Characterization and Treatment of Titanium Dioxide via Ultrasonic Process with Melastoma malabathricum as Sustainable Sensitizer for Photovoltaic Solar Cell

Generation from the existing commercial devices costs about ten times more than the conventional methods.Therefore, this paper presents a thin-film dyed solar cell (DSC) of natural dyes fromMelastoma malabathricum fruits which consist of the carbonyl and hydroxyl groups of anthocyanin molecule that influences the performance of photosensitized effect due to its bound on the surface of filler. Experimental results comparing engineering grade (>99% purity) of metal oxide; U1 and U2 with treated metal oxide; U3 and U4 using ultrasonic process, which is to break the particle agglomeration from 0.37μm down to 0.15μm; this treatment led to a more “sponge-like” consistency with high porosity, enabling enhanced absorption and anchorage of the dye sensitizer. The microstructures of metal oxide were observed using Field Emission Scanning Electron Microscope (FESEM) and Atomic Force Microscope (AFM). Along with the highest performance of I-V measurement given by U4 with open circuit, Voc = 0.742V, short circuit, Isc = 0.36mA, fill factor, FF = 57.012 gives 0.039% efficiency the examples for the first outdoor application upon sunlight illumination of such DSC were also reported. Therefore, this ultrasonic treatment and novel dye from Melastoma malabathricum fruit are reliable to be used for further application.

If we look at photosynthesis organisms, the best solar cells on earth, we find that there is no waste in these natural systems; everything is biodegradable, reusable or regenerative. The earth remains as healthy after the organism dies as it was before the organism was born. If we can invest in renewable energy technologies that similarly create a complete loop between the source and the end of the technology cycle, so the condition of the environment is at least as good after we use the energy as it was before we used the energy, then we can achieve a truly sustainable energy source (Ali, S., Spring 2007).
The issues of sustainable energy source has aroused public awareness significantly for the past few decades especially in high demand or developing countries. As the energy usage keep on increasing, so do the climatic disruption such as Greenhouse effect which caused by the fossil fuel combustion and changed in global climate accompanied by depletion of fossil fuel reserves. These undesirable activities have enhanced the growth of renewable energy (wind turbine, biomass, water and etc.) to overcome the dearth.
One of the promising renewable energy which makes used of free source of energy, the sun (3 X 10 24 J per year), is solar cells. According to Gratzel, M. (2005) which is anchored to the surface of a wide band gap semiconductor (commonly TiO 2 ).
Then, the dye is generated by electron donation from the electrolyte such as the iodide / triiodide couple (Murali, S., 2011). The operating principle of DSSC is complete once it receives back an electron from the external circuit.
In this research, a thin film using Engineering grade of TiO 2 >99 %, organic material from wild plant, Melastoma Malabathricum (Senduduk) fruit as dye sensitizer and Carbon black as the counter electrode is disclosed. It is known that most of the efficiency of organic dyes are typically <1 % (Zhou, H. et al., 2010), but rapid research has been done recently as it promises cost reduction and environmental friendly.

Problem Statement
Running our daily life with oil or natural gas or using electricity (from power plants generated with oil and coal) enhanced Greenhouse Effect, the cause of global warming and climate disruption which are becoming more common in recent years. Besides, it also contributes to the fuel crisis whereby affect the daily expenses (foods and clothes) as it is in the same pyramid chain. Many believed that solar cell is one of the best answers of solving the problems apart from other renewable energy such as water, wind, biomass, geothermal and etc.
In the first generation of solar cell, the Silicon based has shown good performance in producing power. However, there are several constraints such as heavy weight per area panels, fragile and also very low return of investment (ROI) since. It is said that nothing in life comes without a price. The management of the hazardous production wastes and the issue of disposing of spent solar cells have yet to be resolved. Therefore, natural dye sensitized photovoltaic metal oxide is produced to overcome the dearth for energy conversion solar cell. Compared to traditional solar cells, this photovoltaic cell has the following differentiation advantages:  Provides additional functionality for energy efficiency and noise reduction

Aim
The aim of this research is to replace DSSC natural dye by using low purity of treated TiO 2 (Engineering grade >99 %) via ultrasonic process and local natural dye, Melastoma Malabathricum (Senduduk) fruit, for future Green Solar Cell Technology.
2) To determine the photovoltaic performance / efficiency (η %) of untreated and treated TiO 2 as semiconductor on ITO substrate and Carbon black as counter electrode.
3) Optimization of ultrasonic process with varying of time to obtain multilayer thickness of a film.

