Fabrication of Platinum coated glass using sputtering system for Dye-Sensitized Solar Cell

Platinum (Pt) coated glass has been successfully fabricated using direct current (DC) sputtering system with different deposition times. Pt films were deposited at 1s, 5s, 10s, 30s and 60s. Surface topography and morphology obtained from atomic force microscope (AFM) and field emission scanning electron microscopy (FESEM) show reducing in surface roughness and particle sizes. Finally, deposition time of 1s give the highest power conversion efficiency of 2.148%.


Introduction
Conventional solar cell with silicon as the major material involves complex preparation and high cost productivity.In contrast, Dye-Sensitized Solar Cell (DSSC) is an efficient device in the aspect of cost and preparation technique without having any harmful side effect of the living things.DSSC consists of two flourine-doped tin oxide (FTO) glasses with metal oxide thin film as electrode and counter electrode (CE), dye and electrolyte.The deposited layers are at both of electrode, a nano-crystalline layer that is titanium dioxide layer (TiO2) at electrode and a layer of platinum (Pt) at CE.
Platinum is a chemical element that has 78 atomic numbers and belongs to group 10.In addition, Pt has grayish white color as transition metal and its characteristic as least reactive metal provided that remarkable resistance to corrosion [1].Thus, this leads to use of Pt as vehicle emissions control devices, catalytic converters, laboratory equipment and electrical contacts [2].The contribution of Pt in DSSC as counter electrode involves study of photo electrochemistry subfield.It becomes a key of interest to provide the development of renewable energy conversion.In detail, the photo electrochemistry process involves the formation of electron and hole that occurs at the interface between a semiconductor and a solution that absorbs light which leads to oxidation or reduction reactions of the solution species [3].
Pt was used as counter electrode in DSSC react as catalyst to decrease the recombination possibility of I3-and electrons in the conduction band of TiO2, restrain the dark current, enhances the charge transfer between the CE and electrolyte surface and consequently improves the open circuit voltage [4][5][6].Deposition of Pt as a thin film on the glass was done by DC reactive sputtering method.The process also gives an improvement to quality of deposited samples because the sputtered atoms embed themselves in the surface layer and gas adsorbed on the coating surface in the vacuum environment.
There are a lot of materials that had been tested to replace or reduced relying on Pt as catalyst such as, graphene, graphene/MWCNT (acid-treated multi-walled carbon nanotubes), PEDOT (poly 3,4-ethylenedioxythiophene) and Pt-graphene [7][8][9][10].At the counter electrode, the mediator was reduced into its original state of the electron collected at TiO2 side of the cell [11][12].However, it shows extremely poor catalytic property for iodine reduction [13][14].This is due to the counter electrode must have high catalytic materials on it.The substitute material of the counter electrode of DSSC must meet requirements to have superior catalytic reactivity, high conductivity and low potential of electrode reaction.Pt film was prepared with various type of method such electro-deposition, thermal decomposition and sputtering processes [15].
In this study, various thicknesses of platinum layers were deposited on top of FTO layers and the power conversion efficiencies of DSSC were been studied.DC reactive sputtering method was used with different deposition time in order to have different thickness of Pt film.Pt film with minimum amount of Pt coated on glass achieved highest photocurrent conversion efficiency of the DSSC.

Experimental 2.1 Fabrication of platinum coated glass
A cleaned FTO with length of 2.0 cm and 1.0 cm was prepared by sequential ultrasonic rinses with a 5 ml equal ratio of ethanol, acetone and distilled water in 10 min.DC sputtering deposition method was performed with 99.99 % pure Pt as target material.Sample of FTO glasses are arranged and fixed with tape on a substrate holder and the distance between the target and sample was 5 cm.Background pressure in this study was fixed at 5.01 x 10 -6 torr and argon flow rate was set to 40 sccm and monitored by mass flow controllers.It is powered by 100 Watt direct current power supply.

Fabrication of TiO2 thin film and Dye-sensitized solar cell
The TiO2 film was prepared from mixture of 5.5 ml acetic acid, 0.4 g of P25, 20 ml of TKC, 30 ml ethanol and 5 drops of Triton X-100 by using spray pyrolysis method on 150 °C hot plate.Then, TiO2 sample was annealed at 600 °C for 3 hours.In order to assemble of the DSSC, the dye adsorption, the annealed TiO2 was immersed in dye solution of RU (II)L2(NCS) 2:2 TBA (L=2,2'-bipyridyl-4,4'-dicarboxylate, TBA=tetra butyl ammonium, N719 Solaronix) overnight in order to ensure enough dye molecules absorb onto the porous TiO2.The electrolyte was injected between the electrodes and both of the electrodes were sandwiched and clipped with crocodile clips at top and bottom of the electrodes.

