History Of Photovoltaic Cells Engineering Essay

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The direct conversion of light into electricity occurring at the atomic level is known as photovoltaics. There are numerous materials that have the ability to absorb photons of light and release electrons. This phenomenon is known as the photoelectric effect and has numerous implications. When the free electrons are captured, an electric current is produced and this electric current in turn can be used as electricity.

When multiple photovoltaic cells are assembled, a photovoltaic module is formed. A photovoltaic array consists of multiple modules wired together. The larger the area of the photovoltaic array, the more electricity will be produced. Direct current is produced by these modules and arrays. The diagrams below illustrate how one photovoltaic cell is combined to form a photovoltaic array and also how photovoltaic modules are arranged to form an array.

http://www.solar-green-wind.com/wp-content/uploads/2009/09/photovoltaic_arrays-6235.jpg

Sometimes the term solar cell is used primarily for devices that are intended specifically to capture energy from sunlight. Some of these devices include solar panels and solar cells. Conversely, the term photovoltaic cell is used when the light source is not limited to sunlight only.

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HISTORY OF PHOTOVOLTAIC CELLS

Photovoltaic technology has been in existence for a very long time. The concept of the photovoltaic effect was initially conceived by French physicist A.E. Becquerel in 1839. Becquerel discovered that some materials produce small amounts of electric current upon being exposed to light. However, it was not until 1883 that the first solar was only around 1% efficient.

Despite the concept being conceived for such a long time, it was not until 1883 that the first solar cell was built by Charles Fritts. The cell he developed was around 1% efficient. In 1905, Albert Einstein won the Nobel Prize in physics for describing the nature of light and the photoelectric effect. In 1954, Bell Laboratories built the first photovoltaic module. It was labeled as a solar battery and at that time was too expensive to gain popularity. However, the space industry in the 1960s was the first to really make serious use of photovoltaic technology. They used the technology to provide power aboard the spacecraft.

Over the years, photovoltaic technology through numerous space programs made significant advancements, established reliability and also through the advancements, its cost declined. Also, photovoltaic technology gained invaluable worldwide recognition as a source of power for non-space applications during the energy crisis in 1970s.

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HOW DO PHOTOVOLTAIC CELLS WORK?

Photovoltaic cells make use of the photovoltaic effect as the main physical process by which sunlight is converted to electricity. Sunlight has particles of solar energy in its rays known as photons. It is these photons that are of major importance as they contain varying amounts of energy. The amount of energy that they contain is dependent on the different wavelengths of the solar spectrum.

When these photons of sunlight (or other source) hit a photovoltaic cell, one of three things can happen.

They can pass straight through the semiconductor (which is usually silicon). Generally speaking, this scenario occurs for photons that have low energy.

The photons can be reflected off the surface.

Once the energy level of the photon is greater than the silicon band gap value, the silicon can then absorb the photon. This scenario creates an electron-hole pair. Also, heat may be generated.

The absorbed photons are the ones that are responsible for generating electricity.

As the photons are absorbed, their respective energies are then transferred to an electron in an atom of the cell. The atom to which the energy is transferred to is the silicon. As the electron of the semiconductor gains energy, it is now able to escape from its original location within the semiconductor and become a part of the current in some external circuit. Before the electron gains energy, it is in the valence band and is tightly held by covalent bonds between neighboring atoms. Hence, the electron in the valence band is unable to move far. However, as the electron gains energy through the photons, it becomes excited and it moves into the conduction band. In the conduction band, the electron is free to move within the semiconductor. Since an electron was removed from the covalent bond to which it was initially a part of, the bond is now void of one electron and a "hole" is formed as a result of the movement of the excited electron. Thus, it is safe to conclude that due to the absorption of photons in the semiconductor, a mobile electron-hole pair is created. The movement of the electrons can only occur in one direction.

