The World Energy Consumption Engineering Essay

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As the world energy consumption is increasing from the past years, the fossil fuel price is increasing more and more. The increase of demand is due to the increase of world population and industrial development by the mankind. The usage of this energy has many effects for the environment. Thus, as the fossil fuel will not last forever and its impact to the environment, much research was initiated to overcome this problem. The Earth itself is a good source for the natural energy source. The energy harvested from natural sources such as sunlight, wind, tides, and geothermal heat are described as the renewable energy. This natural source energy is an alternative to the fossil fuel energy. The rapid growing new renewable energy is the solar energy power. Malaysia is a country which gains sunlight throughout the year. Thus, solar energy will be good alternative energy sources in the upcoming years in our country. In addition, solar energy is a clean energy which does not lead to the environmental damages. The ability to constantly increase the efficiency of this solar energy through enhancement and ingenuity has been continually reduced its cost. A solar cell is a device that converts photon energy into electric current, and most of the current solar cell is based from silicon which is currently the most efficient one approximately 25% of efficiency[3]. The major setback of this technology is that, the cost is much higher than the fossil fuel energy price. This makes it to be fairly unpopular among the users.

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From the development of the nanotechnology, a new technology in the solar cell was introduced. From the continuous research on improvising the solar cell is the third generation solar cells. The dye-sensitized solar cell (DSC) provides a technically and economically credible alternative concept to present day p-n junction photovoltaic devices [1]. This solar cell was developed by Michael Gratzel, thus it's also been referred as the Gratzel cell. As a developing county, this technology can be adopted in our country as well. The development of this solar cell is promising as the construction of the solar cell is not much expensive because it only requires low-cost materials and does not require very complex equipment [3].

In a simple way, the DSC can be explained as solar cell which absorbs light by the dye molecules, and the separation of the electrical charge is done by the nanocrystal inorganic semiconductor that has a wide band gap, and a overall efficiency of 10% have been reached[1,3].

1.2 Objective of the Project

To find an alternative for the Titanium Oxide which have been widely used for the development of DSSC's.

To perform the sol-gel route method to obtain the Zinc Oxide (ZnO) nanopowder as it was chosen as an alternative for the Titanium Oxide (TiO2).

To fabricate the dye-sensitize solar cell (DSSC).

1.3 Zinc Oxide Nanopowder for DSSC

Zinc Oxide appears as a white powder, an inorganic composite. The chemical bonds that form Zinc Oxide are borderline between ionic and covalent bonds though they lean towards being organic, has a low solubility in water (1.6 x 10-6 g/cm3) and decomposes at 1975 [2].

In materials science, ZnO is often called an II-VI semiconductor because zinc and oxygen belongs to the 2nd and 6th groups of the periodic table [2, 4]. The favorable properties of this semiconductor as an alternative for DSSC is the good transparency, high electron stability, wide bandgap, strong room-temperature luminescence and etc[4].

The reasons for choosing the Zinc Oxide as an alternative are:

Crystal Structure

Electrical Properties

Crystal Structure

ZnO crystallizes in three forms which are the hexagonal Wurtzite, zincblende and cubic rock salt [2]. The hexagonal Wurtzite is the predominant form since the structure is the most stable at ambient temperature and pressure [2, 4]. The zincblende form can be stabilized by growing ZnO on substrates with cubic lattice structure where in both cases, the zinc and oxide are tetrahedral. The rock salt NaCl-type structure is only observed at relatively high pressures.

These three forms of ZnO have different iconicity due to its different ion arrangment. Thus, this iconicity makes zinc and oxygen planes bear electric charge where positive and negative respectively [4]. Therefore, the electrical neutrality is maintained as its surfaces are atomically flat.

