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Au Schottky barrier solar cell
Highly efficien betanin dye based ZnO and ZnO/Au Schottky barrier solar cell Aparna Thankappan, S. Divya, Anju K. Augustine, C. P. Girijavallaban, P. Radhakrishnan, Sheenu Thomas, V.P.N. Nampoori PII: DOI: Reference:
S0040-6090(15)00268-0 doi: 10.1016/j.tsf.2015.03.052 TSF 34210
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
7 May 2014 20 March 2015 23 March 2015
Please cite this article as: Aparna Thankappan,S. Divya, Anju K. Augustine,C. P. Girijavallaban, P. Radhakrishnan, Sheenu Thomas, V.P.N. Nampoori, Highly efficient betanin dye based ZnO and ZnO/Au Schottky barrier solar cell, Thin Solid Films (2015), doi: 10.1016/j.tsf.2015.03.052
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ACCEPTED MANUSCRIPT Highly efficient betanin dye based ZnO and ZnO/Au Schottky barrier solar cell Aparna Thankappan1, 2*,S. Divya 1, Anju K Augustine1, C.P Girijavallaban 1, P. Radhakrishnan 1, Sheenu Thomas1,V.P.N. Nampoori 1
Inter University Centre for Nanomaterials and Devices (IUCND), Cochin University of Science and Technology, Kochi, India
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International School of Photonics (ISP), Cochin University of Science and Technology
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Phone: 0091-484-2575848, Fax: 0091-484-2576714 *[email protected]
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Abstract:
Performance of dye sensitized solar cells based on betanin natural dye from red beets with various nanostructured photoanodes on transparent conducting glass has been investigated. In four different
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electrolyte system cell efficiency of 2.99% and overall photon to current conversion efficiency of 20 % were achieved using ZnO nanosheet electrode with iodide based electrolyte in acetonitrile solution.
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To enhance solar harvesting in organic solar cells, uniform sized metal nanoparticles (gold (Au) of ~8 nm) synthesized via microwave irradiation method were incorporated into the device consisting of
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ZnO. Enhanced power conversion efficiency of 1.71 % was achieved with ZnO/Au nanocomposite
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compared to the 0.868 % efficiency of the bare ZnO nanosheet cell with ferrocene based electrolyte. Keywords: betanin Introduction Since Grätzel et al. in 1991 developed a type of solar cells, viz. dye-sensitized solar cells (DSSCs) [1]; they have drawn considerable attention due to their several attractive properties like DSSC’s an environmental friendly and excellent low-cost alternative to conventional inorganic photovoltaic devices. A typical DSSC consists of a redox electrolyte sandwiched between a working electrode and a counter electrode. Platinum film is widely used in fabricating a high performance DSSC since it has good catalytic activity. In these systems a semiconductor film is sensitized with a monolayer of dye molecules. Photo excitation takes place in the dye molecules, and fast electron transfer occurs through 1
ACCEPTED MANUSCRIPT the electrolyte to the platinum counter electrode. In order to achieve efficient light harvesting; charge separation and transport, the choice of nanostructured oxide and corresponding transparent conducting oxide films becomes a critical issue in cell design [2]. Solar energy harvesting is a comparative
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fledged technology for outdoor applications. For indoor applications it is necessary to note that the efficiency of the cell is very low as of its low light luminous intensity which is less than 10 W/m2 as
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compared to 100-1000 W/m2 under outdoor conditions depending on the type of light source and
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distance.
A TiO2 nanoparticle film has been widely investigated for DSSCs due to its high power conversion
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efficiency [3,4,5,6,7,8], ZnO has similar band gap and electron affinity values to those of TiO2 and it is also an environmentally friendly oxide semiconductor. DSSCs assembled from ZnO nanoparticles show the second highest efficiencies after TiO2 [9].Many researchers have focussed their studies on
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improving the efficiency of the DSSC. To date this field has been dominated by solid state junction devices [10,11] which are now being challenged by the emergence of third generation of cells based
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on the nanocrystalline oxide and conducting polymer films [12]. Ruthenium- containing metal organic complexes have been favoured in DSSCs due to their strong absorption of visible light, favourable
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spatial separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
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orbital (LUMO) and their ability to get repetitively oxidized and reduced without degradation [13]. The best reported devices of this class which convert solar energy to electrical energy has an efficiency of 10–11 %[14-16].Recently, the efficiency of DSSCs has improved to 12.3 % by the combination of co-sensitization of porphyrin dyes and a cobalt-complex redox mediator[17]. However, synthetic dyes have offered problems as well, such as complicated synthetic routes, whereas natural dyes are more organic, or better for the environment, and are safer to use compared with synthetic dyes. Due to their cost efficiency, non-toxicity and environmental benefits, DSSCs based on natural dyes have been at the centre of intense research. Natural pigments from plants such as chlorophyll [18], anthocyanin[19] have been extensively examined as sensitizers for the DSSC.Betalains are a class of vegetal pigments, have favourable light absorption characteristics. They exists in nature in association with various co-pigments such as the 2
ACCEPTED MANUSCRIPT betalamic acid, indicaxathin, betanidin, and these can modify their light absorption properties and are capable of forming complex metal ions, possessing the requisite functional group (–COOH) to bind also to ZnO [20]. Although the betalain dyes fulfil the criteria mentioned above, they have not been
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suitably studied as dye sensitizers [21-24]. The betalain pigments comprise the red-purple betacyanin, betanin (I) and betanidin (II), with maximum absorptivity at λmax about 535 nm, and the yellow
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betaxanthins with λmax near 480 nm [25]. Within these 2 categories, many subcategories are also
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present [26]. Although the betanin sensitized DSSC have shown the highest efficiencies up to 2 % - 3 %,further improvement is required to be competitive with inorganic solar cells.[23]
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Metal-doped semiconductor nanocrystallites have attracted much attention, because of their effects on improving their photo activity by changing its photo physical mechanism i.e. the formation of shallow charge trapping on the semiconductor surface due to the replacement of Zn II by metal ions and the
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metal ion introducing could enhance the photocatalytic activity of ZnO in some cases. Several reports have been previously published in this direction [27-30].
