Study on conventional carbon characteristics as counter electrode for dye sensitized solar cells

Activated carbon (AC), black carbon (BC), and graphite were deposited onto ITO (Indium Tin Oxide) glass for counter electrode application in Dye-Sensitized Solar Cells. SEM-EDX was used to observe and analyse the morphology and composition of electrodes. The results showed that the particle distribution of the graphite electrode observed was approximately 34% with a size of 1 to 2 µm and BC electrode about 20% have a size of 0.5 to 1 µm, while AC electrode has a size of 0 – 0.5 µm observed around 20%. AC electrode has a more porous and uniform particle aggregates compared to BC and graphite electrodes. The efficiency of the counter electrode was measured using the solar simulator. The highest efficiency was at 0.011516% for the counter electrode that was fabricated by AC. Meanwhile, black carbon and graphite electrodes were achieved at 0.008744% and 0.010561%, respectively. The results proved that the porosity and the uniform aggregate of the particles were the most significant factors to improve the performance of DSSC.


Introduction
Recently, DSSC has gained extensive attention due to their environment-friendliness, simple fabricating procedure, and low-cost compared to conventional Si solar cells [1][2]. A typical DSSC is composed of a nanoporous TiO 2 photoanode and a Pt counter electrode separated by an iodide/triiodide electrolyte [3]. The photoanode is usually prepared from TiO 2 nanoparticles on a transparent conducting oxide (TCO), while the counter electrode is a thin layer of Pt deposited on another TCO substrate [2].
As one of the most important components, counter electrode should possess mesoporous [4], high catalytic activity [5] and chemical stability against the corrosive electrolyte in the DSSCs [6]. Pt counter electrode has been extensively used as an efficient catalyst for reduction of I 3ions in DSSC. Pt exhibits high electrochemical catalytic activity for triiodide reduction. However, Pt is scarce, expensive metal and has poor stability in corrosive I -/I 3redox system [2] [7].
Recently, a various carbon-based material including graphene [5], graphite [8], and carbon nanotubes [9] have been studied as a low-cost replacement for Pt counter electrode. Carbon materials attracted much attention due to their high porosity, high electrochemical catalytic activity, low cost, and excellent corrosion resistance against the redox couple [10][11][12]. In this present study, we reviewed some carbonaceous material including graphite, activated carbon, and black carbon used as counter electrode and compared their performance in DSSC system.

Preparation of carbon counter electrodes
The carbon powders that were used in this research are graphite, black carbon, and activated carbon. Each carbon paste was prepared by mixing carbon powder with ethanol and was then stirred for 1 hour. The carbon pastes were deposited onto a conductive glass substrates-ITO glass using doctor blade method with a thickness around ∼10 μm. The electrode was heated at 250°C for 30 minutes to remove the organic residue and stabilize the material.

DSSC's fabrication
TiO 2 photoanode was prepared according to the procedure previously reported by Lee et.al [12]. 0.7 g of TiO 2 powder is finely ground in a mortar, then 1.4 mL of distilled water were added. After10 minutes, 0.3 g of PEG, 0.7 mL of acetic acid, along with 1 mL and 0.7 mL Acetylacetone triton X-100were added. The TiO 2 paste was coated on 2.5 cm 2 area of ITO by a doctor blade technique with a thickness around10 µm, and was then sintered at 450°C for 30 minutes. The electrode was immersed overnight in a 0.05 mM ethanol solution of the N749 dye at room temperature. A sandwich-type DSC was fabricated with the dye-sensitized TiO 2 photoanode, electrolyte gel, and carbon as the counter electrode. Figure 1 shows the structure of the DSSC's cell. The electrolyte consisted of 7 g PEG 1000, 25 mL chloroform and a liquid electrolyte homogeneously mixed and stirred with a magnetic stirrer while heated at80°C for 1 hour.

Figure 1. DSSC's formation
The surface morphologies and compositions of the counter electrodes were observed and analyzed using SEM-EDX whilst the current density-voltage (J-V) characteristics of assembly DSSC were measured using an I-V Keithley meter, under illumination at 100 mW/cm 2 Figure 2 shows the top view SEM images of activated carbon (AC), black carbon (BC), and graphite electrode. These images confirmed that particle shape of all samples is spherical irregular particles. It also can be seen that porous structure was observed in activated carbon. The morphology of BC and graphite electrodes showed somewhat large intensity inhomogeneous agglomerations. These agglomerations were decreased by electrical contact, making the resistance of charge transfer increase [13].  Figure 2. SEM images of AC, BC, and Graphite counter electrode Figure 3 shows the particle size distribution observed in any carbon materials used in the present study. The distribution of particles with a size between 1 to 2 μm around 34% observed in graphite electrode and the size of 0.5 to 1 μm for 20% occurred in BC electrode, whilst a size of 0 to 0.5 μm for 20% was observed in AC. The distribution of particle gave large surface area so that the surface contact between carbon materials and electrolyte will increase the transport of electrons to the dye contained in photoanode [10]. The large particles resulted in limited transport of electron process from the outside toward the inner circuit and affect the performance of DSSC.  Table 1, it can be deduced that AC electrode gave the highest efficiency compared to BC and graphite electrodes. This result agrees with the result that obtained by [14][15]. The best values for fill factor was shown by AC electrode. Fill factor is the performance values from DSSC obtained from short circuit current density and open circuit voltage. The fact proved that the performance of AC electrode is better than other carbons used in this study. This is because large particles would slow the rate of I3 reduction due to the high charge transfer resistance (RCT). The high RCT has a negative influence on the fill factor (FF) and shows poor energy conversion efficiency (η) [16].

Conclusion
The study successfully demonstrated the counter electrode that was fabricated from activated carbon, black carbon, and graphite for DSSC applications. The activated carbon has the best performance compared to the black carbon and graphite electrodes due to the efficiency of dye-sensitized solar cells (DSSCs). The results showed that the activated carbon has more porous structure, smallest particle size, and highest fill factor compared to the other two. Due to this fact, we were able to deduce that the morphology of the surface on the counter electrode provides an effect on the performance of DSSCs.