For an electron to be excited from the valence band to the conduction band, the photon needs to have energy that is greater than the energy of the band gap of the semiconductor used. A large percentage of solar radiation coming to the Earth consists of photons possessing energy levels that meet the minimum criteria. Thus, these photons will be absorbed by the PV cell. It can clearly be seen that the solar radiation from the sun is a great medium for photons.

PV cells (short for photovoltaic cells) have special electrical properties. One such property is that they have a built-in electrical field. This electrical field serves to drive current through an external load whatever that load may be. It is able to do this because the field provides the voltage necessary to drive the current. [4] [5]

INDUCING THE ELECTRIC FIELD

Two separate semiconductors are placed together in order to induce the electric field of the PV cell. These two semiconductors are known as the "p" and "n" types of semiconductor and their respective names corresponds to positive (p) and negative (n). The "p" layer consists of an abundance of "holes" that happens as a direct result of the movement of electrons away from the layer. On the other hand the "n" layer has an abundance of electrons.Holes illustration

Figure Showing how the p and n layers are arranged

The junction seen at the interface of the two layers is known as a p/n junction. A direct result of "sandwiching" these two layers and in the process creating the p/n junction at the interface results in the electric field being induced. Both materials of the p and n layers are electrically neutral despite the "n" layer having an abundance of electrons.

The photons from the sunlight are absorbed in the p layer of the PV cell. The "p" layer must be designed in such a way so as to ensure maximum absorption of the incoming photons which in the end would free more electrons.

ELECTRON FLOW

An electron gradient is created by the p-type silicon and the n-type silicon as long as the two layers are placed in close proximity. There is a region of high electron concentration in the n layer and a region of low concentration in the p layer. Due to this gradient, diffusion of electrons occurs with the electrons moving from the n layer across the p-n junction to the p layer. After the electrons diffuse across the junction, they are recombined on the p layer with holes.

This diffusion of the carriers however, does not occur indefinitely because an electric field is created. An electric field is created because charges build up on both sides of the p-n junction. A charge flow known as drift current is promoted by a diode that the electric field creates. [4] [5]

The drift current is what is responsible for opposing and balancing out the diffusion of the electrons and holes.

The two semiconductors (n-type silicon and p-type silicon) act as a battery as a result of the flow of electrons and holes. The electric field that is created at the n-p junction effectively causes the electrons to move from the semiconductor out toward the surface. This movement makes the electrons available for the external electrical circuit. Occurring simultaneously to the movement of electrons is the movement of the holes in the opposite direction. The holes move towards the positive surface and at this point, they await incoming electrons.

In PV cells there is the challenge of preventing the electrons from meeting up with the holes. If this occurs, then it would lead to a recombination of the electrons and holes before the electrons can escape the PV cell.

In order to prevent the recombination of the electrons and holes, the cell is designed in such a way that the electrons are released as close as possible to the n-p junction. This would enable the electric field to send the electrons through the n layer (which is also the conduction layer) and then finally out the cell to the external electric circuit. [4] [5]

Figure showing a typical solar cell

http://www.biggreensmile.com/graffiti/files/media/Solar%20Cell.gif

The cover glass is used to seal the cell from the external environment. 

The contact grid of the cell above is made of a good conductor, such as a metal. It serves as a collector of electrons.

 

The Antireflective Coating is primarily used to guide light into the cell. It makes use of a combination of a favourable refractive index and thickness of the glass used to guide the light. Without the use of this layer, a significant percentage of the light coming to the cell would simple bounce off the surface.

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HOW DO THE "HOLES" AND EXTRA ELECTRONS COME ABOUT?

Most solar cells are made using silicon. Two pieces of silicon are brought together to make the cell. Similar to carbon, silicon has four electrons in its valence shell and hence, would form four bonds to complete its octet. Due to the fact that silicon is a poor conductor of electricity since it has no non-bonding electrons, a technique known as doping is utilized to alter the electrical properties of silicon. This technique brings an atom of a different element into the structure of silicon. Thus, the electrical properties of silicon are altered.