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In this work we report the incorporation of Au nanoparticles (NPs) prepared by a simple and quick
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solution process, microwave rapid heating process that can control the density and size of the NPs, on ZnO to improve the device performance as the result of the enhanced optical absorption of incident
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photon within the active layer. Compared to the other fabrication processes, the microwave rapid heating process has many advantages such as the controllability of particle size and density and uniform morphology, it is a promising method to fabricate the metal nanostructures. Here we also explore the charge transfer process in a ZnO/Au nanocomposite photoelectrode for DSSC application comprising betanin natural dye. Furthermore the effect of electrolyte with different ion composition onto the bare ZnO has also been studied. 2. Experimental details All chemicals used in this study were analytical grade and were used without any further purification. Zinc acetate and NaOH were used as the materials for the growth of ZnO. A low temperature solvothermal method was used for the preparation of ZnO NSs according to the method described by 3
ACCEPTED MANUSCRIPT Pacholski [31]. Briefly, 1 M zinc acetate in methanol was added drop-wise to a well–stirred solution of 3 M NaOH in methanol maintained at 60°C. After a two hour growth, the solution was centrifuged to separate the nanosheets, which are characterized through X-ray diffraction method and scanning
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electron microscopy (figure 1a). The fresh methanol was infused to suspend them.
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ZnO/Au nanocomposite photo electrodes have been prepared by wetting 1 g of ZnO with 0.5 ml of colloidal Au NPs. Microwave rapid heating, which is a simple and quick solution process, was used to
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synthesize the Au NPs. A 0.5mM aqueous solution of auric acid (HAuCl4) and 3.8 mM aqueous solution of sodium citrate were reacted together in the presence of microwave radiations. After
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cooling down to room temperature, products were collected, are characterised through transmission electron microscope (TEM) and these are shown in figure (1b). In order to prepare a working electrode of DSSC, the conductive transparent glass plates (Indium tin
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oxide(ITO) 8-12 Ω /sq) were first cleaned in a detergent solution using an ultrasonic bath for 10 min, rinsed with water and ethanol, and then dried. The photo anodes were prepared by depositing the
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synthesized structures on to the ITO conducting glass: two edges of the ITO glass plate were covered
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with a layer of adhesive tape. To control the thickness of the film and to mask electrical contact strips. Film thickness was measured using a digimatic micrometer and was found to be 33µm.The
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nanostructure paste was then spread uniformly on the substrate by sliding a glass rod along the tape spacer. The as-prepared samples were then subjected to an annealing process at 300 °C for 30 min to ensure a good contact of the Au nanoparticles with the ZnO. Bare ZnO and ZnO/Au composite photo electrodes were impregnated with filtered fresh juice from red beets without further purification. The visible range absorption(525-565 nm) of the dye is attributed to betanin which was described in detail in our previously published paper[26]. Dye adsorption was carried out for 24 h at room temperature, after which the photo electrodes were removed from the dye solution and rinsed several times with water in order to remove weakly adsorbed dye molecules and finally dried on a hot plate. The platinised ITO glass was used as the
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ACCEPTED MANUSCRIPT counter electrode. For this, ITO conducting glass was dipped into a 10 mM chloroplatinic acid hexahydrate solution in ethanol and then annealed at 450 °C for 15 min.
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The counter electrode was then placed on top of the photo electrode, and filled with electrolyte by using capillary force through the sides. Binder clips were used to hold the electrodes together. We
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have used different ion compositions as the redox electrolyte: they are 0.3 g iodine and 1.0 g potassium iodide in distilled water and the same in acetonitrile with a motivator of 0.5 M 4- tert-
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butylpyridine (4-TBP). The third one is the ferrocene based electrolyte in which 0.05M ferrocenium hexafluorophosphate mixed with 0.1 M ferrocene and 0.5 M tert-butylpyridine in acetonitrile. This
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Fe2+/Fe3+ redox couple does not induce corrosion because it does not interact with oxygen during the fabrication process [32].Because of the easy volatile and decomposing properties of the organic liquid electrolyte we have modified the liquid electrolyte on the addition of polymer and developed quasi
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solid state DSSC. For this, 0.3 g iodine 1.0 g potassium iodide and 0.5 M 4-TBP was dissolved into polyvinyl alcohol solution to give a homogeneous solution and was heated up to 60 °C to accelerate
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the polymerisation. After 1h a polymer gel electrolyte was obtained.
