Solar Cells Rawa’a Fatayer

Introduction Energy is very important in our life and can be found in a number of different forms. It can be chemical energy, electrical energy, heat (thermal energy), light (radiant energy), mechanical energy, and nuclear energy, Fossil Fuels - Coal, Oil and Natural Gas energy. Energy is defined as: "the ability to do work."

The most common kind of these energies is fossil fuel and oil energy but we also know that this kind of energy going to disappear by 22th century so the scientist started to look about some alternative sources of energy. So they started to think about the solar energy because we all know that the sun is our nearest star and no life will be exist on our planet with out it.

How they used the solar energy?! in 1950s they developed a device called solar cell which is depend on solar energy. They first use it on U.S. space satellites.

What is the solar cells? Solar cells are also called photovoltaic cells – or PV cells for short – and can be found on many small appliances, like calculators.

How does it work? Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity. When the light incident on the surface of the solar cell as the photoelectric effect there will be an electron which is freely moves to external circuit this external circuit will convert the solar energy to electric energy.

Solar cells are made from kind of material called semiconductors In these semiconductors there is to bands filled band (valence band) and the band in which electrons are free to move (conduction band) are separated by a potential difference of about 1 volt.

Hence, light coming in can push an electron from the valence band into the conduction band if it has an energy of about 1 electron volt (1 eV). The electron in the conduction band is free to move. If it is kept from recombining, it can give up its energy in an external circuit before coming back to the material

Light consists from photons so the incident energy is equal: E=h C/λ

Doping of semiconductors To prevent recombination, two different types of doped semiconductor are grown together to make the solar cell. Pure silicon is grown in a furnace in the presence of silicon vapor. The silicon vapor is doped with acceptors or donors (p-type and n-type semiconductors)

to deposit layers of p-type or n-type material to deposit layers of p-type or n-type material. On occasional lattice sites in p-type semiconductors are atoms or compounds with fewer electrons in the outermost shell than in the rest of the atom. Thus, there are occasionally “vacancies,” or holes for electrons, within the lattice of a p-type semiconductor. An n-type semiconductor has occasional lattice sites occupied by an atom or compound with more electrons in the outermost shells than in the rest of the atoms.

There are occasionally excess electrons in the lattice in n-type semiconductors. The extra electrons can move around in the n-type material in response to an external potential; in the p-type material, the holes move around in response to an external potential (an external electric field that is applied). Such materials therefore conduct electricity better than would be guessed from the band gap

Solar cells act in a way similar to the diode, so that current can flow in only one direction when the cell is exposed to light.

Cells are assembled into modules, which are further assembled into arrays.

Types of solar cell A. Material used in solar cells The most popular choice for solar cells is silicon (Si), with a band gap of 1.1 eV, production cell efficiencies of about 12%,(110-113) and a maximum efficiency of about 15%, and gallium arsenide, with a band gap of 1.4 eV and a maximum efficiency of about 22%.

The maximum theoretical efficiency for a single cell is 33% The maximum theoretical efficiency for a single cell is 33%. For multiple cells, the theoretical maximum is 68%. Both of these materials must be grown as single crystals under very precisely controlled conditions to minimize imperfections, which can cause recombination.

The material gallium arsenide (GaAs) is also very popular for solar cells. Gallium and arsenic are exactly one atomic higher and lower than silicon, so the system has many similarities to a silicon-based semiconductor. Additionally, only very thin films of gallium arsenide need be used since it is so effective at absorbing light

Silicon, gallium arsenide, and aluminum gallium arsenide have different band gaps. They therefore absorb light of different energies

B. Crystalline silicon cells Crystalline silicon cells are manufactured very carefully. In the original setup, a starter was dipped into a vat of molten silicon (~ 1400 °C), and a single crystal slowly formed as the crystal was drawn out over a long period of time. It was essential to the process that uniformity be maintained.

Nowadays, the process is more automated and requires less care Nowadays, the process is more automated and requires less care. The cells need to be at least 100 µm thick because of problems with absorption; the thickness helps allow the light to be absorbed. Typical thickness is about 300 µm for sawn silicon, but it can be made as thin as about 170 µm using wire-cutting techniques. Both silicon and gallium arsenide are easily eaten away by chemical reactions with the holes.

C. Single and multijunction cells Most current photovoltaic materials are made of a single layer of light absorber. However, given the differences among solar cells in terms of the energy they absorb, it can be advantageous to “stage” or layer them. Cells of different band gaps stacked atop one another are known as multijunction cells.

A multijunction cell functions by absorbing sequentially lower energy light from the incident light. Because there are several band gaps, more energy from the light is absorbed. Here, the band gap for material 1 is greater than that for material 2, which in turn is greater than that for material 3.

D. Amorphous cells Materials that have no crystal structure are classed as amorphous, from the Greek, meaning “lack of structure.” Amorphous silicon has no crystal structure, and its atoms are ordered over only a very short distance; small pieces of silicon crystal abut one another at random orientations in such a way that no long-distance structure exists.

Amorphous silicon solar cells in thin films exhibit better absorption than pure silicon (40 times as efficiently as crystalline silicon),(104) but because of the many structural defects, they are only about 11% efficient at maximum, and most cells are about 4% to 8% efficient. Amorphous silicon cells can degrade on exposure to sunlight

Amorphous silicon is much easier to make than grown silicon crystals, and by using several layers, each set for a different band gap “tuned” to a different part of the spectrum, a greater part of the visible spectrum can be used However,solar cell efficiency falls if too much material is added

Point-contact cells The point-contact solar cell has a textured surface that reduces the reflection of incident light and a rear-surface mirror. It absorbs 90% of the incident light. Such cells need be only 100 µm thick

Other types of solar cells Organic solar cells Gallium indium nitride Plastic solar cells

Thank you