Dssc Performance of Zinc - Tin - Vanadium Oxide Nanocomposite Using Beetroot (Beta Vulgaris) as Dye Sensitizer

In this present work, zinc - tin - vanadium oxide (ZTVO) nanocomposite was prepared using hydrothermal route and was subjected to calcination at 600 0 C. The sample was systematically characterized by Powder X-ray Diffractometer (XRD), Attenuated Total Reectance (ATR), Field Emission Scanning electron Microscope (FE-SEM), Transmission Electron Microscope (TEM) and Ultraviolet- Diffuse Reectance Spectroscopic techniques. From the investigations, it is observed that this composite possess the combination of both individual and binary phases. The elongated nanostructures obtained due to the binary phases and spherical shaped nanostructures obtained due to the individual phase were observed from the FE-SEM image. The formation of the nanocomposite has further been conrmed from TEM and HRTEM images. ZTVO nanocomposite possess large surface area of 167.3 m 2 /g and pore size value around 11 nm. Also, the band gap of the material has been found to be 1.97 eV. Dye-sensitized solar cell (DSSC) has been fabricated using this ZTVO nanocomposite as the photoanode and betalain dye extracted from beetroot (Beta vulgaris) as the natural dye. This simple protocol was formulated at a low cost for the rst time for DSSC fabrication and it has attained the eciency of 3.41%. This better eciency of ZTVO might be due to larger surface area, presence of pores in addition to smaller band gap.


Introduction
The low-cost, high-e ciency and non-pollution aspects of Dye-sensitized solar cells (DSSCs) has considered as one of the most promising alternatives to silicon based photovoltaic cells in the last two decades [1]. DSSCs became more interesting since after the phenomenal work carried out by O'Regan and Gratzel in 1991. In the present scenario, natural dyes are employed as light harvesting elements to provide the charge carriers. These natural dyes provide a feasible alternative to expensive organic based DSSCs (rare metal complexes) owing to its low cost, non-toxicity, ease of extraction using simple chemical procedures, environmentall friendly, easily biodegradable, abundancy and large absorption coe cient. Especially the pigments like anthocyanin, carotenoids, avonoids, chlorophyll and betalain present in natural dyes have been emphasised nowadays as these pigments initiated different maximum absorption peak within the solar spectrum wavelength (λmax) which support the electrical performance of DSSC. Also these pigmented dyes such as chlorophyll from g leaves exhibited an e ciency of 0.64 %, anthicyanin ectracted from eggplant and that of pomegrante records an e ciency of 0.64 % and 2.00 % respectively, betalain from red turnip gives out 1.7 % e ciency etc. Also, efforts on betalain extracted from purple wild sicilian prickly pear dye, Red Bougainvillea glabra ower, Opuntia dillenii have recorded the energy conversion e ciency of 2.06 %, 0.98 % and 0.47 % respectively. Among them in this present work, betalain pigment extracted from beetroot has been chosen for its abundant availablity and large absorption coe cient with an intention of achieving better e ciency.
The next critical component in such DSSCs is the photoanode with porous structured materials [2][3][4] over which dye molecules get adsorbed onto its surface. To achieve high performance, the photoanode needs to possess large surface area and good electron transport capability [3]. It has been reported [5] by that the performance of DSSC could be improved by doping or sensitizing the material which in turn enhance the absorption of solar light. Also, nanocomposites made of metal oxides and multi-structured semiconductor metal oxides [5][6][7] provide e cient charge separation for effective path of electron transport and promotes larger surface area for dye adsorption. Amongst these various tools, composites using metal oxides have a remarkable impact due to the implementation of n-type metal oxide semiconductors in heterojunction formation.
Also, different morphologies such as nanoparticles, nanowires and nano owers in DSSCs afford improved light absorption and electron collecting e ciency in the devices [7,8]. Thus, this made extensive research work to focuss on the metal oxide nanocomposite photoanodes using TiO 2 , ZnO, SnO 2 etc., than individual metal oxide semiconductors (MOS) [9]. Among these MOS, zinc oxide (ZnO), tin oxide (SnO 2 ) and vanadium oxide (V 2 O 5 ) has been chosen in this present work to carry out studies on DSSC.
ZnO, the MOS with different nanostructure morphologies, carrier mobility, direct band gap and high electron mobility has been considered as a promising candidate for DSSCs [10]. DSSCs based SnO 2 photoanode materials [11] are also reported owing to its larger band gap and their ability to create fewer oxidative holes in the valence band under UV illumination. This SnO 2 material minimizes dye degradation rate and improves the long term stability of DSSC. Vanadium oxide (V 2 O 5 ) is a transition metal oxide which has important applications in various device fabrication and it shows metal-semiconductor transitions implying sudden change in electrical and optical properties [12]. All these individual MOS has been employed as DSSC materials by various researchers but as a single system, they possess low open circuit voltage (Voc) due to high recombination kinetics with the electrolyte resulting in low conversion e ciency. To overcome these di culties and to enhance the probability of photoconversion, band gap narrowing needs to be achieved. Therefore, to achieve this band gap modulation, synthesis of mixed oxides using these combinations has been identi ed. Hence, in this present work, zinc -tin -vanadium oxide nanocomposite (ZTVO) has been prepared via hydrothermal route. Powder X-ray Diffractometer (XRD), Attenuated Total Re ectance (ATR), Field Emission Scanning electron Microscope (FE-SEM), Transmission electron microscope (TEM), speci c surface area analysis and Ultraviolet-Diffuse Re ectance Spectroscopic techniques have been carried out for this sample. DSSC studies has been carried by fabricating the cell using this nanocomposite aided by the betalain dye extracted from beetroot (Beta vulgaris) and the results are presented herein.