The element phosphorous consists of five electrons in its valence shell as opposed to four in silicon. By introducing phosphorus into the structure of the silicon lattice, a non-bonding electron is added to the lattice. The newly introduced phosphorous atom occupies the very same place where the silicon atom that it replaces formerly occupied. Four of the five valence electrons of the phosphorous atom effectively take over the bonding responsibilities of the silicon valence electrons that they have replaced. But, more importantly, one free non-bonding electron from the phosphorous atom remains. As more and more phosphorous atoms are substituted for silicon in the silicon crystal layer, more free electrons become available. These free electrons can now move about in the crystal with relative ease. The silicon crystal with the newly introduced phosphorous atom in it is called n-type silicon. The atom boron on the other hand has three valence electrons and is used for doping p-type silicon. So, when an atom of boron is added to a silicon crystal, a bond missing an electron occurs in the silicon crystal. This missing electron creates a "hole" as mentioned earlier. Also, as mentioned earlier, this silicon crystal with the hole is known as the p-type silicon.

The figures above show the formation of the "hole" for the p-type silicon as boron is added to the silicon crystal and the addition of the extra non-bonding electron as phosphorous is added to the n-type layer of silicon.

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SINGLE JUNCTION AND MULTI-JUNCTION PV CELLS

Presently, the more common PV devices employ the use of a single junction or interface in order to create an electric field within a PV cell. In these single junction PV cells, only the photons whose energies are greater than or equal to the band gap of the cell material used are able to free electrons for an external circuit. So as a result, for single junction cells, the photovoltaic response is limited to the portion of the sun's spectrum whose energy is above the band gap of the absorbing material. Lower-energy photons cannot be used in this case.

However, there are ways to get around the limitations posed by the single junction cells. One such way is to use multi-junction cells. Multi-junctions cells consist of two or more different cells that have more than one band gap or junction that are used to generate a voltage. Multi-junction devices are capable of achieving higher conversion efficiencies than single junction devices due to the fact that they convert more of the energy spectrum of light to electricity.

Multi-junction PV cells typically have numerous individual single-junction cells comprising of varying energy band gaps stacked on top of one another. The materials with the largest band gaps are placed at the top of the stack. Sunlight falls on these materials first and they are responsible for absorbing the highest-energy photons from the sunlight. The photons that were not absorbed in the first cell then continue onto the second cell. The second cell would then absorb the higher-energy portion of the solar radiation remaining. The second cell would be transparent to the lower-energy photons and hence, would not absorb those specific photons.

Typically, a multi-junction device employs the use of gallium indium phosphide as its top cell, a tunnel junction used to aid the flow of electrons between the cells and also gallium arsenide as its bottom cell.

As can be seen from the diagram, gallium

indium phosphide and aluminum indium

indium phosphide are used as the top layer.

The tunnel diode is used to aid the flow of

electrons between the cells and the bottom

layers consists of gallium arsenide.

http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/

Diagram showing the arrangements of the different layers of cells in a multi-junction cell

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SUMMARY OF HOW PV CELLS WORK

A brief description of the major process and events that occur for a PV cell to generate electricity is as follows:

PV cells are dependent on semiconductors. In their pure form, semiconductors cannot conduct electricity (insulators). They can conduct electricity only when combined with other materials.

The process known as "doping" is employed to introduce these new materials into the semiconductor.

When the semiconductor is doped with phosphorous atoms, an excess of free electrons are developed. This procedure forms the n-type semiconductor.

When the semiconductor is doped with boron, an excess of "holes" are developed. These holes are effectively spaces in the semiconductor lattice that accept electrons. This procedure forms the p-type semiconductor.

The PV cell joins the n-tpye and p-type materials. The layer formed between the two materials is known as the n-p junction.

Even without the presence of light entering the PV cell, there is a small number of electrons that effectively move across the n-p punction from the n-type semiconductor to the p-type. This movement of electrons produces a small voltage.

In the presence of light however, the photons of the light effectively dislodge numerous electrons. These dislodged electrons then flow across the junction and in the process, an electrical current is created.

