The Basics of Solar Cell

The Basics of Solar Cell

The meaning of Solar Cell or PV

Solar Cell, PV and photovoltaic cell are different terms that are used to describe the same word, photovoltaic. The term photovoltaic comes from two words photo (light) and voltaic (voltage). The photovoltaic effect is the transformation of the sun's rays into electricity. The scientific investigation of the photovoltaic effect started in 1839, but the efficient solar cell could not be made. Until 1954, a silicon solar cell was developed and this kind of solar cell was used in specialized applications as satellites from 1959.

Conclusion

Solar cell is a device that is made up of semiconductor materials such as silicon, gallium arsenide, indium phosphide, cadmium telluride and copper indium diselenide, etc. that absorbs sunlight and converts them into electrical carrier, then separate to electrons and holes to produce energy at positive/negative junctions. If the positive and negative junctions of solar cell are connected to DC electrical equipment, it is workable.

Types of solar cell

There are three main cell types that classified by its manufacturing technology and the semiconductor.


Single Crystalline Silicon Solar Cell	Polycrystalline Silicon Solar Cell	Amorphous Silicon Solar Cell

1.	Two types of crystalline silicon solar cell are single crystalline silicon solar cell, which is known as monocrystalline silicon solar cell, made by slicing wafers and polycrystalline silicon solar cell, made by sawing a block of silicon first into bars and then wafers. The crystalline silicon solar cell is very hard and thin.
2.	Amorphous silicon solar cell is the best developed of the thin film technology, which its thickness is 0.5 micron (0.0005 millimeter) and lightweight with its efficiency of 5-10%.
3.	Other semiconductors such as gallium arsenide, cadmium telluride and copper indium diselenide, etc. have both of single crystalline and of polycrystalline. The solar cell, which made by gallium arsenide is high efficiency about 20-25%.

The solar cell structure

The most type of semiconductor currently in use is silicon crystal, which is the inexpensive and most available semiconductor. Because silicon crystals are laminated into p-type and n-type region, it is used to make the solar cell. The silicon has been smelt and purified until it becomes crystals. Then doping atoms are to create a p-type and n-type region. P-type doping, the creation of excess holes, is achieved by incorporating and atoms with the boron and n-type doping, the creation of excess electrons, is achieved by incorporating an atom with the phosphorus. Once a p-n junction is created, electrical contacts are made to the front and the back of the cell by evaporation or screen-printing metal onto the wafer. The rear of the wafer can be completely covered by metal, the front has a grid pattern or thin lines of metal and the back has the metal, which block out the sun from the silicon.

How solar cell works

When the sunlight strikes the solar cell surface, which consists of two types of materials, p-type and n-type semiconductor, and the cell creates charge carrier as electrons and holes. The internal field produced by junction separates some of the positive charges (holes) from the negative charges (electrons). The holes are swept into the positive or p-layer and the electrons are swept into the negative or n-layer. When a circuit is made, the free electrons have to pass through the load to recombine with the positive holes, current can be produced from the cells under illumination.

For example, -a silicon solar cell, which its diameter is 4 inch typically, produces from 2 to 3 amperes and approximately 0.6 volts. Because the output current of each cell is very low, a number of cells are wired together to make a module to reach the required current. And these modules are wired into large arrays. The solar arrays wiring depend on current and voltage requirement.

1.	If the solar arrays are wired in parallel, the output current increases.
2.	If the solar arrays are wired in series, the output voltage increases.

Manufacture solar cells

The single crystal or monocrystalline solar cell has the process as below:

1.	The melt silicon has been grown at 1400 °C then drawn the crystal from the melted silicon by slowly decreasing the temperature until become a large single crystal ingot. The large single crystal ingots are sliced into the single crystal wafers.
2.	The single crystal wafers were doping atoms to create a p-type and n-type region and thereby producing a p-n junction. This doping can be done by high temperature diffusion (900-1000 °C), where the wafers are placed in a furnace with the dopant to introduced as a vapour.

The polycrystalline solar cell has the process as below:

1.	The molten silicon is poured into a mould and allowed to set. Then it is sliced into the wafers.
2.	The polycrystalline wafers were doping to create a p-type, n-type region and p-n junction same as the single crystal solar cell process.

