Fabrication Dye Sensitized Solar Cells by Using Natural Dye Beet Root ( Beta Vulgaris ) as Photosensitizer and Nano Electrodes

In this study different types of Dye Sensitized Solar Cells (DSSCs) were fabricated using two kinds of titania (TiO2) semiconductors as photo anodes and four kinds of counter electrodes, sensitized natural dye Beta vulgaris (beetroot) dye (Betalin). And the electrolyte (I/I3 ). TiO2 semiconductors includes; Nano sized powder (deposited on Indium doped Tin oxide (ITO) glass) and anodized nanotubes template on titanium foil. The counter electrodes include; platinum, electropolymerized polyaniline (PANI), polythiophene (PTh) and polypyrrole (PPY) (also all deposited on ITO glass). The surface structure of prepared photo anodes and counter electrodes were analysis by AFM, SEM,XRD and FTIR. The absorption spectra of natural dye was investigated by UV-Visible spectroscopy which showed absorbed light at 479 nm and 536 nm. The efficiency of prepared DSSCs were estimated through I-V characterization, the fall factors (ff) and electrical conversion efficiencies (ղ %) were measured using potentiostat. The efficiencies of all DSSCs follow the order ;


Introduction
Dye-sensitized solar cells (DSSCs) is promising to global energy and environmental problems because its clean ,low cost, easy preparation, good durability and high conversion efficiency (1- 5) .DSSCs were first proposed by Grӓtzel et al (3) it is the third generation photovoltaic device for low cost conversion of solar energy into electrical energy.The principle of operation of DSSCs is based on sensitization of a wide band-gap metal oxide semiconductor to the visible light region by an adsorbed molecular dye ,when the metal oxide semiconductor is immersed in the dye, a monolayer of the dye is anchored onto its surface.Excitation of dye by sun light and the photo generated charges are separated at the interface between the dye and metal oxide.
Nano metal oxide semiconducting used in the fabrication of DSSCs (6) due to the large surface area they offer for dye anchoring (7) .The charge transport is highly affected by the crystalline quality of metal oxide ,therefore it is important to reduce the charge traps in the films to speed up charge transport (8) .
The ideal dye should strong absorb all light below a threshold wavelength of about 920 nm(broad range of wavelength and high molar extinction coefficient) ,strong binding to the semiconductor oxide surface (it carry attachment groups usually carboxylate or phosphonate, energy levels at the proper positions (LUMO high enough in energy for efficient electron injection and HOMO low enough for efficient regeneration).Stability over many years of exposure to sunlight, simple and reproducible and purification ,low cost, small reorganization energy for excited and ground state electron transfer to minimize energy losses (9) .In the attempt to find dyes satisfying these requirements, a large number of different molecules have been synthesized and characterized of new families of organic dyes and of metal complexes .The most efficient dye is found to be Ru (II) and Os(II) (10,11) .These complexes have good absorption ,long excited lifetime, and highly efficient metal-to-ligand charge transfer.The disadvantages of these complexes are high cost and sophisticated preparation techniques.
Therefore, alternative organic dyes such as nature dyes have been studied intensively .The main features of natural dyes are their availability, environmental friendly and low cost.In this work ,natural dye was extracted from beetroot and characterized by UV-VIS absorption spectrum.
Different types of DSSCs were fabricated using two kinds of titania (TiO2) semiconductors as photo anodes and four kinds of counter electrodes, and the electrolyte (I -/I3 -) are the same.TiO2 semiconductors includes; Nano sized powder (deposited on ITO glass) and anodized nanotubes template on titanium foil.The counter electrodes include; platinum, electropolymerized polyaniline(PANI), polythiophene (PTh) and polypyrrole(PPY) (also all deposited on ITO glass).

Experimental (Materials and instruments)
All chemicals used were of the highest purity available.Different types of instruments and Fabrication of TiO2 Nano particles photo anode; 1gm of TiO2 Nano powder (assay 99,9%MTI,USA) was treaded with few drops of (0.1M) HNO3 to give a viscous paste.The paste was applied on a conductive side of cleaned ITO glass which determined by avometer via the doctor blade method (12) .It then annealed in furnace at 400°C for 2 hours, the annealed TiO2 Paste is cooled down to room temperature gradually and then soaked into a nature dye solution 3 hr and washed with ethanol after soaking.

Fabrication of TiO2 Nano tube photo anode (TNT):
Cleaned Titanium foil with thickness 0.25mm was used for anodic growth of (TNT).The anodization reaction was carried out in two electrode electrochemical cell Ti foil which served as the anode -working electrode and Pt electrode as the counter electrode .The anodization electrode preparation from 0.4 wt% NH4F and 3vol % deionized water first mixed for 5 mints in order to dissolve NH4F .The volume was completed with ethylene glycol then stirred by magnetic stirrer for 30 mints.The anodization process was operated under a constant potential of 60 V for 1 hr at room temperature ,after anodization the Ti foil was washed carefully first with deionized water and second with ethanol to remove the surface residual electrolyte in the Nano tube arrays and dried by heat gun and annealed at 400°C for 1 hr in order to transform the TNT from amorphous to anatase phase.

