Solar Cell

SOLAR CELL

P.Parkavi, S.Saranya, IFET College of engineering, IFET nagar, Ganagarampalayam Villupuram-605108

Introduction You’ve probably seen calculators that have solar cell-calculators that never need batteries and in some cases don’t even have an off button. as long as you have enough light, they seem to work forever. You may have seen larger solar panelson emerging road signs or call boxes. On buons, even in parking lots to power lights. Although these larger panels aren’t as common as solar powered calculators. They‘re out there and not that hard to spot if you know where to look. You have also seen solar cells arrays on satellites, where they are used in powers the electrical system.

You have probably also been hearing about the “solar revolution” for the last 20 years – the idea that one day we will all use free electricity from the sun. this is a seductive promise – on a bright, sunny day the sun shines approximately 1000 watts of energy per square meter of the planet’s surface, and if we could collect all of that energy we could easily power our homes and offices for free

In this section, we will examine solar cells to learn how they convert the sun’s energy directly into electricity. in the process you will learn why we are getting closer to using the sun’s energy on a daily basis, but we still have more research to do before the process becomes cost effective.

A solar cell or a photovoltaic cell is a device that converts photons from the sun’s radiation into electricity. In general, a solar cell that includes both solar and non solar sources of light (such as photons from incandescent bulbs) is termed as photovoltaic cell.

Principle Solar cells or photovoltaic cells are working on the principle “photovoltaic effect” i.e. Creation of emf across the pn junction during irradiation of light. Thus the basic function of a solar cell is the conversion of electromagnetic energy into electrical energy.

Working If a piece of p – type silicon is placed in intimate contact with a piece of ntype silicon, then a diffusion of electrons occurs from the region of high electron concentration (the n-type side of the junction) into the region of low electron concentration (p-type side of the junction). When the electrons diffuse across the p-n junction, they recombine with the holes on the p-type side. The diffusion of carriers does not happen indefinitely however, because of an electric field which is created by the imbalance of charge immediately either side of the junction which this diffusion creates. The electric field established across the p-n junction creates a diode that promotes current to flow in only one direction across the junction. Electrons may pass from the n-type side into the p-type side, and holes may pass from the p-type side to the n-type side. This region where electrons have diffused across the junction is called the depletion region.

The action of the light (shower of photons) falling on the junction is to create electron-hole pairs which move under the influence of this built in field such that the electrons migrate to n-region and the holes migrate to p-region. This charge separation will create an electric field opposite to the electric field created by the diffusion. If the number of absorbed photons is enough, these two fields will cancel

each other, leading to an open circuit voltage between the n and p regions. If these created electrons and holes are made to flow through an external load, electrical energy will be obtained from the absorbed photons.

The DC output from the solar arrays enters an inverter. The inverter turns DC electricity into 120-240 volt AC (alternating current) electricity needed for home appliances. The AC power enters the utility panel in the house. The electricity is then distributed to appliances or lights in the house. The electricity that is not used will be recycled and reused in other facilities. (Solar panels have a sheet of tempered glass on the front, and a polymer encapsulation on the back).

CHARACTERISTIC OF A SOLAR CELL:

A good photo voltaic material should have a large absorption coefficient at low temperature and optimum value of energy gap. If the photon energy is equal to (or) greater than the band gap leads to a large intrinsic carrier concentration and the possibility of photon absorption is less. The photon can be absorbed by the silicon, can generate heat if the photon energy is higher than the silicon band gap value. The construction of a solar cell is difficult because of the fact that one of the crystal (usually p –type) has to be in the form of a single crystal with a controlled impurity. The thickness of p layer is very small when we compare with n-layer to avoid recombination of charge carriers.

Production of solar cell using bulk materials Bulk technologies are often referred to as wafer-based technologies. In each of these approaches, self-supporting wafers between 180 to 240 micrometers thick are processed and then soldered together to form a solar cell module.some techniques are given below:

Silicon: Bulk silicon is separated into multiple categories according to crystallinity and crystal size in the resulting ingot, ribbon, or wafer.

1. Monocrystalline silicon (c-Si): often made using the CZOCHRALSKI process. Single crystal wafer cells tend to be expensive, and because they are cut from cylindrical ingots, do not completely cover a square solar cell module without a substantial waste of refined silicon. Hence most c-Si panels have uncovered gaps at the corners or four cells.

2. Poly-or multicrystalline silicon (poly-Si or mc-Si): made from cast square ingots-large blocks of molten silicon carefully cooled and solidified. These cells are less expensive to produce than single crystal cells but are less efficient. These are suitable for large scale production.

3. Ribbon silicon: formed by drawing flat thin films from molten silicon and having a multicrystalline structure. These cells have lower efficiencies than poly-Si, but save on production costs due to a great reduction in silicon waste, as this approach does not require sawing from ingots.

Thin film solar cell: The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. This can lead to reduced processing costs from that of bulk materials. Many multi-layer thin films have efficiencies above those of bulk silicon wafers.

Cadmium telluride: CdTe is an efficient light-absorbing material for thin-film solar cells. Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large scale production. This is the only technology (apart from amorphous silicon) that can be delivered on a large scale, as shown by First Solar and Antec Solar. There is a 40 megawatt plant in Ohio (USA) and a 10 megawatt plant in Germany. First Solar is scaling up to a 100 MW plant in Germany.

COPPER INDIUM GALLIUM ARSENIDE:

They are multi-layered thin-film composites. These cells are best described by a more complex heterojunction model. The best efficiency of a thin-film solar cell as December 2005 was 19.5% with CIGS. As of 2006, the best conversion efficiency for flexible CIGS cells on polyimide is 14.1%. The use of Gallium increases the band gap of the CIGS layer as compare to CIS thus increase the voltage. In another point of view, gallium is added to replace as much indium as possible due to gallium’s relative availability to indium. Approximately 70% of Indium currently produced is used by the flat-screen monitor industry.

