The future of solar power
covered in solar cells. While this is a hypothetical scenario, the difference in energy that can potentially be harvested is still substantial.
Solar cells, also known as PV (Photo Voltaic) cells, were first introduced in the 1950s. Photons (light) are converted into electricity via the use of semiconducting materials which exhibit the photovoltaic (similar to photoelectric) effect. A solar panel is composed of much smaller solar cells, which are made out of silicon (being the second most abundant element on earth) in a crystalline structure. The cells are made out of two types of silicon, a p- type silicon, which contains an excess of ‘holes’, and an n -type silicon, which contains an excess of electrons in its outer shell. They are known as ‘p’ and ‘n’ type silicon because they hold a negative and positive charge, respectively. This is done by ‘seeding’ or ‘doping’ each silicon with another element, namely phosphorus into the top layer of silicon (adding extra electrons) and boron into the bottom layer of silicon (resulting in fewer electrons, ergo a positive charge). Because one of the types of silicon has an excess of negatively charged electrons, and the other has a deficit, they have opposite charges, creating an electrical field. The border between these two different types of silicon is known as the p-n junction. By understanding this, it is possible to see how through the photoelectric effect the solar cells can create a circuit and generate electricity.
When a photon (a particle of light) strikes a material that exhibits the photoelectric effect, it gives an electron enough energy to free itself from the material (which, thereafter, is commonly known as a photoelectron), as shown in figure 1. This can also be shown where:
Energy (of the photon) – Energy taken to release the electron = Kinetic Energy of the electron
The above equation can be changed into something that is more standard by employing Planck’s constant, a number that links the amount of energy a photon carries to its respective electromagnetic wavelength. This const ant is denoted by the letter ‘ h ’ and is equal to 6.62607015×10 − 34 Js (Joule- seconds). To calculate the photon energy, one must multiply the frequency of the light in hertz by Planck’s constant. Because the frequency ( f ) of light (or other repeated, continuous actions) is equal to its velocity (speed) divided by its wavelength (distance of one ‘repetition’), the equation for photon energy results in h • c/λ , where c is the speed of light, and λ is the wavelength. The energy taken to release the electron is known as the ‘work function’, or theminimumenergy (from the photon) required to release an electron from a specific surface (different elements have different work functions) and is denoted by the Greek symbol ‘ ф ’. Therefore, the Kinetic Energy ( Ek max ) of the photoelectron is equal to h • c/λ – ф , or hf – ф . When an electron is freed from the outer shell in the silicon, it is drawn to the n-side of the silicon, pushed by the electric field created by opposite charges. This electron is collected in the top layer of the solar cell, (which is made out of a conductive material) and flows through an external circuit, where it can power appliances or be stored as electrical energy in batteries. The electron (now without charge)
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