Photovoltaic cell

Photovoltaic cell

The invention: Drawing their energy directly from the Sun, the

first photovoltaic cells powered instruments on early space vehicles

and held out hope for future uses of solar energy.

The people behind the invention:

Daryl M. Chapin (1906-1995), an American physicist

Calvin S. Fuller (1902-1994), an American chemist

Gerald L. Pearson (1905- ), an American physicist

Unlimited Energy Source

All the energy that the world has at its disposal ultimately comes

from the Sun. Some of this solar energy was trapped millions of years

ago in the form of vegetable and animal matter that became the coal,

oil, and natural gas that the world relies upon for energy. Some of this

fuel is used directly to heat homes and to power factories and gasoline

vehicles. Much of this fossil fuel, however, is burned to produce

the electricity on which modern society depends.

The amount of energy available from the Sun is difficult to imagine,

but some comparisons may be helpful. During each forty-hour

period, the Sun provides the earth with as much energy as the

earth’s total reserves of coal, oil, and natural gas. It has been estimated

that the amount of energy provided by the sun’s radiation

matches the earth’s reserves of nuclear fuel every forty days. The

annual solar radiation that falls on about twelve hundred square

miles of land in Arizona matched the world’s estimated total annual

energy requirement for 1960. Scientists have been searching for

many decades for inexpensive, efficient means of converting this

vast supply of solar radiation directly into electricity.

The Bell Solar Cell

Throughout its history, Bell Systems has needed to be able to

transmit, modulate, and amplify electrical signals. Until the 1930’s,

these tasks were accomplished by using insulators and metallic conductors. At that time, semiconductors, which have electrical properties

that are between those of insulators and those of conductors,

were developed. One of the most important semiconductor materials

is silicon, which is one of the most common elements on the

earth. Unfortunately, silicon is usually found in the form of compounds

such as sand or quartz, and it must be refined and purified

before it can be used in electrical circuits. This process required

much initial research, and very pure silicon was not available until

the early 1950’s.

Electric conduction in silicon is the result of the movement of

negative charges (electrons) or positive charges (holes). One way of

accomplishing this is by deliberately adding to the silicon phosphorus

or arsenic atoms, which have five outer electrons. This addition

creates a type of semiconductor that has excess negative charges (an

n-type semiconductor). Adding boron atoms, which have three

outer electrons, creates a semiconductor that has excess positive

charges (a p-type semiconductor). Calvin Fuller made an important

study of the formation of p-n junctions, which are the points at

which p-type and n-type semiconductors meet, by using the process

of diffusing impurity atoms—that is, adding atoms of materials that

would increase the level of positive or negative charges, as described

above. Fuller’s work stimulated interested in using the process

of impurity diffusion to create cells that would turn solar energy

into electricity. Fuller and Gerald Pearson made the first largearea

p-n junction by using the diffusion process. Daryl Chapin,

Fuller, and Pearson made a similar p-n junction very close to the

surface of a silicon crystal, which was then exposed to sunlight.

The cell was constructed by first making an ingot of arsenicdoped

silicon that was then cut into very thin slices. Then a very

thin layer of p-type silicon was formed over the surface of the n-type

wafer, providing a p-n junction close to the surface of the cell. Once

the cell cooled, the p-type layer was removed from the back of the

cell and lead wires were attached to the two surfaces. When light

was absorbed at the p-n junction, electron-hole pairs were produced,

and the electric field that was present at the junction forced

the electrons to the n side and the holes to the p side.

The recombination of the electrons and holes takes place after the

electrons have traveled through the external wires, where they do useful work. Chapin, Fuller, and Pearson announced in 1954 that

the resulting photovoltaic cell was the most efficient (6 percent)

means then available for converting sunlight into electricity.

The first experimental use of the silicon solar battery was in amplifiers

for electrical telephone signals in rural areas. An array of 432

silicon cells, capable of supplying 9 watts of power in bright sunlight,

was used to charge a nickel-cadmium storage battery. This, in

turn, powered the amplifier for the telephone signal. The electrical

energy derived from sunlight during the day was sufficient to keep

the storage battery charged for continuous operation. The system

was successfully tested for six months of continuous use in Americus,

Georgia, in 1956. Although it was a technical success, the silicon solar

cell was not ready to compete economically with conventional

means of producing electrical power.