This electrical current can then be used to power numerous electrical devices.

EFFICIENCY OF PV CELLS

There is an inverse relationship between the energy of a photon and its wavelength.

A photon that has a wavelength of 1.12 microns has 1.11 electron volts of energy. Photons that have wavelengths less than 1.12 microns carry approximately 77% of energy from the sun and as a result, these photons can move a valence electron in silicon into the conducting band. However, despite this fact, the efficiency of silicon in converting these photons to electrons is not 77%.

Energy loses occurs in numerous ways that reduce the efficiency of silicon.

Photons that have more than 1.11 electron volts heat the crystal of the PV cell. Also, about 43% of average absorbed photon energy is used for heating. In addition to this there are photons that are reflected by the exposed surface of the crystal. In the silicon crystal, there exists some internal resistance that inhibits the flow of electrons. As the crystal undergoes heating, this internal resistance also increases.

The efficiency of a PV cell is significantly dependent on temperature for as the temperature is increased, there is a subsequent increase in the internal resistance and a decrease in electrical conductivity.

Silicon has an efficiency of 24% at 0 oC but at room temperature, this efficiency drops to 12%.

This decrease in efficiency as the temperature of the cell is increased poses the basic physical limitation when producing PV cells. This temperature effect makes PV cells made of silicon have operating efficiencies between 5 and 15% and makes the efficiency of PV arrays unlikely to ever be greater than 20%.

The relatively low efficiency of PV cells does not ultimately mean that production is impossible, but what it does mean is that high capital costs will be incurred as large collection areas must be obtained. Without significant subsidization of these high costs, then it is unlikely that PV arrays would be really competitive in the global commercial energy market.

Mixing the impurities (doping) of phosphorous and boron into silicon is one way in which the efficiency of PV cells can be increased.

ENERGY CONVERSION EFFICIENCY OF PHOTOVOLTAIC CELLS

The energy conversion efficiency of a photovoltaic cell is calculated as the percentage of power that is converted and collected when the photovoltaic cell is connected to an external electrical circuit.

The energy conversion efficiency is calculated using the ratio of the maximum power point (Pm) divided by input light irradiance (E) multiplied by the surface area of the photovoltaic cell (Ac). It is calculated under standard test conditions (STC).

The energy conversion efficiency is calculated using the following formula:

\eta = \frac{P_{m}}{E \times A_c}

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TYPES OF PHOTOVOLTAIC CELLS

There are numerous different types of PV cells currently in use. Their specific levels of popularity range from costs, availability, efficiency and stability just to name a few factors.

CRYSTALLINE SILICON CELLS

Crystalline silicon PV cells are the most popular and prevalent type of PV cells currently in use. The two main types of crystalline silicon PV cells are:

Mono-crystalline Silicon abbreviated using (c-Si)

Poly-crystalline Silicon abbreviated using (poly-Si)

THIN-FILM CELLS

Thin-film cells are not as popular as the crystalline cells due to their low efficiencies among other reasons. However, there usage is rapidly increasing as their technology has gone through significant advancements. There are different materials that are used to form thin-film cells. Some of these are:

Amorphous silicon thin-films

Micro-crystalline thin-films

CdTe (Cadmium Telluride)thin-films

CIS/CIGS (Copper indium gallium diselenide)

CdTe and CIS/CIGS are two chemical based thin-film cells.

NEW MATERIALS

Some materials used in the creation of PV cells that are relatively new to this technology are:

Organic polymer photovoltaic cells

Dye sensitized PV cells

Hybrid PV cells

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MONO-CRYSTALLINE SILICON CELLS

For mono-crystalline silicon cells, the crystalline framework of the cell is homogenous. This composition can be recognized by an even external coloring of the cell. The silicon crystal lattice has certain specific characteristics. The lattice is continuous and it is unbroken with no grain boundaries present. A grain boundary is known as the interface between two grains in a polycrystalline material. They are manufactured by the Czochralski process. The ingot growth technique is used.

