Characterization of Flower Based Organic Dyes and Titanium Dioxide Thin Film for Dye Sensitized Solar Cell

In the present study, we described the organic dye extraction process from local flowers employing different methods and solvents (a mixture of acetic acid and ethanol, another is only acetic acid); especially we completed our research by exploring dye from red gerbera flower as it showed higher absorbance. Both dry and fresh petals of the flower were used for extracting dyes. The results revealed that the mixture-based dye extraction from dry red gerbera yielded a higher absorbance value of 4.579 a.u., whereas the acetic acid-based extraction from the same source exhibited a lower absorbance value of 2.778 a.u. The choice of solvent for dye extraction was found to significantly influence the absorption spectrum. Additionally, the Beer-Lambert law was employed to analyze the relationship between the absorbance and the concentration of the extracted dyes. For the electrode, a TiO2 film was created using the spin coating technique. From the XRD graph, we found the band gap energy of TiO2 which is 2.81eV (annealed at 450°C), the type of nanoparticle of the thin film. Also found a Crystallite size of 32.95nm and a specific Surface Area (SSA) of 43.048 m2.g−1, Microstrain (ε) of 5e-3 and Dislocation Density (δ) of 9.2e-4cm−2.


Introduction
Global warming is the most concerning issue of this modern age.The main reason for global warming is greenhouse gases such as carbon dioxide, Sulphur dioxide, and vapor.These greenhouse gases are emitted by burning fossil fuels to generate electricity and run industries, and automobiles.It is related to our energy consumption rate.After a few decades, these sources of energy will be finished.So, it is high time to find an alternative source of energy like renewable energy.Solar energy is expected to be a good alternative for future renewable energy sources.Hourly provided energy by the sun is larger than annual Global energy consumption [1].However, it is a big challenge to capture solar energy and convert it into electrical or chemical energy at a low cost.At present crystalline silicon-based photovoltaic cells are widely used to convert solar energy into electrical energy [2].Multicrystalline silicon solar cells have conversion energy above 20% [3].There are several types of solar cells; crystalline solar cells which are most common all over the world.Another solar cell is thin film solar cells which have high production, material, and purification costs.Thin film solar cells are made by the layers of various semiconductor materials such as cadmium telluride (CdTe) [4], copper indium gallium di-selenide (CIGS) [5], amorphous silicon [6].Less amount of materials are required to 1305 (2024) 012036 IOP Publishing doi:10.1088/1757-899X/1305/1/012036 2 manufacture thin film solar cells but the materials of multilayer complex thin film solar cells are expensive and limited [7].Recently nanotechnology has been involved in the field of photovoltaic energy conversion.By using nanotechnology, we can make cells on a nanometer scale and generate new materials for solar cells that can be used in large amounts and at low cost in the future [8].Synthetic organic materials [9], inorganic nanoparticles [10], and non-particle systems [11] are included in these types of materials, and solar cells made of these materials are called organic solar cells.At present the efficiencies of organic solar cells are less than inorganic solar cells [12] [13].An alternative solar cell technology was invented by Professor Gratzel in 1991 dye sensitized solar cell (DSSC) is the subject of the present work [14].It is an organic material-based photovoltaic cell.A dye-sensitized semiconductor material presents there where dye molecules are anchored onto the surface of the semiconductor material which absorbs sunlight.Usually, titanium dioxide (TiO 2 ) is widely used as a semiconductor material.As titanium dioxide absorbs light in the ultra-violate (UV) range so, the efficiency of solar cells based on titanium dioxide is very low than silicon-based solar cells [15].Though it is not commercialized yet, it has a bright future as its production cost is low and has a huge space for development [16].The dye-sensitized solar cell that has the highest efficiency is a ruthenium-based complex sensitizer solar cell [17].On the other hand, organic dyes like natural dyes with the same characteristics (high absorption coefficient, efficiency) can be the alternative solution.Natural dyes are used for their easy availability and low-cost production [18].Natural dyes are found in plants body, leaves, flowers, and fruits.They are mostly found as anthocyanin, carotene, tannin, and chlorophyll in plants [19].These constituent molecules are in the form of carbonyl and hydroxyl.group occurs naturally in fruit, leaf, and flowers and is responsible for the exhibition of the types of color observed in the visible red-to-blue spectrum [20] [21] [22] [23].Also, it can be used in devices we use daily such as calculators [24].The study focuses on dye characterizations that are extracted from some common flowers and the manufacture of efficient titanium dioxide-based thin films.Natural dyes are extracted from some common flowers that are available locally.Besides, polyethylene glycol (PEG) is used with TiO 2 to make a layer of thin film.PEG is a synthetic polymer that has benefits in this area of producing efficient DSSCs.It has stable properties and soluble in water and organic solvents which helps to make a homogeneous solution with TiO 2 and helps TiO 2 particles into a uniform thin film on the substrate surface.We can control the thickness of thin film.It also can bind dyes which increases the light absorbing area and leads to increased overall efficiency of DSSCs [25] [26] [27].The objectives of this research work are to find out natural dye with high absorbance and to find characteristics of TiO 2 thin film.

