Solar Cells:
The Photoelectric Effect
The following document will discuss the photoelectric effect, or the ability of
matter to emit electrons when light is shone on it. Because of this naturally
occurring process, solar cells can transform solar into electrical energy. These
devices can have very complex structures when changing layers, adding layers,
doping different materials, and altering other characteristics to adapt to a specific
application. These applications can range from small calculators to big satellites
orbiting the earth. Since they use the same principle to function, we will describe
a simple silicon p-n junction to reduce complications. Therefore, we will use this
practical application to fully describe the conversion of energy.
The replacement of fossil fuels is an issue that increases in importance
every year. Fossil fuels damage the environment, and their production cannot
keep up with our energy needs. For this reason, scientists have dedicated time
and research to discover ways to produce energy other than using fossil fuels,
such as: wind farms, nuclear power, and hydraulic generators. Although a good
start to alternative energy, none of these sources can produce efficient energy
density per cost in comparison to petroleum. Currently scientists from all parts of
the world believe that solar cells (photovoltaic cells) can be a promising
alternative energy source due to its abundance of fuel (the sun), its cheap
fabrication, and its environmentally sensitive approach.
This document is directed to prospective engineers looking to gain a
working knowledge of the physical concepts governing photovoltaic applications.
To fully take advantage of this information you need a basic knowledge in the
physics related to magnetism, atomic theory, and wave behavior.
How solar cells work
A solar cell is a multilayered material connected to a circuit by two contacts. It
consists of an oxide antireflection coating, a semiconductor emitter, and a
semiconductor base, as we can see in Figure 1. These do not have mechanical
motion but rather move charges in the material. They follow three basic steps:
1. Absorption of light: Electrons and holes1 are created due to light hitting the
surface.
1
Holes are impurities in an atom. This occurs when an electron leaves its site to go to another
atom.
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Solar Cells: The Photoelectric Effect
2. Separation of charge carriers: The electrons and holes will go in opposite
directions. One to the bottom, and the other to the top.
3. Extraction of carriers to an external circuit: Loops connected to the top and
bottom contacts will generate a voltage drop.
Figure 1: Solar cell structure; light incident from the top (Sun).
Absorption of light
The first step in the photoelectric process is the incidence of light in a
material. When the packets of energy that constitute light, or photons,
penetrate the coating, they are trapped, and continue to move through the
layers. Inside, they collide with electrons, and if they have sufficient kinetic
energy will excite electrons knocking them out of position. This creates
electron-hole pairs (charge carriers) in the emitter-base layer, consisting of
semiconductor materials. As a semiconductor, this layer can act as an
insulator, or a conductor depending on the conditions.
This kinetic energy is unique to every material, and can be measured
through the work function. Taking that into account, in order to absorb
sunlight we need to absorb waves from 300 to 780 nm in wavelength
since they carry the majority of energy emitted by the sun. Furthermore,
semiconductors are partially doped to create more electrons (N-type
doping) or holes (P-type doping), like we see in Figure 2.
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Solar Cells: The Photoelectric Effect
Figure 2: N-type doping using antimony (left). P-type doping using boron (right).
Separation of charge carriers
After the creation of an electrical charge in the form of electron-hole pairs,
a relative movement of charges begins. Electrons begin to diffuse thereby
creating a diffusion current. From the N-doped emitter new free electrons
create a negative bias that moves in response to the holes located in the
P-doped base. As a result it gives rise to a current inside the material. This
is the second step in the photoelectric effect. Inside the material, the
difference in charge will generate an electric field moving electrons to the
bottom of the base, and holes to the top. This imbalance in the net electric
field will continue as long as there is a moving charge in the material.
With time there will be a recombination of electrons. This can happen
with three mechanisms: auger, radiative, and defect recombination. The
most common in solar cells is the auger recombination. It consists of
electrons looking to become stable after the material has been cycled,
which fill the physical, and energetic holes necessary for carrier separation.
This reduces the ability of the material to conduct a charge, and its utility
as a solar cell.
Extraction of carriers
Finally, a voltage drop is created. By connecting both contacts the current
flows from bottom to the top of the cell through an external circuit. This
produces a net current, and a constant supply of electrical energy that is
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Solar Cells: The Photoelectric Effect
proportional to the level of light absorbed. This energy is harvested, and
stored. The photoelectric effect keeps the current flowing in the same
direction by repeating these steps.
Conclusion
The photoelectric effect follows three steps, and makes solar powered
devices possible. Solar cells trap light, shinning it onto the semiconductor
layer. Free electrons are then emitted by the photon electron collisions
leaving holes behind. Since there is no charge neutrality in the material, a
charge migration starts to balance the material. Additionally, this
generates an electric field guiding the diffusion current to the bottom of the
cell. Finally, the electrons reach the conductor layers, and move through
the circuit producing a voltage drop with a net current. This is the energy
stored or harvested from the process. From there, the cycle will continue
as long as sufficient light or solar energy is supplied.
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Solar Cells: The Photoelectric Effect
Sources and Related Links
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http://www.physics.org/article-questions.asp?id=51
http://www.pveducation.org/pvcdrom/solar-cell-operation/solar-cellstructure (Figure 1)
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/dope.html (Figure 2)
http://www.pveducation.org/pvcdrom/pn-junction/types-of-recombination
http://www.britannica.com/technology/solar-cell
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Solar Cells: The Photoelectric Effect