Synthesis of zinc -tin -vanadium oxide nanocomposite using hydrothermal route
The reagents used for this present work were of analytical grade purchased from Rankem and were used as such. 0.1 M of 100 ml aqueous solutions of ZnCl 2 , SnCl 2 .2H 2 O and VCl 3 were taken as precursors and 0.3 M glyoxalic acid (surfactant) was added under constant stirring. The gel was obtained by adjusting the pH to 9 by the addition of ammonia under stirring. It was then transferred into a Te on -lined stainless -steel autoclave and maintained at 160ºC for 3 h. Aerogel collected was washed several times, dried in air and calcined at 600 ºC and was named as ZTVO. The protocol for the synthesis of this composite has been reported earlier [13] and it has been proved to be a better ethanol sensor at room temperature.

Fabrication of Dye Sensitized Solar Cell (DSSC) using the composite and the natural dye
Initially, Fluorinated tin oxide (FTO) glass substrates were subjected to cleaning using ethanol and double distilled water and were subjected to sonication. The nanopowders obtained were mixed with triton X and is grounded well and coated on FTO substrates. The lm was calcined at 500 o C for 15 min. 100 g of beetroot (purchased from local market) was taken, grinded well and it is treated with 50 ml of ethanol, kept in a hot plate at 100 C for 1h at room temperature. The crude solution was tered using Whatman No. 41 lter paper to remove the solid residues. Finally, the concentrated dye solution was shielded from exposure to direct light and stored in a refrigerator and the extract has been used as the sensitizer. This The DSSC was assembled using dye sensitized ZTVO as the working electrode and graphite as the counter electrode. These two electrodes were assembled using a semi-closed DSSC method and the electrolyte was led into the cells and the photovoltaic measurements were carried out systematically.

Characterisation Techniques
ATR spectrum of the sample was recorded in the 400-4000 cm -1 region using a Perkin Elmer RX1 FT-IR spectrophotometer by KBr pellet technique. The crystallinity of the sample was studied by X-ray powder diffraction (XRD) using PANALYTICAL X -Ray diffractometer (Cu-Kα radiation, λ = 1.54Å) in 2θ ranging from 20° − 80°. The morphology of the sample was studied using JEOL JSM Field emission scanning electron microscope (FE -SEM) and the elemental composition was obtained using JEOL Model JED − 2300 Energy Dispersive X-ray (EDAX) Spectrometer analysis. JOEL JEM 2100 Transmission microscope was used to record the Transmission electron microscopic (TEM) image and Selective area electron diffraction pattern (SAED) was also recorded using the same instrument. The optical property was studied using UV-visible diffuse re ectance spectrum (DRS) by JASCO -UV VIS spectrophotometer. The speci c surface area was analysed using the Brunauer-Emmett-Teller (BET) equation based on the nitrogen adsorption isotherm. DSSC studies was carried out using LOT-LS0104 solar simulator under under xenon lamp irradiance of 100 mW/cm 2 and AM 1.5. The radiation exposure area for the fabricated DSSC was 1 cm 2 , which has been achieved by covering the cell with a thin black paper mask having an aperture of 1 cm 2 . The I -V measurements of the fabricated DSSC was recorded using electrochemical workstation (Metrohm, Autolab 302NFRA2).

Functional group analysis
From the ATR spectrum of ZTVO (not shown), it has been observed that the absorbance band around were found to exist in sample in the proportion of 28%, 19%, 36% and 17% respectively. The formation of binary phases such as zinc stannate and zinc vamadate along with the individual SnO 2 is con rmed from the XRD pattern [13].

