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SOLAR CELLS


General Information

Research conducted by Panasonic over many years on solar cells and the application of this new technology culminated in 1984 with the successful development of the world’s first thin-film solar cell using compound semiconductors. The company named these cells Sunceram II. The Sunceram II cells have good weatherproof properties and high spectral sensitivity characteristics over a wide wavelength range. Furthermore, since the entire film-forming process involves only screenprinting and since belt sintering is employed, these cells are very amenable to mass production. It also means that high-voltage type solar cells can be formed at a high density on a single glass substrate, and that it is easy to produce them with larger surface areas. Besides developing compact and lightweight Sunceram II modules for outdoor use which maintain a stable performance over prolonged periods, Panasonic has developed compact, high-performance Sunceram II sign units which are used in combination with the company's own coin-type rechargeable batteries. With its sights firmly fixed on power sources for the new forms of soft energy which will be abundant in the twenty-first century, Panasonic is committed to developing new products which will fill the needs of the market.

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“DEVELOPMENT OF SOLAR CELLS”
Introduction:
 Photovoltaic cells or solar cells are the devices used in photovoltaic conversion i.e. when solar radiation falls on these devices, it is converted directly into dc electricity. The first solar cell was built by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions.
Advantages:
 No moving parts, require little maintenance, and work quite satisfactorily with beam or diffuse radiation.
 Readily adapted for varying power requirements.
 Environmentally friendly source of electricity.
Description:
 Single crystal silicon cell.
 p-type, n-type silicon and a junction.
 Metal electrodes (Ti-Ag solder).
à Front metal electrode.
à Back metal electrode.
Anti-reflection coating and a thin transparent encapsulating sheet on the top surface.
Solar Module:
 In order to obtain higher voltages and currents, individual cells are fixed side by side on a suitable back-up board and connected in series and parallel to form a module or solar panel
 In turn a number of PV modules are interconnected to form an array.
Principle of working of a solar cell:
Creation of pairs of positive and negative charges (electrons-hole pairs) in the solar cell by absorbed solar radiation.
 Photons of sunlight.
 Semiconductor materials.
 Energy bands-valence and conduction bands.
 Band gap energy.
 Electron-hole pairs.
(2) Separation of the positive and negative charges by a potential gradient within the cell.
 Electron-hole pair.
 Separated if potential gradient exists.
 Obtained by sandwiching of p-type & n-type silicon.
 p-type_silicon doped with boron. n-type_silicon doped with phosphorous.
Energy levels, jump in energy levels.
Existence of potential gradient, flow of direct electric current.
Efficiency factors:
(1) Maximum power point: A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing the resistive load on an irradiated cell continuously from zero (a short circuit) to a very high value (an open circuit) one can determine the maximum-power point, that is, the load for which the cell can deliver maximum electrical power at that level of irradiation. Vm x Im = Pm in watts.
(2) Energy conversion efficiency: The maximum conversion efficiency of a solar cell is given by the ratio of the maximum useful power to the incident solar radiation.
(3) Fill factor: Another defining term in the overall behavior of a solar cell is the fill factor (FF). This is the ratio of the maximum power point divided by the open circuit voltage (Voc) and the short circuit current (Isc).
Materials and efficiency:
 Various materials have been investigated for solar cells. There are two main criteria - efficiency and cost. Efficiency is a ratio of the electric power output to the light power input. By far the most common material for solar cells is crystalline silicon. Crystalline silicon solar cells come in three primary categories:
àSingle crystal or monocrystalline wafers: Most commercial monocrystalline cells have efficiencies on the order of 14%; the SunPower cells have high efficiencies around 20%. Single crystal cells tend to be expensive, and because they are cut from cylindrical ingots, they cannot completely cover a module without a substantial waste of refined silicon. Most monocrystalline panels have uncovered gaps at the corners of four cells. Sunpower and Shell Solar are among the main manufacturers of this type of cells.
 Poly or multi crystalline made from cast ingots - large crucibles of molten silicon carefully cooled and solidified. These cells are cheaper than single crystal cells, but also somewhat less efficient. However, they can easily be formed into square shapes that cover a greater fraction of a panel than monocrystalline cells, and this compensates for their lower efficiencies.
 Ribbon silicon formed by drawing flat thin films from molten silicon and has a multicrystalline structure. These cells are typically the least efficient, but there is a cost savings since there is very little silicon waste since this approach does not require sawing from ingots.
These technologies are wafer based manufacturing. In other words, in each of the above approaches, self supporting wafers of ~300 micrometres thick are fabricated and then soldered together to form a module.
 Thin film approaches are module based. The entire module substrate is coated with the desired layers and a laser scribe is then used to delineate individual cells. Two main thin film approaches are amorphous silicon and CIS:
à Amorphous silicon films are fabricated using chemical vapor deposition techniques, typically plasma enhanced (PE-CVD). These cells have low efficiencies around 8%.
 CIS stands for general chalcopyrite films of copper indium selenide (CuInSe2) While these films can achieve 11% efficiency, their costs are still too high.
There are additional materials and approaches. For example, Sanyo has pioneered the HIT cell. In this technology, amorphous silicon films are deposited onto crystalline silicon wafers.
Cost analysis:
 The US retail module costs are in the $3.50 to $5.00/Wp range. Additional installation costs for a residential rooftop retrofit in California (CA) is around $3.50/Wp or more. So on the low side, installed system costs are about $7.00/Wp in CA, and probably higher in places with less experience. Federal, state, utility, and other subsidies combined pay about half the cost. So CA rule of thumb is that the installed system PV will cost you at the low end, $3.50/Wp.
Cost reduction:
 Developing innovative manufacturing techniques (like the EFG process), which speed up the production process, reduce material wastage & yield large size cells.
 Development of thin film solar devices which require much less material and, if possible, use material which is inherently inexpensive. For example Cadmium sulphide-cadmium telluride and Copper Indium Diselenide solar cells.
 Significant cost reduction is achieved by the use of concentrators to focus the sunlight on high efficiency solar cells. The concentration is achieved by using either linear or circular Fresnel lenses or parabolic or paraboloid concentrators which focus along a line or at a point, concentration ratios ranging from 10 to 1000 being used.
Current research:
There are currently many research groups active in the field of photovoltaics at universities and research institutions around the world.
Much of the research is focussed on making solar cells cheaper and/or more efficient, so that they can more effectively compete with other energy sources, including fossil energy. One way of doing this is to develop cheaper methods of obtaining silicon that is sufficiently pure. Silicon is a very common element, but is normally bound in silica sand. Another approach is to significantly reduce the amount of raw material used in the manufacture of solar cells. The various thin-film technologies currently being developed make use of this approach to reducing the cost of electricity from solar cells.
The invention of conductive polymers may lead to the development of much cheaper cells that are based on inexpensive plastics, rather than semiconductor grade silicon. However, all organic solar cells made to date suffer from degradation upon exposure to UV light, and hence have lifetimes which are far too short to be viable.
Applications:
In spite of the high initial cost, photovoltaic systems are being used increasingly to supply electricity for many applications requiring small amounts of power. Their cost-effectiveness increases with the distance of the location (where they are to be installed) from the main power grid lines. Some applications for which PV systems have been developed are,
(1) Pumping water for irrigation and drinking and electrification for remote villages for providing street lighting and other community services.
(2) Telecommunication for the post and telegraph and railway communication network.
In addition, in developed countries solar cells are being used extensively in consumer products and applications.