Multiwall Carbon Nanotube Coated with Conducting Polyaniline Nanocomposites for Quasi-Solid-State Dye-Sensitized Solar Cells

1 Center of Excellence for Research in Engineering Materials (CEREM), College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia 2 Photovoltaic Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan 3Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia 4Department of Industrial Engineering, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia


Introduction
In general, a typical DSSC is composed of three adjacent thin layers: a mesoporous oxide �lm, such as TiO 2 , supported on transparent conducting glass dye molecules, such as ruthenium bipyridyl derivatives which are sensitive to visible light in the solar spectrum, and an electrolyte solution containing iodide and triiodide ions as a redox mediator to reduce the oxidized dye molecules.e three layers are sandwiched together by a second conducting glass covered with platinum [1,2].However, leakage and the volatilization of organic solvent-based electrolytes have restricted practical applications of DSSC.Room-temperature ionic liquids (RTILs) have become attractive candidates for replacing organic solvents because of their negligible vapor pressure, high thermal stability, wide electro-chemical window, and high ionic conductivity [3][4][5][6][7].Nevertheless, the viscosity of RTIL is still much higher than that of organic solvents, including acetonitrile (ACN) and 3-methoxypro-pionitrile (MPN) and has resulted in lower power conversion efficiencies because of the RTIL poor ionic diffusion ability.
Carbon nanotube coated by conjugated conducting polymer nanocomposites is worthy conducting hybrid materials, which are o�en used in organic �eld effect transistor, solar cells, sensors, electrochromic devices, and light emitting diodes [8][9][10][11][12].Here, the MWNT-PAni nanocomposites were utilized to ionic liquid-based electrolyte to form the extended electron transfer surface (Scheme 1) from the counter electrode's surface to the bulk electrolyte, in order to facilitate electron transfer and, thereby, decrease the dark current from the working electrode to the electrolyte.In the present study, MWNT-PAni composites have been used in DSSC, and the effect of MWNT-PAni composites addition in the solvent-free ionic liquid electrolyte without the incorporation of iodine was studied.
e conducting MWNT-PAni hybrid composites were synthesized as per our reported work [13].e instruments used for this work included a �eld emission scanning electron microscope (FE-SEM) (Hitachi Model S-4300), a transmission electron microscope (TEM) (Philips model CM 200) with an Acc.Voltage of 200 kv.e room-temperature conductivity of the pressed pellets was measured by the fourpoint probe method using a Jandel engineering instrument, Model CMT-SR1060N.e composite electrolyte was prepared by mixing the solid powder of MWNT-PAni, PMII, and ACN in a weight ratio of 1 : 7: 7. ACN was added to the composite to improve the mixing and was removed on a hot plate at a temperature of 90 ○ C.
A double-layer TiO 2 photoelectrode (10 + 5) mm in thickness with a 10 mm thick nanoporous layer and a 5 mm thick scattering layer (area: 0.25 cm 2 ) was prepared by screen printing on a conducting glass substrate.A dye solution of 3 × 10 −4 M concentration in acetonitrile-tert-butyl alcohol (1/1, v/v) was used to uptake the dye onto the TiO 2 �lm.Deoxycholic acid (DCA) (20 mM) as a coadsorbent was added into the dye solution to prevent aggregation of the dye molecules.e TiO 2 �lms were immersed into the dye solution and then kept at 25 ○ C for 30 h.Photovoltaic measurements were performed in a two-electrode sandwich cell con�guration.A 30 m thick surlyn spacer was put on the dye-deposited TiO 2 electrode and attached by heating.e MWNT-PAni/MPII hybrid composite electrolyte was then put onto the dye sensitized TiO 2 �lm at 85 ○ C to ensure that the PMII can penetrate well into the porous structure and remove the residual ACN.e dye-deposited TiO 2 electrode with the MWNT-PAni/MPII hybrid composite electrolytes was assembled with a platinum-coated conducting glass electrode and sealed by heating the polymer frame.e electrolytes used for liquid cell were composed of 0.6 M dimethylpropylimidazolium iodide (DMPII), 0.05 M I 2 , and 0.1 M LiI in acetonitrile.

