Photovoltaics

The name photovoltaics refers to a material or a system that possess the ability to process and directly convert solar energy into an electrical current. A solar panel consisting of a number of solar cells is known as a photovoltaic system. Solar cells have been heavily studied for many decades and the research continues to improve the efficiency and the cost-effectiveness of these. Solar cells are considered together with wind and water a clean sustainable energy technology which only driving force is the sun. Since the energy source is free everywhere around the globethe price of solar cells will only decrease compared to the price of oil since it is only the technology and the advantages made that will affect the price. Solar energy have also been predicted to play a crucial role in the future energy production.

The industry of solar cells and have greatly benefited and expanded because of results from all the research and today the easy recognizable blue solar panels are a common thing in our everyday life whether it is on roof tops, traffic light, parking meters, watches, calculators etc. The growth in the solar panel industry points in the direction of moving away from the small private solar panels into big fields of solar panels connecting to the energy grid producing kilowatts (or more) of energy.

A solar cell consists in general of two semiconducting materials (one N- and one P-type) on top of each other creating a P-N junction. On top of each semiconducting materials a thin conducting layer (electrode) is added that will be able to transport electrons away from the solar cell and finally a top layer of glass is added for protection. When sun light hits the semiconducting material it will knock loose electrons and hence also create positive holes where the electrons used to be. The electrons will be able to move through the P-type material and the positive hole through the N-type to the conducting layer and hence create a flow of electrons and positive holes (electrical current).

The most common materials for commercially available solar cells are silicon which has been doped (semiconducting material) to achieve a material that exhibit the photovoltaic effect. One part of the silicon has been doped with boron, which has more electrons than silicon, resulting in an excess of electrons (N-type) while the other have been doped with phosphorus, less electrons than silicon, which results in electron deficiency (P-type). Different types of silicon is available in the fabrication of the solar cell; monosilicon crystal, polycrystalline, and amorphous thin film. The monosilicon solar cells show the highest efficiency (~25%) [3] but because of the need for a mono crystal these cells are expensive to manufacture. The polycrystalline silicon cells are made from molten silicon which are afterward cooled down causing the silicon to crystallize in different domains and because of this the efficiency will be lower compared to the monosilicon solar cells but the manufacturing cost will likewise be lower which makes them almost as cost effective. Amorphous silicon can be applied to a glass surface in a thin layer (hence the name thin-film cells) which results in low production coast because of the small amount of material needed but unfortunately the efficiency will be lower but cost effective it is comparable to the other two fabrication methods.  Other materials which are also used for commercially available thin-film cells are copper indium gallium selenide and cadmium telluride.

Many countries have seen the possibilities of solar panels and the environmental friendly energy production and have created different governmental funds to support the installations of solar panels and offer the seller a higher price for the green produced electricity.  The only downside of solar energy is that it can only be harvested during the day when the sun is up so if a large energy grid system should only rely on this an efficient way of storing large amounts of energy is needed.Another type of thin film solar cells is the dye sensitize solar cell also known as the Grästcel Cell which is considered cheaper to manufacture than the silicon solar cells. In principal the process of this cell mimics that of the photosynthesis of plants. The cell consists of two transparent conducting glass plates (electrodes) which are sandwiched around the actual cell. A metal organic dye based on ruthenium is absorbed onto the surface of the semiconducting titanium dioxide which is in contact with an iodine electrolyte solution.  The organic dye gets excited when light hits it and release an electron that will be carried by the titanium dioxide to the electrodes. The dye reaches it ground state by oxidizing the electrolyte solution which will be regenerated by the released electrons  that has travelled through the circuit and back to the counter electrode.  Because a dye is used these cells can be made colorful and since they are semitransparent and flexible they could be used in e.g. windows or as beautiful decorations on facades of buildings (see picture). These types of cells are not commercially available yet but because of their low fabrication cost an efficiency of 10 % could still be cost effective.

Videos

Solar cell

 

Dye sensitized solar cell

 

References

[1]           Solarwirtschaft.de.WBCU 2003

[2]           https://forceofthesun.com/solar-parking-lots-sound-like-a-great-idea-why-arent-they-catching-on-csmonitor-com

[3]           Photovoltaics Report, Fraunhofer ISE, July 28, 2014, pages 18,19

[4]           https://solarpower.com/blog/2011/08/

[5]           https://techon.nikkeibp.co.jp/english/NEWS_EN/20081212/162752/?SS=imgview_e&FD=1465256612