Source of Natural Dyes
We collected different types of flowers such as roses and two types of gerberas.However, we conducted further research with red gerbera as it has shown higher absorbance than others.Each time we took 2.16gm gerbera flower petals to make natural dyes.

Materials and Equipment
The materials used for our experiment: Other apparatuses were used such as a petri dish, beaker, porcelain mortar and pestle, electronic weighing balance, filter paper, and hot plate.
The equipment used for characterization is listed below:

Mixture preparation:
For dye extraction, we used two solvents.One was acetic acid and another was a mixture of ethanol and acetic acid.The required solution was made with the same amount of acetic acid and ethanol with the help of a magnetic stirrer.

Dye from fresh flower:
We took the fresh flower in two beakers in equal amounts and mixed them with 10ml acetic acid and mixture.We kept the beakers in a dark place for two days as light could not sensitize them.After two days we separated the dye using the filter paper and kept it in the dark place for further experiments.
Fresh Flower + Acetic Acid → filter → stored in a dark place.
Fresh Flower + mixture (Acetic Acid + Ethanol) → filter → stored in a dark place.

Dye from dry flower:
For this process, we took the same amount of fresh flower samples as before.Then cut the flower petals into small pieces and dehydrate them at 45ᵒC in the oven for two days.After drying, we took samples in two beakers in equal amounts and mixed them with 10ml acetic acid and mixture.We kept the beakers in a dark place for two days as light could not sensitize them.After two days we separated the dye using filter paper and kept it in the dark place for further experiments.
Dry Flower + Acetic Acid → filter → stored in a dark place.
Dry Flower + mixture (Acetic Acid + Ethanol) → filter → stored in dark place

Results and discussion
By the methodology described in the preceding section, samples were prepared and then underwent various analyses and characterization methods.The details of these analyses and characterization techniques will be elaborated upon in this.The graphical representation of the spectrometer of fresh red gerbera dye extracted with acetic acid is shown in Figure 1(a), which represents the absorbance versus wavelength curves at different concentrations (50%, 25%, 12.5%, and 6.25% respectively) of fresh red gerbera dye extracted with acetic acid.The absorbance and noise of the dye decrease with the concentration.Dye has followed the bare-lambert law shown in Figure 2(b).Noise has been found only at 50% concentration for this dye.This dye absorbs a wide range of sunlight from 350nm to 600nm and the maximum light spectrum 4.077a.u. has been found at 510.08nm wavelength for 50% concentration.The absorbance of dry red gerbera dye extracted with acetic acid has been shown in Figure 2(a).In Figure 2(b) absorbance increases with increasing dye concentration.No noise has been found for this dye.A maximum light spectrum of 2.778a.u. has been found at 509.65nm wavelength for 50% concentration with less than the absorbance of fresh red gerbera dye extracted with acetic acid.Fresh red gerbera dye also has been extracted with a mixture of acetic acid and ethanol.Figure 3(a) shows the absorbance data of the dye.Absorbance increases with increasing dye concentration shown in Figure 3(b).Some noise has been found at the peak for 50% concentration of this dye.Maximum light spectrum 3.718a.u. has been found at 511.36nm wavelength for 50% concentration is higher than the absorbance of fresh red gerbera dye extracted with acetic acid.The absorbance of dry red gerbera dye extracted with the mixture of acetic acid and ethanol is shown in Figure 4(a).A nonlinear property has been observed in Figure 4(b), as its absorbance increases with increasing dye concentration but absorbance does not increase linearly.At 50% and 25% concentration some noises have been found in the curves.Maximum light spectrum 4.579a.u. has been found at 510.51nm wavelength for 50% concentration which is higher than the absorbance of the previous three gerbera dyes.UV-Vis Spectrometer analysis has been reported in Figure 5, absorbance 4.579 a.u.measured at 510nm for dry red gerbera dye extracted with mixture solvent.On the other hand, dry red gerbera dye extracted with acetic acid has shown a lower absorbance of 2.778 a.u. at 509 nm.The other two fresh dyes extracted with acetic acid and mixture have achieved an absorbance of 4.077 a.u. at 510nm and 3.718 a.u. at 511nm.After dye adsorption, the absorbance of remaining dyes has been measured.After comparing the absorbance of dyes, significant changes have been noticed (Figure 6).The absorbance of the dyes is decreased after the dye adsorption process.The absorbance of red dry gerbera extracted by the mixture has shown absorbance of 4.579 a.u. and after the adsorption process, the absorbance decreases at 3.321 a.u. and for the fresh red gerbera dye extracted by acetic acid, absorbance decreases from 2.778 a.u. to 2.389 a.u.Among the dyes, the absorbance of dry red gerbera dye with the mixture decreased extremely after the adsorption process than other dyes.That means, molecules of this dye attached in a higher amount on the surface of TiO 2 film than other dyes.So that, much sunlight can be absorbed by TiO 2 film.

