Multi-Crystalline Wafers & Solar Cells Assessment Answer

It is well known that, “Sun furnishes an ultimate source of solar energy”. This quiet form energy being in the form of radiations fabricates an affordable and ample amount of electricity. At present; it is evaluated to be the most prevailing forms of energy as it has minimal detrimental consequences for nature. Hence, eventually turns out to be an indispensable element of Sustainable development. The light and heat from sun’s radiations are harnessed by photovoltaic or PV cells, which further convert it into a more usable form of electricity. The prime advantage subsumes zero operation costs, free power production purified power generation, job creation in the solar energy sector and prolonged lifetime (of about more than 20 years) of its associated technologies.
The Solar Boom accelerated globally over the past 5 years. In accordance with the belief, power begets power, solar energy not only produces electrical energy, but also oils up the wheels of a country’s economy. Germany, being the solar leader, alone generates 3.8 GW of PV solar energy followed by China and Italy. (Pletzer, et al., 2011)

Multi-crystalline Wafers- First Generation Cells

The Energy Agency has adopted a methodology to name a solar cell after the material being used in its production. Out of the three generations of solar cell, the traditional one is wafer-based cells made up of crystalline silicon. This material is considered as a most prevalent bulk material for solar cells. The two variants of this type of cells are, Mono-crystalline and Poly-Crystalline. The former one is more efficient than other types, but is quite expensive and the latter one, although being less expensive and less crystalline, is the most preferred type being used in photovoltaic. 
Poly-crystalline or Multi-crystalline silicon (multi-Si) cells are made from cast square ingots. These ingots are nothing but simply large blocks of molten Silicon periodically cooled and then solidified. The cell contains small crystals imparting it a typical metal flake effect. The multi-crystalline solar panels have cells that are grown from multifaceted crystalline material, which is nothing but just a crystal that has grown in multiple directions. The efficiency difference in both the cousin wafers arises due to two peculiar reasons. Firstly, due to the multi-faceted crystals from which the cells of multi-crystalline wafers are grown and Secondly, during the manufacturing phase, the cells of multi-crystalline wafers are cut into a square shape rather than a round shape, as in case of mono-crystalline wafers. The researchers, all around the globe, are working on this aspect to have an efficient yet cheap supply of this renewable source of energy. (Mulvaney, 2005)