COPPER INDIUM SELENIDE:

The materials based on CuInSe2, that are of interest for photovoltaic applications include several elements from groups I,III,VI in the periodic table. These semiconductors are especially attractive for thin film solar application because of their high optical absorption co-efficients and versatile optical and electrical characteristics which can in principle can be manipulated and tuned for a specific need in a given device. CIS is an abbreviation for general chalcopyrite films of copper indium selenide(CuInSe2), CIGS mentioned above is a variation of CIS. While these films can achieve 13.5% efficiency, their manufacturing costs at present are high when comparing to silicon solar cell but continuing work is leading to more cost-effective production processes.

GALLIUM ARSENIDE (GaAs) MULTIJUNCTION:

High-efficiency cells have been developed for special applications such as satellites and space exploration which require highperformance. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP2. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly the entire spectrum, thus generating electricity from as much of the solar energy as possible.

GaAs multifunction devices are the most efficient solar cells to date, reaching as high as 39% efficiency.

SHELL SOLAR: It is the renewable energy company producing and marketing solar panels for homes, business and remote use.

NANOCRYSTALLINE SOLAR CELLS: These structures make use of some of the same thin-film light absorbing materials but are overlain as an extremely thin absorber on a supporting matrix of conductive polymer or mesoporous metal oxide having a very high surface area to increase internal reflections (and hence increase the probability of light absorption).

THE SPACE AGENCY (NASA):

The space agency (NASA) awards a two year grant to use nanotechnology to improve the efficiency and radiation - resistence of solar cells for space craft on January 5th, 2007.

NANO SOLAR-HOMEPAGE: It is a world leader in solar power innovation. They are setting the standard for affordable clean electricity with solar cell technology of distinctly superior cost efficiency, versatility and availability.

ORGANIC OR POLYMER SOLAR CELLS:

Organic solar cells and polymer solar cells are built from thin films of (typically 100 nm) organic semiconductors such as polymers and small- modules compounds like polyphenylene vinylene, copper phthalocyanine (a blue or green organic pigment)and carbon fullerenes. Energy conversion efficiency achieved to date using conducting polymers are low at 4 to 5 % efficiency for the best cells to date. However, these cells could be beneficial for some applications where mechanical flexibility and disposability are important.

MORE POWERFUL SOLAR CELLS: Today, solar cells are not a very efficient way to produce electricity, even if the source of energy is free. They can be manufactured from thick crystalline silicon wafers (300 microns thick) or thinner non-crystalline ones(about 2 microns thick) .But thin cells built are less efficient than thick ones, even if they are cheaper to produce. But now, researchers have found a way to boost the performance of solar cells by 40 % to 50%.

SOLAR CELL EFFICIENCY FACTORS: A solar cell may operate over a wide range of voltages (V) and currents (I). By increasing resistive load (voltage) in the cell from zero(indicating a short circuit ) to infinitely high values (indicating open circuit) one can determine the maximum power point (the maximum output electrical power Vmax * Imax ; or Pm, in watts). Pm = Vmax * Imax.

ENERGY CONVERSION EFFICIENCY:

A solar cell’s energy conversion efficiency (η, ’eta’),is the percentage of power converted (from absorbed light to electrical circuit) and collected, when a solar cell is connected to an electrical circuit. This term is calculated using the ratio of Pm, divided by the input light irradiance under standard test conditions (E, in Weber / meter square) and surface area of the solar cell.

η = Pm/( E* Ac) “STANDARD” solar radiation (known as the “air mass 1.5 spectrum”) has a power density of 1000 watts per square meter. Thus, a 12% efficiency solar cell having one meter square of surface area in full sunlight at solar noon at the equator during either March or September.

MERITS AND DEMIRITS:

SILICON PROCESSING: Silicon is very common element, but it is normally bound in silica, silica sand. Processing silica (SiO2) to produce silicon is a very high energy process, and more energy efficient methods of synthesis are not only beneficial to the solar industry, but also to industries surrounding silicon technology as a whole. The current industrial production of silicon is via the reaction between carbon (charcoal) and silica at a temperature around 1700 degrees Celsius. In this process, known as carbothermic reduction, each tone of silicon (metallurgical grade, about 98% pure) is

produced with the emission of about 1.5 tones of carbon dioxide. Solid silica can be directly converted (reduced) to pure silicon by electrolysis in a molten salt bath at a fairly mild temperature (800 to 900 degree Celsius). This is the only technology (apart from amorphous silicon) that can be delivered on a large scale, as shown by First Solar and Antec Solar. There is a 40 megawatt plant in Ohio(USA) and a 10 megawatt plant in Germany. First solar is scaling up to a 100 MW plant in Germany. The perception of the toxicity Ce Te is based on the toxicity of elemental Cd. However, it is possible for toxic elements to combine to form a harmless compound, as in the example of NaCl, better known as common salt, which consists of highly reactive metal Na and the corrosive and toxic gas Chlorine. Scientific work, particularly by researchers of the National Renewable Energy Laboratories (NREL) in the USA. Has shown in that the release of Cd due to the atmosphere is lower with CdTe-based solar cells than with silicon photovoltaic and other thin films solar cell technologies. Some investors in solar technology worry that production of CIGS cells would use about 10% of the indium produced in 2004. Indium can easily be recycled from decommissioned PV modules. The recycling program in Germany would be one good example to follow.

CONCLUSION: It is still in its infantancy but is likely to grow rapidly from 2006 onwards. It has the potential to become one of the world’s important PV markets. This is just one of the conclusions in the markets survey from solar plaza entitled “THE ITALIAN PV MARKET”.