How Do Solar PV Panels Work?

The simplified answer Silicon is mounted beneath non-reflective glass to produce photovoltaic panels. These panels collect photons from the sun, converting them into DC electrical power. To make a PV system the power created then flows into an inverter. The inverter transforms the power into AC electrical power. That is a very brief explanation, but some may prefer a more in depth answer to the question, “How do solar panels work?” Here is the more detailed response, though it still remains fairly basic:

The different panel types: There are other materials other than doped silicone that ex-hibit the ability to move electrons . Amorphous Glass Panels have a thin substrate coated onto glass and flexible panels can be made by coating other materi-als. This technology gives lower performing panels but are cheaper in some circumstances. Silicone based collectors come in two distinct types, Poly Crys-

taline and Mono-Crystaline. Both poly-crystalline and mono-crystalline solar panels are made from the same material, silicon. However the difference is that the poly-crystalline material is made up of millions or billions of small silicon crystals while the mono-crystalline material is actual just that, one large singe crystal of silicon. Single crystal silicon is more efficient at converting photons to electrons for electricity, the poly-silicon its much less efficient because electrons are captured or generated less efficiently where the crystals of silicon touch. However, even though poly solar panels are not as efficient, they are cheaper to manufacture so they can still be competi-tive on a £/watt basis. They would just need more area to produce the same amount of electricity as the mono-crystalline panels.

Panel Performances: For any given set of operational conditions, cells have a single operating point where the values of the current (I) and Volt-

age (V) of the cell result in a maximum power output. These values correspond to a particular load resistance, which is equal to V / I as specified by Ohm's Law. The power P is given by P=V*I. A photovoltaic cell has an ap-proximately exponential relationship between current and voltage. From basic circuit the-ory, the power deliv-ered from or to a de-vice is optimized where the derivative (graphically, the slope) dI/dV of the I-V curve is equal and opposite the I/V ratio (where dP/dV=0). This is known as the maximum power point (MPP) and corre-sponds to the "knee" of the curve.

The Physics

To start off, it is crucial that silicon be better explained. Silicon has four electrons in its outer shell. However, it has the capacity to hold eight. By sharing these four electrons with other silicon atoms and their four shell electrons, the capacity of eight is filled. When they combine with each other in this way, silicon atoms develop a strong, stable bond. This structure is known as pure, crystalline silicon. Of course, this pure silicon is a poor conductor of electricity, as there are no electrons free to move about. In other words, the silicon is better off with impurities. To create these impurities, silicon is combined with something else. When silicon combines with an element that has five electrons to share, such as phosphorus, a negative charge is created. Silicon can only take four of the five electrons. This leaves one free electron looking for a spot. These addi-

tional electrons are known as free carriers; they carry an electrical current. On the other hand, when silicon is combined with an element that has three electrons a positive charge is created. Boron is a material which suits this purpose. When silicon and boron are combined, holes are created. These silicon combinations and their differing charges are used to make solar panels. As photons come down from the sunlight and strike the sili-con, it shakes everything up. The free electron that was hanging onto the silicon/phosphorous combination is now forced to the outer ring. From here, it gets sucked up to the outer ring of the silicon/boron combination. This is how electricity is created.

Today's most common PV devices use a single junction, or in-terface, to create an electric field within a semiconductor such as a PV cell. In a single-junction PV cell, only photons whose energy is equal to or greater than the band gap of the cell material can free an electron for an electric circuit. In other words, the photovoltaic response of single-junction cells is limited to the portion of the sun's spectrum whose energy is above the band gap of the ab-sorbing material, and lower-energy photons are not used. One way to get around this limi-tation is to use two (or more) different cells, with more than one band gap and more than one junction, to generate a volt-age. These are referred to as "multi-junction" cells (also called

"cascade" or "tandem" cells). Multi-junction devices can achieve a higher total conver-sion efficiency because they can convert more of the energy spectrum of light to electricity. As shown above, a multi-junction device is a stack of indi-vidual single-junction cells in descending order of band gap. Eg the top cell captures the high-energy photons and passes the rest of the photons on to be absorbed by lower-band-gap cells. Much of today's research in multi-junction cells focuses on gal-lium arsenide as one (or all) of the component cells. Such cells have reached efficiencies of around 35% under con-centrated sunlight. Other materials studied for multi-junction devices have been amorphous silicon and copper indium di-selenide. As an example, the multi-junction device below uses a top cell of gallium indium phosphide, "a tunnel junc-tion," to aid the flow of electrons between the cells, and a bottom cell of gallium arsenide.

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