Operation of Solar Cell

How much sunlight energy does Solar cell absorb? Unfortunately, the most that simple solar cell could absorb is around 25 percent, and more likely is 15 percent or less. Why so little?

Visible light is only part of the electromagnetic spectrum. Electromagnetic radiation is not monochromatic. It is made up of a range of different wavelengths, and therefore energy levels. (See How Special Relativity Works for a good discussion of the electromagnetic spectrum.)

Light can be separated into different wavelengths, and we can see them in the form of a rainbow. Since the light that hits solar cell has photons of a wide range of energies, it turns out that some of them won't have enough energy to form an electron-hole pair. They'll simply pass through the solar 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 1.1 eV for crystalline silicon), is required to knock an electron loose. 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 (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 account for the loss of around 70 percent of the radiation energy incident on solar cell.

Why can't we choose a material with a really low band gap, so we can use more of the photons? Unfortunately, band gap also determines the strength (voltage) of 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 1.4 eV for a solar cell made from a single material.

We have other losses as well. Electrons have to flow from one side of the solar 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 contacts only at the sides of cell, then the electrons have to travel an extremely long distance (for an electron) 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, solar cell is 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.

There are a few more steps left before we can really use a solar cell. Silicon happens to be a very shiny material, which means that it is very reflective. Photons that are reflected can't be used by the cell. For that reason, an antireflective coating is applied to the top of the cell to reduce reflection losses to less than 5 percent.

The final step is the glass cover plate that protects the solar cell from the elements. PV modules are made by connecting several cells (usually 36) in series and parallel to achieve useful levels of voltage and current, and putting them in a sturdy frame complete with a glass cover and positive and negative terminals on the back.