These particular types of cells are one of the oldest and most efficient types of cells of photovoltaic technology. They are very dependable and are one of the most efficient commercially viable cells in use.

A disadvantage of this type of PV cell lies in its cost. Due to its arrangement, each module of a monocrystalline photovoltaic array is made from a single silicon crystal. As a result, they are more expensive when compared to other types of photovoltaic cells such as polycrystalline types because the process of making these cells from a single crystal is very complex. However, due to the precise arrangement, this type of cell is very efficient compared to the others.

An advantage of using these cells is the efficiency with which they are associated with. Also, in terms of wattage produced per square foot of solar panel, these cells are mostly unrivalled. An average 175 watt solar panel incorporating these types of cells is about sixty inches in length and thirty inches in width. The height of the panel is about one inch and the entire solar panel structure weighs about thirty pounds (if an aluminum frame is used).

Monocrystaline cells have a minimum lifespan of about twenty-five years. Some of these cells work efficiently up to and over forty-five years. So, it is safe to assume that an investment in these types of cells for long term use is very much worthwhile. However, on the down side, the panels containing these cells are extremely fragile and hence, would require a rigid mounting.

Similar to the other types of photovoltaic cells, monocrystalline cells also endure a significant reduction in output as the temperature of the light it receives reaches about 50 oC. Approximately 12-15% reduction in electricity output can be expected when such an increase in temperature is observed.

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ORGANIC PHOTOVOLTAIC CELLS

Organic photovoltaic cells (OPVC) use organic electronics. Organic electronics is a branch of electronics that is involved with conductive organic polymers for the purposes of light absorption and charge transport. The use of organic materials for photovoltaic applications has been a subject of intense investigation over the past years. This particular type of PV cell involves the use of polymers or plastics.

The plastics used in the manufacture of these cells have low production costs in relatively high volumes. This factor combined with the added flexibility of organic molecules make the use of organic materials in the manufacture of PV applications a potentially valuable asset. Also, the energy gap of the polymers used can be chemically altered through molecular engineering procedures such as changing the length and functional groups of the polymers. Another beneficial factor of this technology is that organic molecules have high optical absorption coefficients. This enables them to absorb a large amount of light using a small amount of material. However, a disadvantage associated with the use of this type of PV cell is their low efficiencies, low stability and strength in comparison to the inorganic PV cells.

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The efficiencies of these cells are very low being less than 5% in most cases. This low percentage is due largely in part to the large band gap of the organic materials used. Other problems experienced with these cells are their instabilities against oxidation and reduction and a decreased performance over a period of time due to temperature variations. The extent of some of the problems listed above occurring is also largely dependent on the composition of the materials used. It is an area that is under development.

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HOW DO ORGANIC PV CELLS WORK?

Organic PV cells operate slightly different from crystalline silicon cells. The absorbing layers of OPV cells are usually designed using materials that consists largely of carbon atoms that is arranged in a conjugated system. When the semiconducting materials come into contact with each other, an interface is formed. This interface allows a one-way transport of either electrons or holes.

When the first layer of the cell absorbs a photon, an exciton is formed. An exciton is an electrically neutral electron-hole pair. The excitons that are formed are not free to travel throughout the layer in this type of cell however. Instead, they move via exciton diffusion to the second layer where they undergo a charge transfer into electrons and holes. The electrons and holes then travel to the cathode and anode respectively.

DIAGRAM SHOWING PROCESS EXPLAINED ABOVE

Even though using polymeric organic thin-film layers for organic PV cells affords the opportunity for deposition onto plastic or flexible substrates, a "bound exciton" system involves certain limitations. One of these limitations is the sensitivity of the thin-films that are used as the absorption media.

A feature of prominence with almost any semiconductor is that the electrons and/or the hole producing layer must be in direct contact with either an anode or cathode. For organic PV cells, the deposition of a metal cathode which is usually Ag or Al can causes numerous defects at the cathode interface. These defects in turn limit the overall efficiency of the cell.

