Different types of solar cells

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- INTRODUCTION:

Any device that directly converts the energy in light into electrical energy through the photovoltaic effect ,are called solar cells. Unlike batteries or fuel cells, solar cells do not utilize chemical reactions or require fuel to produce electric power, and, unlike electric generators, they do not have any moving parts. Solar cells can be arranged into large groupings called arrays. These arrays, composed of many thousands of individual cells, can function as central electric power stations, converting sunlight into electrical energy for distribution to industrial, commercial, and residential users. Solar cells in much smaller configurations, commonly referred to as solar cell panels or simply solar panels, have been installed by homeowners on their rooftops to replace or augment their conventional electric supply. Solar cell panels also are used to provide electric power in many remote terrestrial locations where conventional electric power sources are either unavailable or prohibitively expensive to install. Because they have no moving parts that could need maintenance or fuels that would require replenishment, solar cells provide power for most space installations, from communications and weather satellites to space stations. Another growing application of solar cells is in consumer products, such as electronic toys, handheld calculators, and portable radios. Solar cells used in devices of this kind may utilize artificial light as well as sunlight. http://www.answers.com/topic/solar-cell

- Different types of solar cell:

There are three main types of solar cells, which are distinguished by the type of crystal used in them. They are monocrystalline, polycrystalline, and amorphous. To produce a monocrystalline silicon cell, absolutely pure semiconducting material is necessary. Monocrystalline rods are extracted from melted silicon and then sawed into thin plates. This production process guarantees a relatively high level of efficiency.

  1. -Monocrystalline silicon: Efficiency in Lab about 24% and Efficiency of production cells 14-17%.
  2. -Polycrystalline silicon: Efficiency in lab about 18% and efficiency of production cell 13-15%.
  3. -Amophas silicon: Efficiency in lab about 13% and efficiency of production cell 5-7%.

The production of polycrystalline cells is more cost-efficient. In this process, liquid silicon is poured into blocks that are subsequently sawed into plates. During solidification of the material, crystal structures of varying sizes are formed, at whose borders defects emerge. As a result of this crystal defect, the solar cell is less efficient. http://www.daviddarling.info/encyclopedia/S/AE_solar_cell.html
If a silicon film is deposited on glass or another substrate material, the result is a so-called amorphous or thin-layer cell. The layer thickness amounts to less than 1µm - the thickness of a human hair for comparison is 50-100 µm. The production costs of this type are lower because of the lower material costs. However, the efficiency of amorphous cells is much lower than that of the other two cell types. As a result, they are used mainly in low power equipment, such as watches and pocket calculators, or as facade elements.
http://www.daviddarling.info/encyclopedia/S/AE_solar_cell.html

- METHDOLOGY: (A).Solar cell structure and operation:

Solar cells, whether used in a central power station or a calculator, have the same basic structure. Light enters the device through an optical coating, or antireflection layer , that minimizes the loss of light by reflection; it effectively traps the light falling on the solar cell by promoting its transmission to the energy-conversion layers below. The antireflection layer is typically an oxide of silicon, tantalum, or titanium that is formed on the cell surface by spin-coating or a vacuum deposition technique. http://www.answers.com/topic/solar-cell

The three energy-conversion layers below the antireflection layer are the top junction layer, the absorber layer, which constitutes the core of the device, and the back junction layer. Two additional electrical contact layers are needed to carry the electric current out to an external load and back into the cell, thus completing an electric circuit. The electrical contact layer on the face of the cell where light enters is generally present in some grid pattern and is composed of a good conductor such as a metal. Since metal blocks light, the grid lines are as thin and widely spaced as is possible without impairing collection of the current produced by the cell. The back electrical contact layer has no such diametrically opposed restrictions. It need simply function as an electrical contact and thus covers the entire back surface of the cell structure. http://www.answers.com/topic/solar-cell

Since most of the energy in sunlight and artificial light is in the visible range of electromagnetic radiation, a solar cell absorber should be efficient in absorbing radiation at those wavelengths. Materials that strongly absorb visible radiation belong to a class of substances known as semiconductors. Semiconductors in thicknesses of about one-hundredth of a centimetre or less can absorb all incident visible light; since the junction-forming and contact layers are much thinner, the thickness of a solar cell is essentially that of the absorber. http://www.answers.com/topic/solar-cell