Current issues being worked on in making of Multi-crystalline Wafers

Having a look at the energy-resources scenario prevailing three decades ago, solar energy became the first choice for those working in the energy field. Why a nation will spend on less efficient cum costly resources if a far cheaper one is available. Hence, the researchers started turning their heads towards this booming energy-source. The Photovoltaic cells were the first acceleration to this discovery. Talking about the efficiency of material being installed in these cells, earlier the scientists preferred the single-silicon crystal cells as they catered high-efficiency energy supply. But, their high-costs paved a way for the introduction of their alternative cousins, multi-crystalline wafer cells. The factories fabricate these kinds of cells from lower purity cast ingots composed of many cheap and small crystals. On comparing both the variants, although the mono-crystalline cells are far better, but its high-cost completely outweighs its efficiency. (Bullis, 2008)
The fundamentals of management often suggest that, in order to make a product or an idea a successful one, the producers should precisely deal with the cost-related issues and incorporate ways that will cut down the unnecessary costs. This will make the product more prevalent among the buyers. The same was done by the silicon wafer producers. The Scientists, from all over the world, are constantly suggesting ways to develop low-cost cum high-energy solar cells. They are suggesting incremental ways to boost the amount of electricity that common photovoltaic (PV) generate from sunlight without any substantial increment in its production costs. As already mentioned, the efficiency and costs involved in the multi-crystalline cells are a bit less than the mono-crystalline ones, hence, the researchers at Massachusetts Institute of Technology (MIT) are finding out ways to increase the conversion efficiency of the multi-silicon wafers. The current statistics show that the researchers there have succeeded in increasing the conversion efficiency of multi-crystalline test cells from the typical 15.5 percent to nearly 20 percent. This kind of reformation could cut down the future PV power costs from the current cost of $1.90 / watt to $1.65 / watt. (Ashley, 2008)
The researchers at the institute have also revealed the conventional fabrication process of the wires installed in the multi-crystalline wafers. In this process, the cell manufacturers use screen-printing techniques and inks that containing silver particles to form the bus wire. Although the process seems to be easy and sound, but the product obtained is a wire of wide diameter and short length (around 20 by 20microns). These wires often include many non-conductive voids that block considerable sunlight and hamper the normal flow of current. This conventional manufacturing system often compromised with the power generation efficiency of the cells. To check this flaw in the manufacturing process, the researchers have introduced and are still working on a proprietary wet process that can fabricate thinner and taller wires (around 20 by 20 microns). The need of these kinds of wire is to use a minimum amount of silver in the wire-fabrication process. These types of wires could be placed together to draw maximum current at a single time. When compared to their counterparts, these wires have fewer chances of non-conductive voids formation, hence, the wires tends to block less incoming lights. 
Another issue in the making of multi-crystalline wafers is in the fabrication of interconnect wires. These wide and flat wires collect current from the silver bus wire and eventually link the adjacent cells electrically. In conventional systems these wires could only shade as much as 5 percent of the area of the cell. In advanced manufacturing units, in order to increase the absorption and transmission capacity of light, textured mirrors are placed on the faces of these interconnect wires. Earlier, the smooth surface of the cell doesn’t allow the light to bend at a required angle. But now, in advanced manufacturing units, these rough mirrors reflect the light at lower angles and hence the light remains inside due to Total Internal Reflection. The longer the light remains inside the wire, the maximum are the chances for its absorption and transformation into electricity. (Alternative Energy Team, 2015)
To significantly improve the efficiency of multi-crystalline solar cells while keeping all the manufacturing costs same, a new startup, called 1366 Technologies, has geared up with all new advanced ideas and technologies. Venture capitalists have already invested around $12.4 million in the company. By the constant innovation in the conventional manufacturing systems of the PV cells, the company has claimed to improve the efficiency of the multi-crystalline silicon wafers by 27%. To be a bit more precise, efficiency in terms of a photovoltaic cell is a measure of the electricity generated from a given amount of sunlight. The 27 percent improvement will bring multi-crystalline cells with efficiencies about the same as single-crystal cells–around 19.5 percent–at lower costs. Summing up, the three key innovations required to increase the efficiency of the cells are: Firstly, by adding a texture to the cell-surface to absorb maximum light from the sun, Secondly, by reducing the width of the silver wires to one-fifth of the original wires so as to reduce the spacing between them when placed together. This closer spacing will eventually make the wires more efficient at collecting the electric current generated by the silicon wafers and thirdly, by using flat wires to collect current from the thin silver wires. (TAI, 2011)
Moving on to the non-technical issues, the PV cell manufacturing factories are facing a new challenge of deteriorating health conditions of the workers in the factories. The high temperatures required to for crystalline-silicon wafer manufacturing makes this process extremely energy sensitive and a highly expensive one too. Also, the process accompanies the production of a great amount of wastes. According to the recent studies, around 80% of the metallurgical grade silicon is lost in the manufacturing process. Another notifying waste product is the silicon dust named as Kerf. These silicon particulate matters pose inhalation problems for the factory workers. Despite the prevailing use of respiratory masks, the workers are constantly overexposed to silicon dust. Silane gas is yet another significant hazard as it is extremely explosive and hence, presents a potential danger to workers as well as the communities residing nearby to factories. The accidental release of this gas has caused many spontaneous explosion incidents every year. There is a long list of other chemicals that require a special handling and disposal procedures. Some of them are: Sodium and Potassium hydroxide that are used to remove sawing damage on the wafer surface often causes irritation in the eyes, lungs and skin. Accidental release of toxic Phosphine or Arsine gas used in doping of the semiconductor may cause numerous occupational risks. (ROSELUND, 2015)

