Solar energy is a promising source of clean energy and one that contributes to the stability of power supply worldwide.
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By Wim Zoomer
Solar cells, or photovoltaic (PV) cells, are devices manufactured to convert solar energy into electrical energy. Two PV technologies are prevalent: crystalline-silicon wafers and thin films. Crystalline silicon, a relatively poor light absorber, is still used as a light-absorbing semiconductor in most solar cells. Thin-film semiconductors require less semiconductor material because they are relatively strong light absorbers. As a result, material costs are substantially reduced. Although today’s market share of thin-film solar cells is less than 10%, this emerging semiconductor technology is finding its place quickly in more applications each day.
First generation
The development of the first generation of solar cells was based on high efficiency and high cost. Exceptionally pure silicon was used to manufacture a mono- or multi-crystalline silicon wafer, resulting in relatively high efficiency levels. However, silicon has become more expensive, as have the requisite production technologies, making it almost impossible to reduce costs of these fragile PV cells and see a return on investment in a reasonable amount of time.
The PV effect is created by a semiconductor—silicon doped with impurities. These can be either phosphor or boron. Phosphor, doped into silicon has an excess of electrons, is an n-type semiconductor. On the other hand, boron doped into silicon creates a shortage of electrons and, therefore, is a p-type semiconductor. Impure silicon photons in solar energy hit the solar cell and are absorbed by the silicon.
Negatively charged electrons knock loose from their atoms, create holes, and flow through the material. Connecting the n-type semiconductor to the p-type semiconductor creates an interesting interaction. Now the negatively charged electrons prefer to move towards the holes in the p-type silicon because the p-type silicon has an excess of electrons. The contact surface between the two silicon layers consists of a negative charge and a positive charge. This contact surface is called p-n junction. The electrons flow just like in a diode, from the p-side towards the n-doped silicon electrode. The electron flow generates electrical current.
Second generation
The second-generation solar cell is based on thin-film-semiconductor technology. Major developments took place during the 1990s and early 2000s. The goal to develop second-generation solar cells was low cost combined with low efficiency, flexibility, and light weight. The cells have a conversion efficiency of 10-15%. Low cost is achieved by using inexpensive materials and simple, fast production processes to create flexible solar cells that are less susceptible to breakage.
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