When light falls on a solar cell, electrons in the absorber layer are excited from a lower-energy “ground state,” in which they are bound to specific atoms in the solid, to a higher “excited state,” in which they can move through the solid. In the absence of the junction-forming layers, these “free” electrons are in random motion, and so there can be no oriented direct current. The addition of junction-forming layers, however, induces a built-in electric field that produces the photovoltaic effect. In efect, the electric field gives a collective motion to the electrons that flow past the electrical contact layers into an external circuit where they can do useful work. http://www.answers.com/topic/solar-cell

The materials used for the two junction-forming layers must be dissimilar to the absorber in order to produce the built-in electric field and to carry the electric current. Hence, these may be different semiconductors (or the same semiconductor with different types of conduction), or they may be a metal and a semiconductor. The materials used to construct the various layers of solar cells are essentially the same as those used to produce the diodes and transistors of solid-state electronics and microelectronics. Solar cells and microelectronic devices share the same basic technology. In solar cell fabrication, however, one seeks to construct a large-area device because the power produced is proportional to the illuminated area. In microelectronics the goal is, of course, to construct electronic components of ever smaller dimensions in order to increase their density and operating speed within semiconductor chips, or integrated circuits. http://www.answers.com/topic/solar-cell

The photovoltaic process bears certain similarities to photosynthesis, the process by which the energy in light is converted into chemical energy in plants. Since solar cells obviously cannot produce electric power in the dark, part of the energy they develop under light is stored, in many applications, for use when light is not available. One common means of storing this electrical energy is by charging electrochemical storage batteries. This sequence of converting the energy in light into the energy of excited electrons and then into stored chemical energy is strikingly similar to the process of photosynthesis. http://www.answers.com/topic/solar-cell

(B). Solar panel design: Most solar cells are a few square centimetres in area and protected from the environment by a thin coating of glass or transparent plastic .Because a typical 10 cm × 10 cm (4 inch × 4 inch) solar cell generates only about two watts of electrical power (15 to 20 percent of the energy of light incident on their surface), cells are usually combined in series to boost the voltage or in parallel to increase the current. A solar, or photovoltaic (PV), module generally consists of 36 interconnected cells laminated to glass within an aluminum frame. In turn, one or more of these modules may be wired and framed together to form a solar panel. Solar panels are slightly less efficient at energy conversion per surface area than individual cells, because of inevitable inactive areas in the assembly and cell-to-cell variations in performance. The back of each solar panel is equipped with standardized sockets so that its output can be combined with other solar panels to form a solar array. A complete photovoltaic system may consist of many solar panels, a power system for accommodating different electrical loads, an external circuit, and storage batteries. Photovoltaic systems are broadly classifiable as either stand-alone or grid-connected systems. http://www.answers.com/topic/solar-cell

Stand-alone systems contain a solar array and a bank of batteries directly wired to an application or load circuit. A battery system is essential to compensate for the absence of any electrical output from the cells at night or in overcast conditions; this adds considerably to the overall cost. Each battery stores direct current (DC) electricity at a fixed voltage determined by the panel specifications, although load requirements may differ. DC-to-DC converters are used to provide the voltage levels demanded by DC loads, and DC-to-AC inverters supply power to alternating current (AC) loads. Stand-alone systems are ideally suited for remote installations where linking to a central power station is prohibitively expensive. Examples include pumping water for feedstock and providing electric power to lighthouses, telecommunications repeater stations, and mountain lodges. http://www.answers.com/topic/solar-cell

- SUMMARY:

(A) .New development on solar cells: The development of solar cell technology stems from the work of the French physicist Antoine-César Becquerel in 1839. Becquerel discovered the photovoltaic effect while experimenting with a solid electrode in an electrolyte solution; he observed that voltage developed when light fell upon the electrode.http://www.britannica.com/EBchecked/topic/552875/solar-cell/45872/Development-of-solar-cells

(a)Printable solar cells on the way:

Researchers from the Victorian Organic Solar Cell Consortium, which includes scientists from government research agency the CSIRO, the University of Melbourne and Monash University, have developed a new technique that could open up the door for cheap, mass-produced solar cells."These solar cells are cutting edge technology and offer advantages over traditional solar technology," Peter Batchelor, Victorian Minister for Energy and Resources, said at the launch. "The production of these film-like solar cells will be literally as easy as printing money."http://www.britannica.com/EBchecked/topic/552875/solar-cell/45872/Development-of-solar-cells

(b)Flexible plastic:

The new cells are printed onto a thin flexible plastic, which (unlike existing silicon solar cells) can be easily crafted to fit any rooftop. Gerry Wilson, a member of the CSIRO team, said humankind has been printing for centuries and this is one of many potential applications for "printable electronics."The active ingredients in the new solar cells are thin-printed layers of light-sensitive inks that absorb energy from the Sun. The researchers said that during an ongoing trial period these inks would be tested for maximum efficiency.

http://www.sciencedaily.com/releases/2007/07/070719011151.htm.

Currently, the printable solar cells are two to five per cent efficient, the experts said, something they are trying to improve it "by tweaking the chemical structure" of the inks. Solar cells currently on the market range from 5 to 24 per cent efficiency. http://www.britannica.com/EBchecked/topic/552875/solar-cell/45872/Development-of-solar-cells

(c)Forefront of polymer technology:

Jai Singh, a physicist from Charles Darwin University, in the Northern Territory, said that while the technology is still in its infancy, it could provide an economical alternative energy source. "They are cost-effective because the expensive indium tin oxide used in traditional solar cells will be replaced by low cost functionalised graphene layers," he said.Australian science minister, Senator Kim Carr, said the trial was an exciting development for the industry. "This research is at the forefront of polymer technology, which has already brought to the world the banknotes used in Australia and 21 other countries. It is an important step in building up the solar industry," he said.Andrew Blakers, Director of ARC Centre for Solar Energy Systems at Australian National University said any investment in the industry is always welcomed and will encourage progress in renewable energy sources. "Australian solar industry needs to be encouraged and well funded in order for Australia to take its place as a world leader in this industry," he said.

http://www.sciencedaily.com/releases/2008/11/081104114427.htm.

(d)Hybrid Materials For Future Solar Cells Under Development:

Semiconductor nanocrystals or also called quantum dots exhibit outstanding optical properties compared to organic dyes. Due to the quantum confinement their emission color can be continuously tuned from the ultraviolet to the near infrared range by changing the size and chemical composition. They exhibit a broad absorption spectrum, a narrow emission band and large absorption cross sections. Their surface can be covered by a few monolayers of different semiconductor materials in such a way that we can either improve their luminescent properties and stability or avoid the fluorescence to obtain charge carriers. The latter effect opens tremendous alternatives in photovoltaics. Due to their optical properties, semiconductor nanoparticles are studied in different disciplines, from optics to biomedicine.

http://www.sciencedaily.com/releases/2008/11/081104114427.htm.

Scientists can now produce nanoparticles of different materials controlling their size, shape, and surface properties. Examples of nanoparticles produced by non hydrolytic colloidal synthetic methods are CdS, CdTe, InP, GaAs, PbS, or PbSe. However, the most studied system is CdSe, with tunable emission from blue to red. Due to the synthetic approach (hot injection method), the surface of these nanoparticles is capped with an organic shell that protects them and makes them stable in non-polar organic solvents. It is also possible to controllably replace the initial organic shell for water compatible ones. The organic shell plays a relevant role in the quantum efficiency of the nanoparticles and their stability in different media. However, this shell prevents high electrical conduction. http://www.sciencedaily.com/releases/2008/11/081104114427.htm.

Carbon nanotubes are another example of nanomaterials with extraordinary electrical properties. They consist of one or several rolled up graphene layers. In the case of a single layer they are called single-wall and multi-wall when several layers are rolled-up. Hybrid materials composed of semiconductor nanoparticles and carbon nanotubes combine the high absorption properties of the former and the high electrical conductivity of the latter. http://www.sciencedaily.com/releases/2008/11/081104114427.htm.

