Solar Cell Workings Silicon The P n Junction Photons

The main component of a solar cell is silicon ' doped' with trace amounts of impurities. In pure silicon, each atom is fixed in a crystal lattice and bonded to other silicon atoms covalently, sharing the 4 valence electrons in their outer shells with them. There are thus few free electrons or positive charge carriers to carry charge, and pure silicon is thus a bad conductor.

In doped silicon, atoms with 3 or 5 valence electrons are introduced to the lattice. Take arsenic or phosphorusPhosphorus is the chemical element in the periodic table that has the symbol P and atomic number 15. A multivalent, nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks and in all living cells but is never naturally fo for example, with 5 valence electrons. Since silicon atoms require only 4 of those electrons to form stable bonds, there will be one free electron which can move and thus carry charge. Since there are so many free electrons in silicon doped with arsenic or phosphorus (compared to pure silicon), this sort of silicon is called " n-typeAn N-type semiconductor is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order to increase the number of free (in this case negative) charges. When the doping material is added, it gives away silicon".

If the silicon is doped with boronBoron is the chemical element in the periodic table that has the symbol B and atomic number 5. A trivalent metalloid element, boron occurs abundantly in the ore borax. There are two allotropes of boron; amorphous boron is a brown powder, but metallic boro, which has 3 valence electrons, when it bonds with silicon it will be short of one electron. This ' holeIn solid state physics, an electron hole (usually referred to simply as a hole is the absence of an electron from the otherwise full valence band. A full (or nearly full) valence band is present in semiconductors and insulators. The concept of a hole is e' is also free to move. Since there are so many positively-charged holes in silicon doped with boron, this sort of silicon is called " p-typeA P-type semiconductor is obtained by carrying out a process of doping, that is adding a certain type of atoms to the semiconductor in order to increase the number of free (in this case positive) charges. When the doping material is added, it takes away ( silicon".

1.2 The p-n junction

In a solar cell, a plate of p-type silicon is placed next to a plate of n-type silicon. At the junction between the 2, electrons in the n-type plate will migrate to the p-type plate, and vice-versa for the holes in the p-type plate. After a while, enough holes and electrons would have combined to form a barrier at the junction, preventing further flow of holes and electrons.

There is now an electric fieldIn physics, an electric field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity. Definition and derivation The mathematical definition of the electric field is developed as follows. Coulomb's law gives the across the p-n junctionA p-n junction is formed by combining N-type and P-type semiconductors together in very close contact. The term junction refers to the region where the two types of semiconductor meet. It can be thought of as the border region between the P-type and N-typ — positive on the n-type silicon plate, since the electrons have crossed over and there are excess protons which do not have corresponding electrons and negative on the p-type silicon plate, because the reverse occurs.

The electric field and the barrier act as a diodeA diode functions as the electronic version of a one-way valve. By restricting the direction of movement of charge carriers, it allows an electric current to flow in one direction, but blocks it in the opposite direction. Applications Radio demodulation T — electrons can move from the n-type plate to the p-type plate easily due to the electric field, but the reverse is difficult.

1.3 Photons

Light energy is transmitted by photons, and its quantity is given by the formula: E = hν — the energy of a photon equals its frequency multiplied by Planck's constant (6.626 × 10⁻³⁴ m²kg/s).

When photons hit the silicon plates, electron-hole pairs are created (with a probability depending on the quantum efficiency) and separated. The electric field across the p-n junction draws the electrons and holes in opposite directions, and they then diffuse to the front and back contacts. If the 2 silicon plates are connected across a load, the electrons and holes can be extracted, driving the load in the process. If nothing is connected, electrons and holes, each being in minority in their respective zone, recombine with the majority carriers. It is the open-circuit voltage (V_oc) condition.

1.4 Practical Solar cells

Because the current and voltage supplied by any one solar cell is small, many cells are typically coupled in series and parallel to produce the desired level of output.

2 Types

The simplest type of solar cell is a silicon diode, but research is continuing into more exotic materials (see below) with greater efficiencies. Modern solar cells are encapsulated in glass-fronted plastic sheets. They have design lifetimes that exceed forty years. Sunlight provides about 1 kilowatt per square meter at the Earth's surface, and most solar cells are between 8 and 12 percent efficient. In desert areas, they can operate for an average of 6 hours per day when mounted in nonrotating brackets.

Solar panels come in four varieties. Most common are rigid monocrystalline and polycrystalline silicon sheets. Monocrystalline silicon provides the highest efficiency of all four types but is also the most expensive. Polycrystalline silicon is lower cost but lower in efficiency. Also available are amorphous silicon solar cells which can be applied to a variety of substrates including flexible ones, like metal foil or plastic foil. The main advantage of amorphous silicon solar cells is that they should eventually be able to be processed at a much lower cost than crystalline solar cells. Nanocrystalline silicon has also been used, in the same processing systems as for amorphous silicon, and it shows slightly higher efficiency due to the increased absorption in the longer wavelengths. Experimental non-silicon solar panels are made of carbon nanotubes embedded in plastic. These have only one-tenth the efficiency of silicon panels but could be manufactured in ordinary factories, not clean rooms which should lower the cost.

