Thursday, 26 March 2009

Solar cells elements, how solar cells work.

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

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:

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:

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.

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 (pectinate electrode), 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 sunlights 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 deposite 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 deposite a thin layer of amorphous silicon on the substrate by LPCVD, then annealing this layer of amorphous silicon, to get larger crystal, and then deposite 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. Polysilicon 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 remarkablly increased it's conversion efficiency.

3.Nanocrystalline chemistry solar cell
Silicon solar cells are undoubtedly the most sophisticated amone all solar cells, but because of it's 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 nanocrystalline solar cells (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 photocurrent generated.
Nanocrystalline TiO2 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-

Nanocrystalline chemistry solar cell application model

4 Hand made dye-sensitized nanocrystalline solar cells

1. TiO2 film Preparation

1)Grinding titanium dioxide powder with adhesive mortars

2)Spread the mixture on TCO (transparent conductive) glass

3)Sinter it on alcohol burner,them cool it down.

2.Color up the TiO2 with natural dyestuff

as shown in the picture, extrude the fresh or freezing black berry, Punica granatum seeds or black tea added with a spoon of water, then put the TiO2 film into it for color up, it's need around 5 minutes till the film become modena, if the color is nonuniform for both side, could dip in for another 5 minutes. afterwards, wash it with ethanol, then dry it with soft paper lightly.

3.Make positive electrode,

The electron outflow from dyed TiO2, means negative electrode. The positive electrode could be the conductive side of the TCO glass(the side depsited with SnO2), it's could be distinguish which side of the glass are conductive by a multi meter, also could distinguish by finger as the conductive side more coarseness. as below picture, mark the non-conductive side with "+", and use a pencil wipe the conductive side with a layer of black lead equably,

4.Join the electrolyte

Use the solution with iodin-hydronium to be the electrolyte for solar cells, mainly for revert and rebirth dyestuff. drop 1 or 2 dripping electrolyte on the TiO2 .

5.Assemble the solar cells

Put the color up TiO2 film on the tale facing up, drop on 1 or 2 dripping of iodin-hydronium electrolyte, then put the positive electrode facing down on the TiO2 film. put the 2 glass slightly staggered, use 2 clamps to nip the solar cell, the 2 glass exposed parts are for connect wires. In this way, you maked the solar cells.

6.Solar cell testing:

put the solar cell under sunshine outdoor, test your solar cell if it could generate electric current.


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  6. you have mentioned here
    N type semiconductor contains more hole, while the P type semiconductor contains more electron??? what is this??
    please correct it.....

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