Scope
In this study, solar cells based on natural dye sensitized photovoltaic, TiO 2 semiconductor material with Engineering grade > 99 %, KI 3 liquid electrolyte and Carbon black as counter electrode were fabricated. Using the ultrasonic process for reducing agglomeration of semiconductor particles which resulting on thin film morphology, it is also emphasis on natural dye extracted from Melastoma Malabathricum (Senduduk) fruit as dye sensitizer. The physical and electrical characteristics were performed to gain maximum efficiency in order to construct a competitive solar cells's panel.

Solar cells development
As early 1839, a French physicist found that certain materials would give electric current characteristic when light hits on them. This phenomenon was described as the "photoelectric effect" until now. Later, the first solar cell was invented in 1877 with around 1% efficiency-the semiconductor selenium was coated with a thin layer of gold and became a demonstrated working principle of solar cells.
Many researches had been done and finally in 1940, the first "silicon" solar cells were produced by the American. By 1954, scientists from Bell Laboratory accidentally discovered that silicon doped with certain impurities would give rather higher efficiency up to 6 % and this resulted solar cells being used in practical application, spacecraft.

Three Generations of Solar Cells
Solar cells are usually divided into three generations. Those generations are a point base on the order of which each became important.
The first generation is currently used in practice. It is relatively expensive to produce and very pure silicon is needed, and due to the energy-requiring process, the price is high compared to the power output (Lund, H. et al., 2008). Single junction silicon devices are approaching the theoretical limiting efficiency of ~31 % as suggested thus become the main reason for the emergence of the second generation of solar cells.
The second generation contains types of solar cells that have a lower efficiency, but are much cheaper to produce, such that the cost per watt is lower than in first generation cells. Most well-known materials in this second generation are cadmium telluride (CdTe), copper indium gallium selenide (CIGS), amorphous silicon (a-Si) and micromorphous silicon (µa-Si) (Lund, H. et al., 2008).
Among major manufacturers there is certainly a trend toward second generation technologies as shown in Table 2.1, however it face difficulty on market acceptance.
The product based has proven hazardous to our health and environment as Cd, As, In, Se and Te are toxic materials (Charles,C.S.,Sunao, S., & Janusz, N., 2005). Eventhough other semiconductors such as GaAs, GaAlAs, GaInAsP, InAs, InSb, and InP arise as an interesting solar cell materials because they have near-optimal band gaps, these materials are extremely expensive, and have found applications only in the space solar cells (Halme, J., 2002)  As refer to Figure 2.2, the semiconductor, TiO 2 and the electrolyte are located between two glass plates, coated with transparent conducting oxide (TCO). The TiO 2 is covered with a monolayer of dye and the counter electrode is coated with Carbon black

Metal oxide semiconductors
Semiconductor or an insulator is a concept where the valence band is completely filled with electrons in bonding states so that the electrons conductivity cannot occur. There   (Hoffmann, M. R.et al., 1994;Wolf, E. L., 2009). Furthermore, the radiation must be equal or lower wavelength than that calculated by the Planck"s equation and listed as in  According to Hoffmann, M. R.et al., (1994), Diebold, U., (2002) and Sumandeep, K., (2007), TiO 2 has proven to be the most suitable for widespread environmental applications compared to other semiconductor as it is biologically and chemically inert; stable with respect to photocorrosion and chemical corrosion, inexpensive material, instead of high photosensitivity and high structure stability under solar irradiation and in solutions.

