Every metal has its own band gap that
describes how strong the electrons are bonded to the atoms. For semiconductors,
such as silicon, the band gap refers to the energy difference between the
valence band and the conduction band. When a negative electron is excited, it
leaves behind a void which is called a positive hole. The presence of a missing
covalent bond allows the bonded electrons of neighbouring atoms to jump into
the hole, leaving another hole behind. Because of this, holes also move through
the lattice. When photons are absorbed in the semiconductor, it can be said
they create mobile electron-hole pairs.
The History Highlight of Solar Sells
(Photovoltaic Cells)
1839 – The photovoltaic effect was
discovered by Alexandre-Edmond Becquerel, who was a French physicist. This was
“the beginning” of the solar cell technology. Becquerel's experiment was done
by illuminating two electrodes with different types of light. The electrodes
were coated by light sensitive materials, AgCl or AgBr, and carried out in a
black box surrounded by an acid solution. The electricity increased when the
light intensity increased.
1873 – The photo conductivity of an
element, selenium, was discovered by Willoughby Smith, who was an English
electrical engineer.
1876 – Selenium produces electrical
current when it is exposed to sun light. William Grylls Adams and Richard Evans
Day proved that it is possible to convert solar energy into electricity
directly, without any moving parts or heat. The solar cell was very
inefficient, and it couldn't be used to run any electrical equipment.
1883 – A description of the first solar
cells made from selenium wafer were made by Charles Fritts.
1894 – Charles Fritts constructed what
was probably the first true solar cell. He coated a semiconductor material
(selenium) with an extremely thin layer of gold. The efficiency were only about
1%, so it couldn't be used as energy supply, but were later used as light
sensors.
1904 – A German physicist, Wilhelm Ludwig
Franz Hallwachs, discovered that a combination of copper and cuprous oxide was
photosensitive.
1905 – Albert Einstein published his
paper about the photoelectric effect. There he claimed that light consists of
“packets” or quanta of energy, which we now call photons. This energy varies
only with its frequency (electromagnetic waves, or the “color of the light”).
This theory was very simple, but revolutionary, and it explained very well the
absorption of the photons regarding to the frequency of the light.
1914 – Goldman and Brodsky noted that it
existed a barrier layer in photovoltaic devices.
1916 – Robert Andrews Millikan provided
experimental proof of the photoelectric effect. He was an American experimental
physicist who later won the Nobel Prize for his work on the photoelectric
effect and for his measurement of the charge of the electron.
1918 – Jan Czochralski, a Polish chemist,
developed a way to grow single-crystal silicon. This increased the efficiency
of the silicon-based cells considerably.
1923 – Albert Einstein received the Nobel
Prize for his theories explaining the photoelectric effect, which he published
18 years earlier.
1930s – Walter Schottky, Neville Mott and
some others developed a theory of metal-semiconductor barrier layers.
1932 – Audobert and Stora discover the
photovoltaic effect in cadmium sulfide (CdS).
1950s – Bell Labs produce solar cells for
space activities.
1951 – A grown p-n junction enabled the
production of a single-crystal cell of germanium.
1953 – Dr. Dan Trivich of Wayne State
University makes the first theoretical calculations of the efficiencies of
various materials of different band-gap widths based on the spectrum of the sun
1954 – Three researchers, Gerald Pearson,
Daryl Chapin and Calvin Fuller, at Bell Laboratories discovered a silicon solar
cell, which was the first material to directly convert enough sunlight into
electricity to run electrical devices. The efficiency of the silicon solar
cell, which Bell Labs produced, were 4%, which later increased to 11%. The
cells were made by hand and cost $1000 per watt.
1954 – A cadmium sulphide p-n junction
was produced with an efficiency of 6%
1958 – Hoffman Electronics achieved 9%
efficient PV cells.
1958 – The first PV-powered satellite,
Vanguard I, was launched. The solar panel had an area of 100cm² and delivered an
effect of approximately 0.1W. The satellite power system operated for 8 years,
and is the world's oldest satellite still in orbit (2007).
1958 – Ted Mandelkorn of U.S. Signal
Corps Laboratories fabricates n-on-p (negative layer on positive layer) silicon
photovoltaic cells,
1959 – Hoffman Electronics achieved 10%
efficient commercially available PV cells and demonstrated the use of a grid
contact to significantly reduce series resistance.
1959 – Explorer-6 was launched with a PV
array of 9600 cells, each only 1 cm x 2 cm.
1960 – Hoffman Electronics achieved 14%
efficient PV cells.
