Why Solar ?

In the long term, solar energy is the best answer to the greenhouse effect. Solar energy is CO2 free and virtually inexhaustible. According to the International Energy Agency (IEA), at present, solar energy constitutes a mere 0.04% of worldwide energy consumption whereas fossil fuels feed over 80% of the world energy supply. However, some key facts on solar energy highlight its significant potential:

• Two billion people in the world have no access to electricity. For most of them, solar energy would be their cheapest electricity source, but they cannot afford it.

• Unlike fossil fuels, which produce significant amounts of pollution and enormous amounts of greenhouse gases, the sun's energy is clean and its supply virtually limitless.

• In just one hour, the Earth receives more energy from the sun than human beings consume during an entire year.

• According to America's Department of Energy, solar panels could, if placed on about 0.5% of USA’s mainland landmass, provide for all of its current electricity needs.

• The Sun has sufficient helium mass to provide the Earth with energy for another five billion years and, every 15 minutes, it emits more energy than humankind uses in an entire year.

• The Earth receives only one half of one billionth of the Sun's radiant energy, but in just a few days it gets as much heat and light as could be produced by burning all the oil, coal and wood on the planet.

In addition to the dramatic benefits to reducing global warming, harnessing solar power can reap significant benefits for individuals and businesses alike. For example, businesses can use solar energy to:

• Reduce the risk of volatile and rising fossil fuel prices, thereby reducing or stabilizing operating costs, particularly as governments move to tax carbon emissions.

• Take advantage of government incentives and rebates which are designed to increase the use of renewable energy sources.

• Reduce the risk and cost of power outages and backup power systems.

• Strengthen relationships with key stakeholders such as customers and the community, by demonstrating sensitivity to climate change issues.

Solar panels absorb sunlight through their silicon membrane and turn the energy absorbed into useable power.

You have probably also been hearing about the "solar revolution" for the last 22 years -- the idea that one day we will all use free electricity from the sun. This is a seductive promise: On a bright, sunny day, the sun shines approximately 1,000 watts of energy per square meter of the planet's surface, and if we could collect all of that energy we could easily power our homes and offices for free.

In this article, we will examine solar cells to learn how they convert the sun's energy directly into electricity. In the process, you will learn why we are getting closer to using the sun's energy on a daily basis, and why we still have more research to do before the process becomes cost effective.

Photovoltaic Cells: Converting Photons to Electrons

The solar cells that you see on calculators and satellites are photovoltaic cells or modules (modules are simply a group of cells electrically connected and packaged in one frame). Photovoltaic’s, as the word implies (photo = light, voltaic = electricity), convert sunlight directly into electricity. Once used almost exclusively in space, Photovoltaic’s are used more and more in less exotic ways. They could even power your house.

How do these devices work?

Photovoltaic (PV) cells are made of special materials called semiconductors such as silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely.

PV cells also all have one or more electric fields that act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

That's the basic process, but there's really much more to it. Let's take a deeper look into one example of a PV cell: the single-crystal silicon cell.

How Silicon Makes a Solar Cell

Silicon has some special chemical properties, especially in its crystalline form. An atom of silicon has 14 electrons, arranged in three different shells. The first two shells, those closest to the center, are completely full. The outer shell, however, is only half full, having only four electrons. A silicon atom will always look for ways to fill up its last shell (which would like to have eight electrons). To do this, it will share electrons with four of its neighbor silicon atoms. It's like every atom holds hands with its neighbors, except that in this case, each atom has four hands joined to four neighbors. That's what forms the crystalline structure, and that structure turns out to be important to this type of PV cell.

We've now described pure, crystalline silicon. Pure silicon is a poor conductor of electricity because none of its electrons are free to move about, as electrons are in good conductors such as copper. Instead, the electrons are all locked in the crystalline structure. The silicon in a solar cell is modified slightly so that it will work as a solar cell.

A solar cell has silicon with impurities -- other atoms mixed in with the silicon atoms, changing the way things work a bit. We usually think of impurities as something undesirable, but in our case, our cell wouldn't work without them. These impurities are actually put there on purpose. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms.

Phosphorous has five electrons in its outer shell, not four. It still bonds with its silicon neighbor atoms, but in a sense, the phosphorous has one electron that doesn't have anyone to hold hands with. It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place.

