Effects of Potential Deposition on the Parameters of ZnO dye-sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) are a feasible option for photovoltaic energy. Zinc oxide is an n-type semiconductor employed as photoanode on DSSCs. ZnO thin films were electrodeposited to study the effects of different potentials applied during deposition. SEM images, XRD and UV-Vis analysis were conducted to reveal the morphologic, structural and optical properties of the films at three potentials. DSSCs were assembled and the photovoltaic parameters were obtained through J-V plots. DSSC with 0.031% of efficiency was demonstrated at -1.4 V of deposition potential.


Introduction
Dye-sensitized solar cells are an alternative to the use of the conventional Si based solar cells due to advantages such as low production cost and relatively high-energy conversion efficiency 1 .This photo electrochemical conversion can be explained by the light harvesting of the dye molecules found on a typical DSSC 2 .In addition, by using dye, DSSCs can be colored or transparent, which increases the commercial use on locations where the appearance of the cell is important, like sunroofs and automobile panels 2 .Overall, a DSSC is composed by a photoanode, an oxide layer for electronic conduction, a monolayer of charge transfer dye, a redox electrolyte, usually an organic solvent that restores the dye by electron donation, and a counter electrode 3 .
In a DSSC, the photo-excited dye oxidizes the mediator, the redox electrolyte, and is oxidized by the photoanode.The electrons circulate through the external circuit and, at the cathode; the electrolyte is regenerated by reduction 2 .Figure 1, adapted from Grätzel (2003), exemplifies the working principle of a DSSC, using titanate as photoanode 3 .
The photoanode plays an important role in the conversion of light into electrical energy 4 .Among some of the metal oxides used as photoanodes, it can be listed TiO 2 , ZnO, SnO 2 , Nb 2 O 5 , SrTiO 3 , Fe 2 O 3 , WO 3 and Ta 2 O 5 .Between them, zinc oxide has many properties than can be used in a DSSC, such as high electron mobility of 115-155 cm2 V -1 s -1 , stability against photocorrosion, large excitation binding energy (60 eV) and band gap near the one of the TiO 2 (~3.2 eV) 5 .Besides, ZnO is found at low-cost and in many forms, like nanorods, nanowires and nanosheets 6 .
In a DSSC, the photoanode is deposited in a conductive glass.ZnO can be deposited by many methods, such as chemical vapor deposition, hydrothermal synthesis, spray pyrolysis, pulsed laser deposition and electrodeposition 6 .Electrochemical deposition (ECD) can fabricate thin layers with specific morphology, high orientation degree and good adhesion to the substrate, besides being cost-effective to the preparation of large area thin ZnO films 7 .
Previous works investigated the electrodeposition of ZnO thin films and its properties, such as uniformity. of the zinc oxide films decreased 9 .Ahmed et al. (2015)  also electrodeposited ZnO at high potentials, varying the duration of the ECD and using n-Si wafers, adding KNO 3 in the precursor solution 7 .In addition, publications reveal studies to improve the efficiency of solar cells, using the zinc oxide as photoanode.2018) also electrodeposited ZnO structures, this time as nanorods, with its respective DSSC presenting 1.76% of efficiency and 0.37 of fill factor 10,13 .
In this work, three dye solar cells were assembled using ZnO electrodeposited at three different potentials to observe the effect these changes had on the conversion efficiency of the cells, besides the effects on the morphology, optical and structural ZnO film characteristics.The electrodepositions were carried out at a low temperature, below the usually tested temperatures, without any catalyst or additive, a simple aqueous zinc nitrate solution, on fluoride doped tin oxide layer synthesized in laboratory and used as the working electrode in the ECD process.

