Manipulation of MWCNT Concentration in MWCNT/TiO2 Nanocomposite Thin Films for Dye-Sensitized Solar Cell

1 Department of Electrical, Electronic & System, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia 2 Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia 3 Department of Civil and Structural Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia


Introduction
Dye-sensitized solar cell (DSSC) is considered as a relatively new type of solar cell being discovered, in 1991, by Grätzel et al.DSSC shows greater promise compared to Si solar cells due to low cost, environmental friendly and simple manufacturing process.DSSC has attracted a lot of attention worldwide.Nevertheless, Grätzel's cell has a solar conversion efficiency of ∼13% [1], which is significantly lower than that of Si solar cells.To improve the performance of DSSC devices, a number of aspects are considered.Electron transport across a TiO 2 electrode is one of the most important factors affecting the conversion efficiency of DSSC; the greater electron mobility is, the higher the DSSC efficiency will be [2].On the other hand, charge recombination processes generally inhibited injected electrons from TiO 2 to the conducting glass substrate, thus decreasing the performance of DSSC.Therefore, the rapid photo induced electron transport in the working electrode, TiO 2, and the suppression of charge recombination processes can ensure a higher conversion efficiency of DSSC [3,4].
In recent years, multiwalled carbon nanotube MWCNT has attracted considerable attention worldwide due to its excellent mechanical properties and electrical and thermal conductivity making it a high potential candidate in various applications, for example, field emission display [5], photo catalysis, photovoltaic devices, and DSSC [6].Several researches [7,8] reported the incorporation of MWCNT within nanocrystalline TiO 2 working electrodes to enhance the solar energy conversion efficiency of DSSC.Furthermore, some studies [9] have revealed that the better performance of DSSC fabricated using MWCNT/TiO 2 electrodes is because of the higher electron mobility at the electrodes than that of conventional TiO 2 electrodes.This resulted in a higher short-circuit photocurrent ( sc ) of DSSC.However, the performance of DSSC dropped when high MWCNT content was applied to the MWCNT/TiO 2 electrode, possibly due to a severe aggregation of MWCNT in the nanocomposite electrode [10,11].
In this study, we aim to synthesize and fabricate MWCNT/TiO 2 dye-sensitized solar cell using sol-gel method with different concentrations of MWCNT.The different MWCNT concentrations were the main key in order to get better photovoltaic efficiency.We investigate the MWCNT concentration effect on the solar cell in terms of their photo-conversion efficiency performance.The samples were analyzed using FESEM, TEM, AFM, and IV curve analysis to see the effect of MWCNT doped into TiO 2 photoelectrode.

Methodology
2.1.Materials.Titanium (IV) tetraisopropoxide (TTIP) (98%) was purchased from Sigma-Aldrich, Belgium, and used as the main material.MWCNT, 98% carbon basis with length of 6 to 13 nm, was purchased from Sigma-Aldrich, USA.The MWCNT will go through the acid treatment process before being used.The MWCNT will be sonicated for 2 h in a beaker containing 50 mL of concentrated nitric acid and boiled at 90 ∘ C on a hotplate.The MWCNT powder achieved after filtering was washed several times using distilled water to remove residual acid and dried in oven for 24 h.In this study, other chemicals were used as ethanol anhydrous I 2 (99.5%), ruthenium 620-1H3TBA dye obtained from Solaronix SA, adult MPN-100 purchased from Solaronix SA, nitric acid (95%), and electrode substrate fluorine tin oxide (FTO) glasses, 30 Ω, obtained from Solaronix SA and used as received.

Preparation of MWCNT/TiO
2 Solution.TiO 2 nanoparticles are produced from titanium (IV) tetraisopropoxide that acts as a precursor solution.The TTIP solution mixed with anhydrous ethanol solution in the ratio of (0.1/2) using magnetic bar stirrer for 30 min.The acid treatment MWCNT powder with different weights of (a) 0.00 g, (b) 0.01 g, (c) 0.02 g, and (d) 0.03 g was added into the TiO 2 colloidal solution paste and dispersed using an ultrasonic horn machine for 60 min.MWCNT/TiO 2 nanocomposite stirred vigorously for another 2 h to get homogeneous solution.The MWCNT/TiO 2 paste coated onto the fluorine tin oxide (FTO) conductive glass using doctor-blade technique to generate 0.25 cm 2 active area is followed by evaporation of ethanol in air at room temperature for a few minutes.All the MWCNT/TiO 2 thin films were annealed in a dry furnace at 300 ∘ C for 30 min forming noncrack and uniform thin film electrode.Figure 1 shows the flowchart of this preparation.

