Photo-Voltaic Panels

Photovoltaic cells.

A solar cell, or photovoltaic cell, is a semiconductor device that converts photons (light) into electricity. Fundamentally, the device needs to fulfill only two functions:

1. Photogeneration of charger carriers (electrons and holes) in a light-absorbing material.
2. Separation of the charge carriers, preferably to a conductive contact that will transmit the electricity.

The conversion is called “photovoltaic effect”: it consists in generation of electromotive force. The most common configuration of this device, the first generation photovoltaic, consists of a large-area, single layer p-n junction diode, which in the presence of sunlight is capable of generating usable electrical energy. These cells are typically made using a silicon p-n junction. However, successive generations of photovoltaic cells that may improve the photoconversion efficiency are currently being developed. The second generation of photovoltaic materials is based on multiple layers of p-n junction diodes. Each layer is designed to absorb a successively longer wavelength of light (lower energy), absorbing more of the solar spectrum and increasing the amount of electrical energy collected. The third generation of photovoltaics is very different, and is broadly defined as a semiconductor device which does not rely on a traditional p-n junction to separate photogenerated charge carriers. They include dye sensitzed cells, organic polymer cells, and quantum dot solar cells. The absorption of photons creates electron-hole pairs, which diffuse to the electrical contacts and can be extracted to power electrical devices. Light generation of carriers Solar cells have many applications. They are particularly well suited to, and historically used in, situations where electrical power from the grid is unavailable, such as in remote area power systems, Earth orbiting satellites, handheld calculators, remote radiotelephones and water pumping applications.

When a photon…

When a photon of light hits a piece of silicon, one of two things can happen. The first is that the photon can pass straight through the silicon (if the energy of the photon is lower than the bandgap energy of the silicon semiconductor). The second thing that can happen is that the photon is absorbed by the silicon (if the photon energy is greater than the bandgap energy of silicon). When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighboring atoms, and hence unable to move far. The energy given to it by the photon "excites" it into the conduction band, where it is free to move around within the semiconductor. The covalent bond that the electron was previously a part of now has one less electron - this is known as a hole. The presence of a missing covalent bond allows the bonded electrons of neighboring atoms to move into the "hole," leaving another hole behind, and in this way a hole can move through the lattice. Thus, it can be said that photons absorbed in the semiconductor create mobile electron-hole pairs.A photon only needs to have energy greater than the band gap energy to excite an electron from the valence band into the conduction band. However, the solar frequency spectrum approximates a black body spectrum at ~6000 K, and as such, much of the solar radiation reaching the Earth is composed of photons with energies greater than the band gap of silicon. These higher energy photons will be absorbed by the solar cell, but the difference in energy between these photons and the silicon band gap is converted into heat (via lattice vibrations - called phonons) rather than into usable electrical energy.

Equivalent circuit of a solar cell To understand the electronic behaviour of a solar cell, it is useful to create a model which is electrically equivalent, and is based on discrete electrical components . An ideal solar cell may be modelled by a current source in parallel with a diode. In practice no solar cell is ideal, so a shunt resistance and a series resistance component are added to the model. The result is the "equivalent circuit of a solar cell" shown in the picture.

Connection to an external load

Ohmic metal-semiconductor contacts are made to both the n-type and p-type sides of the solar cell, and the electrodes connected to an external load. Electrons that are created on the n-type side, or have been "collected" by the junction and swept onto the n-type side, may travel through the wire, power the load, and continue through the wire until they reach the p-type semiconductor-metal contact. Here, they recombine with a hole that was either created as an electron-hole pair on the p-type side of the solar cell, or swept across the junction from the n-type side after being created there. Usually, solar cells are electrically connected, and combined into "modules", or solar panels. Solar panels have a sheet of glass on the front, and a resin encapsulation behind to keep the semiconductor wafers safe from the elements (rain, hail, etc). Solar cells are usually connected in series in modules, so that their voltages add.