Consequences

One of the immediate applications of the solar cell was to supply

electrical energy for Telstar satellites. These cells are used extensively

on all satellites to generate power. The success of the U.S. satellite program prompted serious suggestions in 1965 for the use of

an orbiting power satellite. A large satellite could be placed into a

synchronous orbit of the earth. It would collect sunlight, convert it

to microwave radiation, and beam the energy to an Earth-based receiving

station. Many technical problems must be solved, however,

before this dream can become a reality.

Solar cells are used in small-scale applications such as power

sources for calculators. Large-scale applications are still not economically

competitive with more traditional means of generating

electric power. The development of the ThirdWorld countries, however,

may provide the incentive to search for less-expensive solar

cells that can be used, for example, to provide energy in remote villages.

As the standards of living in such areas improve, the need for

electric power will grow. Solar cells may be able to provide the necessary

energy while safeguarding the environment for future generations.

Photoelectric cell

The invention: The first devices to make practical use of the photoelectric

effect, photoelectric cells were of decisive importance in

the electron theory of metals.

The people behind the invention:

Julius Elster (1854-1920), a German experimental physicist

Hans Friedrich Geitel (1855-1923), a German physicist

Wilhelm Hallwachs (1859-1922), a German physicist

Early Photoelectric Cells

The photoelectric effect was known to science in the early

nineteenth century when the French physicist Alexandre-Edmond

Becquerel wrote of it in connection with his work on glass-enclosed

primary batteries. He discovered that the voltage of his batteries increased

with intensified illumination and that green light produced

the highest voltage. Since Becquerel researched batteries exclusively,

however, the liquid-type photocell was not discovered until

1929, when the Wein and Arcturus cells were introduced commercially.

These cells were miniature voltaic cells arranged so that light

falling on one side of the front plate generated a considerable

amount of electrical energy. The cells had short lives, unfortunately;

when subjected to cold, the electrolyte froze, and when subjected to

heat, the gas generated would expand and explode the cells.

What came to be known as the photoelectric cell, a device connecting

light and electricity, had its beginnings in the 1880’s. At

that time, scientists noticed that a negatively charged metal plate

lost its charge much more quickly in the light (especially ultraviolet

light) than in the dark. Several years later, researchers demonstrated

that this phenomenon was not an “ionization” effect because

of the air’s increased conductivity, since the phenomenon

took place in a vacuum but did not take place if the plate were positively

charged. Instead, the phenomenon had to be attributed to

the light that excited the electrons of the metal and caused them to

fly off: Aneutral plate even acquired a slight positive charge under the influence of strong light. Study of this effect not only contributed

evidence to an electronic theory of matter—and, as a result of

some brilliant mathematical work by the physicist Albert Einstein,

later increased knowledge of the nature of radiant energy—but

also further linked the studies of light and electricity. It even explained

certain chemical phenomena, such as the process of photography.

It is important to note that all the experimental work on

photoelectricity accomplished prior to the work of Julius Elster

and Hans Friedrich Geitel was carried out before the existence of

the electron was known.

Explaining Photoelectric Emission

After the English physicist Sir Joseph John Thomson’s discovery

of the electron in 1897, investigators soon realized that the photoelectric

effect was caused by the emission of electrons under the influence

of radiation. The fundamental theory of photoelectric emission

was put forward by Einstein in 1905 on the basis of the German

physicist Max Planck’s quantum theory (1900). Thus, it was not surprising

that light was found to have an electronic effect. Since it was

known that the longer radio waves could shake electrons into resonant

oscillations and the shorter X rays could detach electrons from

the atoms of gases, the intermediate waves of visual light would

have been expected to have some effect upon electrons—such as detaching

them from metal plates and therefore setting up a difference

of potential. The photoelectric cell, developed by Elster and Geitel

in 1904, was a practical device that made use of this effect.

