Clean, Green Energy: The Search for a Free Lunch

You’ve heard it all before, we need to find a new, sustainable, clean source of energy, and fast. People are looking to wind, solar, nuclear, and biofuels. But what do these things mean, really?

Today, I will discuss solar.

The sun is the main source of energy for this entire planet. We get much of our energy from coal and oil which is made of millions-of-years-old plant and animal matter.  Animals get their energy from plants, which get their energy from the sun.  The wind blowing your hair in your face and turning the wind turbines?  That’s from the difference in temperature in different locations (due to the sun) and the rotation of the earth.  Hydroelectric power like that made at the Hoover Dam?  That’s from rainwater filling up a high altitude river source.  What causes rain?  The evaporation of water by heat from the sun, of course.  As you can see, there are few sources of energy (nuclear and geothermal being the primary exceptions) which are not directly related to energy recently emitted from the sun. The problem with all of these forms of energy is that there is a “middle man” between the sun’s energy and usable electricity.  Solar cells, which have been around since the mid-1950’s, attempt to dispose of the middle man allowing us to directly harness the power of the sun.

A solar cell, also called a photovoltaic cell, can be simply described as a cell which absorbs a photon, the fundamental unit of light, and emits and electron, the movement of which is called electric current or electricity.  The simplest solar cells are based on a phenomenon discovered by Heinrich Hertz and explained by Albert Einstein called the photoelectric effect.  The photoelectric effect is the name for the ability of light to hit a metal and cause an electron to be released.  You can think of this in terms of a billiards table.  The cue ball is the photon of light, and the rack is the piece of metal, with each colored ball representing an electron. When the cue ball hits the rack, one of the colored balls will be knocked out of the group, like an electron being knocked off of the metal.  The analogy is imperfect, but you get the idea!

So if the photoelectric effect causes electrons to be released when light hits a metal, shouldn’t making a solar cell be easy?  Well, yes and no.  Creating a tiny amount of electric current is fairly simple, yes, but there’s a fly in our proverbial ointment.

To make a solar cell, we use semiconductors.  Semiconductors are materials which can act like metals or non-metals.  The most commonly used of these is silicon.  For a semiconductor to produce current, impurities, called dopants, need to be present.  Depending on what type of impurity you add, the semiconductor will either conduct negative charges, electrons, or positive charges, holes.  The first is called an n-type semiconductor and the second is called p-type.  We can make use of this property and sandwich an n-type and a p-type semiconductor together, applying an electric field which allows electrons to enter the n-type conductor but not go back to the p-type conductor.  When light hits this system it can produce an electron-hole pair, called an exciton, close enough to the interface between the n-type and p-type conductors that it will be effected by the electric field.  If it is, the electron will be pulled toward the n-type conductor, and the hole will move to the p-type conductor. Once the electron is in the n-type semiconductor, it will be whisked away to the electrode, and current will have been created.

This still seems good, where’s that darned fly?  Well, the problem is that the sun puts out lots of photons with lots of different energies, and only some of those photons have energies that are high enough to knock an electron out of its atom.  Think again about the billiards table.  Imagine all the balls, except the cue ball, are slightly magnetic.  If you hit the cue ball lightly, the balls in the rack will hold on to each other.  You need to whack the cue ball hard enough to overcome the magnetic attraction of the colored balls in order to break.  The same is true for light.  If it doesn’t have enough energy, the electrons will stay in their atoms.  A property of semiconductors called their bandgap is what determines how much energy a photon needs to knock off an electron.  Higher bandgaps means more higher energy photons are needed, and this in turn means that fewer photons from the sun are usable.  Unfortunately, lowering the bandgap also requires reducing the applied electric field strength inside the cell.  If we lower that field, the charges will recombine and not drift enough to reach the electrodes and we won’t get any current.  So we need a high bandgap to make current, and a low bandgap to free the electrons.  How confusing!  In the end, the solution is a compromise.  A bandgap of about 1-2 eV, which is the energy required to move an electron over a electric potential difference of  1-2 volts, is sufficiently low and high to take care of both of our requirements respectively, but it is still imperfect.  Energy is still lost resulting in low efficiencies.

As if that weren’t problem enough, silicon cells are expensive, fragile, and heavy.  Solar energy is a clear contender in the race to be our next great energy supply, but it needs a lot of work.  In a future post, I will explore possible alternatives to silicon cells, and what we can do to make solar energy cheaper and more flexible.