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The measurement of light absorption at room temperature was carried out using a spectrophotometer (Jasco V-570 UV/VIS/IR). The morphologies and sizes of the as-prepared samples were examined by
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transmission electron microscopy and x-ray diffraction method. The morphology and sizes of the ZnO sample was examined by scanning electron microscopy (JEOL/EO, and JSM6390) and the X-ray diffraction data were collected on an AXS Bruker D% diffractometer using Cu Kα-radiation (λ=0.1541 nm, the operating conditions were 35mA and 40 kV at a step of 0.0200 and step time of 29.5 s in the 2ϴ range from 30 to 70 0. The electrical characteristics (J–V) of the prepared solar cells were examined by using a standard solar irradiation of 1000 W xenon arc lamp as light source. The cell surface was exposed to light through a water column to minimize heating effect. The input intensity was measured using a light meter (metravi 1332). The incident light intensity in the range 4200-8000 Lx (6 W/m2-12 W/m2) was measured and then converted into W/m2 using the simplified relationship 1 Lx=1.46 mW/m2 [33,34] Incident monochromatic photo-to-current conversion
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ACCEPTED MANUSCRIPT efficiency (IPCE) measurements were carried out using small band-pass filters to provide monochromatic light.
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The degree of the squared shape of the J-V curve is given by the fill factor (FF), which measures the ideal nature of the device and is defined as the ratio of the maximum power output per unit area to the
[1]
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product of open-circuit voltage (Voc) and short circuit current (Jsc).
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The solar to electric power conversion efficiency ƞ is given by the ratio of the maximum extractable power to the incident solar power (Pin) given by equation
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[2]
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Where Pin is the incident power, Pout is the output power, FF is the fill factor, ƞ is the efficiency, Jsc is the short circuit current density, and Voc is the open circuit voltage.
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Incident photon-to-electron conversion efficiencies, IPCE(λ), were recorded as a function of incident
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wavelength for the betanin-sensitized solar cell, it is the ratio of the number of electrons produced in the external circuit to the number of photons incident on the cell and is calculated using the formula [35]
[3]
Where λ is the wavelength of the incident light in nm, Pin is the incident power in watts, and Jscis the maximum current in amps. The shape of IPCE (λ) is somewhat different from the absorption spectrum of the dye sensitized film which may be due to the adsorption of more than one kind of pigment. 3. Results and discussion 3.1 Effect of electrolyte
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ACCEPTED MANUSCRIPT The solar cells with ZnO with different electrolytes in the presence of betanin natural dye were initially prepared in order to study the effects of electrolyte on the photovoltaic behaviour of the system. The morphology of ZnO nanostructure (like nanosheets) was identified by SEM. The X – ray
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diffraction (XRD) pattern of ZnO film is also shown in figure 1. The current–voltage (J-V) data for DSSC containing ZnO are shown in figure 2 and summarised in table 1. In this study, the cells using
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acetonitrile (ACN) solution have a higher efficiency 2.99% than reported [23], and the overall photon
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to current conversion efficiencies (IPCE) of 20% was achieved. The iodide (I2) electrolyte in ACN has been shown to be the most efficient organic solvent due to its viscosity (0.34 cp, 25 °C)[36] and
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good solubility to dissolve organic components and salts, as well as additives in the electrolytes, at the same time it has good long term stability. When ferrocene is replaced by other electrolyte iodine in polyvinyl alcohol (PVA) Eredox-EHUMO is reduced, which will affect Voc and Jsc..In the case of PVA
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based electrolyte the performance drops down due to the poor mobility of charge carriers as well as poor wetting ability. Its incomplete penetration into the semiconductor to regenerate the dye plays a
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detrimental role.
Even though the iodide based electrolyte showed better Jsc than ferrocene based electrolyte, the
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ferrocene based electrolyte exhibit better Voc and FF attributed to lower charge recombination at the
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photo electrode. Dye degradation by ferrocene is a simple one-electron transfer reaction (D+ → D + Fc) that does not involve the cleavage or reformation of chemical bonds or the formation of highenergy intermediate radical species and has more favourable redox potential than the two electron I3-/Iredox system (0.62 V )which should result in a higher Voc[37].The nature of the resonant HOMO→LUMO transition, vibrations of the aromatic ring and conjugated framework are strongly boosted by using iodide based electrolyte in ACN. 3.2 Effect of Au nanoparticles When the frequency of the incoming light is in resonance with the collective oscillations of the electron density i.e. surface plasmons, metal NPs show a strong enhancement in the local electromagnetic field. As a result, unique optical properties known as localized surface plasmon
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ACCEPTED MANUSCRIPT resonance (LSPR) have been observed which are depends on the size, shape, density and local dielectric environment of the NPs [38]. To enhance solar harvesting in organic solar cells, uniform sized metal nanoparticles (gold (Au) of ~8 nm) synthesized via microwave irradiation method which
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is a kind of simple and quick solution process that can control the size of metal nanoparticles, were incorporated into the device, the J-V characteristics of the sandwiched cell structures with both bare
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ZnO and gold mixed ZnO were examined. Even though the iodide based electrolyte showed better Jsc
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than ferrocene based electrolyte, the ferrocene based electrolyte exhibit better Voc and FF attributed to lower charge recombination at the photo electrode, therefore, these cells are betanin natural dye
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sensitized and use a ferrocene based electrolyte and an active area of 0.6 cm2 with Pin= 8 k lx. The optical absorptions of ZnO NS on ITO, Au incorporated ZnO on ITO and betanin sensitized ITO of the same are shown in figure 3.The absorption spectrum of the ITO/ZnO NS structure incorporated
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with gold NPs is characterized by a large absorbance over the measured spectral range and a clear broad peak in the range 380 - 488 nm, and the observed spectral change (red shift) in this system is
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attributed to the plasmon resonance absorption arising from electron deficiency on the gold surface and the interfacial coupling between the Au and ZnO.
