Dye-sensitized solar cells using natural dye as light-harvesting materials extracted from Acanthus sennii chiovenda flower and Euphorbia cotinifolia leaf

Abstract Natural dyes are environmentally and economically superior to ruthenium-based dyes because they are nontoxic and cheap. In this study, dye-sensitized solar cells (DSSCs) were fabricated using natural dyes light harvesting materials. The natural dyes were extracted from Acanthus sennii chiovenda flower and Euphorbia cotinifolia leaf. In the as-prepared DSSC, a quasi-solid state electrolyte was sandwiched between the working electrode (photoanode) and counter electrode (PEDOT-coated FTO glass). The photoelectrochemical performance of the as-prepared quasi-solid state DSSCs showed open-circuit voltages (V OC ) varied from 0.475 to 0.507 V, the short-circuit current densities (J SC ) ranged from 0.352 to 0.642 mA cm −2 and the fill factors (FF) varied from 47 to 60% at 100 mWcm −2 light intensity. The dye extracted from A. sennii chiovenda flower, using acidified ethanol (in 1% HCl) as extracting solvent, exhibited best conversion efficiency with a maximum open-circuit voltage (V OC ) of 0.507 V, short-circuit current density (J SC ) of 0.491 mA cm −2 , fill factor (FF) of 0.60 and an overall conversion efficiency (η) of 0.15%. On the other hand, the maximum power conversion efficiency of the dye extracted from E. cotinifolia leaf was 0.136%. This is the first study that reports the fabrication of DSSC using natural dye sensitizers extracted from these plants in the presence of quasi-solid state electrolyte and PEDOT as a counter electrode.