TiO 2 properties
The crystallite TiO 2 comprises of three structures as shown in Figure

Nanopowder production
One of the well-known modifications is through developed particle size from the scale range of 10 -9 m and this is called Nanostructured materials.

Ultrasonic process
Manipulating the physical, biological and chemical properties means TiO 2 semiconductor can be made to be better "sponge" leading to higher solar cell performance. Basically, conventional methods of powder particle size reduction that can be used in this application are milling (Yamamoto, Y. et al., 2011), grinding, jet milling, crushing, and air micronization. Milling, due to impact and high shear fields however produced irregular particles size and the particles might be contaminate from milling media. Therefore, new advance in processing-ultrasonic approached as it generates in liquids by implosive bubble collapse and associated shock waves as shown in Figure (Chang, W. O., et al., 2004;Sumandeep, K., 2007) which is illustrated in Figure 2.7 (Mandzy, N., Grulke, E., & Druffel, T., 2005) with different parameters involved, Jin & Suslick (2010) summarized results showing that ultrasonic approach has more advantages over conventional methods in the synthesis of nanostructured materials such as metals, alloys, oxides, sulfides, carbides, carbons, polymers, and even biomaterials. The versatility of the ultrasonic process when performed in a solvent produces a more uniform size distribution, contributes to a higher surface area, a faster reaction time, and improved phase purity. Degussa, and (d) titania from TAL Materials, Inc. (Mandzy, N., et al., 2005).

Dye sensitizer
Dye sensitizers used in photovoltaic solar cells can be divided into two types, (1) inorganic dyes and (2) organic dyes. Inorganic dye includes metal complex, such as polypyridyl complexes of ruthenium and osmium, metal porphyrin, phytalocyanine and inorganic quantum dots (Kong et al., 2007). The three dyes shown in Figure 2.8 can be considered as the backbone of currently applied sensitizers with efficiency >10 %. Generally, transition metal coordination compounds (ruthenium polypyridyl complexes) are used as effective sensitizers due to their intense charge-transfer absorption in the whole visible range and highly efficient metal-to-ligand charge transfer (MLCT) (Hao, S., 2006). However, the synthesis process of this complex is very complicated, expensive and contains heavy metals which make it unpopular from environmental value.
The use of natural stuff as sensitizing dye for the energy conversion solar cell is very interesting due to significant benefits from economical aspect as it inexpensive because they do not contain noble metals like Ru, Pt, and Os. In addition, the natural dye can be easily extracted from fruits ( Tanihaha

Anthocyanin
The anthocyanin derivatives as shown in Figure 2.10, most prominent among the flavonoids (a large class of phenolic compounds) which commonly refer to natural dyes are responsible for the color in the red-blue range of the fruits, flowers and leaves of plants. Therefore, from aforementioned, it is significantly shows that anthocyanin from various plants gave different sensitizing performances.

Absorbed dye Dye in ethanol
As far as noted, there is no evidence of study that has been conducted on the anthocyanin pigment in Senduduk"s fruit for dye sensitizer of photovoltaic solar cell application. The closest been reported is for anthocyanins"s stability in petals at different stages of Senduduk"s flower development by Janna, O. A. et. al. (2005).She noted that the highest ancthocyanins been found in S3 stage that is when the petals are sonication. This has shown marked enhancement for cotton dyed fabric (as refer to Figure 2.11) since it reducing specific water and contribute to energy consumption. As aforementioned, anthocyanin in Senduduk not only serves as food and fabric dyed, it is also being studied in biomedical science (Susanti, D. et al., 2008).