Results and Discussions
The absorption characteristic is a very important property in DSSCs as it directly reflects the optical transition probability (13) .Figure( 1) exhibit the UV-VIS absorption spectrum of dye which show that absorbed light in the region visible and s how two peaks at 479 nm and 536 nm.

Figure (1) The absorption spectrum of beet root dye
The FT-IR of PANI shown in figure (2) ,the spectrum show a typical infrared spectrum of the PANI ,the main absorption peaks was at 3468cm -1 it is possible assign asymmetric stretching vibration of NH2.The N-H stretching in benzoid -NHbenzoid (B-NH-B) ,the broad band at 3310cm -1 to hydrogen bonded NH and 3178 cm -1 to terminal quinoid =NH(Q= NH ) .The 2943 cm -1 represented C-H stretching region.The absorption peak observed at 1647.21 cm -1 was attributed to C=C stretching in atomic nuclei .Absorption bands at 1477.4 cm -1 evidenced to C=N stretching in aromatic compounds .Absorption bands at 1300-1200 which confirms the C-N stretching of primary aromatic amines.The 1141 cm -1 band is vibrational mode of (B + -NH =Q) and (B + =NH -Q) which is formed in doping reaction ,this band is very intense and broad which may be attributed to an existence of positive charges (13) .The absorption peak at 1546.91 cm -1 assigned to quinoid structure does not reveal any significant changes for all polymer samples, that concludes that the polymers were prepared using di and tri basic acids .It has been reported that H2SO4 may interact with PANI by donating either hydrogen sulfate (HSO4) -or sulfate (SO4) -2 anions as dopant anions .Many authors agreed that (HSO4) -dopant anions are present in PANI / H2SO4 (14,15) .(19) .The small peaks at 3522cm -1 , 3415cm -1 is assigned presence of N-H stretching vibrations.

Figure (4) FT-IR spectrum of electro polymerized PPY electrode XRD Measurement of TiO2 and TNT Electrodes
The electrodes (TNT and TiO2) were studied by XRD techniques to identify the crystalline phases .Fig (5) shows XRD spectra of TiO2 Nano tube and TiO2 revealed the titania with Rutile phase.TNT in Anatase phase with high crystalline .
The TNT electrode was doped into dye 3 hr and again wased carefully with distilled water then dried by heat gun Fabrication of Pt counter electrode: Few drops of solution (H2PtCI6) (44098ppm) on the was applied on a conductive side of cleaned ITO glass which determined by avometer by using few drops of zahi wasd solution and dried by heat gun and annealed at 450°C for 2 hr then washed carefully with distilled water and ethanol then dried by heat gun .Vol: 13 No:3 , July 2017 DOI : http://dx.doi.org/10.24237/djps.1303.303CP-ISSN: 2222-8373 E-ISSN: 2518-9255 Fabrication of polyaniline (PANI) counter electrode: The aniline polymerization was occurred with electrochemical method ,using two electrodes (ITO glass as working electrode and Pt as counter electrode) under a constant potential of 12 V for 5 mints at room temperature at the same method and conditions prepared polypyrrol and polythiophene.The polyaniline was prepared by mixing 0.3 M of aniline with 1M H2SO4 for 30 mints using magnetic stirrer then washed with deionized water and with ethanol and dried .The poly thiphene was prepared by mixing 0.1M thiophene with 0.5M sodium perchlorates .The polypyrrol prepared by mixing 0.1M of pyrrol with 0.25M oxalic acid.Preparation of cell electrolyte: The cell electrolyte composed by dissolving( 0.5M) of KI and (0.05 M ) of I2 in Ethylene Glycol to obtain an (I -/ I3 -) ions which served as acceptor and donor for electrons through an oxidation reduction reaction with anode and counter electrode.Dye extraction: The dye was prepared by extraction the juice from the small pieces of clean beet root by electric mixer and the resulting extracts were filtered to remove any solid residue and were used for sensitization.Solution were protected from direct light and stored in the refrigerator.Assembling the dye sensitive solar cells (DSSCs) : The cell assembly was performed by adding a few drops of (I -/ I3 -)electrolyte on the photo anode and fix both electrodes (anode and counter) facing each other using a clips binder .The space between two electrodes was obtained by using two sides scotch as a spacer.Evaluation of the fabricated DSSCs: The evaluation system for DSSCs equipment's [light source (Xe lamp) ,cables, potentiostat ] connect solar cell with potentiostat using cables ,shine the light ,control the voltage value through potentiostat which give a table and diagram for I -V characterization.The I-V curves were measured at 100 m W/cm 2 irradiations using Xe lamp.Vol: 13 No:3 , July 2017 DOI : http://dx.doi.org/10.24237/djps.

Figure( 13 )Fig ( 14 )
Figure(13) I -V tests of the four assembled DSSCs from TiO2/ITO glass as anode and different catalyzed cathodes with beetroot dye

Fig 15 :
Fig 15 : I-V tests of the two assembled DSSCs from TNT/Ti glass TiO2/ITO as anode and Pt/ITO catalyzed cathodes with beet root dye.