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Upon illumination with simulated sunlight, the device with ZnO mixed with gold nanoparticles can
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deliver improved photocurrent as shown in figure 4.Also the power conversion efficiency improved from 0.868 % to 1.71 % and exhibits IPCE of 14 %, which is depicted in figure 5. These changes apparently results from the enhanced visible absorption of the active layer via the high electromagnetic field strength in the vicinity of the excited surface plasmons provided by the Au NP embedded to the ZnO NS, leading to an increase in the exciton generation. An enhancement over the visible range is related to the combined effect of betanin natural dye and the LSPR which induce more photo generated charge carriers. The formation of Schottky barrier and the possible electron transfer from the excited dye molecules to the ZnO is schematically represented in figure 6. Upon irradiation the electrons from the LUMO band of the dye are transported to the conduction band (CB) of ZnO through the tunnelling effect of the thin layer of gold NPs. It is further believed that another way of electron transfer from the electron 8
ACCEPTED MANUSCRIPT accumulation in the Au nanoparticles, raising their Fermi energy level to further closer to the CB of ZnO and a quick transport of electrons from Au to ZnO through the physical necking network takes place to establish charge equilibrium in the system. The electrons transferred to ZnO are collected at
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the bottom ITO electrode, and thereby generate an anodic photocurrent. The Schottky barrier at the Au-ZnO interface reduces charge recombination by blocking the transfer of electrons from the ZnO
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conduction band to the dye and/or electrolyte thus improving the efficiency of DSSCs due to the large
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work function of Au compared with the electron affinity of ZnO. Normally, when the charge transport was improved the FF and Jsc will be improved simultaneously. But here the FF was decreased from
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0.35 to 0.29 is probably caused by an increase in the resistive loss due to the increased flux [39,40] and the charge recombination at the photo electrode coming from the Au nanoparticle agglomeration. These accumulated electrons recombine with the oxidised dye molecules or with the redox electrolytes
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before they can be transfer to the CB of ZnO [41,42].
The presence of carboxylic groups in the betalain presents together with the higher oxidation potential
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boosts the anchoring. The formation of the oxidation product, cyclo- dihydroxyphénylalanine along with the possible formation of indicaxanthin and the heat-induced cleavage of betanin into betalamic
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acid may play a crucial role in the cell performance. To assess the stability of the assembled cell they
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have been wrapped in tissue and left for 1 day and for 1 month. Stability of the ZnO/Au nanoparticles with fresh ferrocene electrolyte was tested by measuring the J–V characteristics of the ZnO/Au solar cell after 24 h, only negligible changes in the cell performance were observed. But for the 1 month dated cell show the photovoltaic action with very less values of Jsc and Voc. Although the efficiencies obtained with betanin natural dye are below the requirements for the large scale production, the results are hopeful and are suitable alternative to inorganic dyes in DSSC. 4. Conclusion Betanin sensitised Dye-sensitized solar cells with a ZnO/Au nanocomposite as the photoelectrode were fabricated and their performances were compared with the bare ZnO NS DSSC and the effect of electrolyte with different ion composition onto the bare ZnO NPs has also been discussed. The
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ACCEPTED MANUSCRIPT photochemical performance of DSSC with NP based photo anode with ferrocene based and iodide in acetonitrile solution exhibit better efficiency of 0.868 % and 2.99 % respectively .We found that, incorporation of Au NPs led to enhanced power conversion efficiency from 0.868 % to 1.71 % and
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IPCE of 14 %,mainly resulting from the formation of the Schottky barrier at the ZnO/Au interface and enhanced optical absorption of ZnO/Au photo electrodes arising from both high electromagnetic
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field strength in the vicinity of excited surface plasmons of Au NPs and high dye excitation .
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Acknowledgement:
This work was supported by the research grants from the DST pursue scheme and the financial
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support from the IUCND. References
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[36] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells,Chem. Rev.
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110 (2010) 6595-6663
[37] Torben Daeneke, Tae Hyuk Kwon, Andrew B. Holmes, Noel W Duffy, Udo Bach, Leone piccia,
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High-efficiency dye-sensitized solar cells with ferrocene-based electrolytes, Nature chem. 966
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(2011)1-5
[38] Seok Soon Kim, Seok In Na, Jang Jo, Dong Yu Kim, Yoon Chae Nah, Plasmon enhanced performance of organic solar cells using electrodeposited Ag nanoparticles, Appl. Phys. Lett. 93, (2008) 073307 [39]Y.B Tang, Z.H Chen, H.S Song, C.S Lee, H.T Cong, H.M Cheng, W.J Zhang, I. Bello, S.T Lee,Vertically Aligned p-Type Single-Crystalline GaN Nanorod Arrays on n-Type Si for Heterojunction Photovoltaic Cells, Nano Lett. (2008) 812 [40] W.J.E Beek, M.M Wienk,, R.A.J. Janssen, Hybrid solar cells from regioregular polythiophene and ZnO nanoparticles, AdV.Funct.Mater. 16,8, (2006) 1112-1116
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ACCEPTED MANUSCRIPT [41] P.V Kamat, Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvestors, J. Phys. Chem. B, 113, (2008) 18737-18753
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[42] M. Haruta, Size- and support-dependency in the catalysis of gold, Catal. Today 36, (1997) 153-
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List of figures
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Figure 1: (a) XRD pattern and SEM image of ZnO film and (b) TEM image of Au NPs Figure2: current density against voltage characteristics of ZnO with different electrolytes (Pin=4.2klx)
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Figure 3 :Absorption spectra of ZnO on ITO film ,ZnO mixed with Au NP on ITO film and
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betanin sensitized ITO film of the same.