Experimental
Fluorine doped tin oxide (SnO:F 2 ) was deposited by spray pyrolysis on blades for microscopy, adapted from the method described by Lima et al. (2015), without adding acid, to create a conductive coat of fluorine doped tin oxide (FTO) on the glass surfaces 14 .The furnace temperature during the pyrolysis operated at 600 ºC.Tin chloride II (NEON) (99.3%) and ammonium fluoride (Sigma-Aldrich) (98%) were the source materials for the FTO deposition.
The electrodeposition consisted of a three-electrode system, where the working electrode was the FTO coated glass (surface resistivity between 20-30 Ω/sqr), the counter electrode was a platinum sheet and Ag/AgCl (3 M) was the reference.An aqueous solution of zinc hexahydrate (Dinâmica Ltda.) at concentration of 0.2 M, kept at constant temperature of 67ºC, was the bath electrolyte for the electrochemical deposition.Before deposition, all FTO blades were cleaned on ultrasonically bath with deionized water (10 minutes) and acetone (Qhemis) (20 minutes).The potentials tested during the electrodeposition were -1.0 (Film A), -1.2 (Film B) and -1.4 V (Film C), on three different FTO blades.Each potential was applied for 30 minutes.The changes were made to observe the effect on the morphology and optical characteristics on the ZnO film.In addition, to verify if the different potentials could alter the performance of the dye sensitized solar cells.This zinc oxide electrodeposition system is similar to the one used by Lima et al. (2015), but, in this study, the concentration of the precursor solution was fixed, the applied potentials were high and varied and the temperature was fixed at 67ºC 12 .
Each one of the photoanode ZnO films were immersed in an ethyl alcohol (0.0003 M) N719 (Solaronix) dye solution for 24 h.Then, three face-to-face cells were assembled, using platinum (Pt) as the counter electrode (CE), due to being a stable and transparent counter electrode with high electrocatalytic activity towards I -reduction.Also, a Pt CE improves the light harvesting efficiency due to its reflective property 15 .The dye-adsorbed ZnO films was the photoanode and, between them, the Iodolyte AN-50 (Solaronix) acted as the electrolyte.The assembled cell, therefore, is similar to a Grätzel cell, replacing the titanate semiconductor by the zin oxide semiconductor 3 .
All the ZnO film samples were structurally characterized through X-ray diffraction.XRD analyses were carried out on a Bruker D8 Advance, using CuKα1 (λ = 0,154 nm) as radiation source.The scattering angle was in the 20º-70º range, operated at 40 kV.The optic measurements were performedin a Cary 100 UV-Vis Spectrophotometer at room temperature with wavelength range of 190-900 nm.The scanning electron microscopy (SEM), using a Quanta 450 FEG-FEI, examined the surface morphology of the films, besides the thickness of each ZnO film.The same equipment realized energy dispersive spectroscopy for the SEM images.
The current-voltage (J-V) data was obtained under illumination of 100 mW cm -2 , light source of Led whiteneutral.The electrical parameters were calculated using model proposed by Li et al 16 .

Electrodeposition process
The reactions which took place during the ECD and are responsible for the formation of the ZnO thin films are generally described by reactions 1, 2 and 3 17 .Reaction (1) describes the reduction of NO 3 -at the working electrode in Zn 2+ aqueous solution, then, those Zn 2+ react with the OH -íons formed in the reductionreaction, resulting in zinc hydroxide (Reaction 2).The Zn(OH) 2 suffers dehydration, and consequently, it occurs the chemical precipitation of ZnO (Reaction 3) 17 .The hydroxylation and the dehydration control the rate of the overall reaction and are affected, mainly, by the temperature, Zn(NO 3 ) 2 .6H 2 O concentration and the applied potential in the electrodeposition 17 .As the temperature and concentration were kept constant for the three films, in this study, the differences on the reaction rate and, by consequence, the morphology of the films were affected by the different potentials. (1) (2)

X-ray diffraction
Fig. 2 shows the XRD patterns of the ZnO films at the different potentials.All the peaks were observed at (100), ( 002) and ( 101), which is in accordance with the hexagonal ZnO wurtzite structure 10 .The slight increase in the intensity of the (002) peak in all the three patterns indicates, as observed by Marimuthu et  al. (2017), that the nanocrystals grow in a preferential or along the c-axis to the surface of the substrate, in this case FTO 10 .

NO H O
e NO H 2 20 . XRD patterns of the three deposited potentials

Morphology
SEM images (Fig. 2) illustrate the effects on the morphology structure caused by the changes on the applied potentials.The presence of porous on the structures differs from -1.0 to -1.4 V. Films A and B present porous between the nanostructures, while, at film C, those same porous are not seen as regularly and the ZnO is more nodular, which can be explained by the higher deposition potential 18 .Mahalingam et al. (2005) reports that those nodular structures are characteristic of the ZnO growth 19 .The ZnO nanosheets in films A and B can be attributed to the concentration of Zn +2 in solution, as observed by Yang et al 20 .When increasing the potential, those nanosheets are no longer noticed.
Since the precursor solution was Zn(NO 3 ) 2 .6H 2 O, the source of Zn 2+ and OH -was the zinc nitrate.At first, the Zn 2+ moves to the FTO-coated cathode, induced by the electrical field, which causes the increase of the OH -concentration and the formation of Zn(OH) 2