Significant progress has been made in the development of an exciton blocking layer (EBL) to act as an intermediate between the light absorbing layer and the cathode.

The exciton blocking layer would not only buffer the sensitive portions of the cell, but when doped with light absorbing materials, can also work to increase the absorption efficiency of the cell.

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http://www.ansercenter.org/images/org_photovoltaic.jpg

DIAGRAM OF AN OPV

POLYCRYSTALLINE SILICON CELLS

Polycrystalline PV cells are less expensive to manufacture than monocrystalline cells. However, they are less efficient due to the fact that the cells are grown into a large block of numerous crystals as opposed to being grown into a single crystal. They are made using a block of silicon that comprises of multiple crystal. This orientation is what gives them a shattered glass-like appearance. Also, similar to monocrystalline cell, polycrystalline cells are then sliced into wafers producing the individual cells which make up the solar panel.

These cells are widely used for commercial PV cell product even though their efficiencies are relatively low when compared to the other types of cells available. The efficiency of a typical cell is about 12-14% however, there have been improvements made to the efficiency of these cells. There are some cells with an overall efficiency of about 19.8%. This improved efficiency is as a direct result of enshrouding the cell surfaces in thermally grown oxides. This reduces the electronic activity that is very detrimental to the cell and it prevents isotropic etching from forming a hexagonally symmetric "honeycomb" surface texture. This surface texture is responsible for reducing reflection losses and also significantly increasing the effective optical thickness of the cell. It does this by causing the absorbed light to be trapped within the cell by total internal reflection.

These cells are easier to produce and also cost less to manufacture than monocrystalline cells. They are very much on par with monocrystalline cells in terms of durability and longevity.

Some brands that produce panels made of polycrystalline modules are Sharp, Kyocera and BP SX. One square meter of a polycrystalline panel that is exposed to direct sunlight is capable of producing 120 watts of power.

As is typical of silicon cells, a rise in temperature contributes to a significant decrease in cell efficiency.

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THIN FILM PV CELLS

Thin-film photovoltaic cells are made when one or more thin layer (thin film) of a photovoltaic material is deposited on a substrate. The deposition occurs at temperatures of around 200 oC. The layers of the cell can be deposited onto various low-cost substrates. These substrates often known as superstrates can be made of materials such as glass, stainless steel, or plastic in virtually any shape. The thickness of this thin layer deposited ranges from about a few nanometers to tens of micrometers.

Thin-film PV cells require a significantly lower amount of silicon material than crystalline silicon PV cells. This would ultimately imply that thin-film cells possess a better cost reduction potential than crystalline silicon cells. Additionally, thin-film cells are advantageous over crystalline silicon cells due to the fact that the time required for the PV module to cover its initial cost is far less than the crystalline silicon cells. Also, the quantity of energy invested during its fabrication is significantly less when compared to its crystalline counterparts.

Thin-film PV cells are also advantageous because they are primarily based on raw materials that are in abundance in the Earth's crust.

A major disadvantage of this particular type of PV cell is its limited stabilized efficiency. It has an average efficiency of about 6-8 percent which is considerably lower than its competitors (crystalline silicon cells).

Thin-film PV cells are categorized based on the photovoltaic material that is deposited on the substrate. Also, various deposition methods exist on a wide variety of substrates. Some of the materials used for the creation of a thin-film PV cell are:

Amorphous Silicon (a-Si)

Cadmium Telluride (CdTe)

Copper indium gallium diselenide (CIS or CIGS)

COPPER INDIUM DISELENIDE (CIS)

Copper indium diselenide (CIS) possesses very high absorptivity. It is used as a semiconductor in thin-film cells to replace the more traditional silicon. The high absorptivity of CIS results in about 99% of light shining on CIS being absorbed in the first micrometer of the material.