Impact of Multi-crystalline wafers on Solar cells

Poly crystalline or Multi-crystalline silicon wafers being a highly pure polycrystalline form of silicon is the most common type of raw material required in the manufacturing of the photo-voltaic cells. Although the efficiency of such type of cells is not up-to-the mark, yet it is highly preferred by maximum of the manufacturers across the globe. There could be numerous reasons behind this. One such reason among them is that if the wafers are handled well and are fabricated using the most advanced technologies, then they may prove to be the best source that could transform the maximum amount of solar energy into electrical current. Cost of production and installation is quite low as compared to the mono-silicon wafers. Polycrystalline silicon wafers are regarded as the crucial feedstock in the crystalline silicon based photovoltaic industry. (Osborne, 2015) This could be further proved by a fact that for the first time in the year 2006, over half of the global supply of the poly-silicon wafers was used by the Photo-voltaic cell manufacturers. The solar industry was severely hindered by a shortage in supply of poly-silicon feedstock and was forced to idle about a quarter of its cell and module manufacturing capacity in 2007. Another interesting statistical report revealed that till 2008 only twelve factories were known to produce solar-grade poly-silicon wafers. This number drastically increased and by 2013, there were 100 manufacturers in the race. (Saga, 2010)
The above statistics shows that, although the multi-crystalline wafers based cells are known as traditional or conventional ones, still no new advance material could replace the importance of these wafers in the manufacturing of solar cells. The reformation as suggested above could readily be adopted to increase the efficiency of the cells, but the key components and their impact on the solar cells could never be altered. The best chosen crystalline cells will judge the efficiency of the solar cells. Hence, it is the responsibility of the manufacturer as well as of the buyer to select the most efficient crystalline wafer that has a beneficiary impact on the solar cells being produced. (GREEN, 2012)

Cost Analysis of the Multi-crystalline wafers

Although poly-crystalline wafers are less efficient than those made up of a single crystal, but they are simpler to produce and cost quite low to the manufacturer. This eventually makes these cells less exorbitant for the buyers too. These cells have made the solar power affordable for the ones who cannot buy the single crystal panels. The production process of the multi-crystalline silicon wafers allows a low quality of silicon as compared to the one involved in the production of mono-crystalline silicon wafers. Hence, the cost of production poly-crystalline cells is quite lower than the other one. During the installation process, a multi-crystalline panel is generally placed on a south-facing roof with minimal shade coverage. Generally, these panels cost between $8.5 and $10 a watt and have about twelve to twelve and a half percent conversion efficiency. In a simpler sense this means that, 12% of sun’s energy that strikes these panels is converted into electricity. (Rogers, 2014)
From the various studies, it has been observed that mono-crystalline solar cells are more expensive than the ploy-crystalline ones. However, after production, when the cells are separately arranged in panels for electricity production than the cost of assembling in $/watt is almost same for both the types of cells. Hence, although manufacturing cost differs, but assembling both the panels are of same costs. After going through few reformations, as suggested by the researchers at the Massachusetts Institute of Technology (MIT), the multi-crystalline solar cell at present cost $2.10 per watt generated. If these technological reformations are installed in the commercial manufacturing units then these cells might cost $1.65 per watt. And if the reformations are widely accepted by the people and succeeds as per decided then this price might also fall to around $1.30 per watt. But instead of comparing with its cousin cells, when the cell is compared to the earth’s energy resources, like coal, then the price has to fall to $1 per watt. This could be achieved only if the above suggested three key innovations are quickly installed in this cheap energy source and also at the same time is readily accepted by the consumers. The demand of the cells has to increase for it to remain in the long run of advance energy sources. (Thantsha, et al., n.d.)
Installation of a multi-crystalline solar panel can be between $15,000 to $29,000 for an average sized systems sized between 4kW and 8kW. The average breakdown of the costs of a residential solar installation could be explained with the following graph:

• Equipment costs include cost of Solar panels, inverter mounting hardware and wire.
• Installation and permits costs include cost of installation, supply chain, permitting and interconnection.
• Sales and Operational costs include monitoring and maintenance costs, repairs, additional operational and overhead.

