How Things Work: Solar Cells
Albert Einstein is best remembered today for his theory of relativity. However, it was his explanation of the photoelectric effect that earned him a Nobel Prize in physics in 1921. The photoelectric effect explained the behavior of light particles in terms of discrete quanta, or amounts of energy. This research greatly accelerated the understanding of the operation of the solar cell, allowing for a number of innovative improvements.
Silicon is the most versatile semiconductor material because operating on a silicon substrate is both easy and well studied. Today, as a result of this ease of use, silicon has become ubiquitous. We see it in our integrated circuits, microprocessors, and also in the solar panels used to harvest the sun’s light energy.
All atoms have a dense nucleus composed of neutrons and protons. This nucleus is surrounded by negatively charged electrons, which are equal in number to the positively charged protons in the nucleus. The atom therefore has no net charge. For most atoms, the outermost orbit of electrons constantly strives to accept or reject electrons so as to contain eight electrons (its most stable state). Silicon occupies a special place in the periodic table of elements, as it has four electrons in the outermost electron orbit. This allows every silicon atom to form bonds with four other silicon atoms by sharing electrons. This sharing of electrons forms a bond known as a covalent bond. By forming four covalent bonds, each silicon atom now has eight electrons.
The simplest solar cell can be devised from just crystalline silicon. When light falls on the surface of the crystalline silicon, the energetic light particles — photons — transfer their energy to the electrons in the covalent bonds of the silicon atoms. Electrons can only leave the bond they are shared in if supplied with sufficient energy. Only photons that are vibrating at a “threshold frequency” or above possess enough energy to knock an electron out of its covalent bond. Any photons at frequencies less than the threshold frequency pass right through the silicon wafer without releasing any electrons. Photons with too much energy release an electron, and the excess energy is dissipated in the wafer as heat.
Releasing electrons from bonds is important because free electrons constitute the flow of electric current. The electrons that receive sufficient energy from incident photons then break free from the covalent bond and are forced to move in one direction, usually toward one electrode.
An escaping electron leaves in its place a “hole,” which simply represents the lack of an electron in a bond. These holes, which are atoms missing an electron, migrate toward the opposite electrode anti-parallel to the direction of motion of the electrons. The movement of electrons generates an electric current, which ultimately is the basis for getting power from solar cells.
Solar cells are made of a number of layers. These layers all serve various purposes. The layers typically include a protective layer, a non-reflective layer, contacts, and doped silicon or other light-reactive material.
Efficiency is defined as the ratio of output electric energy to the incident light energy. A silicon wafer alone produces very little electric energy, usually on the order of less than 5 percent. This efficiency can be improved by a number of methods. The best option is to introduce certain impurities into the silicon crystal. These impurities can have their electrons freed relatively easily, and are usually trace amounts of boron or phosphorous. Reducing the amount of incident light reflected by the surface of silicon (crystalline silicon is highly reflective) also improves the efficiency of the cell. The first solar cell (made of selenium instead of silicon) had an efficiency of around 1 percent. As more was learned about the chemistry involved, better materials began to be used. With time, the doped silicon cells reached efficiencies of around 25 percent, as stated on www.PhysOrg.com. While this might seem like a low efficiency when compared with other energy sources, this is a considerably high efficiency among the solar cell technologies. Recently, other elements and combinations of elements have been used to manufacture cells. These materials include complex compounds such as gallium arsenide and cadmium telluride.
Today, the highest efficiency achieved is 43 percent, as reported by www.PhysOrg.com. These cells consist of a number of different light-capturing layers, allowing the cells to capture a broader spectrum of incident light. This would mean that photons having lower frequencies may also be used to generate electron-hole pairs
According to Nature, the Earth receives as much energy in one hour as is required by humans for an entire year. Efforts by nations around the world to harness this “free energy” are picking up. The advantages of this source of energy are plenty; for example, there is no fuel spent and there are no noxious exhaust gases to damage the environment. With our fast-dwindling fossil fuel reserves, it is becoming increasingly clear how important a renewable source of energy is for the survival of energy-dependent mankind. Today, of the many renewable energy sources available, solar energy is the most researched, owing largely to solar panels. At the rate research is progressing, it seems comfortably positioned at the top of the stack of renewable energy sources.