There are different types of solar of photovoltaic cells available in the market. Each of them has its own technical and commercial advantages as well as disadvantages. The solar cells are mainly categorized on the basis of the materials used as semiconductor in the cell. Different types of solar cell have different technical parameters but they can be chosen for use in different applications depending upon the optimum conditions of the services. The main row material used to construct a solar cell is semiconductor material and silicon is widely used semiconductor for that. There is plenty of availability of silicon on the earth. Silicon is available in normal sand. But extracting pure silicon from the natural sources is quite expensive. Lots of efforts are required for that work. In the process silicon is purified, melted, and crystallized. Continuous researches and developments are going on to reduce the quantity of silicon used in the solar cell without affecting its performance. This ensures the availability of cheaper solar cells. Two most common techniques of producing silicon wafers are the chzrolaski (CZ) and floating zone (FZ) techniques.
Let us discuss some common techniques of solar or photovoltaic cells.
Thin Film Solar Cell
This is the cheapest technology available for solar cell. Most commonly used thin-film solar cell is Amorphous solar cell. Here, a very thin film of amorphous silicon of few microns thick is used as active layer of the cell. Reliability and lifespan of these cells are quite poor. The main advantage of this technology is that it can be manufactured in different shaped hence can be mounted in any form of surface. Amorphous silicon is basically a silicon substance but in non crystalline form. It is specially made by deposition of silicon based gases on subtract. The chemical bonding between atoms in amorphous silicon is quite similar to that of crystalline silicon but only difference is that the material has no regular lattice structure. In amorphous silicon a significant number of silicon atoms create covalent bonds with three neighbor silicon atoms and rest one valence electron of these silicon atoms creates bond with hydrogen atom. While a mixture of silicon containing hydrogen gas (SiH4) and raw hydrogen gas pass through between two electrodes in a vacuum chamber and an RF voltage is applied between these electrodes, hydrogenised silicon will be deposited on a warm subtract attached to the electrodes. In this way very thin film of hydrogenised silicon can be created.
The thickness of the film may be less than 1 micron. The main advantage of amorphous silicon is that, it is quite cheaper than crystalline silicon. For a given thickness, the light absorption capacity of amorphous silicon is about 2.5 times more than that of crystalline silicon. As the quantity of required silicon for producing micro thin film, is quite low compared to crystalline silicon, the cost as well as the weight of amorphous silicon solar cell or a-Si:H solar cell is much low. Amorphous silicon is flexible in nature hence can be easily deposited on flexible and roll way type subtracts. In addition to Amorphous solar cells there are also Cadmium Telluride (CdTe) and Copper-Indium-Gallium-Selenide (CIGS) thin film solar cells available in the market. The materials used for making the active layer of amorphous cells is amorphous silicon, Cadmium and Tellurium are used for Cadmium Telluride (CdTe) solar cells and Copper, Indium, Gallium, Selenium are used for Copper-Indium-Gallium-Selenide solar cells. The efficiency of amorphous cell is about 6 to 8 % and this low efficiency of amorphous cells is some extend improved in Cadmium Telluride (CdTe) and Copper-Indium-Gallium-Selenide (CIGS) thin film solar cells. The efficiency of Cadmium Telluride (CdTe) and Copper-Indium-Gallium-Selenide (CIGS) thin film solar cells is about 8 to 11 %. Open circuit voltage of amorphous cells is 0.7 to 1.1.volt and same for the Cadmium Telluride (CdTe) and Copper-Indium-Gallium-Selenide (CIGS) thin film solar cells are 0.8 to 1 volt and 0.5 to 0.7 volts respectively.
Crystalline Silicon Solar Cell
Crystalline Silicon Solar Cell has most advantageous against other available technologies of solar cell manufacturing. But these cells require purified silicon as row material which is quite expensive. Here naturally available silicon quartzes are melted, purified and then crystallized in ingots. Then these ingots then further cut into smaller sized ingots and then these ingots are sliced into number of thin silicon wafers. These wafers were generally made 0.3 mm thick in early days but now technology has developed to make them about 0.15 mm thick. This thickness of the silicon crystal wafer is important in the view of cost optimization of the cell. Then these wafers are chemically treated for doping with impurities to create p-n junction across them.
Then negative and positive metallic contacts are fabricated on top and bottom the cells by screen printing techniques.
The cells typically produce 0.5 to 0.6 volts across them at standard working conditions hence numbers of such cells are wired in series to produce standard 6 or 12 volts. Numbers of such series combinations are then arrayed in parallel on a glass substrate or some kind of insulated reinforced base substrate to increase the power capacity of the module. Then the module is covered with none conducting transparent encapsulating resin and then a protective transparent film. The whole module is then framed with aluminium channel to provide ultimate reinforcement to it. The required power output is achieved by interconnecting suitable number of such modules in an array.