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History of the Solar Cell

In 1839, the French physicist Alexandre-Edmond Becquerel
discovered that light can influence electrical processes. He
determined that metal electrodes immersed in acid produce
more electricity when they are placed in the sunlight. Later,
scientists named this the photoelectric effect, or photovol-
taic effect. This was a very important discovery because it
clearly showed that under certain circumstances, light can
be converted into electrical energy.

In 1883, an American inventor named Charles Fritts built

the first solar cell, but it was very inefficient. A lot of time
passed and a lot of research was done before the first mod-
ern solar cell was patented by another American named
Russell Ohl. In 1958, the U.S. sent the first solar-powered
satellite into space, showing how much photovoltaic tech-
nology had progressed in just over 100 years.

How Does a Solar Cell Work?

A solar cell is a flat device that uses an electronic compo-
nent called a semiconductor to convert photons, or the
massless particles of light energy, into electrical energy. The
semiconductor creates a voltage, or difference in electrical
potential energy, between two surfaces when it is exposed
to light. You can think of it like a battery, which also has a
voltage between to points. This is just a brief explanation
of how a solar cell works. To explain it more thoroughly, we
must first discuss electricity.

The phenomenon of electricity is nothing more than the

movement of negatively charged particles, called electrons,
through a material, called a conductor. Electricity flows eas-
ily through some materials, like metal, and poorly or not at
all through other materials, like plastic. We have discovered
materials, like silicon, which are naturally poor conductors
in pure form, but can be treated to become better conduc-
tors under special conditions. These are semiconductors.

In a solar cell, electrons are excited into motion by

exposure to energy from light. The solar cell is designed to
make the electrons flow in a specific direction, creating a
negative pole on the side where there are more electrons
and a positive pole on the side where there are fewer elec-
trons, or more “empty holes” for electrons. To achieve this
electron flow, pure silicon must be treated to become a bet-
ter conductor. This is done by adding impurities, or other
elements, to the silicon, in a process called doping.

There are two layers of treated silicon in the solar cell.

Phosphorus is added to the first layer, resulting in an abun-
dance of free electrons. Because electrons have a negative
charge, this layer has a negative charge, and is thus called
n-type doped silicon. The other layer is doped with Boron,
resulting in an absence of electrons, or more holes for elec-
trons. This gives the layer an overall positive charge, and is
thus called p-type doped silicon (Figure 1).

The n-type silicon layer is positioned right next to the

p-type layer (Figure 2). All of the free extra electrons in the
n-type layer flow to fill up the holes in the p-type layer, creat-
ing an electric field. Right along the line where the two lay-
ers meet, something interesting happens: electrons are able
to move from the n-type layer into the p-type layer, but not
from the p-type layer into the n-type layer. This area where

the two layers meet is called the p-n junction. You can think
of it like a hill, where electrons can easily roll down the hill
(to the p-type layer) but it is very hard for them to go back
up the hill (to the n-type layer).

When the cell is exposed to light, the energy from

the light excites the electrons in the p-type layer, and they
break free from their holes (Figure 3). With contacts and
wires attached to conduct electrons out of the p-type layer,
through a load (such as a light bulb or motor), and back to
the n-type layer, we now have a complete solar cell circuit.
When an electron moves, the hole it was previously sitting
in becomes empty, and another electron can easily move
into its position (Figure 4). And because electrons can only
travel in one direction through the p-n junction, they must
pass through the wire and load to get back to the n-type
layer. This creates the electric current.

This basic solar cell with one p-type layer, one n-type

layer, and one p-n junction is called a first generation pho-
tovoltaic. A second generation photovoltaic has many layers
and multiple p-n junctions, to absorb more light. There is
also a third generation photovoltaic, which does not use the
traditional p-n junction at all.

This is a simplified description and we encourage you

to find out more about how solar cells work from books, the
internet, and your science teacher.

cover glass
antiglare layer
top contact layer
n-type silicon
p-type silicon
bottom contact layer

load

(motor)

electron flow

p-n junction

n-type layer

p-type layer

n-type layer

p-type layer

n-type layer