1962 – The Telstar communications
satellite, launched by Bell Labs, is initial powered (14W) by solar cells.
1963 – A Japanese electronics
manufacturer, Sharp Corporation, produces a viable photovoltaic module of
silicon solar cells.
1970 – First highly effective GaAs
heterostructure solar cells are created by Zhores Alferov (a Russian physicist)
and his team in the USSR.
1972 – The Institute of Energy Conversion
is established at the University of Delaware to perform research and
development on thin-film photovoltaic and solar thermal systems, becoming the
world’s first laboratory dedicated to photovoltaic research and development.
1976 – David Carlson and Christopher
Wronski of RCA Laboratories produced the first amorphous silicon photovoltaic
cells, which could be less expensive to manufacture than crystalline silicon
devices. The efficiency was of 1.1%.
1980 – At the University of Delaware, the
first thin-film solar cell exceeds 10% efficiency. It's made of copper sulfide
(Cu2S) and cadmium sulfide (CdS).
1981 – Paul MacCready builds the first
solar-powered aircraft, the Solar Challenger, and flies it from France to
England across the English Channel. The aircraft had over 16,000 solar cells
mounted on its wings, which produced a power of 3kW.
1982 – Hans Tholstrup, an Australian,
drives the first solar-powered car, the Quiet Achiever, 4,000km between Sydney
and Perth in 20 days. That was 10 days faster than the first gasoline-powered
car to do so. The maximum speed was 72 km/h, and the average speed was 24 km/h.
1984 – The IEEE Morris N. Liebmann
Memorial Award was presented to Drs. David E. Carlson and Christopher R.
Wronski at the 17th Photovoltaic Specialists Conference, "for crucial
contributions to the use of amorphous silicon in low-cost, high-performance photovoltaic
solar cells."
1985 – The University of South Wales
breaks the 20% efficiency barrier for silicon solar cells under one sun
conditions.
1989 – Reflective solar concentrators are
first used with solar cells.
1991 – Efficient Photo electrochemical
cells (PEC) are developed. Each cell consists of a semiconducting photo anode
and a metal cathode immersed in an electrolyte. The Dye-sensitized solar cell
(DSC), also called Grätzel cells, is invented. It was a new class of low-class
DSC.
1992 – University of South Florida
develops a 15.9% efficient thin-film photovoltaic cell made of cadmium
telluride, breaking the 15% barrier for the first time for this technology.
1994 – The National Renewable Energy
Laboratory develops a solar cell, made from gallium indium phosphide and
gallium arsenide that becomes the first one to exceed 30% conversion
efficiency.
1996 – Renewable Energy Corporation (REC),
a Norwegian solar energy company established.
1996 – EPFL, the Swiss Federal Institute
of Technology in Lausanne, achieves 11% efficiency with the DSCs.
1999 – Spectrolab, Inc. and the National
Renewable Energy Laboratory develop a photovoltaic solar cell that converts
32.3 percent of the sunlight that hits it into electricity. The high conversion
efficiency was achieved by combining three layers of photovoltaic materials
into a single solar cell. The cell performed most efficiently when it received
sunlight concentrated to 50 times normal. To use such cells in practical
applications, the cell is mounted in a device that uses lenses or mirrors to
concentrate sunlight onto the cell. Such “concentrator” systems are mounted on
tracking systems that keep them pointed toward the sun.
1999 – The National Renewable Energy
Laboratory achieves a new efficiency record for thin-film photovoltaic solar
cells. The new measurement is of 18.8 percent efficiency.
2000 – Two new thin-film solar modules,
developed by BP Solarex, break previous performance records. The company’s
0.5-square-meter module achieves 10.8 % conversion efficiency—the highest in
the world for thin-film modules of its kind. And its 0.9-square-meter module
achieved 10.6% conversion efficiency and a power output of 91.5 watts — the
highest power output for any thin-film module in the world.
2001 – TerraSun LLC developes a method of
using holographic films to concentrate sunlight onto a solar cell
2003 – REC Solar started production.
2007 – The University of Delaware achieves
a 42.8% efficiency solar cell technology.
The photovoltaic effect
Photovoltaic is a term in solar
technology that describe a solar cells ability to convert light from the sun
directly into electric power. When photons in the sun light collide with the
silicon solar cell, one of three things can happen:
- The photon can be reflected at the surface of the silicon
- The photon can be absorbed by the silicon
- The photon can pass right through the silicon
As the photons hits the atoms in the
silicon, the energy is absorbed by the electrons and excited into a higher
state of energy. When these free electrons flow through the material,
electricity arises.