Electrolyte
Between the two glass substrates, a typical electrolyte is encapsulated whereby functionalize as to reduce the dye cation and following electron injection hence completing the cycle of electron in DSSC. Roughly, an electrolyte can be divided into three types; liquid electrolyte, quasi-solid state electrolyte (Wu, J. et al., 2008) and solid-state electrolyte. For liquid electrolyte, it also can be split into two; organic solvent electrolyte (acetonitrile, ethylene carbonate, and etc.,) and ionic liquid electrolyte. Figure 2.12: Structure and the viscosity of several ionic liquids (Kong, F. T. et al., 2007).
In complex system of DSSC, one of crucial part for electron transport is the rate of the electron injection from the electrolyte to the oxidized dye sensitizer. Yip, C. T. (2010) in his thesis state that the faster the rate of reaction, the less recombination of electrons, and thus, the more chance for the electrons to leave the "sponge like" TiO 2 thin film and contribute to photocurrent. When unmatched redox couple, I -/I 3 is used, counter electrolyte, nitrogen-containing heterocyclic such as 4-ter-butylpyridine (TBP) are needed. Different counter electrolyte or additive gives different optimization of the photovoltaic performance. However, only small amount of additive is required or it will jeopardize the system performance (Wu, J. et al., 2008).

Counter electrode
The function of the counter electrode is to transfer electrons from the external circuit back to the redox electrolyte. The plain Indium Tin Oxide (ITO) layer does this rather poorly. Therefore, platinum (Pt. ) mostly being used as counter electrode. As Ru complexes, Pt. would be replaced by other alternative catalyst as it is expensive for mass production. Furthermore, Pt. was found to diminish on exposure to dye solution and dissolve in the electrolyte by oxidation and complex formation with I -/I 3 -. Therefore, a low cost alternative, porous carbon from graphite powder was proposed (Kay, A., & Gratzel, M., 1996). Figure 2.13 below shows the I-V curves with various mixtures of Carbon black and graphite for counter electrode. Yang, S.C.et al., (2007) reported that as the ratio of Carbon black in counter electrode increased, the shortcircuit current and the open-circuit voltage get improved. This is due to the larger area/ volume ratio and better adhesion of carbon black than the graphite.

Basic operating theory
A schematic presentation of the operating principles of DSSC is given in Figure 2.14.
The conversion of light into electric starts when the light (photon) hits the surface of electrode and absorbed by the dye molecules which is attached to the high surface area, a sponge like oxide thin film. As a results, the dye is been oxidized (D*) and photoexcitation occurs. Photoexcitation is the state where the electron from the valence band injected to the conduction band of the oxide, leaving a hole (D + ) behind. Excited state conduction band electrons and valence band holes can recombine and dissipate the input energy as heat, get trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface or within the surrounding electrical double layer of the charged particles (Hoffmann, M. R.et al., 1995).
Next, the excited electron are transported by diffusion along the oxide thin film network toward the external conducting glass and consequently reach the platinum counter electrode through the external load (Kee, E. L. et al., 2009). At the time being, the oxidized dye (D + ) back to its normal state (D) as it received electron from the electrolyte which is usually containing a redox system an iodide/ triiodide couple. In turn, the electrolyte is regenerated via the electron from the external load. The operating cycle due to schematic diagram above can be simplified as follows: Anode: Absorption TiO 2 ‫|‬D + hv → TiO 2 ‫|‬D* (2.2) Electron injection TiO 2 ‫|‬D* → TiO 2 ‫|‬D + + e cb (2.3) TiO 2 ‫|‬D + → e cb +TiO 2 ‫|‬D (2.4) Dye regeneration TiO 2 ‫|‬D + + 3/2 I -→ TiO 2 ‫|‬D + 1/2 I 3 -(2.5) Cathode: Electrolyte regeneration 1/2 I 3 -+ e c → 3/2 I -(2.6) e c + hv→ 3 I -(2.7)

Introduction
A single cell of natural dye sensitized photovoltaic metal oxide was fabricated by using a doctor blading technique including the preparation of engineering grade >99 % metal oxide with and without via sonochemical ultrasonic process treatment. Meanwhile, the natural dye, Melastoma Malabathricum's fruit was collected somewhere around Parit Raja area (at the roadside, bushes and etc.).
The materials, apparatus and equipment being used in this experiment are listed in Table 3.1 and Figure 3.1 respectively in this section. As to ensure the quality and safety while carrying out this experiment, some safety rules should be taken and herein, flowcharts in Figure 3.1 as guideline of the research.