Introduction
Dye sensitized solar cell (DSSC) is a promising alternative to conventional silicon solar cells, which effectively utilizes a property of nanocrystalline wide band gap semiconductor (metal M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 2 oxide) porous electrode [1]. It provides a prime attention as an alternative source of clean and green energy due to its several advantages, such as low price to performance ratio, low processing cost, and low intensities of incident light, mechanical robustness, light weight and aesthetically appealing transparent design [2,3]. A DSSC consists of a fluorine-doped SnO 2 (FTO) layer, a nanocrystalline wide band gap metal oxide semiconductor porous electrode, dyes, an electrolyte, and a counter electrode [4,5] as shown in scheme 1. In the assembly of DSSC, the dye plays an important role in harvesting solar energy and converting it to electrical energy with the aid of a semiconducting photoanode [1,6,7]. Therefore, the cell performance is mainly dependent on the type of dyes used as a sensitizer [8]. Many metal complexes and organic dyes [1,6,9] have been synthesized and used as sensitizers. Ruthenium based complexes are considered as good sensitizers for DSSCs because their intense charge transfer absorption over the entire visible range and highly efficient metal to ligand charge transfer [10]. These complex dyes are capable of delivering DSSCs with high conversion efficiency [11] as compared to natural DSSCs [1,12,13]. On the other hand, natural dyes have several advantages over rare metal complexes (ruthenium based complexes) because ease of extraction with minimal chemical procedures, large absorption coefficients, low cost, non-toxicity, environmentally friendly, easily biodegradable and wide availability [8,[15][16][17]. Moreover, synthetic organic dyes have been fraught with problems, such as complicated synthetic routes and low yields [14]. Thus, several dyes extracted from natural pigments including anthocyanins, carotenoids and chlorophylls have been used as sensitizers in DSSC [12,[16][17][18][19]. Chlorophyll is the well-known and dominant natural pigment in terms of absorbing specific wavelengths of the visible light, converting sunlight to chemical energy [20]. The common types of chlorophyll are "chlorophyll a" present in all photosynthetic plants and "chlorophyll b" found widely in higher plants and algae [19]. It possesses a common basic structure that is a porphyrin structure consisting of four pyrrole rings [19]. The presence of magnesium ion in the center is the unique feature of the chlorophyll structure and it plays an important role in the absorption of light energy. Chlorophyll in its raw form is not an efficient sensitizer for DSSC applications because lack of binding sites to TiO 2 [21]. Hence different approaches have been adopted to improve the photoelectrochemical performances [19]. Anthocyanin is one of such flavonoid compound present in many fruits, flowers, leaves and is responsible for the red, violet and blue colors [1,19]. The advantage of anthocyanin is the binding of carbonyl and hydroxyl groups of the chlorophyll to the surface of a M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 3 porous TiO 2 film. This helps excess for anchoring the dye on the surface of TiO 2 film and also provides easy electron transfer from the anthocyanin molecule to the conduction band of TiO 2 [1].
In this study, DSSCs were assembled using locally available natural dyes extracted from Acanthus sennii chiov. flower and Euphorbia cotinifolia leaf using a very simple extraction technique. The optical properties of the extracted dyes were characterized by UV-Vis absorption spectroscopy. Those dyes that have the best optical properties were used as a light harvesting material for the construction of DSSC. To the best of our knowledge, no report has been shown for dyes extracted from these plants for DSSC application using quasi-solid state electrolyte and PEDOT as a counter electrode.
The blue-doped PEDOT film on FTO glass has a potential application for electrodes and has been used as an alternative to platinum due to its good conductivity, remarkable stability and a comparatively lower price than platinum [22]. A quasi-solid state electrolyte is a particular state of matter, neither liquid nor solid, or conversely both liquid and solid [23]. Because of the unique network structure of polymers, quasi-solid state electrolytes show better long-term stability, higher electrical conductivity, and better interfacial contact when compared to liquid electrolytes [24,25]. The conductivity of the quasi-solid state electrolytes depends on the molecular weight and the morphology of the polymer because of the higher mobility of charges in the amorphous phase of polymers when compared to the crystalline phase. The photoelectrochemical performances of the as prepared DSSCs assembled from these dyes were measured.
A schematic representation of the as prepared DSSC is shown in scheme 1. The working principles or operating mechanisms of the as prepared DSSC begin with illumination of light energy. The following are the key operating steps of a typical DSSC. vi. The recombination of the injected electrons with the oxidized mediator (tri-iodide, I 3 -) at the interface between TiO 2 or FTO and electrolyte before the electron has been collected and passed through the load and reached to the counter electrode [26]. This process limits the efficiency of the DSSC.
vii. The recombination of the injected electron with the oxidized dye [27] at the interface between the dyes and TiO 2 .This is also another limiting reaction for the performance of DSSC.
The as prepared natural dye DSSC represented in scheme 1 uses the aforementioned processes/mechanism to convert solar energy in to electrical energy.

Preparation of natural dye sensitizers: Acanthus sennii chiov. flower and
Euphorbia cotinifolia leaf were collected from Bahir Dar city of Ethiopia and these samples were washed with pipe water to get ride off the dust particles. After washing, a sample was dried in the laboratory room in dark place for four weeks at room temperature. After that, the samples

Preparation of photoanode:
The TiO 2 film on the FTO glass was prepared using a doctor blade technique [28]. The mesoporous titanium dioxide (TiO 2 ) paste was prepared with similar methods described elsewhere [29]. active area for dye absorption. Titanium dioxide paste was deposited on the FTO glass between the two pieces of tape and was coated by "doctor blade" method (i.e. sliding a paste with a glass rod on the substrate) to spread the paste across the plate. This process was continued until the layer became homogenous. After the film dried at room temperature, the tape was removed carefully without scratching the TiO 2 coating. The as prepared TiO 2 film was sintered at 450°C for 30 minutes to enhance the film compactness and crystallinity. After sintering, the films were allowed to cool naturally and immersed into the extracted dyes for about 24 hrs until the TiO 2 film covered with the dyes. After the dye adsorbed, the film was taken out of the dye solution and was rinsed with ethanol to remove unabsorbed dye and any other residues available on the surface. Finally, it was dried with an air gun and ready to combine with the counter electrode for DSSCs device preparation. properties of dyes was also studied.

