Solar Power: Photovoltaic Cells - Research Paper Example

Solar Power Human beings have utilized energy from the sun for many centuries. For instance, ancient cultures utilized the sun’s energy to start fires so as to keep warm. Their buildings were designed to trap the sun’s heat during the day and release the heat at night as a way of keeping them warm. With the recent awareness of global warming, instability in fossil fuel prices and the need for green energy, solar energy has emerged as a viable source of energy to power most of man’s inventions when it is converted to electric power. This article will mainly focus on the solar energy and will be mainly biased on photovoltaic (PV) cells. History of Photovoltaic Cells The photovoltaic cells got their name from the photovoltaic effect which is the process by which photons are converted to voltage which can then be utilized to power machines. The discovery of the photovoltaic effect can be traced back to the 19th century when Edmond Becquerel (Harper, 2007), a French physicist first demonstrated that the exposure to light of certain materials increases the electric current within them. Several years later, Albert Einstein clearly illustrated the photovoltaic effect and even received a Nobel Prize for his efforts. The advantages of using PV cells for electricity production by far outstrip the disadvantages as shown in the table below: The practical use of solar cells however was made possible in the 1950s with the discovery of silicon’s ability to generate electric current on exposure to light. The first usable silicon solar cells had an efficiency of six percent but the percentage today is much higher at about 20 percent. Solar power usage is also on the increase with more people turning to solar power as a source for their energy needs both in their homes and at their businesses and the technology is also being utilized in large scale by utility companies. The typical small scale solar panels used domestically are made of solar cells clustered into modules that can accommodate up to 40 cells. Structure and Working of Solar Cells The solar cell contains semiconductors which in turn contain weakly bonded electrons in their valence bands (Luque and Hegedus, 2005). These valence bonds break on application of band gap energy on the valence electrons therefore releasing the electrons to move to the conduction band. Electrons moving in the conduction band enable the material to conduct electricity. The distance separating the conduction band and the valence can be measured and is represented in electron volts (eV). Particle of light, better known as photons can provide the energy required to free the electrons (band gap energy). On exposure to sunlight, the electrons within a solar cell get hit by photons and on absorbing the energy they move to the conduction band where they are collected and driven to an external circuit (Luque and Hegedus, 2005). These electrons will lose their energy by driving machines such as water pumps, fans, sewing machines, a light bulb, mentioning but a few. A diagrammatic representation of a p-n junction is shown below: Source: University of Durham Afterwards the electrons are returned to the solar cell using the return loop through a second selective contact that will return them to the valence band. The electrons go back to the valence band while possessing their original energy. It is the movement of the electrons within the exterior circuit that is referred to as electric current. The amount of energy electron posses while at the external circuit is normally less than the energy that they absorbed to move to the conduction bands. Thus, if it took a photon of two eV to excite an electron, it (the electron) will have energy of about one eV at the external circuit. The energy output is normally a product of the voltage and the current or in other terms, the free electrons’ potential multiplied by their number. Sunlight contains a band of photons spread out over an energy range. Photons containing energy that is greater than the threshold energy excite the electrons to the conduction band but photons possessing less energy than the band gap energy cannot excite the electrons and their energy moves through to the back where it is converted to heat (Luque & Hegedus, 2005). For instance, high-band-semiconductors cannot absorb red light photons, meanwhile, when low-band-gap semiconductors are hit by excess energy of the blue light photons, the unconverted energy turns to heat, which is a waste. This is the reason why solar cells in direct sunlight are usually warmer than the surroundings. The first generation solar cells’ most important features are their layered structures according to Luque and Hegedus (2005). The “P-type” silicon makes the bulk of the solar cell and it is mostly made of pure silicon containing minute amounts of an impurity, usually Boron, which makes the material have a shortage of electrons, or holes. Above the “P-type” junction is the “N-type” function which is also almost of pure silicon but contains a different type of impurity, usually phosphorus that enables the layer possess extra electrons (Harper, 2007). The interface between the two layers is what is referred to as the P-N junction and is the most vital part of the solar cell. The P-N junctions only conduct power in one direction (Preuss, 2002). The p-n junction can either be homo-junction or hetero-junction. Solar cells are traditionally manufactured from silicon and are normally flat-plate. The silicon used in most solar cells is normally in crystalline form though there are other semiconductors that have been discovered to be better at absorbing solar energy. Solar cells of the second generation are referred to as thin-film solar cells since they are manufactured from non-silicon materials or amorphous silicon. Thin-film solar cells utilize layers of semi-conductors that are of a few micrometers in thickness and therefore can also serve as roofing materials (Hamakawa, 2004). These solar cells are manufactured by depositing a single or multiple thin-film layers on a substrate by mostly the plasma-enhanced chemical vapor deposition (PE-CVD). The chemical vapor deposition involves the substrate being exposed to a single or multiple volatile precursors which after the reaction, decomposes on the surface of the substrate, producing the desired deposit, in this case silicon. The reaction also results in volatile byproducts which are gotten rid off by a gas that is made to flow across the chamber. The thin-film cells are classified according to the photovoltaic materials utilized in manufacturing them. Examples of these solar cells include amorphous silicon (a-Si:H or a-Si), cadmium telluride (CdTe), Copper Indium Selenide (CIS), organic and Dye-sensitized solar cells (DSC) (Hamakawa, 2004). The mobility of electrons within the a-Si: H is approximately one or two orders larger in