When energy is added to pure silicon, for example in the form of heat, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case. These electrons then wander randomly around the crystalline lattice looking for another hole to fall into. These electrons are called free carriers, and can carry electrical current.

There are so few of them in pure silicon, however, that they aren't very useful. Our impure silicon with phosphorous atoms mixed in is a different story. It turns out that it takes a lot less energy to knock loose one of our "extra" phosphorous electrons because they aren't tied up in a bond -- their neighbors aren't holding them back. As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon. The process of adding impurities on purpose is called doping, and when doped with phosphorous, the resulting silicon is called N-type ("n" for negative) because of the prevalence of free electrons. N-type doped silicon is a much better conductor than pure silicon is.

Actually, only part of our solar cell is N-type. The other part is doped with boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type silicon ("p" for positive) has free holes. Holes really are just the absence of electrons, so they carry the opposite (positive) charge. They move around just like electrons do.

The interesting part starts when you put N-type silicon together with P-type silicon. Remember that every PV cell has at least one electric field. Without an electric field, the cell wouldn't work, and this field forms when the N-type and P-type silicon are in contact. Suddenly, the free electrons in the N side, which have been looking all over for holes to fall into, see all the free holes on the P side, and there's a mad rush to fill them in.

Anatomy of a Solar Cell

Solar energy is renewable energy for humans. It's also clean energy, do not generate any environmental pollution. Solar photovoltaic (PV) was the most watched item in the researching of solar energy utilize.

Solar PV Panel

The production of solar cells based on semiconductor materials, and its working principle is photovoltaic materials photoelectron conversion reaction after absorb light energy, according to different materials, solar cells can be divided into: 

1, silicon solar cells; 

2 multi-material cells using inorganic salts such as gallium arsenide III-V compounds, cadmium sulfide, copper indium selenium compounds; 

3, polymer materials solar cells; 

4, nano-crystalline solar cells. 

etc.

1.Silicon solar cells
Silicon solar cell's structure and working principle,
Solar cells' elements is the photoelectric effect of semiconductors, normally simiconductors have below structure:

Solar Cell

 As shown in the picture, 

positive charge(+) means silicon atom, 

negtive charge(-) means electron around the silicon atom.


A hole will exist in the crystalline silicon when the 

cyrstalline silicon mixed with boron, 

it's shape as below picture:

Silicon Crystalline

 In the picture, Positive charge (+) means silicon atom, Negetive charge(-) means electron around the silicon atom. and the yellow means mixed boron atom, as only 3 electron around the boron atom, it's bring the hole as in blue, this hole is unstable as it's without electron, easily absorb electron to neutralize to be a P(positive) type semiconductor.

Sameness, when mixed with phosphor atom, it's become highly active as the phosphor atom have 5 electron, it's comes the N(negative) type semiconductor. as shown in below picture, the yellow means Phosphor atom, the red means superfluous electron.

yellow means Phosphor atom, the red means superfluous electron.

N type semiconductor contains more hole, while the P type semiconductor contains more electron, in this way, the electric potential difference will be formed when the P and N type semiconductor combine, that comes the PN junction.


When the P and N-type semiconductor combine, the two types of semiconductors at the interface region will form a special thin-layer, the P side contains negative electron, N side contains positive electron. This is because P-type semiconductor have many hole, N-type semiconductor have many free electrons. Electron from N-zone will be spread to the P-zone, hole from the P-zone will spread to the N-zone.

When the lights reach the crystalline silicon, the hole from N-type semiconductor move to P zone, and electron from P-zone move to N-zone, that formed the electric current from N-zone to P-zone, then formed the electric potential difference, that comes the electricity source. (shown in below picture)

Because the semiconductor is not a good conductor of electricity, the electron will waste very much when passed the P-N junction and flow in semiconductor as it's large resistance. However, if painted a metal upper, sunlight can not going through, electric current will not be able to produce, so in general with a metal mesh covering the p-n junction, in order to increase the size of the incident light.

In addition, the silicon surface is very bright, will reflect many of the sun lights, could not be used by the solar cells. Therefore, scientists painted it with a very small reflectance film, to decrease the sunlight reflection loss below 5% or eve less. A single solar cell can provide only a limited current and voltage, so people join many pieces of solar cells (usually 36) in parallel or series to become the solar modules.

