One of the basic building blocks of organic life is the energy derived from the sun.  In fact, until very recently, we believed that life on Earth was not possible at all in places where extreme conditions did not allow the energy of the sun to reach.  Largely, the energy from sunlight, in all its different wavelengths, drives more life processes than even the multitude we already know about.  Even under the heaviest cloud cover, it is indisputably the oldest source of energy on Earth.

In the 18th century, a new phenomenon was noticed, whereby sunlight photons free electrons from common silicon. The process was named “photovoltaic”, now commonly referred to as PV, combining the Greek words photo (light) and voltaic (electricity).  The photovoltaic or “solar” cells we now think of were developed at Bell Labs in 1950, initially for uses in space exploration.  Photovoltaic (PV) cells are devices that convert sunlight to electricity, bypassing thermodynamic cycles and mechanical generators.

Generally, PV cells create electrical energy when radiant energy passes through glass and reflective layers to two layers of silicon. The layers create a sandwich, where the silicon atoms are arranged in a cubic pattern.  The top, or “n”, layer has silicon atoms with all of their electrons and some more to spare.  The bottom, or “p”, layer has silicon atoms missing some their electrons, creating positively charged pairs or “holes”.  The two layers of the sandwich are separated by a permanent electric field called a junction acting as a diode, an electronic device that restricts current flow chiefly to one direction.  This diode is created by the diffusion of electrons from the top-n side to the bottom p-side because of an imbalance of charge between the two layers.  Electrons are able to move from the top n-side to the bottom p-side, and “holes” from the bottom p-side vice versa.  Metal wires give these electrons a place to go, and they are carried out of the top n-layer along the metal wires back to the bottom p-layer.  This flow of electrons creates a DC electric circuit.  As long as there is photon energy to knock electrons loose, there will be an electric current.  Obviously, the more photons (the more radiant energy from the sun), the stronger the electric current will be.

To use this electricity, we deliver that current to a load, such as a light bulb, always making sure we return the current to the bottom p-layer.  Solar arrays work by delivering all of the current from a bunch of solar cells (tying them together) to the same load in a house or building.  This load is usually an inverter, the same piece of equipment usually used to capture the energy coming from a wind-powered generator.  Utility-scale solar arrays operate no differently, just tying together a larger number of solar cells to a bunch of inverters.

A second, way to convert sunlight to electrical energy utilizes the oldest property of the sun’s energy known to man, its heat.  There are currently numerous technologies and types of solar arrays which reflect and then concentrate the sun’s radiant heat energy onto a medium that absorbs that highly focused heat (sometimes focused to create thousands of degrees Fahrenheit) and then uses that heat to drive some type of generator.  This technology has advanced rapidly in the last few years to the point where, in some cases, there is promise that the medium can hold the heat long enough to generate electricity through the night.

The third major technology on the market uses elements other than silicon to create solar cells that are thin enough to be a flexible, light film.  Though the photovoltaic principles are essentially the same, this “thin film” technology allows for much lighter, but generally less efficient, solar cells and is the third major solar energy technology on the market.  The possible applications for this product, beyond the ubiquitous roof-top or open field solar array, are constantly advancing both in the laboratory and in production.  The efficiencies of different thin-film technologies are steadily gaining on those of standard silicon PV cells.

The decision to use one technology versus another comes down to several factors:  efficiency, expense, speed of production and supply chain, site and space requirements, maintenance required, and the amount of electricity desired.  

The picture below depicts in general terms how we generate electricity from the sun using a photovoltaic module.  It should be mentioned that there are other 'collection' devices that can be used to harness the sun's energy.