Effects of acidification of clove fruit dye extracted in water and ethanol for performance enhancement of DSSCs

Dye extracted from flesh of Clove fruit (Syzgium aromaticum) was used as the sensitizer in Dye Sensitized Solar Cells (DSSCs) of the configuration TiO 2 /dye/electrolyte/Pt. Pigments of Clove fruit was extracted by soaking in distilled water and ethanol and again boiling in both the solutions. Since DSSCs made using dye extraction in ethanol produced higher efficiency than in water, glacial acetic acid was added to dye extractions in ethanol at different ratios to examine any further enhancements. The Clove fruit extract absorbed appreciable solar radiation in the wavelength range 500 - 700 nm that makes it suitable for a DSSC. FTIR spectra of the dye extracts revealed the hydroxyl groups attached to dye is prominent after acidification with acetic acid that helps to anchor on TiO 2 surface. DSSC fabricated using mixture of fruit dye in ethanol and glacial acetic with the ratio of 1:3 produced the highest efficiency of 0.69 % with the photo current of 2.50 mAcm -2 and fill factor of 61.57 %. The electron life time calculated from bode phase plots of the cells also supported the above observations. The enhancement with acetic acid treatment of dye is attributed to intensification of absorption of visible light and strong coupling of the dye with TiO 2 due to the presence of anchoring groups in acidic form is evidence from UV visible and FTIR spectroscopy.


INTRODUCTION
Currently available solar cells compose of single-crystalline, polycrystalline and amorphous forms of inorganic and organic materials. Foremost popular solar cells are made of silicon and their success is due to moderate efficiency and durability relative to organic solar cells.
Dye Sensitized Solar Cell (DSSC) is a kind of solar cell with both organic and inorganic hybrid. The manufacturing cost of DSSC is however, lower than that of silicon based solar cells. A DSSC is a device fabricated using a high-band gap semiconductor to transform light into electrical energy by sensitization of an organic dye [1]. Due to the advantage of lower production costs and simple and easy development process, DSSCs are of considerable concern. They can be assembled at room temperature in normal environments, and their cost is just about 1/10 th of the conventional solar cell. This study was intended to investigate the performance of DSSC sensitized with a dye extracted from Clove fruit into water and alcohol and also to examine the performance of DSSCs by acidification of those extracts with acetic acid.

Preparation of dye solutions
30 g of Clove (Syzygium aromaticum) fruit fresh was ground in an electric blender and dye was extracted by soaking in 100 ml of distilled water and ethanol for 10 minutes. Dye was also extracted by boiling aforementioned bend in distilled water and alcohol at 100 0 C and 80 0 C respectively for 10 minutes. Dye solutions were separated by filtration and the filtrates were stored in a dark coloured bottle in a refrigerator until use.

Preparation of TiO2 films
Fluorine-doped Tin Oxides (FTO) plates were cut into the size of 1 cm × 2 cm. The glass plates were cleaned respectively with tap water, detergent and distilled water by using ultrasonic cleaner for 5 minutes in each case. Washed glass plates were boiled in ethanol, in a beaker on a hot plate at 80 ˚C for 15 minutes. 0.5 g of P25 Degussa Titanium dioxide (TiO2) powder was mixed with 0.1 g of citric acid in a mortar using a pestle with 7 ml ethanol, one drop of triton-X-100 and one drop of PEG-400. The mixture was ground for 15 minutes to obtain a uniform paste. The prepared TiO2 paste was coated on cleaned FTO glass plates by using the doctor blade technique and the area of the TiO2 films on FTO was 1 cm 2 .
Finally, the TiO2 plates were heated in a furnace at 450 ˚C for 60 minutes.

Preparation of dye coated photo-anodes
Dye solutions of 2 ml that extracted by soaking Clove fruit in distilled water and alcohol and also boiling in those two solvents were taken into four test tubes. Another 2 ml set of solutions were prepared by adding previously extracted dye in ethanol with glacial acetic acid at different ratios according to volume ratios of 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, 1:5.
Afterwards TiO2 films on FTO glass were soaked separately in above dye solutions. Each sample was dipped in the dye solutions for 30 minutes and washed with distilled water or alcohol accordingly. Afterwards TiO2 films were dried carefully using a hair drier.

Assembling of the DSSC and characterization
The conducting side of the platinum counter electrode and dye absorbed TiO2 film were sandwiched and held with clamps. The capillary space between two electrodes was filled with an electrolyte which was prepared by using 0.83 g of potassium iodide (KI) and 0.127 g of iodine (I2) added into a solution containing acetonitrile and ethylene carbonate at 8:2 ratio. After that the assembled cells were characterized for photovoltaic measurements under the irradiance of 1000 Wm -2 illumination using a computer coupled galvanostat/potentiostat and ScienceWorkshop 750 Interface. UV-Visible spectra of extractions and acidified dyes were measured by using Multiskan Sky Microplate Spectrophotometer for optical characterizations. Infrared spectra of dye samples were obtained by using Bruker Tensor 27 FTIR spectrometer. Electrochemical Impedance Spectroscopy (IES) was measured by using galvanostat/potentiostat Metrohm Autolab PGSTA T204. Impedance measurements were performed at frequency range of 100 mHz to 100 kHz.

UV Visible spectra of Dyes
The absorption peak can be explained by the chemical structure and color of the dye.
Physically, it can be explained by the difference between energy levels of the materials.
Generally, all the dyes cannot be used for DSSCs, nevertheless they need to matches with the bands of the semiconductor to produce photocurrent when illuminated.    which have high absorption, are responsible for higher short circuit photocurrent densities [3]. This could happen because photon-to-current efficiency depends on the light absorption of the dye and the interaction between the dye and the TiO2. Furthermore, hydroxyl groups facilitate the strong chelation between the dye skeleton andTiO2 surface for efficient transfer of electron.   (fmid), the electron lifetime in the conduction band can be determined [4]. Therefore, the lifetime of the electron (τe) was determined using the following relationship,   The pH values of dye extracted to distilled water and ethanol are 4.86 and 6.15 respectively.

EIS Characterization of DSSCs
Despite the low pH of dye extracted to water, dye extracted to ethanol readily adsorb on TiO2 because of the lower polarity of ethanol than water. The adsorption of the dye is strongly depended on the polarity of the solution. Therefore, the recombination of electrons in DSSC prepared using dye extracted to ethanol is lower than the dye extracted to distilled water due to better adsorption of the dye. As a result, electron life time is higher in the DSSC prepared by dye extracted in ethanol. Subsequently, alteration of the pH of the dye extracted to ethanol also stimulated the electron lifetime as evident from table 02 where the highest life time was recorded for 1:3 volume ratio of dye in ethanol to acetic acid.

CONCLUSION
A DSSC with the configuration, TiO2/dye/electrolyte/Pt was fabricated using Clove fruit dye as the sensitizer. Photovoltaic performance was high when the extract was in ethanol than in water where the adsorption of the pigments is strongly determined by the polarity of the solvent. After acidification, the light absorption at 510 -520 nm and 650 -700 nm ranges in UV-visible spectra attributes to high current density and efficiency of those DSSCs. The most suitable ratio of clove fruit dye and acetic acid was found as 1:3 to obtain the highest efficiency of the DSSCs. The electron life time calculated from bode phase plots also supported the above observations. Acidic form of the dye intensifies the light absorption and also the functionalization of the dye with hydroxyl groups facilitate strong coupling of the dye with TiO2. Therefore, both the effects simultaneously contribute for these enhancements.