2. Crystalline silicon solar cell manufacturing process.

Usual crystalline silicon solar cells are made up from the high-quality silicon at thickness of 350 ~ 450μm, such silicon wafers are cutted from Czochralski or casted silicon ingot

The above method consum more silicon material. In order to save materials, the current preparation of polycrystalline silicon thin-film solar cells using chemical vapor deposition method, including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD) process. In addition, liquid phase epitaxy (LPPE) and sputtering deposition method can also be used to prepare poly-silicon thin-film battery.

Chemical vapor deposition mainly the SiH2Cl2, SiHCl3, SiCl4 or SiH4, as the reaction gas, It's react at a certain protection atmosphere and deposit silicon atoms at the heated substrate, the general substrate materials are Si, SiO2, Si3N4, etc.. But the researching found that it's difficult to form the large crystal on the amorphous silicon (a-si) substrates and easy to cause interspace between crystal. Solutions for this problem is to deposited a thin layer of amorphous silicon on the substrate by LPCVD, then annealing this layer of amorphous silicon, to get larger crystal, and then deposited a more thick poly-crystalline silicon film at this layer, therefore, re-crystallization technology is no doubt a very important aspect, the current technology used is solid-phase crystallization and recrystallization in the FZ method. Poly-silicon thin-film solar cells not onlyi use the re-crystallization process, also used almost all of the mono-crystalline silicon solar cells preparation technology, the solar cells made by this way have a remarkably increased it's conversion efficiency.

3. Nanocrystalline chemistry solar cell

Silicon solar cells are undoubtedly the most sophisticated atom all solar cells, but because of its high cost, can not meet the requirements of large-scale application. Therefore, Peoples always explore in process, new material and thin film solar cells etc, among this, the newly developed nano TiO2 crystalline chemistry solar cells get a great importance from home and abroad scientists.

For example, the dye-sensitized nano-crystalline solar cell (DSSCs), such solar cells mainly includes a glass substrate deposited with trasparent conductive film, dye-sensitized semiconductor materials, electrode and electrolyte etc.

As shown in below picture, the white ball means TiO2, red ball means dye molecules. Dye molecules transite to excited state after absorb solar energy, excited state unstable, the electron rapidly injected into the nearby TiO2 conduction band, Dye lost the electron is quickly be compensated from the electrolyte, electron enter the conduction band of TiO2 and eventually enter the electric conductive film, and then through the outer loop photo-current generated.

Nano-crystallineTiO2 solar cells have it's advantages of cheap cost, simple production process and a stable performance. Photoelectric efficiency stability at 10%, and the production costs is only 1 / 5 ~ 1 / 10 of silicon solar cells. Life expectancy can achieve more than 20 years. However, because of such a solar cell researching and development still in its infancy, it is estimated to be in the market gradually.

 

Anode: dye-sensitized semi-conductive thin film ( TiO2 film)

Cathode: TCO glass deposted with platinic

Electrolyte: I3-/I-

How do solar panels work?

Solar panels collect solar radiation from the sun and actively convert that energy to electricity. From a solar-powered calculator to an international space station, solar panels generate electricity using the same principles of electronics as chemical batteries or standard electrical outlets. With solar panels, it's all about the free flow of electrons through a circuit.
To understand how solar panels generate electrical power, it might help to take a quick trip back to high school chemistry class. The basic element of solar panels is the same element that helped create the computer revolution -- pure silicon. When silicon is stripped of all impurities, it makes a ideal neutral platform for the transmission of electrons. Silicon also has some atomic-level properties which make it even more attractive for the creation of solar panels.

Solar CCTV System

Silicon atoms have room for eight electrons in their outer bands, but only carry four in their natural state. This means there is room for four more electrons. If one silicon atom contacts another silicon atom, each receives the other atom's four electrons. This creates a strong bond, but there is no positive or negative charge because the eight electrons satisfy the atoms' needs. Silicon atoms can combine for years to result in a large piece of pure silicon. This material is used to form the plates of solar panels.

