What do photovoltaics do

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. Only a certain amount of energy, measured in electron volts eV and defined by our cell material about 1. We call this the band gap energy of a material.

If a photon has more energy than the required amount, then the extra energy is lost. That is, unless a photon has twice the required energy, and can create more than one electron-hole pair, but this effect is not significant. These two effects alone can account for the loss of about 70 percent of the radiation energy incident on our cell. Why can't we choose a material with a really low band gap, so we can use more of the photons?

Unfortunately, our band gap also determines the strength voltage of our electric field, and if it's too low, then what we make up in extra current by absorbing more photons , we lose by having a small voltage.

Remember that power is voltage times current. The optimal band gap, balancing these two effects, is around 1. We have other losses as well. Our electrons have to flow from one side of the cell to the other through an external circuit. We can cover the bottom with a metal, allowing for good conduction, but if we completely cover the top, then photons can't get through the opaque conductor and we lose all of our current in some cells, transparent conductors are used on the top surface, but not in all.

If we put our contacts only at the sides of our cell, then the electrons have to travel an extremely long distance to reach the contacts. Remember, silicon is a semiconductor -- it's not nearly as good as a metal for transporting current. Its internal resistance called series resistance is fairly high, and high resistance means high losses. To minimize these losses, cells are typically covered by a metallic contact grid that shortens the distance that electrons have to travel while covering only a small part of the cell surface.

Even so, some photons are blocked by the grid, which can't be too small or else its own resistance will be too high. Now that we know how a solar cell operates, let's see what it takes to power a house with the technology. We use an inverter. A solar inverter takes the DC electricity from the solar array and uses that to create AC electricity.

Inverters are like the brains of the system. Along with inverting DC to AC power, they also provide ground fault protection and system stats, including voltage and current on AC and DC circuits, energy production and maximum power point tracking. Central inverters have dominated the solar industry since the beginning. The introduction of micro-inverters is one of the biggest technology shifts in the PV industry. Micro-inverters optimise for each individual solar panel, not for an entire solar system, as central inverters do.

This enables every solar panel to perform at maximum potential. Another option to consider is using micro-inverters on each of the panels. If one solar panel has an issue, the rest of the solar array still performs efficiently. First, sunlight hits a solar panel on the roof. The panels convert the energy to DC current, which flows to an inverter. The inverter converts the electricity from DC to AC, which you can then use to power your home.

And what happens at night when your solar system is not generating power in real time? A typical grid-tied PV system, during peak daylight hours, frequently produces more energy than one customer needs, so that excess energy is fed back into the grid for use elsewhere.

Electrons within the boron-doped silicon can jump around to fill in the hole. Alternatively, the holes themselves can be thought of as moving in the opposite direction to the electrons as the electrons hop from one bond to another. This movement also constitutes an electrical current. We now have a material with an overall deficiency of electrons, making a positive p-type material.

This gives each doped material an overall slight preference to either give or receive electrons. This is known as electronegativity—a measure of how strongly an atom or material hangs on to its electrons. Through doping, and the resulting changes in electronegativeity, silicon is turned into a conductor of electricity albeit not a particularly good one. However, when we put the p-type and the n-type materials in contact with each other, something interesting and useful happens. At the point where the two types meet—the junction—electrons from the n-type layer diffuse over into the p-type layer, leaving behind an area with a slight positive charge in the n-type layer.

The reverse happens in the p-type layer—holes diffuse into the n-type layer, leaving behind a slight negative charge in a region of the p-type layer. This creates an electric field, which will direct the flow of electric current.

Now, consider a photovoltaic cell made from a wafer-thin combination of p-type silicon laid over a layer of n-type silicon. Controlled by the force of the electric field, the electrons travel to the n-type side, and the holes to the p-type side. Sometimes, these electron-hole pairs will simply pair up again recombine with the extra energy emitted as heat.

But if they find themselves near the electric field at the junction of the p- and n-type layers, the electric field will send electrons to the n-type layer, and holes to the p-type layer.

If you create an external connection using electrodes and a wire between the two layers, the electron will then travel back through the wire to the p-type layer, as an electric current that does useful work. The whole thing acts rather like an electric battery, continually recharged by the sunlight. Interactive How we harness the sun Next Reset. Out of all the elements in the periodic table, why use silicon as the main component of a solar cell, and why dope with phosphorus and boron?

Silicon is the most common go-to material for a photovoltaic cell because the maximum wavelength of energy it absorbs is around nanometres, which is close to the peak of the radiation emitted by the Sun. The Sun emits a spectrum of radiation, ranging from around nanometres to 2, nanometres, but by far the majority of it is within the range of to nanometres.

The reason why phosphorus and boron are most often used as the doping agents is a bit more complicated. This amount of energy is equivalent to the difference in electronegativity between the two layers this is called the band gap. Solar energy can help to reduce the cost of electricity, contribute to a resilient electrical grid, create jobs and spur economic growth, generate back-up power for nighttime and outages when paired with storage, and operate at similar efficiency on both small and large scales.

Solar energy systems come in all shapes and sizes. Residential systems are found on rooftops across the United States, and businesses are also opting to install solar panels.

Utilities, too, are building large solar power plants to provide energy to all customers connected to the grid. Learn more about the innovative research the Solar Energy Technologies Office is doing in these areas. In addition to this basic information about solar energy, you can find more solar energy information resources here. Solar Energy Solar radiation is light — also known as electromagnetic radiation — that is emitted by the sun.

Solar Radiation Basics.