About This QuizSolar cells are on everything from pocket calculators to the roofs of buildings. So exactly how do they turn light into energy?
Solar cells use _____ to turn light into energy.
When light hits a solar cell, semiconductors such as silicon absorb the light and eventually transform it into an electrical current.
Silicon works as a semiconductor because:
Added impurities allow electrons to move freely.
In its ordinary, crystalline form, silicon is not a good conductor because none of its electrons can move freely. But if you add tiny amounts of impurities, electrons can move, creating a tiny current.
Its molecular structure is a lot like that of copper.
It forms conductive crystals.
The phosphorous in a solar cell's semiconductor:
provides free electrons
Phosphorous has five electrons in its outer shell. Silicon has four. An atom of phosphorous will bond with silicon, leaving one unpaired electron. Phosphorous-laced silicon is called N-type silicon because it carries a negative charge.
bonds with unpaired electrons
stabilizes the silicon matrix
The boron in a solar cell's semiconductors:
produces extra electrons
Boron has three electrons in its outer shell to silicon's four. So when boron bonds with silicon, there's room for those unpaired electrons coming from N-type silicon's phosphorous. Boron-laced silicon is P-type, or positive, silicon.
When you combine P-type and N-type silicon in a solar cell:
Electrons rush from N to P, leaving the N side empty.
Electrons at the N-P junction eventually form a barrier.
It may seem like all the free electrons from N-type silicon would make a mad dash for the available empty spaces in P-type silicon. In reality, the electrons near the borders of each type mix, gradually creating an effective barrier -- an electrical field.
Once equilibrium is reached between the N and P sides of a cell, the cell acts as:
Once the N and P sides of a cell reach equilibrium, the system acts like a diode. It allows electrons to move from the P side to the N side, but not the other way around.
When light hits a solar cell, electrical current comes from:
electrons traveling from the N side to the P side along a circuit
When sunlight hits a solar cell, it loosen electrons, creating both free electrons and spaces for the electrons to go. Add a path from N to P, and the electrons will follow it, creating an electrical current along the path.
electrons traveling from the N side to the P side across the N-P junction
electrons traveling from the P side to the N side along a circuit
A solar cell can only absorb 15 to 25 percent of the sun's energy because:
Not all wavelengths of light create the right reaction in the cell.
There's too much resistance inside the semiconductor.
both A and B
When a photon of light hits a solar cell, it can free an electron, simultaneously making a place for another electron to go. But if the photon doesn't have enough energy, nothing happens. On top of that, silicon isn't as efficient at moving electrons as a conductor.
A typical photovoltaic array has 36 cells. If one is shaded, power production drops by:
If you put solar cells on your roof, it's important to make sure that the whole array gets full sun. If one cell is shaded, power output is cut in half.
To use solar energy in your home, you need:
An inverter converts direct current (DC), which comes from your solar cells, into alternating current (AC), which the appliances in your house can use. Without an inverter, there's not much you can do with your solar panels.