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The Crystalline Silicon Solar Cell
PV cells can be made of many different semiconductors. But
we'll use crystalline silicon as an example, for three reasons.
First, crystalline silicon was the material used in the earliest
successful PV devices. Second, and more important, it's still the
most widely used PV material. And third, although other PV
materials and designs exploit the photoelectric effect in slightly
different ways, if you know how the effect works in crystalline
silicon, then you'll have a basic understanding of how it works in
all PV devices.
An Atomic Description of Silicon
All matter is composed of atoms, which are made up of
positively charged protons, negatively charged electrons, and
neutral neutrons. Protons and neutrons, which are about the
same size, are in the close-packed, central nucleus of the
atom. The much lighter electrons orbit the nucleus. Although
atoms are built of oppositely charged particles, their overall
charge is neutral because they contain an equal number of
positive protons and negative electrons whose charges offset
one another. |
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As depicted in this
simplified diagram, silicon has 14 electrons. The four
electrons that orbit the nucleus in the outermost
"valence" energy level are given to, accepted from, or
shared with other atoms. |
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| Electrons orbit at different distances from
the nucleus, depending on their energy level. For example, an
electron with less energy orbits close to the nucleus, whereas
one with greater energy orbits farther away. The higher energy
electrons farthest from the nucleus are the ones that interact
with neighboring atoms to form solid structures. |
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In the basic unit of a
crystalline silicon solid, a silicon atom shares each of
its four valence electrons with each of four neighboring
atoms. |
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A silicon atom has 14 electrons, but their
natural orbital arrangement allows only the outermost four
electrons to be given to, accepted from, or shared with other
atoms. These outermost four electrons, called
valence electrons, play a
very important role in the photoelectric effect.
Large numbers of silicon atoms bond with each
other by means of their valence electrons to form a crystal. In
a crystalline solid, each silicon atom normally shares one of
its four valence electrons in a covalent bond with each of four
neighboring silicon atoms. The solid thus consists of basic
units of five silicon atoms: the original atom plus the four
other atoms with which it shares valence electrons.
The solid silicon crystal is thus made up of a
regular series of units of five silicon atoms. This regular,
fixed arrangement of silicon atoms is known as the
crystal lattice. -
Reference U.S. Department of Energy
Bandgap Energies of Semiconductors and Light
When light shines on crystalline silicon,
electrons within the crystal lattice may be freed. But not all
photons — as packets of light energy are called — are created
equal. Only photons with a certain level of energy can free
electrons in the semiconductor material from their atomic bonds
to produce an electric current.
This level of energy, known as the "bandgap
energy," is the amount of energy required to dislodge an
electron from its covalent bond and allow it to become part of
an electrical circuit. To free an electron, the energy of a
photon must be at least as great as the bandgap energy. However,
photons with more energy than the bandgap energy will expend
that extra amount as heat when freeing electrons. So, it's
important for a PV cell to be "tuned"—through slight
modifications to the silicon's molecular structure—to optimize
the photon energy. A key to obtaining an efficient PV cell is to
convert as much sunlight as possible into electricity.
Crystalline silicon has a bandgap energy of 1.1
electron-volts (eV). (An electron-volt is equal to the energy
gained by an electron when it passes through a potential of 1
volt in a vacuum.) The bandgap energies of other effective PV
semiconductors range from 1.0 to 1.6 eV. In this range,
electrons can be freed without creating extra heat.
The photon energy of light varies according to
the different wavelengths of the light. The entire spectrum of
sunlight, from infrared to ultraviolet, covers a range of about
0.5 eV to about 2.9 eV. For example, red light has an energy of
about 1.7 eV, and blue light has an energy of about 2.7 eV. Most
PV cells cannot use about 55% of the energy of sunlight, because
this energy is either below the bandgap of the material or
carries excess energy.
Note:
Different PV materials have different energy band gaps.
Photons with energy equal to the band gap energy are absorbed
to create free electrons. Photons with less energy than the
band gap energy pass through the material. -
Reference U.S. Department of Energy
Built-In Electric Field
Light shining on crystalline silicon may free electrons
within the crystal lattice. But for these electrons to do
useful work—as in providing electricity to light a light
bulb—they must be separated and directed into an
electrical circuit. To separate the electrical charges,
the silicon solar cell must have a built-in electric
field.
To create this electric field within a photovoltaic
(PV) cell, two separate semiconductors are sandwiched
together. P-type (or "positive") semiconductors have an
abundance of positively charged holes, whereas n-type (or
"negative") semiconductors have an abundance of negatively
charged electrons.
When n- and p-type silicon come into contact, excess
electrons move from the n-type side to the p-type side.
The result is a buildup of positive charge along the
n-type side of the interface and a buildup of negative
charge along the p-type side.
