The photovoltaic effect
is closely related to the photoelectric effect, with a key difference. In the photoelectric effect, electrons are emitted into space. But, in the photovoltaic effect, electrons enter what we call the conduction band of the material
. Since the photovoltaic effect doesn’t require breaking an electron completely free of a material, it requires less energy, and thus can occur more often.
What do we mean by the “conduction band”? Well, there are two main bands of energies that electrons can have. One range of energies, where an electron is bound around a single atom or molecule, is called the valence band. The other, where the electron is free to move from one atom or molecule to another, is called the conduction band. We’ll discuss this a bit more later, but the general idea is that when an electron is in the conduction band, we can conduct electricity, or have electrons move about more freely.
There’s a second requirement for the photovoltaic effect, though. Once an electron is in the conduction band, for the photovoltaic effect to occur, the electron has to move under the influence of an electric potential (or voltage). We’ll see later that this potential can be produced in semiconductors by putting two different, specific materials in close contact.
Electrons that move away from their parent material create a charge difference between where they are,
and where they were.
This charge difference generates a voltage, much like that across the terminals of a battery, and the moving electrons make up an electrical current. Current and voltage give us electrical power, or energy over time. The photovoltaic effect is the more practical way we convert solar energy into electrical energy. It’s what solar cells rely on.
The first photovoltaic cell
was made at Bell labs in 1954, and it could only convert about 4% of sunlight into electricity. Today, we’re doing much better, with commercial conversion efficiencies
closer to about 20%
, and research efficiencies pushing much higher, as shown in the NREL graph below. They’ve also gotten a lot cheaper, too, with cost decreasing by about 10% every year
. There are many growing advantages to using solar cells.