3 Implementations

In late 2001, with batteries to provide power at night, desert climates can get power for about 8 cents per kilowatt hour using solar cells, batteries and electronic inverters. By contrast, nuclear and hydroelectric power plants can provide power at 1.5 to 3 cents per kilowatt hour. Solar power is already cheaper than internal combustion generators that use natural gas, diesel or gasoline, and is becoming competitive with the costs of coal power in some areas. Homes in the U.S. use between 5 and 20 kilowatt hours per day, depending on whether they use electricity for lighting, or heating, cooling and cooking as well.

If a roof is required for other reasons, and the solar cells are chosen and fabricated to form a weather-resistant roof, the value of the roof reduces solar costs considerably. A well-designed solar cell roof will simply be constructed at the optimal angle to collect power. This saves the cost of mounting brackets to raise the cells to an optimal angle, saving quite a bit of money. Solar panel shingles which replace ordinary shingles have recently become available.

The least expensive way to buy a solar power system for a home or small business is as part of a cooperative. Periodically, a group will form on the internet to purchase solar equipment cooperatively.

4 Current research

Some of the most efficient solar cell materials are cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). Unlike the basic silicon solar cell, which can be modelled as a simple p-n junction (see under semiconductor), these cells are best described by a more complex heterojunction model. The best efficiency of a bare solar cell as of April 2003 was 16.5% [Dr IM Dharmadasa, Sheffield Hallam University, UK]. Higher efficiencies (around 30%) can be obtained by using optics to concentrate the incident light.

Polymer or organic solar cells are built from thin layers of organic semiconductors such as polyphenylene vinylene and fullerene. The p/n junction

model is only a crude description of the functioning of such cells, as

electron hopping and other processes also play a crucial role.

They are potentially cheaper to manufacture than silicon or inorganic cells, but efficiencies achieved to date are low and cells are highly sensitive to air and moisture, making commercial applications difficult. The technology has however already successfully been commercialised in organic LED's and organic displays , also called polymer displays

Graetzel cells (sometimes called photoelectrochemical cells ) have been around for two decades or so. A p/n junction is used

here too in the form of a doped solid (normally titanium dioxide) in contact with a solid or liquid electrolyte (for example CuI ). In contrast to the classical solar cell not the semiconductor but a dye placed at the p/n interface is used for absorption of radiation, mimicking the process of photosynthesis. As a result, this type of cell allows a more flexible use of materials. Like organic solar cells, Graetzel cells can be manufactured under "dirty" conditions. Commercial applications have failed to appear however due the fast degradation occurring in Greatzel cells.

5 See also

6 External links

Pennicott, Katie, "Solar cell edges towards endless energy". December 7, 2001. PhysicsWeb.
Dye Sensitized Solar Cells (DYSC) based on Nanocrystalline Oxide Semiconductor Films
News searching: Solar Cell, Photovoltaic
Historical: Photovoltaic Solar Energy Conversion: An Update
Wladek Walukiewicz, Materials Sciences Division, Berkeley Lab.: Full Solar Spectrum Photovoltaic Materials Identified. Quote: "... Maximum, theoretically predicted efficiencies increase to 50%, 56%, and 72% for stacks of 2, 3, and 36 junctions with appropriately optimized energy gaps, respectively...."
CNET: 5/12/03 SunPower Announces World's Most Efficient, Low-Cost Silicon Solar Cell Quote: "...The National Renewable Energy Laboratory (NREL) has verified 20.4 percent conversion efficiency for the A-300...."
SunPower A-300 (pdf), SunPower
March 29, 2002, Scientists Create New Solar Cell Quote: "...semiconducting plastic material known as P3HT... 1.7 percent for sunlight..."
15 February 03, 'Denim' solar panels to clothe future buildings Quote: "... Unlike conventional solar cells, the new, cheap material has no rigid silicon base..."
Examples of Photovoltaic Systems

6.1 Yield data

http://www.tectosol.staticip.de/index_en.htm electricity yield of a solar power system
http://www.sunny-portal.de Yield Portal for solar power system users

6.2 Theory

6.3 Cost-Benefit

PVWATTS - A Performance Calculator for Grid-Connected PV Systems

6.4 Do-It-Yourself

6.4.1 PEC (Photo Electro Chromic)

How to Build Your Own Solar Cell
DIY (Do It Yourself): Nanocrystalline Dye-Sensitized Solar Cell Kit Quote: "... sunlight-to-electrical energy conversion efficiency is between 1 and 0.5 %..."

6.4.2 Cuprous oxide solar cells

6.5 Indexes

Google: Solar, DMOZ: Solar

6.6 Newsgroups

alt.solar.photovoltaic

Electrical components Renewable energy

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1 Workings

1.1 Silicon