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Figure 4: current density against voltage characteristics of Au incorporated ZnO (Pin=8 klx) Figure 5: IPCE (λ) for (a) betanin-sensitized NP with ferrocene based and iodide electrolyte
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and (b) betanin-sensitized Au NP with ferrocene electrolyte. Figure 6: Energy level diagram and mechanism of photocurrent generation in the cell with ITO/ZnO/Au/Dye as the photo anode.
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Area of cell Jsc(mA/cm2) Voc (V)
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0.87
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0.84
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Table 1: comparison of DSSC with different electrolyte (Pin=4.2k lx)
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The influence of electrolytes has been studied
Cell efficiency of 2.99% was achieved by ZnO
Enhancement of efficiency with incorporation of Au nano
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S0040-6090(15)00268-0 doi: 10.1016/j.tsf.2015.03.052 TSF 34210
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
7 May 2014 20 March 2015 23 March 2015
Please cite this article as: Aparna Thankappan,S. Divya, Anju K. Augustine,C. P. Girijavallaban, P. Radhakrishnan, Sheenu Thomas, V.P.N. Nampoori, Highly efficient betanin dye based ZnO and ZnO/Au Schottky barrier solar cell, Thin Solid Films (2015), doi: 10.1016/j.tsf.2015.03.052
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Highly efficient betanin dye based ZnO and ZnO/Au Schottky barrier solar cell Aparna Thankappan1, 2*,S. Divya 1, Anju K Augustine1, C.P Girijavallaban 1, P. Radhakrishnan 1, Sheenu Thomas1,V.P.N. Nampoori 1
Inter University Centre for Nanomaterials and Devices (IUCND), Cochin University of Science and Technology, Kochi, India
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2
International School of Photonics (ISP), Cochin University of Science and Technology
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1
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Phone: 0091-484-2575848, Fax: 0091-484-2576714 *[email protected]
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Abstract:
Performance of dye sensitized solar cells based on betanin natural dye from red beets with various nanostructured photoanodes on transparent conducting glass has been investigated. In four different
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electrolyte system cell efficiency of 2.99% and overall photon to current conversion efficiency of 20 % were achieved using ZnO nanosheet electrode with iodide based electrolyte in acetonitrile solution.
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To enhance solar harvesting in organic solar cells, uniform sized metal nanoparticles (gold (Au) of ~8 nm) synthesized via microwave irradiation method were incorporated into the device consisting of
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ZnO. Enhanced power conversion efficiency of 1.71 % was achieved with ZnO/Au nanocomposite
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compared to the 0.868 % efficiency of the bare ZnO nanosheet cell with ferrocene based electrolyte. Keywords: betanin Introduction Since Grätzel et al. in 1991 developed a type of solar cells, viz. dye-sensitized solar cells (DSSCs) [1]; they have drawn considerable attention due to their several attractive properties like DSSC’s an environmental friendly and excellent low-cost alternative to conventional inorganic photovoltaic devices. A typical DSSC consists of a redox electrolyte sandwiched between a working electrode and a counter electrode. Platinum film is widely used in fabricating a high performance DSSC since it has good catalytic activity. In these systems a semiconductor film is sensitized with a monolayer of dye molecules. Photo excitation takes place in the dye molecules, and fast electron transfer occurs through 1
ACCEPTED MANUSCRIPT the electrolyte to the platinum counter electrode. In order to achieve efficient light harvesting; charge separation and transport, the choice of nanostructured oxide and corresponding transparent conducting oxide films becomes a critical issue in cell design [2]. Solar energy harvesting is a comparative
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fledged technology for outdoor applications. For indoor applications it is necessary to note that the efficiency of the cell is very low as of its low light luminous intensity which is less than 10 W/m2 as
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compared to 100-1000 W/m2 under outdoor conditions depending on the type of light source and
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distance.
A TiO2 nanoparticle film has been widely investigated for DSSCs due to its high power conversion
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efficiency [3,4,5,6,7,8], ZnO has similar band gap and electron affinity values to those of TiO2 and it is also an environmentally friendly oxide semiconductor. DSSCs assembled from ZnO nanoparticles show the second highest efficiencies after TiO2 [9].Many researchers have focussed their studies on
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improving the efficiency of the DSSC. To date this field has been dominated by solid state junction devices [10,11] which are now being challenged by the emergence of third generation of cells based
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on the nanocrystalline oxide and conducting polymer films [12]. Ruthenium- containing metal organic complexes have been favoured in DSSCs due to their strong absorption of visible light, favourable
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spatial separation of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
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orbital (LUMO) and their ability to get repetitively oxidized and reduced without degradation [13]. The best reported devices of this class which convert solar energy to electrical energy has an efficiency of 10–11 %[14-16].Recently, the efficiency of DSSCs has improved to 12.3 % by the combination of co-sensitization of porphyrin dyes and a cobalt-complex redox mediator[17]. However, synthetic dyes have offered problems as well, such as complicated synthetic routes, whereas natural dyes are more organic, or better for the environment, and are safer to use compared with synthetic dyes. Due to their cost efficiency, non-toxicity and environmental benefits, DSSCs based on natural dyes have been at the centre of intense research. Natural pigments from plants such as chlorophyll [18], anthocyanin[19] have been extensively examined as sensitizers for the DSSC.Betalains are a class of vegetal pigments, have favourable light absorption characteristics. They exists in nature in association with various co-pigments such as the 2
ACCEPTED MANUSCRIPT betalamic acid, indicaxathin, betanidin, and these can modify their light absorption properties and are capable of forming complex metal ions, possessing the requisite functional group (–COOH) to bind also to ZnO [20]. Although the betalain dyes fulfil the criteria mentioned above, they have not been
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suitably studied as dye sensitizers [21-24]. The betalain pigments comprise the red-purple betacyanin, betanin (I) and betanidin (II), with maximum absorptivity at λmax about 535 nm, and the yellow
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betaxanthins with λmax near 480 nm [25]. Within these 2 categories, many subcategories are also
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present [26]. Although the betanin sensitized DSSC have shown the highest efficiencies up to 2 % - 3 %,further improvement is required to be competitive with inorganic solar cells.[23]
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Metal-doped semiconductor nanocrystallites have attracted much attention, because of their effects on improving their photo activity by changing its photo physical mechanism i.e. the formation of shallow charge trapping on the semiconductor surface due to the replacement of Zn II by metal ions and the
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metal ion introducing could enhance the photocatalytic activity of ZnO in some cases. Several reports have been previously published in this direction [27-30].
