Department of EEEEEE-445, Sec-1, Spring-2010

ASSIGNMENT

On

Different kinds of Solar cell and their merits and demerits

Prepared by

Student Name: SANIULISLAM

Student ID: 2006-3-80-004

Date of Submission: 24.02.10

Course Title: Renewable Energy

Introduction:

A solar cell is a device that converts the energy of sunlight directly into electricity by the photovoltaic effect. Sometimes the term solar cell is reserved for devices intended specifically to capture energy from sunlight such as solar panels and solar cells, while the term photovoltaic cell is used when the light source is unspecified. Each solar cell is made of a semiconductor material, such as silicon. When a photon from sunlight hits a solar panel, it can do one of three things: pass through the silicon, reflect off the surface, or become absorbed. Photon absorption is the basis for generating solar heat and creating electricity. The energy brought in from the photons in light cause electrons in the semiconductor to flow freely. This, in turn, creates an electric current, which can be captured by metal contacts placed on the borders of the solar cell. Once captured, the electric energy can be converted, or used externally. There are many types of solar cells. In this paper different types of solar panels their merits and demerits will be described.

Multiple-junction solar cells

Multijunction solar cells are a new technology that offers extremely high efficiencies compared to traditional solar cells made of a single layer of semiconductor material. Depending on the particular technology, multijunction solar cells are capable of generating approximately twice as much power under the same conditions as traditional solar cells made of silicon. Unfortunately, multijunction solar cells are very expensive and are currently only used in high performance applications such as satellites due to their cost.

We use multi-junction solar cell to remove loss and increase absorption. The latest multi-junction cells already offer efficiencies of 40%.

Hetero-Junction Solar CellHetero junction solar cells are supposed to be cost-effective devices with high conversion efficiencies. A hetero-junction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps as opposed to a homojunction. It is often advantageous to engineer the electronic energy bands in many solid state device applications including semiconductor lasers, solar cells and transistors to name a few. The combination of multiple hetero-junctions together in a device is called a hetero-structure although the two terms are commonly used interchangeably. The requirement that each material be a semiconductor with unequal band gaps is somewhat loose especially on small length scales where electronic properties depend on spatial properties. A more modern definition may be to say that a hetero-junction is the interface between any two solid state materials including crystalline and amorphous structures of metallic, insulating, fast ion conductor and semiconducting material.

Multi-junction solar cells work by layering semiconductor materials that have different bandgaps. Light enters through the layer that has the largest bandgap. Depending on the energy of the photon, it penetrates the solar cell until it reaches a layer that has a smaller bandgap than the photon’s energy. Using this concept, multi-junction solar cells are more efficient than single layer solar cells. This is because less of the energy of a photon exceeds the bandgap of the absorbing semiconductor, which is energy lost to heat.

Figure 1: Multi junction Solar cell

Figure 2: Band diagrams for AlGaAs/GaAs heterostructure solar cells: (a) structure with p-n junction in GaAs and frontal wide-gap p-AlGaAs ''window;'' (b) structure with backside potential barrier in n-region; (c) structure with an AlGaAs frontal layer o

Advantage of hetero-Junction Solar Cell is Potential for high efficiency solar cells. Low temperatures throughout the process (< 200ºC) that will allow technological development with thin or low quality crystalline silicon wafers. The fabrication time is less than that usually required for commercial standard cells. Low energetic cost (low temperature process) and less resources used. Better response with operating temperature. Hetero junction solar cells are less affected by high operating temperatures than conventional solar cells. This is a great advantage; since the decrease in the module efficiency due to high operating temperatures is still one of the most important problems to solve in photovoltaic installations

When a hetero-junction is formed by two different semiconductors, a quantum well can be fabricated due to difference in band structure. In order to calculate the static energy levels within the achieved quantum well, understanding variation or mismatch of the effective mass across the hetero-junction becomes substantial.

Hetero-junction manufacturing generally requires the use of molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) technologies into order to precisely control the deposition thickness and create a planar interface. MBE and CVD tend to be very complex and expensive compared to traditional silicon device fabrication.

Thin-film solar cellsA Thin-film solar cell (TFSC), also called a thin-film photovoltaic cell (TFPV), is a solar cell that is made by depositing one or more thin layers (thin film) of photovoltaic material on a substrate. The thickness range of such a layer is wide and varies from a few nanometers to tens of micrometers.

Many different photovoltaic materials are deposited with various deposition methods on a variety of substrates. Thin-film solar cells are usually categorized according to the photovoltaic material used:

Amorphous silicon (a-Si) and other thin-film silicon (TF-Si) Cadmium Telluride (CdTe)

Copper indium gallium selenide (CIS or CIGS)

Dye-sensitized solar cell (DSC) and other organic solar cells

Figure 3:Thin film solar cell

The cells, called thin-film solar cells, are 100 times thinner and potentially lighter than today's silicon cells. Because they require less semiconductor material than other solar cells, lots of thin film solar cells can be made for less money. However, the new cells have a much more complex structure and are more difficult to make which so far has limited their production and commercialization.

The primary advantages of thin film panels lie in their low manufacturing costs and versatility. Because amorphous silicon and similar semiconductors do not depend on the long, expensive process of creating silicon crystals, they can be produced much more quickly. Because they can be applied in thin layers to different materials, it is also possible to make flexible solar cells.The advantage of thin film solar cells is their ability to produce electrical power without harmful emissions.

Thin film panels do have several significant drawbacks. They are the least efficient type of solar panel currently available. Thin-film technology also uses silicon with high levels of impurities. This can cause a drop in efficiency within a short period of time.