Fabrication of Dye-Sensitized Solar
Cell.The annealed MWCNT/TiO 2 electrode thin films were then being immersed in 0.5 M N719 ruthenium dye for 1 day to make sure that all the MWCNT/TiO 2 particles were covered with the N719 dye particle.The electrodes were then sandwiched with another electrode glass called counter electrode which was covered with platinum (Pt) thin film on top of the FTO glass.Both electrodes separated using parafilm barrier  to place electrolyte between the electrodes.DSSC based MWCNT/TiO 2 nanocomposite is completed and is ready to be analyzed.Figure 2 shows the flowchart of this preparation.

Field Emission Scanning Electron Microscopy (FESEM).
By referring to other researches and studies [12], we expect to improve the solar cell energy conversion efficiency with the presence of the MWCNT as favorable electrical conductivity on the metal oxide nanocomposite.MWCNT nanoparticles can extend the electron lifetime and enhance the electron transport rate in the photovoltaic metal oxide electrode.In addition, we notice that the MWCNT nanoparticles that go through acid treatment process in the concentrated nitric acid could produce MWCNT with terminal COOH group.This phenomenon can improve the solar cell electron collection due to better interconnection between MWCNT and TiO 2 nanoparticles.Figure 3: (a) 0.00 g, (b) 0.01 g, (c) 0.02 g, and (d) 0.03 g show the morphological images of MWCNT/TiO 2 thin films, where else for (e) and (f) images show the film cross-section and EDX graph.From FESEM images, we found that the MWCNT and TiO 2 nanoparticles there are in good contact.MWCNT nanoparticles are well dispersed and highly compact after annealing at 300 ∘ C for 30 min.The porosity can be observed from the morphological structure of all thin films.The thin films are connected randomly which in lack dissemble long range order for the pore arrangement.We also found that, with every additional MWCNT, the porosity of the thin film became bigger and larger, while the TiO 2 nanoparticles become smaller and thinner [13,14].The thin films also show, with increasing the amount of MWCNT, the structural morphology of TiO 2 nanoparticles changes from spherical  to oval nanoparticles structure, which existed around the long range MWCNT nanoparticles.The average thickness of TiO 2 and MWCNT/TiO 2 thin film samples around 14.88 m and 18.79 m, respectively.The addition of MWCNT can improve the structural and morphological design of the films.However, the amount of MWCNT added must be controlled and optimized to produce high quality electrode thin film.The amount of MWCNT can affect the films in terms of large crack and inhomogeneous arrangement.Referring to others [15], with the increase in the amount of MWCNT, the number of cracks on the surface of the films is increased subsequently.It is thought that the cracks generated on the surface could be reducing the number of adsorption sites on TiO 2 film as well as causing the discrimination in the conversion efficiency of DSSC.
Table 1 shows the data collected for energy dispersive Xray (EDX) spectroscopy analysis.The EDX data prove that every additional MWCNT added into the samples, the carbon weight percentage in the samples increased from 0.00%, 4.15%, and 2.25% to 6.55% of samples (a) 0.00 g, (b) 0.01 g, (c) 0.02 g, and (d) 0.03 g, respectively.These results confirm that the sample (d) 0.03 g MWCNT added has a larger MWCNT weight percentage compared to the other samples.Figure 4 shows the MWCNT weight percentage in graphical diagram.Both oxygen and titanium compound weight percentage data were decreased in small amounts from 40.09% to 38.84% and from 59.91% to 54.61%, correspondingly.