Morphological studies
FE-SEM image of the nanocomposite ZTVO is shown in Fig. 2 (a). It revealed ower like microstructures made up of nano akes and these microstructured owers consist of enormous petals and minute pores. The nano akes which is clearly depicted as thin nanorods in TEM image (Fig. 2 (c)) has around 320 nm length and 13 nm width. These elongated stuctures are due to the formation of the binary phases ZnV 2 O 6 and ZnV 3 O 8 which has been further proved from the EDAX data. The EDAX recorded over the nano akes reveal the composition of vanadium to be 11.72 atomic% which is greater than tin and zinc. FE-SEM image also reveals the presence of nanoparticles of uniform grain size whose TEM image in Fig. 2 (b) also shows the same with grain size around 11 nm. The EDAX recorded over this area also supports the presence of higher composition of tin (9.09 atomic %) than vanadium and zinc.
High magni cation HRTEM image of ZTVO is depicted in Fig. 3 (a) and the d-spacing for SnO 2 is found to be 0.332 nm and that of ZnV 2 O 6 is found to be 0.312 nm and the values matches well with the corresponding standard values. From the SAED pattern shown in Fig. 3 (b), the polycrystalline nature is proved from the orientation of atoms in different planes and the presence of different phases has been indexed.

Optical studies
From the diffuse re ectance spectrum of ZTVO shown in the inset of Fig. 4, a broad absorption edge is noticed around 600 nm. This might be due to the synergistic effect of the composite material and the band gap for ZTVO has been calculated to be 1.97 eV from the band gap determination plot in Fig. 4. Band gap (E g ) plays a crucial role in assisting the wavelength range for light absorption. It has reported [15][16][17] that all photons with energies larger than E g are absorbed and contribute to photocurrent, those with smaller energies are transmitted and not used. So, smaller the band gap, larger will be the probability of photo conversion and hence ZTVO possessing the band gap of 1.97 eV could show better e ciency.

Surface area analysis
The performance of DSSC greatly depends on the surface area and also on the nature of pores. The nitrogen adsorption-desorption isotherm obtained for ZTVO exhibited type IV with type H3 hysteresis loop for the relative pressure P/Po in the range of 0.1-1 [13,18]. Which indicates the mesoporous nature of the material. The speci c surface area of ZTVO calculated using BET analysis was found to be 167.3 m 2 /g and pore size of 11 nm is obtained from the PSD plot. The synergistic effect [19][20][21] of the hierarchical nanostructured ZTVO offers large surface area which could favour better performance of the photoanode and henceforth DSSC performance.

DSSC measurements
The fabricated DSSC was subjected to V-I characteristics from which power was calculated using the equation P max = V max x I max . Current -Voltage characteristics of DSSCs fabricated using ZTVO photoanodes is presented in Fig. 5. The ll factor (FF) is the ratio between the maximum output power density available ( J m . V m ) and the maximum power combining short -circuit and open -circuit situations [19][20][21]. This nanocomposite serving the role of photoanode has been reported for the rst time in which ZTVO executes an conversion e ciency of 3.41% with a ll factor of 0.15.
The schematic representation of the contribution of the dye molecules over the ZTVO nanocomposite in DSSC fabrication is depicted in Fig. 6.
A very strong parameter which favours the conversion e ciency is the smaller band gap [23][24][25] and in which the band gap of ZTVO (1.97 eV) proves to be the better photon converter. Since, porosity of the nanocomposite is also an important factor to be taken into account in which adsorption of dye molecules could be favoured so that better performance of DSSC could be achieved.
The e ciency of DSSC using the beetroot as a natural dye with ZTVO as photoanode seems to be better than reported by M. Mazhar et al (0.30 %) and S. Sathyajothi et al (1.3 %) but further investigations are in progress, to optimise the proportion of the individual precursors to achieve uniform porous morphology and also to monitor the thickness of the lm.

Conclusion
Zinc-tin-vanadium oxide nanocomposite prepared via hydrothermal method was implemented as a photoanode and extract from beetroot was used as the natural dye in fabricating a low cost DSSC for the rst time. The photoanode material was systematically characterised by various state-of-the art facilities. It is observed that the nanocomposite material possess both elongated microstructures contributed by the presence of binary phases and the spherical nanostructures corresponding to the individual phase.

Con icts of interest/Competing interests
There is no con ict of interest

Availability of data and material
There is no availability of data and material

Code availability
There is no code availability    Band gap determination plot of ZTVO (inset : Diffuse re ectance spectrum) Figure 5 Current-Power characteristics of the fabricated DSSC