magnitude than the holes and therefore, the movement rate of electrons from the p-type to n-type layer (p-i-n junction-type) is faster than the other way round (n-i-p junction). For this reason, the p-type layer of amorphous silicon is placed on the top part of the solar cell where there is a stronger light intensity. A typical illustration of a hetero-junction solar cell (with a p-n junction) is shown below: Source: University of Durham The glass is usually of approximately two to four millimeters in thickness and its main function is to protect the solar cell’s active layers from environmental damage. Its exterior surface has a coating that is anti-reflective so as to enhance the cell’s properties. The transparent conducting oxide is normally of indium tin oxide and its purpose is to reduce the device’s resistance that would otherwise be present due to the cadmium sulphide (CdS) layer. The CdS layer is n-doped and is therefore one half of the p-n junction while the cadmium telluride (CdTe) layer makes the other half as it is p-doped. The back contact is normally manufactured from aluminum or gold so as to provide an electrical connection of low resistance to the CdTe (Hamakawa, 2004). Third-generation solar cells are manufactured from silicon and an assortment of materials including solar dies, solar inks and conductive materials. Some of them use mirrors or plastic lenses to concentrate rays of sunlight on a tiny piece of a photovoltaic material with high efficiency. The PV material may be expensive but the overall system cost is reduced since on a small piece is utilized. However, the concentrating collectors are best utilized in sunny locations as the lenses have to be pointing at the sun. The single and multi-junction a-Si:H technology has been utilized for large scale electricity such that by the year 2000, it had a global electricity production of approximately 35 megawatts annually. After the a-Si:H solar cells, the CdTe-based solar cell is the next cell that can be utilized for large production since it had a global production capacity of slightly over one megawatt per year by 2000. The CIGS thin film solar cells is a leader among the thin-film solar cells with a global capacity of producing 100 kilowatts per year, also by 2000 (Hamakawa, 2004). The a-Si:H thin film PV devices are superior to the other thin film devices since they have a higher optical absorption coefficient of about 10,000 per centimeter and a flexible band gap (1.1eV-2.5 eV) achievable by alloying. They in addition are easily fabricated using varying techniques at temperatures as low as 300 degrees centigrade. The a-Si:H solar cells multi-junction device has the ability to efficiently utilize the solar spectrum making it not only very efficient but cost-effective as well. Their limitations include efficiency degradation caused by the Staebler-Wronski effect, electronic properties’ degradation by a few Angiosperms per second and overall low module efficiency. a-Si:H- based solar cells differ from the other thin-film cells since they require p-i-n-type devices while the others use the p-n junction devices. This is mainly because the amorphous silicon and its alloys posses intrinsically elevated defect densities and related lower minority-carrier lifespan and, therefore, for an efficient gathering of photogenic carriers, they require field assistance (Hamakawa, 2004). There are however some research being made to seek ways of reducing these limitations (Hamakawa, 2004) and seeking better materials to use for manufacturing future solar cells. Research on the use of indium nitride as a semiconductor has shown that its band gap is about 0.7 eV and as thus, alloys of nitrogen, gallium and indium can convert almost the full sunlight spectrum to electric current (Preuss, 2002). If solar cells are to be manufactured from this alloy, there will be benefits in terms of lower costs and improved efficiency. Silicon may be cheap but it wastes most of the light it is supposed to convert to electricity in terms of heat. The elements belonging to the third group of the periodic table make the most efficient semi-conductors when combined with group five elements. The maximum efficiency achievable by a solar cell manufactured from a single substance is approximately 30 percent though the actual highest value ever achieved is 25 percent. To increase the performance of solar cells, different substances are stuck together to form multi-junction cells. In order to trap photon across the spectrum, dozens of differing layers could be stacked on one another resulting in a very high efficiency though this comes with its numerous challenges. Crystals are damaged by strain when their lattices vary too much. The most efficient solar cells have only two layers. The table below shows the merits and demerits of solar cells. Table 1: The merits and demerits of PVs Merits of photovoltaic cells Demerits of photovoltaic cells Their source of fuel is vast and available almost everywhere on earth They produce no emissions or dangerous radioactive wastes hence environmentally safe They have a very low operating and maintenance costs since they posses no moving parts They operate at ambient temperatures meaning less safety issues They have high reliability and can operate for more than ten years They can be quickly and easily installed They integrate well into buildings and can even serve as roofing materials They have a perfect safety record and have high acceptance among the public They can be installed almost anywhere There is no limit on how many cells can be connected together Their fuel source is of low density energy Their initial cost (installation) is high There is still no efficient way of storing their energy Conclusion The most versatile energy form available today is electricity and its access and consumption is can be related to the quality of life one lives. There may be several ways of producing this energy but the recent calls for renewable production ways had driven more research to be done on solar power. Currently the major hurdle facing solar power is its high initial cost. However with the research being done to address that issue, it could soon emerge as our main source of electricity production. References Hamakawa, Y. (2004) Thin-Film Solar Cells: Next Generation Photovoltaics and its Applications. Berlin: Springer. Harper, D.J. (2007) Solar Energy for the Evil Genius. New York: McGraw-Hill Professional. Luque, A. and Hegedus, S. (2005) Handbook of Voltaic Science and Engineering. West Sussex: John Wiley and Sons Limited. Preuss, P. (2002) An Unexpected Discovery Could Yield a Yield Spectrum Solar Cell. Retrieved on 10th April, 2010. Available at: http://www.lbl.gov/Science-Articles/Archive/MSD-full-spectrum-solar-cell.html University of Durham (n.d.) Semiconductors and Electroceramics. Retrieved on 9th April, 2010. 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