In 1888,Wilhelm Hallwachs observed that an electrically charged

zinc electrode loses its charge when exposed to ultraviolet radiation

if the charge is negative, but is able to retain a positive charge under

the same conditions. The following year, Elster and Geitel discovered

a photoelectric effect caused by visible light; however, they

used the alkali metals potassium and sodium for their experiments

instead of zinc.

The Elster-Geitel photocell (a vacuum emission cell, as opposed to

a gas-filled cell) consisted of an evacuated glass bulb containing two

electrodes. The cathode consisted of a thin film of a rare, chemically

active metal (such as potassium) that lost its electrons fairly readily; the anode was simply a wire sealed in to complete the circuit. This anode

was maintained at a positive potential in order to collect the negative

charges released by light from the cathode. The Elster-Geitel

photocell resembled two other types of vacuum tubes in existence at

the time: the cathode-ray tube, in which the cathode emitted electrons

under the influence of a high potential, and the thermionic

valve (a valve that permits the passage of current in one direction only), in which it emitted electrons under the influence of heat. Like

both of these vacuum tubes, the photoelectric cell could be classified

as an “electronic” device.

The new cell, then, emitted electrons when stimulated by light, and

at a rate proportional to the intensity of the light. Hence, a current

could be obtained from the cell. Yet Elster and Geitel found that their

photoelectric currents fell off gradually; they therefore spoke of “fatigue”

(instability). It was discovered later that most of this change was

not a direct effect of a photoelectric current’s passage; it was not even

an indirect effect but was caused by oxidation of the cathode by the air.

Since all modern cathodes are enclosed in sealed vessels, that source of

change has been completely abolished. Nevertheless, the changes that

persist in modern cathodes often are indirect effects of light that can be

produced independently of any photoelectric current.

Impact

The Elster-Geitel photocell was, for some twenty years, used in

all emission cells adapted for the visible spectrum, and throughout

the twentieth century, the photoelectric cell has had a wide variety

of applications in numerous fields. For example, if products leaving

a factory on a conveyor belt were passed between a light and a cell,

they could be counted as they interrupted the beam. Persons entering

a building could be counted also, and if invisible ultraviolet rays

were used, those persons could be detected without their knowledge.

Simple relay circuits could be arranged that would automatically

switch on street lamps when it grew dark. The sensitivity of

the cell with an amplifying circuit enabled it to “see” objects too

faint for the human eye, such as minor stars or certain lines in the

spectra of elements excited by a flame or discharge. The fact that the

current depended on the intensity of the light made it possible to

construct photoelectric meters that could judge the strength of illumination

without risking human error—for example, to determine

the right exposure for a photograph.

A further use for the cell was to make talking films possible. The

early “talkies” had depended on gramophone records, but it was very

difficult to keep the records in time with the film. Now, the waves of

speech and music could be recorded in a “sound track” by turning the sound first into current through a microphone and then into light with

a neon tube or magnetic shutter; next, the variations in the intensity of

this light on the side of the film were photographed. By reversing the

process and running the film between a light and a photoelectric cell,

the visual signals could be converted back to sound.