Because of the flow of electrons and holes, the two
semiconductors behave like a battery, creating an electric
field at the surface where they meet—what we call the p/n
junction. The electrical field causes the electrons to
move from the semiconductor toward the negative surface,
making them available for the electrical circuit. At the
same time, the holes move in the opposite direction,
toward the positive surface, where they await incoming
electrons. - Reference U.S. Department
of Energy |
Although both materials are electrically
neutral, n-type silicon has excess electrons and p-type
silicon has excess holes. Sandwiching these together
creates a p/n junction at their interface, thereby
creating an electric field. |
Absorption and Conduction
In a PV cell, photons are absorbed in the p-layer. And
it's very important to "tune" this layer to the properties
of incoming photons to absorb as many as possible, and
thus, to free up as many electrons as possible. Another
challenge is to keep the electrons from meeting up with
holes and recombining with them before they can escape
from the PV cell. To do all this, we design the material
to free the electrons as close to the junction as
possible, so that the electric field can help send the
free electrons through the conduction layer (the n-layer)
and out into the electrical circuit. By optimizing all
these characteristics, we improve the PV cell's conversion
efficiency, which is how much of the light energy is
converted into electrical energy by the cell. -
Reference U.S. Department of Energy |
To
make an efficient solar cell, we try to maximize
absorption, minimize reflection and recombination |
Electrical Contacts
| Electrical contacts are
essential to a photovoltaic (PV) cell because they
bridge the connection between the semiconductor
material and the external electrical load, such as a
light bulb. The back contact of a cell — on the side
away from the incoming sunlight — is relatively
simple. It usually consists of a layer of aluminum or
molybdenum metal. But the front contact — on the side
facing the sun — is more complicated. When sunlight
shines on the PV cell, a current of electrons flows
all over its surface. If we attach contacts only at
the edges of the cell, it will not work well because
of the great electrical resistance of the top
semiconductor layer. Only a small number of electrons
would make it to the contact.
To collect the most current, we must place contacts
across the entire surface of a PV cell. This is
normally done with a "grid" of metal strips or
"fingers." However, placing a large grid, which is
opaque, on the top of the cell shades active parts of
the cell from the sun. The cell's conversion
efficiency is thus significantly reduced. To improve
the conversion efficiency, we must minimize these
shading effects. |
Grid contacts on the top surface of a
typical cell are designed to have many thin,
conductive fingers spreading to every part of the
cell's surface. |
Another challenge in cell design is to minimize the
electrical resistance losses when applying grid contacts
to the solar cell material. These losses are related to
the solar cell material's property of opposing the flow of
an electric current, which results in heating the
material.
Therefore, in designing grid contacts, we must balance
shading effects against electrical resistance losses. The
usual approach is to design grids with many thin,
conductive fingers spreading to every part of the cell's
surface. The fingers of the grid must be thick enough to
conduct well (with low resistance), but thin enough not to
block much of the incoming light. This kind of grid keeps
resistance losses low while shading only about 3% to 5% of
the cell's surface.
Grids can be expensive to make and can affect the
cell's reliability. To make top-surface grids, we can
either deposit metallic vapors on a cell through a mask or
paint them on via a screen-printing method.
Photolithography is the preferred method for the highest
quality, but has the greatest cost. This process involves
transferring an image via photography, as in modern
printing.
An alternative to metallic grid contacts is a
transparent conducting oxide (TCO) layer such as tin oxide
(SnO2). The advantage of TCOs is that they are nearly
invisible to incoming light, and they form a good bridge
from the semiconductor material to the external electrical
circuit.
TCOs are very useful in manufacturing processes
involving a glass superstrate, which is the covering on
the sun-facing side of a PV module. Some thin-film PV
cells, such as amorphous silicon and cadmium telluride,
use superstrates. In this process, the TCO is generally
deposited as a thin film on the glass superstrate before
the semiconducting layers are deposited. The
semiconducting layers are then followed by a metallic
contact that will actually be the bottom of the cell. As
you can see, the cell is actually constructed "upside
down," from the top to the bottom.
But the construction technique isn't the only thing
that determines whether a metallic grid or TCO is best for
a certain cell design. The sheet resistance of the
semiconductor is also an important consideration. In
crystalline silicon, for example, the semiconductor
carries electrons well enough to reach a finger of the
metallic grid. Because the metal conducts electricity
better than a TCO, shading losses are less than losses
associated with using a TCO. Amorphous silicon, on the
other hand, conducts very poorly in the horizontal
direction. Therefore, it benefits from having a TCO over
its entire surface. - Reference U.S.
Department of Energy
Antireflective Coating
Silicon is a shiny gray material and can act as a
mirror, reflecting more than 30% of the light that shines
on it. To improve the conversion efficiency of a solar
cell, we want to minimize the amount of light reflected so
that the semiconductor material can capture as much light
as possible to use in freeing electrons.
Two techniques are commonly used to reduce reflection.
The first technique is to coat the top surface with a thin
layer of silicon monoxide (SiO). A single layer reduces
surface reflection to about 10%, and a second layer can
lower the reflection to less than 4%.
A second technique is to texture the top surface.
Chemical etching creates a pattern of cones and pyramids,
which capture light rays that might otherwise to deflected
away from the cell. Reflected light is redirected down
into the cell, where it has another chance to be absorbed.
- Reference U.S. Department of Energy
• The Photo Electric Effect • Light and the PV Cell • Crystalline Solar Cell • PV Performance • |
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