natural photosensitizers extracted from date fruits and gambier branches in DSSCs. Remarkably, the gambier branch extract in Zamzam water yielded the highest recorded conversion efficiency in this series, achieving an η of 3.85 %, a Jsc of 1.89 mA/cm², and a Voc of 0.53 V, as shown in Table 3. This result is particularly noteworthy as it marks the first successful application of Zamzam water as a solvent in DSSCs. The promising performance observed suggests that Zamzam water could be a valuable candidate for further investigation, especially regarding its potential effects on DSSC performance. The unique properties of Zamzam water, which is revered in Islamic tradition and believed to possess beneficial qualities, make it an intriguing solvent for DSSC technology. The high conversion efficiency achieved with the gambier branch extract in Zamzam water highlights the critical role of solvent choice in optimizing DSSC performance. The enhanced light absorption and charge transfer efficiency observed may be attributed to specific interactions between the natural photosensitizers and the unique composition of Zamzam water. These results underscore the need for further studies to better understand the underlying mechanisms and to refine the use of Zamzam water in DSSC fabrication.

From Tables 1 to 3, it is evident that the fill factor (FF) of all-natural photosensitizers is inconsistent. This variability is likely due to the complex compositions and unstable carbonyl and hydroxyl functional groups present in natural dyes. Such factors may lead to inconsistent dye adsorption on the TiO₂ surface, resulting in weak adhesion, poor electron transfer, and increased recombination losses, ultimately impacting the FF negatively (Mishra et al., 2009; Seithtanabutara et al., 2023).

Overall, our DSSCs utilizing natural photosensitizers demonstrate superior performance compared to those reported in previous studies (Al-Alwani et al., 2016; Chang et al., 2010; Chumwangwapee et al., 2023; Conrad-Fletemeyer et al., 2023; Cruz et al., 2022; Ghann et al., 2017; Gómez-Ortíz et al., 2010; Hendi et al., 2023; Khammee et al., 2023; Nan et al., 2017; Patunrengi et al., 2019; Prakash et al., 2023; Rajaramanan et al., 2023; Seithtanabutara et al., 2023; Suyitno et al., 2018). The highest efficiencies achieved in our work were 4.71 % with gambier leaves, 3.85 % with gambier branches, 3.31 % with Tectona petioles, and 3.12 % with Ajwa dates, highlighting the significant potential of these natural photosensitizers for further exploration in DSSC applications. Additionally, we emphasize the critical role of solvent selection, as it directly influences the interaction between dye molecular configurations and solvents, thereby affecting the optical and structural properties of the dyes. Notably, the novel use of Zamzam water as a solvent in DSSC technology points to alternative and sustainable approaches for enhancing solar energy conversion. Future research should delve deeper into the mechanisms behind the performance enhancements observed with mixtures of different dyes, as these insights could be key to advancing more efficient and sustainable solar cell technologies.