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In this work we report the incorporation of Au nanoparticles (NPs) prepared by a simple and quick
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solution process, microwave rapid heating process that can control the density and size of the NPs, on ZnO to improve the device performance as the result of the enhanced optical absorption of incident
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photon within the active layer. Compared to the other fabrication processes, the microwave rapid heating process has many advantages such as the controllability of particle size and density and uniform morphology, it is a promising method to fabricate the metal nanostructures. Here we also explore the charge transfer process in a ZnO/Au nanocomposite photoelectrode for DSSC application comprising betanin natural dye. Furthermore the effect of electrolyte with different ion composition onto the bare ZnO has also been studied. 2. Experimental details All chemicals used in this study were analytical grade and were used without any further purification. Zinc acetate and NaOH were used as the materials for the growth of ZnO. A low temperature solvothermal method was used for the preparation of ZnO NSs according to the method described by 3
ACCEPTED MANUSCRIPT Pacholski [31]. Briefly, 1 M zinc acetate in methanol was added drop-wise to a well–stirred solution of 3 M NaOH in methanol maintained at 60°C. After a two hour growth, the solution was centrifuged to separate the nanosheets, which are characterized through X-ray diffraction method and scanning
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electron microscopy (figure 1a). The fresh methanol was infused to suspend them.
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ZnO/Au nanocomposite photo electrodes have been prepared by wetting 1 g of ZnO with 0.5 ml of colloidal Au NPs. Microwave rapid heating, which is a simple and quick solution process, was used to
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synthesize the Au NPs. A 0.5mM aqueous solution of auric acid (HAuCl4) and 3.8 mM aqueous solution of sodium citrate were reacted together in the presence of microwave radiations. After
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cooling down to room temperature, products were collected, are characterised through transmission electron microscope (TEM) and these are shown in figure (1b). In order to prepare a working electrode of DSSC, the conductive transparent glass plates (Indium tin
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oxide(ITO) 8-12 Ω /sq) were first cleaned in a detergent solution using an ultrasonic bath for 10 min, rinsed with water and ethanol, and then dried. The photo anodes were prepared by depositing the
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synthesized structures on to the ITO conducting glass: two edges of the ITO glass plate were covered
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with a layer of adhesive tape. To control the thickness of the film and to mask electrical contact strips. Film thickness was measured using a digimatic micrometer and was found to be 33µm.The
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nanostructure paste was then spread uniformly on the substrate by sliding a glass rod along the tape spacer. The as-prepared samples were then subjected to an annealing process at 300 °C for 30 min to ensure a good contact of the Au nanoparticles with the ZnO. Bare ZnO and ZnO/Au composite photo electrodes were impregnated with filtered fresh juice from red beets without further purification. The visible range absorption(525-565 nm) of the dye is attributed to betanin which was described in detail in our previously published paper[26]. Dye adsorption was carried out for 24 h at room temperature, after which the photo electrodes were removed from the dye solution and rinsed several times with water in order to remove weakly adsorbed dye molecules and finally dried on a hot plate. The platinised ITO glass was used as the
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ACCEPTED MANUSCRIPT counter electrode. For this, ITO conducting glass was dipped into a 10 mM chloroplatinic acid hexahydrate solution in ethanol and then annealed at 450 °C for 15 min.
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The counter electrode was then placed on top of the photo electrode, and filled with electrolyte by using capillary force through the sides. Binder clips were used to hold the electrodes together. We
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have used different ion compositions as the redox electrolyte: they are 0.3 g iodine and 1.0 g potassium iodide in distilled water and the same in acetonitrile with a motivator of 0.5 M 4- tert-
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butylpyridine (4-TBP). The third one is the ferrocene based electrolyte in which 0.05M ferrocenium hexafluorophosphate mixed with 0.1 M ferrocene and 0.5 M tert-butylpyridine in acetonitrile. This
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Fe2+/Fe3+ redox couple does not induce corrosion because it does not interact with oxygen during the fabrication process [32].Because of the easy volatile and decomposing properties of the organic liquid electrolyte we have modified the liquid electrolyte on the addition of polymer and developed quasi
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solid state DSSC. For this, 0.3 g iodine 1.0 g potassium iodide and 0.5 M 4-TBP was dissolved into polyvinyl alcohol solution to give a homogeneous solution and was heated up to 60 °C to accelerate
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the polymerisation. After 1h a polymer gel electrolyte was obtained.