Typically, cells that are made from CIS are multi-junction structures. As explained earlier, these are structure having the n-p junction being formed between semiconductors that have varying bandgaps. Commonly, cadmium sulfide (CdS) is used as the material for the top layer in CIS devices. The addition of minute amounts of gallium to the lower layer of the CIS device boosts the bandgap of the device from its normal value of 1.0 electron-volts. This would improve the voltage of the device and ultimately the efficiency of the device. CIS devices are also commonly called copper indium gallium diselenide (CIGS PV cells). Films of CIS can be manufactured using low-cost techniques.

CADMIUM TELLURIDE (CDTE)

Another material used in the creation of thin-film cells is cadmium telluride (CdTe). Similar to CIS, CdTe also has a high absorptivity. Its bandgap is 1.44 electron-volts. CdTe can be used in PV devices without being alloyed. However, it can still be easily alloyed with zinc and mercury in order to vary its properties. Another similarity to CIS is that films of CdTe can also be manufactured relatively cheaply when compared to other cells.

CdTe cells also use a multi-junction interface. Once again, cadmium sulfide is used as the material for the top layer of the cell. Materials such as tin oxide are commonly used as an antireflective coating and as a transparent conducting oxide.

Varying methods are used in the manufacture of CdTe cells. Some of these include chemical vapor deposition, electrode deposition and closed-space sublimation.

Certain health concerns have arisen over the use of cadmium in thin-film PV cells as cadmium is a very toxic substance. Not only is it very toxic but it has a tendency to accumulate in food chains. This scenario poses a severe hindrance to the continuous development of cadmium thin-film cells.

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AMORPHOUS SILICON THIN-FILM CELLS

Amorphous silicon thin-film cells as the name suggests comprise of an amorphous type of silicon as opposed to one or more crystals. These cells have the capabilities of replacing monocrystalline and polycrystalline PV cells in the future.

The word "amorphous" refers to an object that does not have a definite shape. The object is defined as a non-crystalline material.

Amorphous silicon cells differ from crystalline silicon cells in their atomic arrangements. In a crystalline silicon cell, the atomic arrangements are regular. However, amorphous cells have an irregular atomic arrangement and a direct result of this peculiar arrangement, the reciprocal action between photons and silicon atoms occurs more frequently than in crystalline silicon cells. The increase in the frequency of the reciprocal action results in much more light being absorbed by amorphous thin-film cells.

Amorphous thin-films can be fabricated using metal or plastics for the substrate. Utilizing these particular materials results in these cells being very flexible. These flexible amorphous thin-film laminates are suitable for integrating into a building roof due to the fact that they are light weight in construction and installation. Also, they offer good performance in high temperatures.

Amorphous thin-film cells are particularly suitable to residential houses, commercial buildings and factories, schools and parking lots to name a few applications. They can also be used for power generation.

HOW DO THIN-FILM PV CELLS WORK?

The basic operations of thin-film cells are very similar to the more traditional crystalline silicon cells.

Traditional PV cells use silicon in the "n" and "p" type layers. However, the relatively new technology known as thin-film PV cells use thin layers of either cadmium telluride (CdTe) or copper indium gallium selenide (CIGS) instead.

A typical modern day thin-film PV cell is shown below. In this cell, the two non-silicon layers used are copper indium gallium selenide and cadmium telluride. The CIGS-on-Foil cell has a thin layer of metal foil acting as the electrode. The layer of zinc oxide assumes the role of the other electrode in the CIGS cell. Cramped between are the CIGS layer and the CdS layer which act as the n-type and p-type materials respectively. The semiconductor used for this cell is CIGS.

The copper indium gallium deselenide (CIGS) solar cell A cadmium telluride (CdTe) solar cell

A (CIGS) solar cell using foil A cadmium telluride (CdTe) solar cell

For a CdTe Thin-film PV cell, one of the electrodes is made using a layer of carbon paste that is infused with copper. The other electrode can be made using tin oxide or cadmium stannate. The semiconductor used for this cell would be CdTe which is used once again with CdS to create the n-p junction.