Results and discussion
Fig. 1 shows topography of atomic force microscope of Pt coated FTO with different deposition time (a) 1 s (b) 5 s (c) 10 s (d) 30 s (e) 60 s.From the topography, there is no significantly different with different deposition times.From table I, surface roughness was increased with the increasing of deposition time.The lowest average roughness was 6.858 nm for one second followed by 5 s, 10 s, 30 s and 60 s deposition times with average roughness that was 7.103 nm, 15.981 nm, 19.729 and 19.214 nm, respectively.In addition, at 60 s of deposition time, shows relatively similar average roughness with 30 s.This is because DC magnetron sputtering has properties of uniform directional deposition.Surface morphology of the Pt film was characterized by FESEM shown in Figure 2. The samples of Pt films were prepared by varying time deposition such as 1 s, 5 s, 10 s, 30 s and 60 s.The FESEM images indicate that the particle of Pt was well deposited and formed.The Pt coating is conformal and reflected the morphology of the underlying FTO glass substrate.This morphology consisted of a considerably textured surface including elongated features of different range of sizes as varying in deposition time.Simultaneously with the increasing deposition time, the growth rate of the particle size also increased.At one second of deposition time, the particles size was from 50 nm to 200 nm.It shows almost no difference with five seconds deposition time as shown in fig. 2 (a) and (b).However, as the deposition time increased, the density of the particle was more packed for five seconds than for one second.The particles size increased from 50 nm to 300 nm in 10 s and from 100 nm to 400 nm in 30 s and 60 s.The biggest size particle was approximately 400 nm which obtained for 30 s and 60 s of deposition time.However, as shown in fig. 3 (d) and (e), at 30 s shows smaller size of particles and the particles are less dense than 60 s deposition time.The particles are more packed and with bigger particle size deposited onto FTO for 30 s and 60 s.At 1s and 5 s, Pt films show less dense and smaller particle size however it has smoother surface and better mirror properties.crystallite face centered for 60 s Pt film.There were five more peaks which contribute to FTO planes at 26°, 35°, 38°, 52° and 62° which confirmed the presence of Pt nanoparticle on FTO.
In order to evaluate the performance of DSSC with the Pt different deposition times, power conversion efficiency measurements were carried out.As shown in Table II, the devices have similar Voc for 1 s, 5 s and 10 s deposition time but different short circuit current density and fill factor (FF).Hence, different photocurrent conversion efficiency was measured for each of the Pt films.The highest Jsc and FF were 5.665 mA/cm2 and 0.488 respectively, at one second deposition time and lead to the highest photocurrent conversion efficiency of 2.148 %.The efficiency of photocurrent conversion for 5 s, 10 s, 30 s and 60 s deposition time that were 1.527 %, 1.523 %, 0.013 %, 0.002 % respectively.From Figure 4, shows that for 10 s deposition time of Pt, there was a slight decrease in FF consequent with significant reducing in Jsc.This is resulting in photocurrent conversion efficiency of 5 s and 10 s to show no significant difference.The samples of Pt film of 30 s and 60 s of deposition time show lower photocurrent conversion efficiency.This is due to the lower surface area of Pt thin film.When the deposition time is increased, the particles size will increased, thus surface area will decreased.Lower in surface area will limited the electron from the counter electrode, thus current density will decreased.As the result, power conversion efficiency will be decreased.Pt thin film of 1 s, 5 s and 10 s deposition time have average thickness of 40 nm to 90 nm show that the surface was conformal and reflected the morphology of the underlying FTO glass substrate.Such a structure was considered give an advantage on surface adhesion and better  mirror properties of Pt film.Also, the Pt film from 40 nm to 80 nm thickness possessed good mirror properties.Lastly, one second of deposition time has the highest current density at Jsc and FF was 5.665 mA/cm2 and 0.448 respectively.This result gives the highest power conversion efficiency of 2.148 %.

Conclusion
The fabrication of the Pt coated glass using sputtering system for DSSC at various deposition times has completed and successfully achieved.The magnetron sputtering system showed a promising method of deposition Pt thin film on FTO for counter electrode of DSSC because of high deposition rate and purity of the film.The surface morphology and topography of the Pt film also have their own properties in contribution as counter electrode of DSSC.This was because of the size of particle and surface roughness that will provide smoother surface and better mirror properties.The highest conductivity Pt film is that of one second deposition time and significantly the photocurrent conversion efficiency, shows one second Pt film has the highest efficiency of 2.148 %.

Fig. 3 .
Fig. 3. XRD pattern of Pt coated glass with different deposition time.

Fig. 4 .
Fig. 4. Graph showing the current density as a function of voltage for varied time of deposited Pt.

Table I .
Roughness counter electrode with different deposition times

Table II .
Roughness counter electrode with different deposition times