High Resolution Transmission Electron Microscopy (HR-TEM).
In Figure 5, the HR-TEM images of TiO From these images, we can find out that the type of CNT used in this research is MWCNT not SWCNT [16,17].This can be confirmed by the multiple thin walls formed along the CNT.The TiO 2 nanoparticles and TiO 2 sphere-like shape are formed and positioned mostly around the MWCNT particles.Another researcher [18] also implies that there is a good contact between MWCNTs and TiO 2 particles.MWCNTs are directly coupled with the uniform anatase shell, which are together embedded in the TiO 2 aggregates.Such a structural feature is beneficial for efficient electron transfer and hole-electron separation, as if there is a "conducting wire" acting as a readily accessible electrontransfer channel.Table 2 illustrates the data size parameter for TiO 2 doped MWCNT nanocomposite and TiO 2 nanoparticles alone.The MWCNT inner and outer structure diameters were approximately 4.49 nm and 17.71 nm in length, respectively.The diameter size for TiO 2 doped MWCNT was slightly smaller with only 3.91 nm compared to the TiO 2 nanoparticles with 12.00 nm.This might be due to the combination with MWCNT nanoparticles that reduce the TiO 2 size and hence produce highly porous thin film as presented and shown in the FESEM analysis.HR-TEM images suggest a good affinity between the TiO 2 and the MWCNT, which is important in view of limiting the MWCNT loading required to improve DSSC performance.MWCNT improves the roughness factor of the electrode and limits the charge recombination of electron/hole (e − /h + ) pairs [19].Another advantage of combining MWCNT with TiO 2 was to get higher photo response due to decrease in resistivity of the thin film and resulting in a higher current collection at the electrode thin film [20,21].
Nevertheless, a high loading of MWCNT causes lightharvesting competition that affects the light absorption of the dye sensitizer and consequently reduces the cell efficiency.Moreover, an excess of MWCNT can result in a less compact TiO 2 layer, in which large pores are formed at the micron scale [22] or in the formation of quite disconnected aggregates of MWCNT covered in conformance by TiO 2 nanoparticles.characterization is an important characteristic to investigate the surface reflection phenomenon in DSSC [23].Due to the increase in roughness average, the enlargement for surface texture angle in the thin film will bounce the light on surface films and causing the light to reflect indirectly back to the electrode surface.This phenomenon can increase the light absorption in the metal oxide photovoltaic and improve the light of electrical conversion energy because the light reflectance had been reduced [24].3.4.I-V Curve Efficiency. Figure 8 shows the -curve graph of MWCNT/TiO 2 thin film photoelectrode dye-sensitized solar cell.This process is performed under 100 mW/cm 2 illuminations using xenon lamp.The measurement of light to electrical process was executed after completing the sandwich-look DSSC between electrode and counter electrode part.The parameters included in Table 4 were opencircuit voltage ( oc ), short-circuit photocurrent density ( sc ), fill factor (FF), and energy conversion efficiency () [25].As stated in Table 4, open-circuit voltage ( oc ) data increased The efficiency data for samples (a) 0.00 g, (b) 0.01 g, (c) 0.02 g, and (d) 0.03 g MWCNT are 1.43%, 2.12%, 2.46%, and 2.80%, respectively.Sample (d) 0.03 g CNT gives the highest efficiency percentage 2.80% with 0.65 V ( oc ), 9.42 mAcm −2 ( sc ), and 45% fill factor.Table 4 demonstrates the solar cell performance trend.It showed that, with every additional MWCNT powder, the efficiency of the solar cell exponentially increased.The MWCNT/TiO 2 nanocomposite combination tends to improve the electrical conductivity of the photoelectrode and also helps to enhance the interconnectivity between the TiO 2 and MWCNT nanoparticles, thus increasing the short-circuit current density and providing an alternative route for efficient electron transfer between the TiO 2 nanoparticles.It shows that the MWCNT can operate as an electrochemical catalyst to improve the energy conversion efficiency of DSSC [28].The overall light to electrical effectiveness of the dye-sensitized solar cell can be efficiently enhanced with every additional MWCNT added.By referring to another researcher [29], the advantages of adding MWCNT in the photoelectrode are that the MWCNT can absorb over almost the entire visible light spectrum and act as photo sensitizers, providing the MWCNT/TiO 2 nanocomposites with an electron transfer mechanism similar to that of DSSC based TiO 2 .As a result, the MWCNT/TiO 2 samples can transfer excited electrons from the MWCNT to the conduction band of TiO 2 when illuminated with visible light, thereby increasing the photocurrent.Second, the conductivity of MWCNTs is superior to that of TiO 2 ; therefore, we can expect a high transport rate of electrons in the CNT/TiO 2 composites.