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EFFICIENCY OF THIN-FILM PV CELLS

Theoretically speaking, crystalline silicon cells have a maximum efficiency of fifty percent. This means that about half of the energy that strikes the PV cell is converted into electricity. However, realistically speaking, crystalline silicon cells have an average efficiency of about 15 to 25 percent. In years gone by, thin-film cells could not compete with the efficiency of crystalline cells. But, significant enhancements to the technology have made them more competitive in this regard. Presently, there are CdTe thin-film PV cells that have an average efficiency of about 15 percent. This figure represents a major increase from efficiencies of about 6-8 percent in times gone by. CIGS cells have improved even more dramatically with average efficiencies ranging from 18-20 percent.

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DEVELOPMENT OF THIN-FILM TECHNOLOGY

Over the years, thin-film technology and moreover, PV technology has made numerous advancements so much so that almost the entire world is involved in this technology in some way or the other. Below is a list of significant accomplishments that thin-film technology in particular has made. Also, some future aspirations of various entities are listed.

As of October 16 2008, the largest thin-film pitched roof system in Germany that was constructed by Riedel Recycling has been in operation. It has been producing solar power using more than eleven thousand cadmium telluride modules delivering a total of 837 kW of electricity.[18]

The company First Solar has recently completed a 2.4 MW rooftop installation. This installation is a part of the Southern California Edison program which aims in installing a total of 250 MW of rooftop solar panels throughout the entire region of Southern California by the year 2013. [19]

On July 2008, the construction of a 10 MW plant located in the Nevada desert commenced. [20]

Installation by Electron Solar Energy plans to install 4.8KW thin film flexible solar panels that are manufactured by Uni-Solar Ovonic on South Beach California. The aim of this project is to publicly illustrate a residential example of thin-film technology. [23]

GENERAL INFORMATION ABOUT PV SYSTEMS

For PV systems, the inverter which is responsible for converting the DC electricity from a PV array to AC is the basic controller for the entire PV system. This component of a PV system is usually the second most expensive component of the entire PV system. Only the PV array is more costly. In terms of maintenance costs, the inverters more often than not, are the components of PV systems that incur the most maintenance costs. This is due to the complexity of the electronic componentry, the software and thermal management of the system.

Numerous involved entities are actively involved in the development of more reliable inverters for PV systems. Other components of a PV system include the mounting hardware, wiring and cable housing, disconnect and fuse.

TRENDS OF PV CELLS

The demand for solar energy has been increasing on a consistent basis by about 20-25% yearly for the past 20 years. Some of the reasons that are primarily responsible for this trend are the continuous rise in efficiency of solar cells and the significant improvements in manufacturing technology.

Just to shed light on the continuous increase in PV cells, in the year 2005, some 1,460 MW of photovoltaics were installed. In the space of 1 year - 2006, this figured ballooned to 1744 MW.

The major manufacturers of solar modules are Sharp, Kyocera, Shell Solar and BP Solar. These four companies combined represents more than 50% of the total solar modules produced worldwide.

[21]

In 2009, the world solar PV market installations grew by 20% over the previous year. The total installations for that particular year reached a record high of about 7.3 GW.

Also in 2009, the PV industry generated an estimated $38.5 billion in total global revenues.

The continuous increase in demand for solar cells has caused the cost of modules made of crystalline silicon to drop significantly by about 38% of the cost in the previous year.

It is estimated that by 2014, the global market will be at a minimum 2.5 times greater than it currently is. Significant growth is expected in at least the next 5 ensuing years. [22]

http://www.solarbuzz.com/Photos/FP-MB105.jpg

http://www.solarbuzz.com/marketbuzz2010-intro.htm

The figure above represents the current market share of the various types of PV cells. As can be seen, the silicon based PV cells such as monocrystalline and polycrystalline represent about 85% of the global market share. It is expected that crystalline silicon cells will continue to dominate the market for the next 10 years or so, however, thin-film technologies have most potential at the long run due to their inherent lower material costs, their potentials of having higher efficiencies due to stacking and integrated manufacturing.

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