Silicon as a photovoltaic material

Silicon is a semiconductor as a solid material, meaning that there are certain bands of energies which the electrons are allowed to have, and other energies between these bands which are forbidden. These forbidden energies are called the "band gap".At room temperature, pure silicon is a poor electrical conductor. In quantum mechanics, this is explained by the fact that the Fermi level lies in the forbidden band-gap. To make silicon a better conductor, it is "doped" with very small amounts of atoms from either group 13 (III) or group 15 (V) of the periodic table. These "dopant" atoms take the place of the silicon atoms in the crystal lattice, and bond with their neighbouring Si atoms in almost the same way as other Si atoms do. However, because group 13 atoms have only 3 valence electrons, and group 15 atoms have 5 valence electrons, there is either one too few, or one too many electrons to satisfy the four covalent bonds around each atom. Since these extra electrons, or lack of electrons (known as "holes") are not involved in the covalent bonds of the crystal lattice, they are free to move around within the solid. Silicon which is doped with group 13 atoms (aluminium, gallium) is known as p-type silicon because the majority charge carriers (holes) carry a positive charge, whilst silicon doped with group 15 atoms (phosphorus, arsenic) is known as n-type silicon because the majority charge carriers (electrons) are negative. It should be noted that both n-type and p-type silicon are electrically neutral, i.e. they have the same numbers of positive and negative charges, it is just that in n-type silicon, some of the negative charges are free to move around, while the converse is true for p-type silicon.

Semiconductor diodes

In a p-n junction, conventional current can flow from the p-type side (the anode) to the n-type side (the cathode), but not in the opposite direction and the current-voltage, or I-V, characteristic curve is ascribed to the behavior of the so-called depletion layer or depletion zone which exists at the p-n junction between the differing semiconductors. When a p-n junction is first created, conduction band (mobile) electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (places for electrons in which no electron is present) with which the electrons "recombine". When a mobile electron recombines with a hole, the hole vanishes and the electron is no longer mobile. Thus, two charges carriers have vanished. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator. However, the depletion width cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a 'built-in' potential across the depletion zone. If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator preventing a significant electric current. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed resulting in substantial electric current through the p-n junction.

Types of Silicon Solar Cells

Crystalline silicon solar cells come in three primary categories:

• Single crystal or monocrystalline wafers made using the Czochralski process (or CZ). Most commercial monocrystalline cells have efficiencies on the order of 16–17%. 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. Additionally monocrystalline panels have uncovered gaps at the corners of four cells.
• Poly or multi crystalline silicon solar cells are made from cast ingots - large crucibles of molten silicon carefully cooled and solidified. These cells are cheaper than single crystal cells and only slightly less efficient (typically ~15–16%). They can also be formed into square shapes that cover a greater fraction of a panel than monocrystalline cells.
• Ribbon silicon is formed by drawing flat thin films from molten silicon having a multicrystalline structure. These cells have even lower efficiencies (~13.5–15%), but there is very little silicon waste, as this approach does not require sawing from ingots.

All three of these technologies are wafer-based manufacturing. In other words, in each of the above approaches, self-supporting wafers 180-240 micrometres thick are processed into solar cells and then soldered together to form a module.

• Amorphous silicon films (a-Si) or thin-film silicon solar cells are fabricated using chemical vapor deposition techniques, typically plasma enhanced (PE-CVD). These cells have low efficiencies of around 8%, however they are much less costly to produce.

Amorphous silicon has a higher bandgap (1.7 eV) than crystalline silicon (c-Si) (1.1 eV), which means it is more efficient to absorb the visible part of the solar spectrum, but it fails to collect an important part of the spectrum : the infrared. As nanocrystalline Si has about the same bandgap as c-Si, the two material can be combined by depositing two diodes on top of each other creating a layered cell called a tandem cell. The top cell in a-Si absorbs the visible light and leaves the infrared part of the spectrum for the bottom cell in nanocrystalline Si. A patented silicon thin film technology being developed for building integrated photovoltaics (BIPV) is semi-transparent solar cells which can be applied as window glazing. These cells function as window tinting while generating electricity.

Links

http://www.fsec.ucf.edu/pvt/pvbasics/ (make it yourself)
http://www.re-energy.ca/pdf/cp_solarcar.pdf
http://home.earthlink.net/~bdewey/EV_solarpower.html (solar glossary), (solar history timeline)
http://www.eere.energy.gov/solar/photovoltaics.html

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