2.1: Absorption of light
The photovoltaic effect can be realized in various systems made from semiconductors, electrochemical substances,
organic materials, dyes and others. In the following pages, you learn how the photovoltaic effect occurs in semiconductors
that have a p-n junction.*
The first thing that happens in the PV effect is that an electron absorbs light and, with this, acquires the energy of the light.
It has taken physicists a long time to understand absorption, because it happens in a way that is unfamiliar to us. We usually expect
that the longer the electron is exposed to light, the more energy it can acquire. This is not the case. Electrons can only absorb a
certain amount (quantum) of energy, regardless of how long they are exposed to light. But this quantum depends on the color (wavelength)
of light, as you will learn in the lesson on photogeneration.
Such a quantum is carried by a photon, and we can consider light as a flux of photons. Blue light consists of photons with a
high energy, red light with a low energy.
On the The PV principle page it was stated that there must be at least a two-level system, where electrons can make a transition
from the lower (relaxed) to the upper (excited) energy state when they absorb light. Such a two-level system can absorb sunlight only
with a certain photon energy Eg (i.e., at a certain wavelength), see the figure below.
In a semiconductor, each atom
brings an energy level in, and as there are about 1022 atoms per cm3, these levels form two (nearly) continuous bands.
The lower band is called valence band, the upper conduction band. Because the bands are nearly continuous, a semiconductor can absorb
sunlight at all photon energies larger than Eg (this is 1.12 eV for Si, see the next page).
At lower energies than Eg, the light cannot be absorbed, because the electron does not find a state into which it could
be excited. This is the reason why silicon solar cells cannot produce much electricity from low-temperature heat alone; they also need visible
Figure 1: A two-level system (left) can absorb light only at a certain energy (or wavelength). With an increasing
number of energy levels (toward the right), a broader energy (or wavelength) range of light can be absorbed. The color of the arrows corresponds
to the color of absorbed light (black is infrared) if the energy gap Eg is the one of silicon (1.12 eV, see the next page).
* A p-n junction is not necessary for the PV effect to happen in a semiconductor. An example are metal–insulator–semiconductor
(MIS) cells. Peter Würfel will explain this in his interview. But nearly all the solar cells fabricated today have a p-n junction.