You've probably seen calculators with solar cells -- devices that never need batteries and in some cases, don't even have an off button. As long as there's enough light , they seem to work forever.
You may also have seen larger solar panels, perhaps on emergency road signs, call boxes, buoys and even in parking lots to power the lights. Although these larger panels aren't as common as solar-powered calculators, they're out there and not that hard to spot if you know where to look. In fact, photovoltaics -- which were once used almost exclusively in space, powering satellites' electrical systems as far back as -- are being used more and more in less exotic ways. The technology continues to pop up in new devices all the time, from sunglasses to electric vehicle charging stations.
This is a seductive promise, because on a bright, sunny day, the sun's rays give off approximately 1, watts of energy per square meter of the planet's surface. If we could collect all of that energy, we could easily power our homes and offices for free. A module is a group of cells connected electrically and packaged into a frame more commonly known as a solar panel , which can then be grouped into larger solar arrays, like the one operating at Nellis Air Force Base in Nevada.
Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely.
PV cells also all have one or more electric field that acts to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off for external use, say, to power a calculator. This current, together with the cell's voltage which is a result of its built-in electric field or fields , defines the power or wattage that the solar cell can produce.
That's the basic process, but there's really much more to it. On the next page, let's take a deeper look into one example of a PV cell: the single-crystal silicon cell. Adding solar panels to an existing home can be expensive -- but there are lots of other ways to make your home greener. Learn more about what you can do to protect the environment at Discovery Channel's Planet Green.
Silicon has some special chemical properties, especially in its crystalline form. The first two shells -- which hold two and eight electrons respectively -- are completely full. The outer shell, however, is only half full with just four electrons. A silicon atom will always look for ways to fill up its last shell, and to do this, it will share electrons with four nearby atoms. It's like each atom holds hands with its neighbors, except that in this case, each atom has four hands joined to four neighbors.
That's what forms the crystalline structure , and that structure turns out to be important to this type of PV cell. The only problem is that pure crystalline silicon is a poor conductor of electricity because none of its electrons are free to move about, unlike the electrons in more optimum conductors like copper.
To address this issue, the silicon in a solar cell has impurities -- other atoms purposefully mixed in with the silicon atoms -- which changes the way things work a bit. We usually think of impurities as something undesirable, but in this case, our cell wouldn't work without them. Consider silicon with an atom of phosphorous here and there, maybe one for every million silicon atoms.
Phosphorous has five electrons in its outer shell, not four. It still bonds with its silicon neighbor atoms, but in a sense, the phosphorous has one electron that doesn't have anyone to hold hands with.
It doesn't form part of a bond, but there is a positive proton in the phosphorous nucleus holding it in place. When energy is added to pure silicon, in the form of heat for example, it can cause a few electrons to break free of their bonds and leave their atoms. A hole is left behind in each case. These electrons, called free carriers , then wander randomly around the crystalline lattice looking for another hole to fall into and carrying an electrical current.
However, there are so few of them in pure silicon, that they aren't very useful. But our impure silicon with phosphorous atoms mixed in is a different story. It takes a lot less energy to knock loose one of our "extra" phosphorous electrons because they aren't tied up in a bond with any neighboring atoms.
As a result, most of these electrons do break free, and we have a lot more free carriers than we would have in pure silicon. The process of adding impurities on purpose is called doping , and when doped with phosphorous, the resulting silicon is called N-type "n" for negative because of the prevalence of free electrons.
N-type doped silicon is a much better conductor than pure silicon. The other part of a typical solar cell is doped with the element boron, which has only three electrons in its outer shell instead of four, to become P-type silicon. Instead of having free electrons, P-type "p" for positive has free openings and carries the opposite positive charge. On the next page, we'll take a closer look at what happens when these two substances start to interact.
That's because without an electric field , the cell wouldn't work; the field forms when the N-type and P-type silicon come into contact.
Suddenly, the free electrons on the N side see all the openings on the P side, and there's a mad rush to fill them. Do all the free electrons fill all the free holes?
If they did, then the whole arrangement wouldn't be very useful. However, right at the junction , they do mix and form something of a barrier, making it harder and harder for electrons on the N side to cross over to the P side. Eventually, equilibrium is reached, and we have an electric field separating the two sides. This electric field acts as a diode , allowing and even pushing electrons to flow from the P side to the N side, but not the other way around. It's like a hill -- electrons can easily go down the hill to the N side , but can't climb it to the P side.
When light, in the form of photons , hits our solar cell, its energy breaks apart electron-hole pairs. Each photon with enough energy will normally free exactly one electron, resulting in a free hole as well.
If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side.
The electron flow provides the current , and the cell's electric field causes a voltage. With both current and voltage, we have power , which is the product of the two. There are a few more components left before we can really use our cell. Silicon happens to be a very shiny material, which can send photons bouncing away before they've done their job, so. The final step is to install something that will protect the cell from the elements -- often a glass cover plate.
PV modules are generally made by connecting several individual cells together to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with positive and negative terminals. How much sunlight energy does our PV cell absorb? Unfortunately, probably not an awful lot. In , for example, most solar panels only reached efficiency levels of about 12 to 18 percent. The most cutting-edge solar panel system that year finally muscled its way over the industry's long-standing 40 percent barrier in solar efficiency -- achieving Department of Energy ].
So why is it such a challenge to make the most of a sunny day? Visible light is only part of the electromagnetic spectrum.
See How Light Works for a good discussion of the electromagnetic spectrum. Since the light that hits our cell has photon s of a wide range of energies, it turns out that some of them won't have enough energy to alter an electron-hole pair. They'll simply pass through the cell as if it were transparent. Still other photons have too much energy. MadRthanevr madrthanevr Rep: 23 1. View the answer I have this problem too Subscribed to new answers.
Is this a good question? Yes No. Voted Undo. Score 1. I replaced mt calculators. Chosen Solution. Was this answer helpful? The Office Depot representative had no idea. Any tips on how I can extract it with no damage to the binder? Ugh, how many of these so-called solar calculators are fakes? I have the same issue with an Office Depot calculator. I'd contact them, they'll probably send you a new one. I'm always amazed at what I can get for just asking.
When it stopped working he handed it to me to fix. There is a little door on the back of the calculator so I figured that it must be a solar assisted, battery powered calculator.
I popped open the door and found a AG10 button cell battery in it. The battery was corroded so obviously it was bad. As you can see, the calculator is only wired to the battery and the so-called solar cell is a FAKE. What do you know?
I wonder if Samsill knew that was what they were getting?? Tags from the story. More from John Mueller I wrote this step-by-step tutorial on how to replace the radiator on You might try warming it with a hair dryer to soften the adhesive.
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