One of the main drawbacks in the formation of such hybrid structures focuses on the type of interaction between them. Most of the existing procedures involve the growth of nanoparticles on previous defect sites provoked on the surface or edges of carbon nanotubes by aggressive chemical means. These aggressive treatments render an oxidized nanotube surface or even structural damage that deteriorates their outstanding electrical, mechanical, and optical properties significantly. Thus, supramolecular or electrostatic functionalisations are better approaches for photovoltaic applications.Although in an initial stage, the results obtained up to now points out the high potential of these composites to build up photovoltaic devices and solar cells. http://www.sciencedaily.com/releases/2008/11/081104114427.htm.

-Solar cell applications:

There is an extensive range of applications where solar cells are used;

a)-Rural electrification:

The provision of electricity to rural areas derives important social and economic benefits to remote communities throughout the world. Power supply to remote houses or villages, electrification of the health care facilities, irrigation and water supply and treatment are just few examples of such applications.

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b)-Water pumping:

More than 10,000 PV powered water pumps are known to be successfully operating throughout the world. Solar pumps are used principally for two applications: village water supply (including livestock watering), and irrigation. Since villages need a steady supply of water, provision has to be made for water storage for periods of low insolation. In contrast, crops have variable water requirements during the year which can often be met by supplying water directly to produce without the need for a storage tank.

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c)- Domestic supply:

Stand-alone PV domestic supply systems are commonly encountered in developing countries and remote locations in industrialised countries. The size range varies from 50 Wp to 5 kWp depending on the existing standard of living. Typically larger systems are used in remote locations or island communities of developed countries where household appliances include refrigeration, washing machine, television and lighting. In developing regions large systems (5 kWp) are typically found for village supply while small systems (20-200 Wp) are used for lighting, radio and television in individual houses.

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d)-Health care:

Extensive vaccination programmes are in progress throughout the developing world in the fight against common diseases. To be effective, these programmes must provide immunisation services to rural areas. All vaccines have to be kept within a strict temperature range throughout transportation and storage. The provision of refrigeration for this aim is known as the vaccine cold chain.

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e)-Lighting:

In terms of the number of installations, lighting is presently the biggest application of photovoltaics, with tens of thousands of units installed world-wide. They are mainly used to provide lighting for domestic or community buildings, such as schools or health centres. PV is also being increasingly used for lighting streets and tunnels, and for security lighting.

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f)-Electric power generation in space:

Photovoltaic solar generators have been and will remain the best choice for providing electrical power to satellites in an orbit around the Earth. Indeed, the use of solar cells on the U.S. satellite Vanguard I in 1958 demonstrated beyond doubt the first practical application of photovoltaics. Since then, the satellite power requirements have evolved from few Watts to several kiloWatts, with arrays approaching 100 kW being planned for a future space station .A space solar array must be extremely reliable in the adverse conditions of space environment. Since it is very expensive to lift every kilogram of weight into the orbit, the space array should also have a high power-to-weight ratio.

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Advantages of solar cells:

There are many advantages of solar energy. Just consider the advantages of solar energy :

  1. -There is no costs in buying the land and preparing the site.

  2. -The transmission losses are much lower because the load is on the same site as the supply.

  3. -The value of the PV electricity is also higher because it is equal to the selling price of the grid electricity which has been replaced, rather that to the cost of generating it.

  4. -Solar energy is a renewable resource. Although we cannot utilize the power of the sun at night or on stormy, cloudy days, etc., we can count on the sun being there the next day, ready to give us more energy and light. As long as we have the sun, we can have solar energy .
  5. -Solar cells are totally silent. They can extract energy from the sun without making a peep. Now imagine the noise that the giant machines used to drill for and pump oil make!
  6. -Solar energy is non-polluting. Of all advantages of solar energy over that of oil, this is, perhaps, the most important. The burning of oil releases carbon dioxide and other greenhouse gases and carcinogens into the air.
  7. -Solar cells require very little maintenance (they have no moving parts that will need to be fixed), and they last a long time.
  8. -Although solar panels or solar lights, etc., may be expensive to buy at the onset, you can save money in the long run. After all, you do not have to pay for energy from the sun. On the other hand, all of us are aware of the rising cost of oil.

i)-Solar powered lights and other solar powered products are also very easy to install. You do not even need to worry about wires.

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-Disadvantages of solar energy:

  1. -Solar cells/panels, etc. can be very expensive.
  2. -Solar power cannot be created at night.

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