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The three forms of ZnO crystals:

Figure 1.3.1: Wurtzite [2] Figure 1.3.3: Zinc Blende [2]

Figure 1.3.2: Rock Salt [2]

Table 1.3.1: Physical Properties of ZnO Property Value Crystal Structure : Wurtzite

Zinc Blende

Rock Salt Lattice Parameter :

ao

co

ao/ co

0.32495nm

0.52069nm

1.60 Density 5.606 g/cm3 Melting Point 1975 Thermal Conductivity 0.6, 1-1.2 Linear expansion coefficient ( /) : ao : 6.5x10 -6

co: 3.0 x 10 -6 Static Dielectric constant 8.656 Refractive Index 2.008, 2.029 Energy gap 3.3eV, direct Intrinsic carrier concentration (per cm3) 1016 to 1028 Exciton binding energy 60meV Electron effective mass 0.24 Electron Hall mobility at 300K for low n-type 200 cm2 /V s Hole effective mass 0.59 Hole Hall mobility at 300K for low p-type 5-50 cm2 /V s Typical impurities H, Al, In, Ga Typical defects : Zinc Interstitials, Oxygen Vacancies,

Zinc Vacancies, Complexes Table 1.3.1: Physical properties of ZnO [2]

Electrical Properties

From the properties table, we can see that the ZnO has a relatively large direct band gap of 3.3eV at the room temperature. Advantages associated with a large band gap include higher breakdown voltages, ability to sustain large electric fields, lower electronic noise, and high temperature and high-power operation [4]. In addition, ZnO is both conductive and transparent which puts it in an important class of materials called transparent conductive oxides, or TCOs [2].

1.4 Sol Gel Route Method

There are several methods in making the ZnO thin film which are sputtering, molecular beam epitaxy, vapor-phase deposition, thermal evaporation, reactive evaporation, chemical vapor deposition, spray pyrolisis and the sol-gel process method. The sol-gel methods would be suitable choice for the ZnO thin films fabrication due to several advantages in comparison with other deposition methods. Through this method, we will be able to prepare high quality thin films in large scale, simplicity, safety, low cost of the apparatus and raw materials [5]. Moreover, with the limited facilities available, the sol-gel process would be good choice to produce the ZnO nanopowder for the fabrication process. The sol-gel process method is explained in the Chapter 3.

Figure 1.4.1: Schematic Diagram of sol-gel route method

1.5 Dye Sensitized Solar Cell

The dye sensitize solar cell (DSSC) is the third generation solar cell which is still under research and have not been commercialized yet. It is another alternative energy for a clean source of solar energy. This technology can be utilized in a big scale because of its advantage which is much more economical and simple in design compare to the current silicon solar cell which is available in the market now. Moreover, the this technology has not reach a efficiency of at least 25%, thus many research are still going to improvise it since introduced in the early 1993. Titanium oxide was the favorite metal oxide which was used for this cell. Through advances of material science, study shown that Zinc oxide shows a good potential to be an electrode material for this cell which is a suitable replacement for Titanium Oxide (TiO2).

The dye-sensitize solar operation is not very complex as the silicon solar cell, where it's actually a process of light absorption and some electron changes over the cell which turns it into current. The material which comprised in this solar cell is the conductive glass, a good potential metal oxide, organic dyes and electrolyte. The materials which are been used for the solar cell shows that it does not require any expensive semiconductor materials. Moreover, the dye may be chemically tailored to capture the full range of the solar spectrum whereas the semiconductor materials used in conventional solar cells are limited to the light spectrum that excites the band gap energy [2].

Figure 1.5.1: The schematic diagram of a dye-sensitize solar cell [6]

The figure above shows the basic operation on how the electricity is generated in the dye sensitize solar cell. When the sunlight energy excited the organic dye molecules, the molecules get excited. The excitation of these molecules will be able to transfer electron to the semiconducting metal oxide layer. This process is called electron injection or sensitization [8]. The electron is transferred from the metal oxide layer to the conductive glass which is the electrolyte to complete the circuit counter electrode [8]. After travelling through the electrical load, the electron collected at the metal oxide From the diagram, we can also observe that the simple operation of this solar cell. This solar cell has a significant reduction of cost on producing a solar cell. Although the efficiency of the cell is not high as the silicon solar cell, through more research the solar cell would be able to be commercialized and be used for as an alternative solar energy.

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