The cost of solar panels is falling, down almost 100% since 1977 and more than 50% since 2007 alone. As the demands are growing day by day, hence to meet this demand more solar providers are entering the market. As a result, the installation costs are also falling. Talking more precisely about household multi-crystalline photo voltaic cells, the prices started falling around 2009 and a substantial fall of 30% was observed between 2010 and 2013. They went from an average of around $32,000 for a five kilowatt system to under $23,000. (Utah Edu, 2013)

Diagnosis, Monitoring and Quantification of Defects

There are a number of defects in multi-crystalline solar wafers. These defects have a huge impact on the solar cells made from this material. Dislocation is a defect that occurs at high temperatures of 5000 C or above. The most harmful part of this defect is that it interacts with other defects such as metallic impurities within the material of solar cell. The impurities tend to settle down in the defect while dislocation.  This reduces the efficiency of the cell. To cure this defect, a researcher at MIT suggested applying a method in wafers of poly-crystalline silicon before processing it into solar cells. This method more precisely involves using a chemical treatment in order to view the dislocations and analyze the geometric variation on the surface. The distribution and concentration of metal impurities can be diagnosed and monitored by Crystallographic Analysis as well as X-rays. After the analysis one can easily deduce the electrical behavior within the material. To quantify this defect the silicon wafer should undergo multiple defect analysis procedures before actual installation in solar cells. (Abdulgafar, et al., 2014)
Apart from the above defect the defects in the multi-crystalline broadly falls under two categories: Intrinsic defect and Extrinsic defect. Grain boundaries are an example of intrinsic defect, while micro-crack falls in the second category of defects. The former type of defect has an impact on the circuit of the cell and diminishes the short-circuit current of the cell. In the latter defect, there are formations of cracks in the silicon wafers that are less than 30 micro-meters and are invisible to naked eyes. These cracks are formed usually by scratches during cell fabrication or can also be due to wafer sawing or laser cutting. These cracks have a tendency of propagating. This propagation causes the detachment or internal breakage of the grainy materials within the cells. Another type of crack is formed due to sharp point impact which induces several line cracks which eventually crosses each other. These types of small sized defects can only be diagnosed and visualized electronically through electro-luminescence (EL) technique and high power cameras. This technique has become a powerful and fast characterization tool providing spatially-resolved information about defects in the material. On applying forward bias, the solar cell emits Infrared Radiation. The intensity of these radiations decreases in the vicinity of intrinsic defects and cracks. The cracks have a tendency of decreasing the efficiency of the solar cells. As a result, the cells absorb less energy from the sun and hence such cells provide less electrical energy to the consumers. The crack defects could be quantified by incorporating precautions while handling the wafers during cell fabrication. If errors at this stage are minimized then there are less chances of propagation of cracks at solar cell installation level. (Anwar & Abdullah, 2014)
A Grain Boundary is the interface between two crystallites in a poly-crystalline silicon wafer. These defects have the potential to decrease the thermal and electrical conductivity of the material. The grain boundaries formed are the preferred sites of corrosion and for the precipitation of impurities. Another notifying impact of such defects is the disruption of the motion of dislocations. The defects can be monitored by the similar techniques being installed in the detection of smaller sized cracks i.e. Electro-luminescence (EL) techniques. In order to remove these defects the only quick and easy method is the reduction of crystallite size just at the time of formation of poly-crystallite wafers. The crystallite size needs to be optimized and the manufacturers could also even standardize the size prior to the commencement of the wafer fabrication. This would definitely minimize the error of varying crystallite size at the production time. 

Conclusion

Multi-crystalline wafers have many pros and cons, but in the end, they can be an inexpensive and easy to manufacture, source of energy that efficiently converts solar energy into electrical energy. They are the cheapest yet a reliable way to put together a solar power generation system. They are much simpler to install and without a long list of disadvantages. These wafers are fitted in much the same way as any other panel and do not account for any extra charges. From all the above discussions, it could be concluded that mono-crystalline solar wafers are slightly more efficient than poly/multi crystalline solar wafers. But the latter being less expensive is more prevalent in the global markets. The easy manufacturing procedure and the less installation charges make these cells to top the list of solar energy manufacturers and buyers. Due to tighter spacing of the ploy-crystalline solar cells in the panels as compared to the mono-crystalline solar panels, the higher efficiency of the latter one becomes negligible. Also, at high temperatures the poly-crystalline cells tends to perform better than the other one. Hence, poly-crystalline solar wafers prove to be the cheapest yet efficient source of solar energy transformation. (Amazon.com, 2014)

Bibliography

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