How is a solar cell powered? Sometimes turning the problem upside-down can be fun and more revealing. So it might be more informative to ask another question: How aren't solar cells powered? Or: Why can the very best solar cell still "only" convert
44.7% of the sun's power into useable power? The following demonstration illustrates these answers and more. For an introduction to solar cells, visit
this post.
In the demonstration below, photon energy is represented by wavelength and color. Electrons are circles with different sizes and colors, with both representing their energy. Small electrons and long wavelength photons are both
red and
low energy. Large electrons and short wavelength photons are both
purple and
high energy.
Click the button! When you fire a photon of a particular energy into our device, it is usually absorbed. This usually results in electrons taking on the same amount of energy which the absorbed photon used to carry. The most favorable course for these excited electrons is to quickly relax, much the way a baseball thrown straight up tends to quickly fall back down. This means that very soon after being excited by the photon, they release energy to their surroundings by some means (bang into another electron or a nucleus, emit a photon, etc.). Unless they can be excited enough to reach a whole new band of states where they can very favorably spread out, they will fully relax to their original state immediately. If they can, however, reach that higher energy band of possible states, then they will be able to dwell upon it for a relatively long period of time. In the demonstration, try to figure out which color/energy photon(s) don't have enough energy to excite the electron to this new band. The fact that this energy is immediately lost limits solar cell performance!
SIDEBAR: Thinking about electron states can be strange. Remember that the electrons tend to spread out and favor doing as many different things as possible in as many different ways as they can, just like gas molecules, just like liquid molecules, really just like everything. You know the trend. Think of how the smell of hot home cooking spreads into neighboring space. Or, think of a few drops of food coloring placed in a thimble of water and stirred. Now think of that same amount of food coloring placed in a drinking glass full of water. The dye relaxes by spreading out, which is accompanied by a more transparent appearance. The electrons that don't make it up to the conducting band of states don't favor getting trapped in a little thimble where they would bump into their neighbors that are like them. So instead they quickly fall back down to the lower band of states (tall glass of water) they had originally occupied.
Now, if a photon has a lot more energy than the gap between bands of electron states, then that energy will quickly be lost to the surroundings as well. In the demonstration, try to find which color/energy photons have so much more energy than the electron band-gap that a lot of it is immediately lost. This also reduces energy conversion efficiency.
Once an electron is excited to this new, higher energy band of states, it becomes free to randomly wander around in space. Because they are no longer confined in space, we call them conducting electrons. But our solar cell is built in a clever way: like a trap. If the conducting electrons happen to randomly wander across the threshold of the trap, then it is favorable for them to start falling down an energetic hill (toward the left). In the demonstration, try to find where this hill is located and to where it leads the conducting electrons as they fall. Much like water at the source of a river forces flow at the river's mouth, this action of falling downhill also causes electron drift everywhere else around the circuit. For this to occur, they must lose some energy throughout their entire journey--much the way rivers only flow down hills. This is another reason we can't convert all the light's energy into usable power. Since this entire chain of events was initiated by incoming photons, energy is input to the circuit, charging our battery. Note that the electrons lose the largest amount of energy/size/color to the battery, so the solar cell is indeed doing its job! Can you think of ways to increase the amount of energy transferred to the battery? Then try to figure out how to make them work and power the world!
[download
this software if the demo doesn't load]
Tips and Things to Notice:
-This is a demonstration--nothing is to scale
-The black box is a 2D representation of the solar cell's innards--we have electrons moving around in 2 dimensions (instead of the real 3D situation)
-Lower energy photons must travel further into the device to be absorbed
-Conducting electrons are always wandering around at random, this is diffusion, and dominates in the right part
-Conducting electrons fall down hill, or drift down a potential energy gradient, in the left part
-Diffusion actually occurs in the left part too, but is dominated by drift
-Electrons in the circuit lose a small amount of energy, this is
resistance
-Electrons re-enter the device on the other side with about as much energy as they had before photon absorption
-If the circuit were not completed there, all the electrons would stop drifting
-This is exactly what occurs when the battery is fully charged (the potential energy gradient forcing electrons to drift disappears)
-An advanced topic concerns the
holes which electrons leave behind in the localized valence band, which are incredibly useful entities, both conceptually and mathematically