Fuel cell

The invention: An electrochemical cell that directly converts energy
from reactions between oxidants and fuels, such as liquid
hydrogen, into electrical energy.
The people behind the invention:
Francis Thomas Bacon (1904-1992), an English engineer
Sir William Robert Grove (1811-1896), an English inventor
Georges Leclanché (1839-1882), a French engineer
Alessandro Volta (1745-1827), an Italian physicist
The Earth’s Resources
Because of the earth’s rapidly increasing population and the
dwindling of fossil fuels (natural gas, coal, and petroleum), there is
a need to design and develop new ways to obtain energy and to encourage
its intelligent use. The burning of fossil fuels to create energy
causes a slow buildup of carbon dioxide in the atmosphere,
creating pollution that poses many problems for all forms of life on
this planet. Chemical and electrical studies can be combined to create
electrochemical processes that yield clean energy.
Because of their very high rate of efficiency and their nonpolluting
nature, fuel cells may provide the solution to the problem of
finding sufficient energy sources for humans. The simple reaction of
hydrogen and oxygen to form water in such a cell can provide an
enormous amount of clean (nonpolluting) energy. Moreover, hydrogen
and oxygen are readily available.
Studies by Alessandro Volta, Georges Leclanché, and William
Grove preceded the work of Bacon in the development of the fuel
cell. Bacon became interested in the idea of a hydrogen-oxygen fuel
cell in about 1932. His original intent was to develop a fuel cell that
could be used in commercial applications.
The Fuel Cell Emerges
In 1800, the Italian physicist Alessandro Volta experimented
with solutions of chemicals and metals that were able to conduct electricity. He found that two pieces of metal and such a solution
could be arranged in such a way as to produce an electric current.
His creation was the first electrochemical battery, a device that produced
energy from a chemical reaction. Studies in this area were
continued by various people, and in the late nineteenth century,
Georges Leclanché invented the dry cell battery, which is now commonly
used.
The work of William Grove followed that of Leclanché. His first
significant contribution was the Grove cell, an improved form of the
cells described above, which became very popular. Grove experimented
with various forms of batteries and eventually invented the
“gas battery,” which was actually the earliest fuel cell. It is worth
noting that his design incorporated separate test tubes of hydrogen
and oxygen, which he placed over strips of platinum.
After studying the design of Grove’s fuel cell, Bacon decided
that, for practical purposes, the use of platinum and other precious
metals should be avoided. By 1939, he had constructed a cell in
which nickel replaced the platinum used.
The theory behind the fuel cell can be described in the following
way. If a mixture of hydrogen and oxygen is ignited, energy is released
in the form of a violent explosion. In a fuel cell, however, the
reaction takes place in a controlled manner. Electrons lost by the hydrogen
gas flow out of the fuel cell and return to be taken up by the
oxygen in the cell. The electron flow provides electricity to any device
that is connected to the fuel cell, and the water that the fuel cell
produces can be purified and used for drinking.
Bacon’s studies were interrupted byWorldWar II. After the war
was over, however, Bacon continued his work. Sir Eric Keightley
Rideal of Cambridge University in England supported Bacon’s
studies; later, others followed suit. In January, 1954, Bacon wrote an
article entitled “Research into the Properties of the Hydrogen/ Oxygen
Fuel Cell” for a British journal. He was surprised at the speed
with which news of the article spread throughout the scientific
world, particularly in the United States.
After a series of setbacks, Bacon demonstrated a forty-cell unit
that had increased power. This advance showed that the fuel cell
was not merely an interesting toy; it had the capacity to do useful
work. At this point, the General Electric Company (GE), an American corporation, sent a representative to England to offer employment
in the United States to senior members of Bacon’s staff. Three scientists
accepted the offer.
A high point in Bacon’s career was the announcement that the
American Pratt and Whitney Aircraft company had obtained an order
to build fuel cells for the Apollo project, which ultimately put
two men on the Moon in 1969. Toward the end of his career in 1978,
Bacon hoped that commercial applications for his fuel cells would
be found.Impact
Because they are lighter and more efficient than batteries, fuel
cells have proved to be useful in the space program. Beginning with
the Gemini 5 spacecraft, alkaline fuel cells (in which a water solution
of potassium hydroxide, a basic, or alkaline, chemical, is placed)
have been used for more than ten thousand hours in space. The fuel
cells used aboard the space shuttle deliver the same amount of power
as batteries weighing ten times as much. On a typical seven-day
mission, the shuttle’s fuel cells consume 680 kilograms (1,500 pounds)
of hydrogen and generate 719 liters (190 gallons) of water that can
be used for drinking.
Major technical and economic problems must be overcome in order
to design fuel cells for practical applications, but some important
advancements have been made.Afew test vehicles that use fuel cells as a source of power have been constructed. Fuel cells using
hydrogen as a fuel and oxygen to burn the fuel have been used in a
van built by General Motors Corporation. Thirty-two fuel cells are
installed below the floorboards, and tanks of liquid oxygen are carried
in the back of the van. A power plant built in New York City
contains stacks of hydrogen-oxygen fuel cells, which can be put on
line quickly in response to power needs. The Sanyo Electric Company
has developed an electric car that is partially powered by a
fuel cell.
These tremendous technical advances are the result of the singleminded
dedication of Francis Thomas Bacon, who struggled all of
his life with an experiment he was convinced would be successful.