Organic Solar cell:An organic photovoltaic cell (OPVC) is a photovoltaic cell that uses organic electronics--a branch of electronics that deals with conductive organic polymers or small organic molecules for light absorption and charge transport.

The plastic itself has low production costs in high volumes. Combined with the flexibility of organic molecules, this makes it potentially lucrative for photovoltaic applications. Molecular engineering like changing the length and functional group of polymers can change the energy gap, which allows chemical change in these materials. The optical absorption coefficient of organic molecules is high, so a large amount of light can be absorbed with a small amount of materials. The main disadvantages associated with organic photovoltaic cells are low efficiency, low stability and low strength compared to inorganic photovoltaic cells.

3D solar cellsThree-dimensional solar cells that capture nearly all of the light that strikes them and could boost the efficiency of photovoltaic systems while reducing their size, weight and mechanical complexity. The new 3D solar cells capture photons from sunlight using an array of miniature “tower” structures that resemble high-rise buildings in a city street grid.

Infrared solar cellsResearchers at Idaho National Laboratory, along with partners at Microcontinuum Inc. in Cambridge, MA and Patrick Pinhero of the University of Missouri, have devised an inexpensive way to produce plastic sheets containing billions of nanoantennas that collect heat energy generated by the sun and other sources, which garnered two 2007 Nano50 awards. The technology is the first step toward a solar energy collector that could be mass-produced on flexible materials. While methods to convert the energy into usable electricity still need to be developed, the sheets could one day be manufactured as lightweight "skins" that power everything from hybrid cars to computers and iPods with higher efficiency than traditional solar cells. The nanoantennas also have the potential to act as cooling devices that draw waste heat from buildings or electronics without using electricity. The nanoantennas target mid-infrared rays, which the Earth continuously radiates as heat after absorbing energy from the sun during the day; also double-sided nanoantenna sheets can harvest energy from different parts of the Sun's spectrum. In contrast, traditional solar cells can only use visible light, rendering them idle after dark.

UV solar cellsJapan's National Institute of Advanced Industrial Science and Technology (AIST) has succeeded in developing a transparent solar cell that uses ultraviolet (UV) light to generate electricity but allows visible light to pass through it. Most conventional solar cells use visible and infrared light to generate electricity. In contrast, the innovative new solar cell uses ultraviolet radiation. Used to replace conventional window glass, the installation surface area could be large, leading to potential uses that take advantage of the combined functions of power generation, lighting and temperature control.

Based on crystal structure there are three types of solar cells. They are monocrystalline, polycrystalline, and amorphous. To produce a monocrystalline silicon cell, absolutely pure semiconducting material is necessary. Monocrystalline rods are extracted from melted silicon and then sawed into thin plates. This production process guarantees a relatively high level of efficiency.

Monocrystalline solar panels:Monocrystalline panels use crystalline silicon produced in a large sheet which has been cut to the size of the panel, thus making one large single cell. Metal strips are laid over the entire cell and act as a conductor that captures electrons.Mono panels are slightly more efficient than Polycrystalline panels but they don't usually cost more than Poly Panels.

Polycrystalline solar panels:Polycrystalline panels use a bunch of small cells put together instead of one large cell. Poly panels are slightly less efficient than mono panels. They are also claimed to be cheaper to manufacturer than mono panels although we have noticed them to be very similarly priced.

Amorphous solar panels:Amorphous technology is most often seen in small solar panels, such as those in calculators or garden lamps, although amorphous panels are increasingly used in larger applications. They are made by depositing a thin film of silicon onto a sheet of another material such as steel. The panel is formed as one piece and the individual cells are not as visible as in other types.

Typically, amorphous silicon thin-film cells use a p-i-n structure, whereas CdTe cells use an n-i-p structure. The basic scenario is as follows: A three-layer sandwich is created, with a middle intrinsic (i-type or undoped) layer between an n-type layer and a p-type layer. This geometry sets up an electric field between the p- and n-type regions that stretches across the middle intrinsic resistive region. Light generates free electrons and holes in the intrinsic region, which are then separated by the electric field.

In the p-i-n amorphous silicon (a-Si) cell, the top layer is p-type a-Si, the middle layer is intrinsic silicon, and the bottom layer is n-type a-Si. Amorphous silicon has many atomic-level electrical defects when it is highly conductive. So very little current would flow if an a-Si cell had to depend on diffusion. However, in a p-i-n cell, current flows because the free electrons and holes are generated within the influence of an electric field, rather than having to move toward the field.

Comments:

Solar Cell Technology shows great promise and even though today’s Solar Cells/Panels cannot be used for large power generation they can be used by homeowners and rv owners to save money on their energy needs at a very affordable price, that adds value to your home. In the future we depend on the renewable energy so photovoltaic cell is one of the most important energy systems. In this Paper many types of solar cell has been described. Except this types of solar cell there are many

types of Solar cell like Carbon Nanotube Solar Cells, Quantum Dot Solar Cells , Hybrid Solar Cells, 3-D Solar Cells etc are proposing solar cell that are under research. The use of home solar power can meet many of your energy needs. It can both heat and cool your home and also operate many appliances. Solar power can also provide lighting, hot water and even heat your pool among other things.

REFERENCE

1. http://www.power-talk.net/solar-panels.html 2. http://en.wikipedia.org/wiki/Solar_cell 3. http://en.wikipedia.org/wiki/Thin_film_solar_cell 4. http://www.answers.com/topic/heterojunction 5. http://www1.eere.energy.gov/solar/solar_cell_structures.html 6. http://solar.calfinder.com/library/solar-electricity/cells/cell-types