Results and Discussion
An ionic liquid usually has favorable properties from the viewpoint of a DSSC, such as negligible vapor pressure, high thermal stability, a wide electrochemical potential window, and high ionic conductivity [14][15][16][17][18]. Conducting polymercoated carbon nanotubes are notable materials, which are being widely studied because of their extraordinary electronic and mechanical properties.Considering these aspects, an incombustible and nonvolatile PMII and MWNT-PAni composites, were incorporated into DSSC for this study (Scheme 1).It is expected that this IL would allow perfect contact at the interface between the dye-coated porous TiO 2 and the extended electron transfer material [19], that is, MWNT-PAni composites.e carbon material in the iodine-free composite electrolyte serves simultaneously as a charge transporter in the electrolyte and as a catalyst for electrochemical reduction of I 3 − ions [20].e iodide anionbased IL can provide sufficient I − for the regeneration of oxidized dye under illumination; I − in turn oxidizes to I 3 − , which can be reduced back to I − at the carbon material.
A typical morphology of MWNT-PAni nanocomposites synthesized by the in situ chemical polymerization method was investigated using scanning electron microscopes (Figure 1).In the Figure 1(a), the FE-SEM image proofs a uniform existence of carbon nanotube and polymer in the MWNT-PAni nanocomposites.A tubular morphology of MWNT-PAni nanocomposites was also identi�ed by using TEM scanning, as shown in Figure 1(b).Microscopic characterizations showed that there was clear indication of interfacial entrapment between the PAni and MWNT; the conducting polymer is coated on the surface of the carbon nanotube.Here, the tubular inner part (core) is mainly the compound of MWNT, and the outer coated surface (shell) is conducting polyaniline with the variable thicknesses (20-50 nm diameters), and their external surfaces are not smooth.e atomic percents of the C, N, and H are 74.28,5.74, and 2.75, respectively, for MWNT-PAni composites by the elemental analysis (EA) which reveal that carbon nanotube and polyaniline both are present in the sample.In general, electrical conductivity may be taken as a function of the conjugation length of the polymer, and the amount of active dopant present in the polymer, as the number of charge carriers depends upon the extent of the dopant concentration, provided that other factors remain unchanged.A powder sample of 0.02 g was loaded and pressed into a pellet 1.2 cm in diameter and a pressure of 170 atm by a manual hydraulic press for 10 min.en, the electrical conductivity of the pellets was measured by a standard four-point probe method, connected to a Keithley voltmeter-constant current source system.e conductivity of the resulting MWNT-PAni composites at room temperature is 1.53 S/cm, which is higher than that of the pristine PAni (∼0.18 S/cm), which is synthesized without MWNT, under the same conditions.e combination of PAni with MWNT has effectively increased the conductivity, an order of magnitude for the MWNT-PAni composites, comparing with its counterpart bulk PAni powders.
e short-circuit photocurrent density ( SC ), opencircuit photovoltage ( OC ), �ll factors (FF), and overall cell efficiencies () of the DSCs under AM 1.5 G simulated solar light at a light intensity of 100 mW cm −2 using MWNT-PAni/PMII composite electrolyte and using bare PMII as electrolyte are summarized in Table 1. Figure 2 shows the an open-circuit photovoltage of 0.560 V, and a �ll factor of 0.57, corresponding to an overall conversion efficiency () of 3.15% under standard AM 1.5 irradiation (100 mW cm −2 ), which is remarkably higher than that of bare PMII device (0.26%) under the same experimental conditions.e low device efficiency of bare PMII device is due to signi�cant decrease in  SC .e presence of MWNT-PAni composite materials in PMII facilitates electron transfer from counter electrode to I 3 − , which enables the I − /I 3 − redox couple to work more efficiently than they would in the absence of MWNT-PAni [20].Under the same experimental device conditions, solar cells with iodine based liquid electrolyte showed higher overall conversion efficiency () of 7.68%.Conversion of iodine containing liquid electrolyte to quasisolid MWNT-PAni/PMII led to decrease in the  SC from 15.21 to 9.87 mA cm −2 , the  OC decrease from 0.70 to 0.56, and the �ll factor decrease from 0.72 to 0.57 because of marked increase in viscosity for MWNT-PAni/PMII device compare to liquid electrolyte device.
Figure 3 shows the monochromatic incident photon to current conversion efficiency (IPCE) for DSCs based on MWNT-PAni/PMII composite.MWNT-PAni/PMII device shows the maximum IPCE of 54% at 550 nm.Here, the integrated  SC value from IPCE was the same value obtained 9.87 mA cm −2 from the I-V measurement.e low photon-to-current conversion efficiency in the MWNT-PAni/PMIIelectrolyte-based device may be due to inefficient charge transport properties in the composite electrolyte.

Conclusion
A quasi-solid-state DSSC was developed using the hybrid MWNT-PAni nanocomposites as an electrolyte layer without adding the conventional iodine electrolytes.A moderately higher efficiency (3.15%) of solid-state DSSC was achieved with the hybrid MWNT-PAni nanocomposites and PMII under AM1.5 full sunlight.It is revealed that the MWNT-PAni nanocomposite electrolyte serves simultaneously as the �ller for physical gelation of electrolyte and as the catalyst for electrochemical reduction of I 3 − .

S 1 :
Schematic design of the charge transport processes in a typical DSSC with a quasi-solid-state composite electrolyte containing the MWNT-PAni hybrid composites and IL.

F 3 :
photocurrent density-voltage performance for DSCs based on MWNT-PAni/PMII device.e MWNT-PAni/PMII electrolyte containing solar cell showed high DSCs performance showing short-circuit photocurrent density of 9.87 mA cm−2 , an open-circuit photovoltage of 0.560 V, and a �ll factor of 0.57, corresponding to an overall conversion efficiency () of 3.15% under standard AM 1.5 irradiation (100 mW cm −2 ), which is remarkably higher than that of bare PMII device (0.26%) under the same experimental conditions.e low device efficiency of bare PMII device is due to signi�cant decrease in  SC .e presence of MWNT-PAni composite materials in PMII facilitates electron transfer from counter electrode to I 3 − , which enables the I − /I 3 − redox couple to work more efficiently than they would in the absence of MWNT-PAni[20].Under the same experimental device conditions, solar cells with iodine based liquid electrolyte showed higher overall conversion efficiency () of 7.68%.Conversion of iodine containing liquid electrolyte to quasisolid MWNT-PAni/PMII led to decrease in the  SC from 15.21 to 9.87 mA cm −2 , the  OC decrease from 0.70 to 0.56, and the �ll factor decrease from 0.72 to 0.57 because of marked increase in viscosity for MWNT-PAni/PMII device compare to liquid electrolyte device.Figure3shows the monochromatic incident photon to current conversion efficiency (IPCE) for DSCs based on MWNT-PAni/PMII composite.MWNT-PAni/PMII device shows the maximum IPCE of 54% at 550 nm.Here, the integrated  SC value from IPCE was the same value obtained 9.87 mA cm −2 from the I-V measurement.e low photon-to-current conversion efficiency in the MWNT-PAni/PMIIelectrolyte-based device may be due to inefficient charge transport properties in the composite electrolyte.