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The measurement of light absorption at room temperature was carried out using a spectrophotometer (Jasco V-570 UV/VIS/IR). The morphologies and sizes of the as-prepared samples were examined by
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transmission electron microscopy and x-ray diffraction method. The morphology and sizes of the ZnO sample was examined by scanning electron microscopy (JEOL/EO, and JSM6390) and the X-ray diffraction data were collected on an AXS Bruker D% diffractometer using Cu Kα-radiation (λ=0.1541 nm, the operating conditions were 35mA and 40 kV at a step of 0.0200 and step time of 29.5 s in the 2ϴ range from 30 to 70 0. The electrical characteristics (J–V) of the prepared solar cells were examined by using a standard solar irradiation of 1000 W xenon arc lamp as light source. The cell surface was exposed to light through a water column to minimize heating effect. The input intensity was measured using a light meter (metravi 1332). The incident light intensity in the range 4200-8000 Lx (6 W/m2-12 W/m2) was measured and then converted into W/m2 using the simplified relationship 1 Lx=1.46 mW/m2 [33,34] Incident monochromatic photo-to-current conversion
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ACCEPTED MANUSCRIPT efficiency (IPCE) measurements were carried out using small band-pass filters to provide monochromatic light.
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The degree of the squared shape of the J-V curve is given by the fill factor (FF), which measures the ideal nature of the device and is defined as the ratio of the maximum power output per unit area to the
[1]
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product of open-circuit voltage (Voc) and short circuit current (Jsc).
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The solar to electric power conversion efficiency ƞ is given by the ratio of the maximum extractable power to the incident solar power (Pin) given by equation
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[2]
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Where Pin is the incident power, Pout is the output power, FF is the fill factor, ƞ is the efficiency, Jsc is the short circuit current density, and Voc is the open circuit voltage.
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Incident photon-to-electron conversion efficiencies, IPCE(λ), were recorded as a function of incident
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wavelength for the betanin-sensitized solar cell, it is the ratio of the number of electrons produced in the external circuit to the number of photons incident on the cell and is calculated using the formula [35]
[3]
Where λ is the wavelength of the incident light in nm, Pin is the incident power in watts, and Jscis the maximum current in amps. The shape of IPCE (λ) is somewhat different from the absorption spectrum of the dye sensitized film which may be due to the adsorption of more than one kind of pigment. 3. Results and discussion 3.1 Effect of electrolyte
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ACCEPTED MANUSCRIPT The solar cells with ZnO with different electrolytes in the presence of betanin natural dye were initially prepared in order to study the effects of electrolyte on the photovoltaic behaviour of the system. The morphology of ZnO nanostructure (like nanosheets) was identified by SEM. The X – ray
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diffraction (XRD) pattern of ZnO film is also shown in figure 1. The current–voltage (J-V) data for DSSC containing ZnO are shown in figure 2 and summarised in table 1. In this study, the cells using
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acetonitrile (ACN) solution have a higher efficiency 2.99% than reported [23], and the overall photon
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to current conversion efficiencies (IPCE) of 20% was achieved. The iodide (I2) electrolyte in ACN has been shown to be the most efficient organic solvent due to its viscosity (0.34 cp, 25 °C)[36] and
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good solubility to dissolve organic components and salts, as well as additives in the electrolytes, at the same time it has good long term stability. When ferrocene is replaced by other electrolyte iodine in polyvinyl alcohol (PVA) Eredox-EHUMO is reduced, which will affect Voc and Jsc..In the case of PVA
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based electrolyte the performance drops down due to the poor mobility of charge carriers as well as poor wetting ability. Its incomplete penetration into the semiconductor to regenerate the dye plays a
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detrimental role.
Even though the iodide based electrolyte showed better Jsc than ferrocene based electrolyte, the
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ferrocene based electrolyte exhibit better Voc and FF attributed to lower charge recombination at the
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photo electrode. Dye degradation by ferrocene is a simple one-electron transfer reaction (D+ → D + Fc) that does not involve the cleavage or reformation of chemical bonds or the formation of highenergy intermediate radical species and has more favourable redox potential than the two electron I3-/Iredox system (0.62 V )which should result in a higher Voc[37].The nature of the resonant HOMO→LUMO transition, vibrations of the aromatic ring and conjugated framework are strongly boosted by using iodide based electrolyte in ACN. 3.2 Effect of Au nanoparticles When the frequency of the incoming light is in resonance with the collective oscillations of the electron density i.e. surface plasmons, metal NPs show a strong enhancement in the local electromagnetic field. As a result, unique optical properties known as localized surface plasmon
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ACCEPTED MANUSCRIPT resonance (LSPR) have been observed which are depends on the size, shape, density and local dielectric environment of the NPs [38]. To enhance solar harvesting in organic solar cells, uniform sized metal nanoparticles (gold (Au) of ~8 nm) synthesized via microwave irradiation method which
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is a kind of simple and quick solution process that can control the size of metal nanoparticles, were incorporated into the device, the J-V characteristics of the sandwiched cell structures with both bare
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ZnO and gold mixed ZnO were examined. Even though the iodide based electrolyte showed better Jsc
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than ferrocene based electrolyte, the ferrocene based electrolyte exhibit better Voc and FF attributed to lower charge recombination at the photo electrode, therefore, these cells are betanin natural dye
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sensitized and use a ferrocene based electrolyte and an active area of 0.6 cm2 with Pin= 8 k lx. The optical absorptions of ZnO NS on ITO, Au incorporated ZnO on ITO and betanin sensitized ITO of the same are shown in figure 3.The absorption spectrum of the ITO/ZnO NS structure incorporated
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with gold NPs is characterized by a large absorbance over the measured spectral range and a clear broad peak in the range 380 - 488 nm, and the observed spectral change (red shift) in this system is
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attributed to the plasmon resonance absorption arising from electron deficiency on the gold surface and the interfacial coupling between the Au and ZnO.