21
. Then, the conversion of Zn(OH) 2 to zinc nitrate, at 67ºC, stimulates nucleation centers of ZnO, specially at the (002) direction, which indicates the low energy barrier imposed at this orientation 21 .The same behavior was observed by Meng et al., (2014) 21 .The cathodic potential controls the nucleation during the deposition process, once all the other parameters were kept constant.At relatively low potentials (Films A and B) the nitrate reduction occurred slowly and not enough OH -is formed, in consequence, more random and less dense ZnO nuclei are formed (Figure 3 (a) and (b)).When the -1.4 V potential was applied, the reduction of NO 3 -and the ZnO nucleation happened at an increased rate, leading to a more dense formation ( Figure 3(c)).At higher potentials, Film C, the zinc oxide growth faces lower energy barriers, leading to high deposition rates, and more dense and packed ZnO film 7 .Entire areas of the SEM for the three samples were scanned, generating the images in Fig. 4 (inserted) and Energy dispersive spectroscopy produced the spectrums (Fig. 4).These EDS analysis allowed to determine the relative proportions (Wt%) of the elements found on a specific area, through characteristic X-rays.The ratio between the zinc and oxygen proportions was 5.16 (film A), 6.42 (film B) and 7.86 (film C), which indicates higher proportion of zinc comparatively to oxygen.The higher element ration in film C is an indication of the increased rate of reduction of nitrate and nucleation of ZnO by the electrodeposition at a high potential.The increase of the absolute value of the potential causes the formation of more dense and packed particles 17 .The spectrums confirmed the presence of atoms of zinc (Zn) and oxygen (O).For the sample C, there was a higher proportion of the Zn atoms, with 77.8%, and a higher number of x-ray counts for the zinc atoms.The presence of the Zn and O elemental peaks at the spectrums confirms the formation of zinc oxide (ZnO) in the electrodeposited samples 22 .

Optical characterization
The ZnO nanostructures optical properties were determined by absorbance measurements in the range of 190-900 nm at room temperature.FTO/glass was used before each measurement, as reference, to subtract the effects of the fluorine tin oxide and the glass 23 .The ZnO thin films exhibited higher absorbance values at short wavelength (Fig. 5), reaching lower values at the IR region 24 .The obtained absorbance peaks were at wavelength less than 380 nm, same as Yang et al 20   Where, A is proportionality constant, hv the incident photon energy and α is the absorption coefficient.The value of ½ is applied due to the direct transition characteristic of the ZnO material.The Eg value was obtained by the extrapolation of the plot of (αhv) 2 as function of the incident energy hv (Fig. 6) 26 .The band gap values were between 3.05 and 3.32 eV, as described in table 1.All the Eg values were below the standard 3.37 eV report in the literature 27 .The lowest potential applied in the ECD deposited the film with the highest band gap energy, while the minimum value for the band gap was obtained for the sample C. As the energy bad gap values decrease, there is a shift in the direction of the visible region of the spectrum (Fig. 5).

= -
The higher potential applied in the ECD deposited the thinner ZnO film.According to Sielmann et al., this effect on the film thickness is a result of the Zn +2 depletion, at high potentials, with formation of H + ions at the counter electrode 28 .These H + ions decrease the overall pH of the solution, which inhibits the film growth.

Photovoltaic performance
The photovoltaic performances of the three dye cells were compared.Figure 7 reports the photocurrent densityvoltage (J-V) plots under the 100 mW/cm 2 illumination.The photocurrent parameters are listed in table 2.
The efficiency (η) was calculated through the following equation 5 : (5 Where Voc is the open-circuit voltage, Jsc is the shortcircuit current density, FF is the fill factor and Pinc the incident light power 5 . The thicker ZnO photoanode yielded an overall efficiency of 0.00023% and Jsc of 0.0016 mA/cm 2 , while the thinner one had 0.031% performance efficiency.These results indicate that larger amounts of N719 dye molecules were adsorbed on the thinner nanosheets (Figure 8), providing a higher    photocurrent density of 0.13mA/cm 2 . 29Figure 8 shows that the film C, sensitized by the N719 dye, presented higher absorption than A and B thin films, with a peak at 350 nm, approximately.This leaded to more photogenerated charge carriers and, by consequence, higher short-circuit current density 30 .All three films absorbed mainly in the ultraviolet spectrum, from 300 to 400 nm, with much lower absorptions in the visible regions 31 .The higher Jsc was the parameter Pinc Voc x jcs x FF h = that contributed to enhance the photovoltaic performance 30 .The photocurrent density of the film C was 80.25% higher than the Jsc of the nanosheet B. In addition, the larger thickness, 650.54 nm, film B, increased the defects inside the film, causing a greater series resistance, which dropped the overall FF value from 0.52 to 0.24 8 .When the Rs is high, there is a voltage drop across the cell and more resistance to the electron transfer process 4 .The increase of the FF value indicates, in the cell C, a reduction of charge recombination between the ZnO photoanode and the electrolyte I -/I - it is still necessary to research for better conditions for the ZnO/dye pair that should avoid the creation of ZnO/ sensitizer aggregations, which lead to the increase of charge recombination at the photoanode interface and the deterioration of the DSSC performance 36 .
In short, the cells lost efficiency especially due to low light harvesting and high electron recombination.
Although the cells with zin oxide acting as the only photoanode did not yield high efficiency, ZnO can be employed in combination with different photoanodes to improve light scattering and electron injection 37 .Niaki et al. ( 2014) designed a double-layer TiO 2 solar cell doped with ZnO, obtaining 6.58% of power conversion efficiency at 0.5% ZnO doped dioxide titanium 37 40 .
The present work can contribute to improve the ECD of zinc oxide, changing the electrodeposition parameters, to be employed in combination with dioxide titanium or a different semiconductor as a dye-sensitized solar cell photoanaode, to increase the cell photovoltaic parameters such as short-circuit current density.