Notice this is the case for
semiconductors in general. Silicon does not have three valence and conduction
bands.
The holes move to the negative layer of
the cell, and the negative excited electrons move to the positive layer. This
will be described by the p-n junction technique later. When placing a circuit
between the two layers, a path of continuous flow of electrons is established.
Due to the concept conservation of
energy, the excited electrons cannot have greater nor less energy than that of
the incident rays from the sun. Photons with less energy than the energy gap
will go straight through the semiconductor and no electrons will be excited.
Photons with greater energy than the energy gap will be absorbed, but the
difference in energy between the photons and the energy gap is converted into
heat by lattice vibration.
HOW SOLAR CELLS WORK
A solar cell, sometimes called a
photovoltaic cell, is a device that converts light energy into electrical
energy. A single solar cell creates a very small amount of energy (about .6
volts DC) so they are usually grouped together in an integrated electrical
panel called a solar panel. Sunlight is a somewhat diffuse form of energy and
only a portion of the light captured by a solar cell is converted into
electricity. The current generation of
solar cells converts only 12 to 15 per cent of the sun's light into
electricity. However in recent years
there have been significant advances in their design. Some new cells on the market now are around
20% efficient and some laboratory prototypes are reaching as high as 30 per cent.
Given this it is likely that their efficiency will continue to improve
over time.
Theory Behind Solar Cells
A solar cell is based upon the
"photovoltaic effect" discovered in 1839 by Edmund Becquerel, a
French physicist. In his experiments he
found that certain materials would produce small amounts of electric current
when exposed to sunlight. Sunlight is
made up of packets of energy called photons.
When the photons strike the semi-conductor layer (usually silicon) of a
solar cell a portion of the photons are absorbed by the material rather than
bouncing off of it or going through the material. When a photon is absorbed the energy of that
photon is transferred to an electron in an atom of the cell causing the
electron to escape from its normal position.
This creates, in essence, a hole in the atom. This hole will attract another electron from
a nearby atom now creating yet another whole, which in turn is again filled by
an electron from another atom. This hole
filling process is repeated a few zillion times and voila, an electric current
is formed.
Structure of a Solar Cell
A typical solar cell is a multi-layered
material. Let's review what the layers are:
Cover Glass - this is a clear glass layer
that provides outer protection from the elements.
Transparent Adhesive - this holds the
glass to the rest of the solar cell.
Anti-reflective Coating - this substance
is designed to prevent the light that strikes the cell from bouncing off so
that the maximum energy is absorbed into the cell.
Front Contact - transmits the electric
current.
N-Type Semiconductor Layer - This is a
thin layer of silicon which has been doped with phosphorous.
P-Type Semiconductor Layer - This is a
thin layer of silicon which has been doped with boron.
Back Contact - transmits the electric
current.
Types of Solar Cells
Because of the extensive research being
done on solar energy there are now many types of solar cells on the
market. All of them follow the
principles described when it comes to generating an electric current. However, many different approaches are now
used to create the structures in order to reduce the costs of production. These
approaches involve a tradeoff between lower manufacturing costs versus lower
efficiency in converting sunlight to electricity. The three most common approaches are summarized
below:
Monocrystalline Silicon - This type of
solar cell uses a single layer of silicon for the semi-conductor. In order to
produce this type of silicon it must be extremely pure which means it is the
most expensive type of solar cell to produce.
Polycrystalline Silicon - To make
polycrystalline silicon cells liquid silicon is poured into blocks that are
subsequently sawed into plates. This type of approach produces some degree of
degradation of the silicon crystals which makes them less efficient. However, this type of approach is easier and
cheaper to manufacture.
Amorphous Thin Film Silicon - This type
of solar cell uses layers of semiconductor that are only a few micrometers
thick (about 1/100th the thickness of a human hair). This lower the material cost but makes it
even less efficient than the other types of silicon. However, because it is so
thin this type of cell has the advantage that it can be placed on a wide
variety of flexible materials in order to make things like solar shingles or roof
tiles.
Another way of looking at solar cells is
in terms of the types of materials they are made with. While silicon is the most commonly used
crystal a number of other materials can be used as well. These include the following:
gallium arsenide
copper indium diselenide
cadmium telluride
Different types of substances perform
better under certain light conditions.
Some cells perform better outdoors (i.e., optimized for sunlight), while
others perform better indoors (optimized for fluorescent light).
