Ecology And Environment

How Solar Powered Batteries Work

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The nomenclature “solar battery,” was first coined by scientists at Bell Laboratories in the early 1960’s,  referring to a semiconductor device that converts solar radiation—specifically photons of ultraviolet light— into an electrical charge differential. This conversion employs the photoelectric effect, first observed in 1877 by Heinrich Hertz and  later explained, or perhaps more appropriately quantified, by Albert Einstein in 1905 (it was for his explanation of the photo electric effect that Einstein was awarded the Nobel Prize in 1921).

While the semiconductor solar cell employs the photoelectric effect, it does so in a significantly different manner than the photoelectric effect had ever been used before. Essentially, the “photovoltaic” cell (aka solar cell or solar battery) uses layers of silicon with different doping materials to form a p-n semiconductor junction (“p” for positive and “n” for negative). The prefix “photo” designates photons as the energy source, and  the suffix “voltaic” infers  the capability to create and hold an electrical charge, or battery function. Since the Sun is the source of the photons, the term “solar battery” became an obvious euphemism. 

Attempting to provide a chemical analysis of exactly how photovoltaic p-n junctions work might be a little bit to technical for this discussion. It suffices to say, that the electro-chemical properties of the silicon and doping materials support ionization of the respective p-n semiconductor layers. The key component of the device is a barrier region or “junction” formed between the p and n type layers. At a low energy level, the junction exhibits dialectic properties resisting the flow of current(electrons) across it.

When ultraviolet photons strike the doped silicon on one side of the p-n junction they cause electrons of the silicon to gain energy and actually ionize the atoms liberating electrons. These “free” electrons congregate along the p-n junction building a static charge. When the charge exceeds a certain threshold, electrons begin to jump across the junction and build up in the silicon substrate on the other side of it. The departing electrons leave behind "wholes"(missing electrons) representative of a net positive electrical charge. The surplus of electrons accumulating on the other side of the p-n junction represent a net negative charge. By bleeding off this built up charge of electrons—establishing current flow through a circuit path which returns electrons to the other side of the junction— a continuous flow of electrons is facilitated, that is, so long as the sun keeps shining on the solar cell.

The physics and chemistry associated with photovoltaic devices may be a little bit complicated, but solar batteries, at a higher level of contemplation, are pretty simple devices. Each cell produces about a half a volt of electric potential, but when you put a whole lot of them together a substantial electric charge can be generated, enough to power the space station and other satellites, or as more contemporaneously popular, augment residential and commercial electricity applications. 

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