Atomic Force Microscopy (AFM). The MWCNT/TiO
However, the concentration of MWCNT should be maintained ∼0.03 g optimum MWCNT to avoid MWCNT agglomeration within the films.This is because higher concentration of MWCNT can cause light-harvesting competition between the dye and MWCNT particles.Thus, increases the charge transport resistance and consequently reduces the solar cell efficiency [30].Besides, excess MWCNT may cause aggregation of TiO 2 grains (as observed in the FESEM result), leading to a decrease amount of dye being adsorbed on the working electrode.In addition, as reported by another researcher [31], excessive amount of MWCNT may cause the working electrode to be less transparent, which leads to reduced efficiency of DSSCs.Furthermore, this condition will decrease the crystallinity of the TiO 2 samples, thereby inhibiting the transport of electrons and increasing the probability of electron trapping by the crystal defects.Moreover, the shielding and scattering effects of excess MWCNT might have prevented the photo absorption of other visible light-active species.Nevertheless, the optimum amount of MWCNT needs to be considered for fabrication of MWCNT/TiO 2 working electrodes being used in DSSC applications.

Conclusion
The MWCNT/TiO 2 nanocomposite DSSC was successfully fabricated using sol-gel method and doctor-blade technique.The films were uniform and highly adherent.Photochemical and structural properties of the thin film were improved by incorporate MWCNT powder into TiO 2 nanoparticles.The 0.03 g is the best and optimum concentration of CNT added to this research.Excessive amount of CNT may cause the working electrode to be less transparent, which leads to reduced efficiency of DSSC.FESEM morphological analysis indicated that the TiO 2 and MWCNT/TiO 2 thin films were in compact alignment and highly porous with thickness around 14.88 m and 18.79 m.HR-TEM inner structural analysis confirms that the thin film is composed of TiO 2 nanoparticles that existed around the multiwalled carbon nanotube particle.The average thickness for MWCNT and TiO 2 nanocomposite was about 17.71 nm and 3.91 nm, respectively.AFM analysis proved that the roughness factor can significantly improve the photoelectrode performance in the solar cell.The AFM topography reveals a very compact and rough surface; the rms values of all films are in the range of 10-25 nm.From - analysis, the highest efficiency

Figure 4 :
Figure 4: Carbon weight percentage graph from EDX spectroscopy analysis.
2 nanoparticles (a) 0.00 g, (b) 0.01 g (c) 0.02 g, and (d) 0.03 g MWCNT/TiO 2 nanocomposite are shown.Images in Figures 5(a), 5(b), 5(c), and 5(d) are carried out to show the comparison between undoped TiO 2 nanoparticles and TiO 2 doped MWCNT nanocomposite in terms of their inner structure design.HR-TEM investigation on selected MWCNT partially covered by TiO 2 aggregates indicates that simple or multiple connections of aggregates of TiO 2 nanoparticles to MWCNT are possible.In Figure 3(a), TiO 2 nanoparticles with low grain density and compact agglomerate configuration were observed.The TiO 2 nanoparticles sizes vary compared to the TiO 2 attached to MWCNT particles.From images (b, c, and d), by using chemical treatment process, we can see the MWCNT particles successfully formed in nanotube structure.
2 thin films upper texture prepared using doctor-blade technique are observed by AFM instrument.Images in Figure 6: (a) 0.00 g, (b) 0.01 g, (c) 0.02 g, and (d) 0.03 g demonstrate that, with increasing MWCNT concentration in the MWCNT/TiO 2 sample, the texture of the thin films
TTIP and P25 TiO 2 nanopowder mixed at molar ratio of 0.1/1 in ethanol solution for 30 min.

Table 1 :
Weight percentage parameter from EDX analysis.

Table 3 :
Average roughness parameter for all samples.The rms values of the MWCNT/TiO 2 thin film are listed in Table3and the data are illustrated with graphical diagram in Figure7.AFM measurement investigates the surface morphological roughness of the film.By AFM roughness analysis, roughness factors obtained are increased as increasing the content of MWCNT in TiO 2 film with the maximum value in 0.03 g carbon-content TiO 2 as listed in Table1.This International Journal of Photoenergy

Table 4 :
[27]r cell efficiency parameter of MWCNT/TiO 2 with different MWCNT concentrations.An increase in gas values implied enhanced electron transfer in DSSC[26].The  oc and  sc data gradually increased for sample (d) 0.03 g, which might be attributed from enlargement of thin film porosity and alignment of MWCNT in the photoelectrode thin film.As referred to other researches[27], MWCNT particles might cause a significant change in efficiency via the fluctuation of the short-circuit photocurrent and the open-circuit voltage for the DSSC.Incorporating MWCNT into TiO 2 nanoparticles electrode might also affect the quantity of dye adsorption and the e − /h + recombination process in this dye-sensitized solar cell.