Here's where science enters the picture. Two plates of pure silicon would not generate electricity in solar panels, because they have no positive or negative charge. Solar panels are created by combining silicon with other elements that do have positive or negative charges.
Phosphorus, for example, has five electrons to offer to other atoms. If silicon and phosphorus are combined chemically, the result is a stable eight electrons with an additional free electron along for the ride. It can\'t leave, because it is bonded to the other phosphorus atoms, but it isn't needed by the silicon. Therefore, this new silicon/phosphorus plate is considered to be negatively charged.
In order for electricity to flow, a positive charge must also be created. This is achieved in solar panels by combining silicon with an element such as boron, which only has three electrons to offer. A silicon/boron plate still has one spot left for another electron. This means the plate has a positive charge. The two plates are sandwiched together in solar panels, with conductive wires running between them.
With the two plates in place, it's now time to bring in the 'solar' aspect of solar panels. Natural sunlight sends out many different particles of energy, but the one we're most interested in is called a photon. A photon essentially acts like a moving hammer. When the negative plates of solar cells are pointed at a proper angle to the sun, photons bombard the silicon/phosphorus atoms.
Eventually, the 9th electron, which wants to be free anyway, is knocked off the outer ring. This electron doesn't remain free for long, since the positive silicon/boron plate draws it into the open spot on its own outer band. As the sun's photons break off more electrons, electricity is generated. The electricity generated by one solar cell is not very impressive, but when all of the conductive wires draw the free electrons away from the plates, there is enough electricity to power low amperage motors or other electronics. Whatever electrons are not used or lost to the air are returned to the negative plate and the entire process begins again.

One of the main problems with using solar panels is the small amount of electricity they generate compared to their size. A calculator might only require a single solar cell, but a solar-powered car would require several thousand. If the angle of the solar panels is changed even slightly, the efficiency can drop 50 percent.
Some power from solar panels can be stored in chemical batteries, but there usually isn't much excess power in the first place. The same sunlight that provides photons also provides more destructive ultraviolet and infrared waves, which eventually cause the panels to degrade physically. The panels must also be exposed to destructive weather elements, which can also seriously affect efficiency.
Many sources also refer to solar panels as photovoltaic cells, which references the importance of light (photos) in the generation of electrical voltage। The challenge for future scientists will be to create more efficient solar panels are small enough for practical applications and powerful enough to create excess energy for times when sunlight is not available.

Energy Loss in a Solar Cell

Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation is not monochromatic -- it is made up of a range of different wavelengths, and therefore energy levels. Light can be separated into different wavelengths, and we can see them in the form of a rainbow. Since the light that hits our cell has photons of a wide range of energies, it turns out that some of them won't have enough energy to form an electron-hole pair. They'll simply pass through the cell as if it were transparent. Still other photons have too much energy. Only a certain amount of energy, measured in electron volts (eV) and defined by our cell material (about 1.1 eV for crystalline silicon), is required to knock an electron loose. We call this the band gap energy of a material.

If a photon has more energy than the required amount, then the extra energy is lost (unless a photon has twice the required energy, and can create more than one electron-hole pair, but this effect is not significant). These two effects alone account for the loss of around 70 percent of the radiation energy incident on our cell.

Why can't we choose a material with a really low band gap, so we can use more of the photons? Unfortunately, our band gap also determines the strength (voltage) of our electric field, and if it's too low, then what we make up in extra current (by absorbing more photons), we lose by having a small voltage. Remember that power is voltage times current. The optimal band gap, balancing these two effects, is around 1.4 eV for a cell made from a single material.

Speed Dome with Solar

We have other losses as well. Our electrons have to flow from one side of the cell to the other through an external circuit. We can cover the bottom with a metal, allowing for good conduction, but if we completely cover the top, then photons can't get through the opaque conductor and we lose all of our current (in some cells, transparent conductors are used on the top surface, but not in all). If we put our contacts only at the sides of our cell, then the electrons have to travel an extremely long distance (for an electron) to reach the contacts.

Remember, silicon is a semiconductor -- it's not nearly as good as a metal for transporting current. Its internal resistance (called series resistance) is fairly high, and high resistance means high losses. To minimize these losses, our cell is covered by a metallic contact grid that shortens the distance that electrons have to travel while covering only a small part of the cell surface. Even so, some photons are blocked by the grid, which can't be too small or else its own resistance will be too high.