Conclusion
ZnO photoanodes were synthesized through electrodeposition at the potentials of -1.0 V (film A), -1.2 V (film B) and -1.4 V (film C).Nanosheets and nodular structures were obtained by changing the potential.Later, these ZnO films were used to assemble three dye-sensitized solar cells.The film with better performance as a photoanode was the nodular structure formed in C. UV-Vis analyses revealed, for the nodular film C, an absorbance shift in the visible region of the spectrum.Also, at this potential, more dye was absorbed by the film, which was indicated by the 0.13 mA/cm 2 short-circuit current density.This parameter helped to increase the overall efficiency of the dye cell.In addition, the nodular nanostructures presented the lower series resistance, which reduced the resistance to the electron transport from the photoanode through the conductive glass.This lower series resistance can be attributed to the thinner film thickness, which decreased with the more cathodic potential applied.The other photovoltaic parameters were shunt resistance, efficiency, fill factor and open-circuit voltage.The cell with zinc oxide electrochemically deposited at -1.4 V provided the best result of efficiency at 0.031% and fill factor of 0.52.All the nanostructures were applied as semiconductor without previous treatment of any kind to investigate the influence of the electrodeposition method on the efficiency of dye-sensitized solar cells.The recombination loss in ZnO based dye solar cells is caused mostly by uncovered oxide surface 32 .These uncovered areas were not reached by the dye and had closer proximity to the electrolyte surface, increasing the recombination between the electron on the conduction band of the ZnO and the holes in the electrolyte 32 .
The overall low efficiency of the dye cells A and B can also be explained by the Zn 2+ /dye agglomerations.Theses accumulations prevent the electron injection from the dye to the conduction band of the semiconductor, causing decrease on the light to current conversion efficiency 33 .This can be avoided by changing the dye concentration or the sensitization time.
The overall efficiency of the ZnO based DSSCs was low, compared to a standard TiO 2 cell.Tasic et al. ( 2016) obtained an efficiency of 4.92% for thermally treated titanium oxide dye cells 34 .The low efficiency reported in this paper can be explained by the lower stability of zinc oxide on acidic dye environments 35 .Furthermore, the semiconductor zinc oxide films were not thermally treated, since the aim of this study is to investigate the influence of the deposition potentials applied, the only varied constant between the cells.
To overcome the zinc instability on acid, different dyes have been tested with ZnO, such as Z907 and N3, but these dyes also present limitations, such as small areas for the J-V curves 31 .Chang et al. (2015) used organic sensitizer W3 with zinc oxide, but obtained 24.7 % decrease of efficiency due to an increase of the charge transfer resistance 36 .Hence,

Figure 3 .
Figure 3. Morphology structures for the three thin ZnO films (a) Film A (b) Film B (c) Film C . Yang et al. suggests those peaks are due to the electronic transitions from the valence band to the conduction band of the ZnO band gap 20 .

Figure 4 .Figure 5 .
Figure 4. EDS data and spectrum for ZnO thin films A, B and C

Figure 6 .
Figure 6.Extrapolation of the Tauc plots to determine bang gap values

Figure 7 .
Figure 7. J-V measurements for the three dye cells (a) Sample A (b) Sample B (c) Sample C

Figure 8 .
Figure 8. Absorbance analysis of the deposited ZnO thin films

Table 1 .
Band gap values and film thickness.

Table 2 .
Photovoltaic parameters for the ZnO based DSSC.