Calculating the band gap of TiO 2
The Tauc plot method is used to find the bandgap of the film [28][29]. (ℎ) Here,  is the absorption coefficient,   energy bandgap,  frequency of radiation, ℎ Plank constant and A is a constant.For different types of transition materials, the value of n is different.As TiO 2 is a direct bandgap material, we take 2 as the value of n.The absorbance of TiO 2 film and dye adsorbed TiO 2 film have been measured by UV-Vis spectrometer and the results are shown in graphical form in figure 7.After dye adsorption the absorbance of the film has increased and the film can absorb spectrum from 300nm to 1200nm wavelength.To determine the bandgap (ℎ) 2 versus ℎ has been plotted.Bandgaps have been found for different annealing temperatures and both spin coating and doctor blade methods in Figure 8 and Figure 9.In Figure 8, the bandgap of TiO 2 which was annealed at 500°C and made by spin coating method is shown.In this process, bandgap of only the TiO 2 substrate is 2.81eV and the substrate adsorbed with dye is 2.7eV.In Figure 9, the TiO 2 substrate has been made by the doctor blade method and annealed at 450°C.The Bandgap of only the TiO 2 substrate is 2.90eV and the substrate adsorbed with dye is 2.81eV.At this time, TiO 2 shows a wider range of bandgap than the first curve and it is closer to the ideal bandgap of 3.2eV of TiO 2 .Component Variations in phase difference between the waves scattered by grains have been caused by different positions and orientations of the grains.Individual intensity of scattering of each grain causes the total intensity of scattering of all grains.On the other hand, lattice strain has been created by varying displacement of atoms regarding their reference potions.This misfit of lattice originates the microstrain (ε).For the highest intensity of XRD peak in Figure 10 and Figure 11, the microstrains are 5e-3 and 3.6e-3 of the TiO 2 thin films annealed at 450°C (doctor blade process) and 500°C (spin coating process) respectively.Dislocations are the defects or imperfections in a crystal lattice.It is no equilibrium imperfections.The dislocation density of thin films is calculated by Williamson and Smallman's relation [31].9.2e-4 cm -2 and 5.8e-4 cm -2 dislocation densities are found shown in table 1.Higher dislocation density has been found for 500°C annealed temperature.From Figure 12, we can see that there is a relation between crystallite size and temperature.Crystallite size increases as temperature increases.We can see at 500°C we get a higher crystallite size of 41.32nm (Table 1).On the other hand, full-width half maximum () is inversely proportional to crystallite size.Agaibeta relates to intensity.If the intensity is high, then the curve width will be narrow, the peak will be sharp and the FWHM will decrease.
Then again, for the low intensity, the peak will be flat and the FWHM become higher [30].So, the XRD graph with higher crystallite size (annealed at 500°C) has shown a narrower and lower FWHM width of 0.2068 than the XRD graph annealed at 450°C with less crystalline size of 32.95nm (figure 12. and Table 1).

Conclusion
Characteristics of red gerbera-based dye have been analyzed.The structural and optical properties of TiO 2 thin films are prepared from the doctor blade and spin coating processes have been studied elaborately in this work.Among various samples, dry red gerbera extracted with the mixture of acetic acid and ethanol exhibits a high absorbance of 4.579a.u. at 50% concentration for 510nm wavelength.Films were found highly crystalline in nature with crystallite size, microstrain, dislocation density, and specific surface area of 32.95nm, 5e-3, 9.2e-4, and 43.048m 2 g -1 respectively.SSA dramatically increases with decreasing crystallite size.A good number of XRD peaks are found for 2θ = 25.36°,27.85°, and 37.88° in the doctor blade process (annealed at 450°C).However, the peak height along the (110) plane is higher for the films grown by the doctor blade process than the spin coating process.

Figure 1 .
Figure 1.(a) Absorbance of Fresh Dye of Red Gerbera extracted with acetic acid at different concentrations; (b) Absorbance increases with increasing concentration (Bear-Lambert Law)

Figure 2 .
Figure 2. (a) Absorbance of Dry Dye of Red Gerbera extracted with acetic acid at different concentrations; (b) Absorbance increases with increasing concentration (Bear-Lambert Law)

Figure 3 .Figure 4 .
Figure 3. (a) Absorbance of Fresh Dye of Red Gerbera extracted with mixture solvent at different concentrations; (b) Absorbance increases with increasing concentration (Bear-Lambert Law)

Figure 5 .
Figure 5. Maximum absorbance of fresh and dry red gerbera with acetic acid and mixture solvent

Figure 6 .
Figure 6.Comparison of maximum absorbance of dye before and after dye adsorption onto the TiO 2 surface

Figure 7 .
Figure 7.Comparison of absorbance of TiO 2 film before and after the adsorption

Figure 12 .
Figure 12.XRD analysis of thin film annealed at 450°C and 500°C

Table 1 .
Comparison of TiO 2 thin film properties annealed at two different temperatures.