Photoelectrochemical Measurements: Photoelectrochemical measurements were
performed by using a computer controlled CHI630A Electrochemical Analyzer. A 250-W tungsten-halogen lamp regulated by an Oriel power supply (Model 68830) was used to illuminate the as prepared DSSC. The measured photocurrent spectra were corrected for the spectral response of the lamp and the monochromatic by normalization to the response of a calibrated silicon photodiode (Hamamatsu, Model S1336-8BK) whose sensitivity spectrum was known [32]. The intensity of the incident light was 100 mW cm -2 . All experiments were carried out at ambient temperature.

Optical absorption measurements of the extracted natural dyes
The UV-Vis absorption spectra for the dye solution extracted using different solvents are shown in Fig. 1 & 2  corresponds to the absorbs region from 400 to 500 nm and 600 to 750 nm respectively [33,34].
It also absorbs very little in the green region of the spectrum from 500 to 600 nm and this also reflected in the absorbance region of anthocyanin since anthocyanin dyes absorb in the region between 500 and 600 nm [1,33]. From Fig. 1 (II), the absorption peaks are 513, 510 and 510 nm for each (different acid concentration) 1, 2 and 3% HCl respectively. These absorption peaks are closely related to the absorption peaks of anthocyanin which indicates anthocyanin is the major components of the observed pigments as reported earlier. As shown from Fig.1 (I) and (II), solutions extracted by both 1% HCl acidified ethanol and distilled water have relatively higher absorbance than solutions extracted by 2% and 3% acidified ethanol and distilled water, due to an increase in the extraction of anthocyanin using an optimal acidification of extracting solvents which leads to a suitable protonation reaction. This indicates anthocyanin pigments are highly soluble in 1% HCl for these studies. A similar finding was reported so far [35]. In this study, extraction of dyes using different acid concentrations was also studied. The dye that was extracted at low acid concentration shows a good interaction with the working electrode results the best cell efficiency compared with the one extracted with higher acid concentration. This is due to the existence of the dye in a stable form with lower acid concentration, where the ions hydrated to form bases (a stable form). The optical properties also assures that the extracted solution have a higher absorbance at lowest acid concentration compared with the highest acid concentration.  Fig.1(I) and (II) above, these absorption regions are also the main characteristics of anthocyanin pigments [1,33].
Acidification leads to a protonation reaction and the equilibrium shifts from the quinonoidal to the flavylium form, which increases the extraction of anthocyanin.

Photoelectrochemical measurements of natural DSSCs
From the current density-voltage (J-V) curves of the as prepared DSSC, the performance of DSSCs was evaluated by short circuit current density (J SC ), open circuit voltage (V OC ), fill factor (FF), and power conversion efficiency (η) [36]. The photovoltaic characteristic of DSSCs is defined as; The overall energy conversion efficiency (η) of DSSCs is calculated using Where P max is maximum output power and P in is the power of incident light.

Conclusion
In this work, natural dyes extracted from two locally available plants such as Acanthus sennii chiov. flower and Euphorbia cotinifolia leaf were used as sensitizers for DSSC. These natural dyes used as a light harvesting material were extracted using different solvents at different acid concentrations. The comparisons of different acid concentrations as an extracting solvent and its effect on the absorption spectra were investigated. The dye solutions extracted from parts of the plant material contains anthocyanin and chlorophyll. The as prepared DSSC were assembled using PEDOT-coated FTO glass as a counter electrode, natural dye anchored TiO 2 film as a photoanode, and quasi-solid state electrolyte as an electrolyte sandwiched in between the two electrodes. The photoelectrochemical performances of the as prepared DSSC were evaluated.
When chlorophyll pigments were used as a light harvesting, did not offer high conversion efficiencies, due to lack of available interaction between the dye and TiO 2 molecules resulting low loading on the surface TiO 2 films. The highest photoelectrochemical performance of the as •Natural dyes were used as a light harvesting pigments for Dye-sensitized solar cell •The effect of dye extracting solvents on the performance of solar energy conversion has been studied.
•Quasi-solid state electrolyte and PEDOT as a counter electrode has been used for DSSC assembly