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Upon illumination with simulated sunlight, the device with ZnO mixed with gold nanoparticles can
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deliver improved photocurrent as shown in figure 4.Also the power conversion efficiency improved from 0.868 % to 1.71 % and exhibits IPCE of 14 %, which is depicted in figure 5. These changes apparently results from the enhanced visible absorption of the active layer via the high electromagnetic field strength in the vicinity of the excited surface plasmons provided by the Au NP embedded to the ZnO NS, leading to an increase in the exciton generation. An enhancement over the visible range is related to the combined effect of betanin natural dye and the LSPR which induce more photo generated charge carriers. The formation of Schottky barrier and the possible electron transfer from the excited dye molecules to the ZnO is schematically represented in figure 6. Upon irradiation the electrons from the LUMO band of the dye are transported to the conduction band (CB) of ZnO through the tunnelling effect of the thin layer of gold NPs. It is further believed that another way of electron transfer from the electron 8
ACCEPTED MANUSCRIPT accumulation in the Au nanoparticles, raising their Fermi energy level to further closer to the CB of ZnO and a quick transport of electrons from Au to ZnO through the physical necking network takes place to establish charge equilibrium in the system. The electrons transferred to ZnO are collected at
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the bottom ITO electrode, and thereby generate an anodic photocurrent. The Schottky barrier at the Au-ZnO interface reduces charge recombination by blocking the transfer of electrons from the ZnO
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conduction band to the dye and/or electrolyte thus improving the efficiency of DSSCs due to the large
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work function of Au compared with the electron affinity of ZnO. Normally, when the charge transport was improved the FF and Jsc will be improved simultaneously. But here the FF was decreased from
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0.35 to 0.29 is probably caused by an increase in the resistive loss due to the increased flux [39,40] and the charge recombination at the photo electrode coming from the Au nanoparticle agglomeration. These accumulated electrons recombine with the oxidised dye molecules or with the redox electrolytes
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before they can be transfer to the CB of ZnO [41,42].
The presence of carboxylic groups in the betalain presents together with the higher oxidation potential
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boosts the anchoring. The formation of the oxidation product, cyclo- dihydroxyphénylalanine along with the possible formation of indicaxanthin and the heat-induced cleavage of betanin into betalamic
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acid may play a crucial role in the cell performance. To assess the stability of the assembled cell they
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have been wrapped in tissue and left for 1 day and for 1 month. Stability of the ZnO/Au nanoparticles with fresh ferrocene electrolyte was tested by measuring the J–V characteristics of the ZnO/Au solar cell after 24 h, only negligible changes in the cell performance were observed. But for the 1 month dated cell show the photovoltaic action with very less values of Jsc and Voc. Although the efficiencies obtained with betanin natural dye are below the requirements for the large scale production, the results are hopeful and are suitable alternative to inorganic dyes in DSSC. 4. Conclusion Betanin sensitised Dye-sensitized solar cells with a ZnO/Au nanocomposite as the photoelectrode were fabricated and their performances were compared with the bare ZnO NS DSSC and the effect of electrolyte with different ion composition onto the bare ZnO NPs has also been discussed. The
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ACCEPTED MANUSCRIPT photochemical performance of DSSC with NP based photo anode with ferrocene based and iodide in acetonitrile solution exhibit better efficiency of 0.868 % and 2.99 % respectively .We found that, incorporation of Au NPs led to enhanced power conversion efficiency from 0.868 % to 1.71 % and
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IPCE of 14 %,mainly resulting from the formation of the Schottky barrier at the ZnO/Au interface and enhanced optical absorption of ZnO/Au photo electrodes arising from both high electromagnetic
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field strength in the vicinity of excited surface plasmons of Au NPs and high dye excitation .
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Acknowledgement:
This work was supported by the research grants from the DST pursue scheme and the financial
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support from the IUCND. References
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List of figures
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Figure 1: (a) XRD pattern and SEM image of ZnO film and (b) TEM image of Au NPs Figure2: current density against voltage characteristics of ZnO with different electrolytes (Pin=4.2klx)
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Figure 3 :Absorption spectra of ZnO on ITO film ,ZnO mixed with Au NP on ITO film and
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betanin sensitized ITO film of the same.
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Figure 4: current density against voltage characteristics of Au incorporated ZnO (Pin=8 klx) Figure 5: IPCE (λ) for (a) betanin-sensitized NP with ferrocene based and iodide electrolyte
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Area of cell Jsc(mA/cm2) Voc (V)
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Ƞ (%)
0.87
0.52
0.02
0.84
0.35
I2 in ACN
0.63
0.99
0.15
0.21
2.99
I2 in water
0.56
0.05
0.25
0.33
0.48
I2 in PVA
0.52
0.01
0.06
0.29
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Ferrocene
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Table 1: comparison of DSSC with different electrolyte (Pin=4.2k lx)
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The influence of electrolytes has been studied
Cell efficiency of 2.99% was achieved by ZnO
Enhancement of efficiency with incorporation of Au nano
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