Hobbies And Interests
The Photovoltaic Effect
Solar cells work based on the properties of semiconductor materials. Silicon and other semiconductor materials have very stable atomic bonds with no free electrons. Engineers render these compounds impure by doping them with known amounts of other elements. These impurities create regions of material with extra electrons, called n-type material, and regions with electron holes, called p-type material. When these two materials are put together, a boundary region called a p-n junction forms. The p-n junction keeps charge carriers from moving across to the other material. When an adequate amount of external power is applied to the junction, however, the semiconductor begins to conduct electricity. In a solar cell, photons from light excite the particles in the p-n junction, allowing a current to flow across the cell.
Wavelength and Energy
Not all light has the same amount of energy. The energy of photons is measured in electron volts, or eV, which is the amount of energy that one electron has when moving in an electric field of one volt. To generate a current in a silicon-based solar cell, a photon must have at least 1.12 eV of energy. This energy will excite an electron in the silicon atom, creating more charge carriers in the semiconductor. Photons of a higher energy state than 1.12 eV will excite electrons, but the rest of the energy will heat the solar cell. Likewise, photons with less than 1.12 eV of energy will only heat the cell. The wavelength of light that has the ideal energy is in the infrared spectrum. Longer wavelengths of light do not have sufficient energy to excite the silicon electrons.
Incandescent Light Bulbs Vs. Sunlight
Both the sun and incandescent light bulbs emit photons with enough energy to create current in a solar cell. The temperature of both the sun and incandescent bulbs is directly related to its emitted power at various wavelengths. The sun, at nearly 6,000 kelvins (approximately 10,340 degrees Fahrenheit), emits most of its power in the visible light spectrum. The average light reaching the Earth's atmosphere from the sun has a power of roughly 1,360 watts per square meter. By contrast, most incandescent bulbs emit light on the order of 40 to 100 watts. The filaments of incandescent bulbs radiate at around 3,000 kelvins (approximately 4,940 degrees Fahrenheit) and also emit most of their power in the visible light spectrum. The difference in power means that there are fewer photons emitted by an incandescent bulb than are available in sunlight.
Disadvantages to Using Incandescent Light
In addition to the drastic power differences between sunlight and incandescent light, incandescent lights also consume power to use. Unless the bulb is already being used for something else -- room lighting, for instance -- charging a solar battery with it would be highly inefficient. If you were to use an incandescent bulb to power the same solar cell from which it draws power, the circuit would eventually lose the power it needs to function.