Solar Cell Efficiency
Solar energy is a wonderful form of renewable energy, but is it completely perfect? The answer to this question is, unfortunately, no. Look at the graph above. The highest efficiency silicon solar cells reach is about 40%. Let’s discuss.
As I mentioned in my previous article, the Science behind Solar Cells (click HERE to read it), light is basically electromagnetic energy or radiation. Basically, electromagnetic radiation is not monochromatic. It is made up of a range of different wavelengths, and therefore energy levels. There are multiple forms of electromagnetic energy, gamma rays, ultraviolet (UV) rays, visible light and so forth with their own specific wavelengths. The specific amount of energy that is required to knock an electron loose is called the band gap energy of a material. Since there is a wide range of energy from the light that hits solar cells, some wavelengths of light don’t have enough energy to alter the electron-hole pair (also discussed in the previous article). The wavelengths of light simply pass through the solar cell as if it were transparent. And some wavelengths of light have too much energy, losing the extra energy when the light hits the solar cell.
Having too much energy or too little energy both account for the loss of about 70 percent of the radiation energy in solar cells. Why shouldn’t we just choose a material with a really low band gap so that we can use for of the photons? Unfortunately band gap also determines the strength of the 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 (Keep in mind that power is voltage x current). The optimal band gap, balancing these two effects, is around 1.4 eV for a cell made from a single material. The graph on the left does a great job of showing the efficiency and voltage of different semiconducting material. But again, note the highest efficiency possible for the cells: 35%.
There are other losses as well. The electrons must flow from one side of the cell to the other through an external circuit. The bottom of the solar cell can be covered with a metal (for good conduction), but the top cannot be covered because photons cannot get through the opaque conductor and we lose all of our current. If all the contacts were put only at the sides of the solar cell, the electrons would have to travel an extremely long distance to reach the contact. Recall from the last article about solar cells that silicon is a semiconductor. It is a put conductor of electricity and has a high internal resistance leading to high losses. To minimize these losses, solar cells are usually covered by a metallic grid that shortens the distance that electrons have to travel while covering only a small part of the cell surface. Despite this the photons are blocked by the grid, which can't be too small or else its own resistance will be too high.
Overall, solar cells are effective, but not as effective as they can be. However, with the progression of science and more and more brilliant minds attacking our biggest environmental problems, there is still hope that this form of renewable energy will be able to perform at its maximum capacity.