The amorphous solar cell has the process as below:

1.	The silane gas (SiH₄) is reacted by Plasma Chemical Vapor Deposition device. The amorphous silicon is made by depositing silicon onto glass or another substrate material from a reactive gas. The layer thickness amounts to less than 1 micron (0.001 millimeter).
2.	When the silane gas reaction the dopants as phosphine and diborane are included to create a p-type,n-type region and p-n junction.
3.	The p-n junction is made up of translucent junction as indium tin oxide.

The gallium arsenide solar cell has the process as below:

1.	The gallium arsenide crystal has been grown by a Liquid Phase Epitaxy furnace.
2.	The p-type and n-type region are created by Molecular Beam Epitaxy device.

Main features of solar cell

	Use the natural power, sunlight, which is clean and non-polluting
	Efficiently use a renewable power source and no limiting
	Able to be installed anywhere to produce and take direct current
	Use no fuel other than sunlight
	No burning and therefore no air and water pollution
	Give off no waste and therefore harmless environment
	No moving parts so there is no mechanical noise being operation
	Minimal maintenance requirement
	Long lifespan and stable efficiency
	Light weight, easy to install and transportable
	With the modular characteristic, it can be constructed any sizes as required
	Reduce collection of gases such as carbon monoxide, sulfur dioxide, hydrocarbon and nitrogen, etc., which generated from fuel, coal and fossil fuel burning power plants. All decrease the impacts of energy on the environment like greenhouse effect, global warming, acid rain and air pollution, etc.

Main components of solar system:

The solar cells produce direct current; therefore they are only compatible with the DC equipment's. If users want to use for AC equipment's or store backup energy for other applications, the solar systems require other components in addition to the solar modules. These components are the followings.

1.	Solar Module is to convert sunlight into DC current and usually stated in watt. If more than one solar module is used, they are interconnected to produce the required current, which called "solar array". The solar arrays can be wired in series to increase the voltage or in parallel to increase the current. The different geographic locations receive the quantities of peak sun hours per day, and as the temperature increase, the output decrease.
2.	Charge controller is to regulate the current from the solar arrays, charge the batteries and prevent the batteries from overcharging. The function of charge controller is to charge the batteries. When the batteries are fully charged, the charge controller will stop or decrease the charging. Most solar charge controllers also include a Low Voltage Disconnect feature, which will disconnect the power supply to the load when the battery voltage drops too far. The solar system needs the charge controller in case of the power storage in the batteries is necessary.
3.	Battery is to store current that produce from the solar arrays for use during the sunlight is not visible, nighttime or other purposes.
4.	Inverter is to convert the DC current that produce from the solar arrays to AC current for operation AC equipment's. There are two types inverter, sine wave inverter is compatible with all AC equipment's, and modified sine wave inverter is used with any electrical equipment, which has no rotating component.
5.	The Lightning Protection is to prevent the electrical equipment damages caused by lightning or induction of high voltage surge. This component is required for the large size and critical solar systems, which include the efficient grounding.

The solar cell applications

The sun is the most attractive renewable power source and more widely use. It can be applied to many purposes in everyday life and environmental-friendly. For example,
Residential homes-Lighting systems, outdoor lighting systems (such as garden lights, garage lights and fence lanterns, etc.), electrical equipments, electric gate openers, security systems, ventilators, water pumps, water filters, and emergency lights, etc.

Water pumping-Consumption, public utility, livestock watering, agriculture, gardening and farming, mining and irrigation, etc.

Lighting systems-Bus stop lightings, telephone booth lightings, billboard lightings, parking lot lightings, outdoor lightings and street lightings, etc.

Battery charging systems-Emergency power systems, Batteries charging centers for rural villages and power supplies for household use and lighting in remote area, etc.

Agriculture-Water pumping and thrashing machines, etc.

Cattle-Water pumping, Oxygen filling systems for fish-farming and insect trapped lightings, etc.

Health centers-Refrigerators and cool boxes for keeping medicines and vaccines, and medical equipment's, etc.

Communication-Air navigational aids, air warning lights, lighthouses, beacon navigation aids, illuminated road signs, road signs, railway crossing signs, street lightings and emergency telephones, etc.

Telecommunication-Microwave repeater stations, telecommunication equipments, portable communication equipments (such as communication radio for service and military exercises, etc.) and weather monitoring stations, etc.

Entertainment and recreation-Power supplies for remote leisure homes, portable battery charging systems and entertainment equipments,
etc.

Remote area-Hills, islands, forests and remote areas that the utility grids are not available, etc.

Space - Satellites