Solar Cells Essay, Research Paper

Solar cells today are mostly made of silicon, one

of the most common elements on Earth. The

crystalline silicon solar cell was one of the first

types to be developed and it is still the most

common type in use today. They do not pollute

the atmosphere and they leave behind no harmful

waste products. Photovoltaic cells work

effectively even in cloudy weather and unlike solar

heaters, are more efficient at low temperatures.

They do their job silently and there are no moving

parts to wear out. It is no wonder that one marvels

on how such a device would function. To

understand how a solar cell works, it is necessary

to go back to some basic atomic concepts. In the

simplest model of the atom, electrons orbit a

central nucleus, composed of protons and

neutrons. each electron carries one negative

charge and each proton one positive charge.

Neutrons carry no charge. Every atom has the

same number of electrons as there are protons, so,

on the whole, it is electrically neutral. The

electrons have discrete kinetic energy levels, which

increase with the orbital radius. When atoms bond

together to form a solid, the electron energy levels

merge into bands. In electrical conductors, these

bands are continuous but in insulators and

semiconductors there is an "energy gap", in which

no electron orbits can exist, between the inner

valence band and outer conduction band [Book

1]. Valence electrons help to bind together the

atoms in a solid by orbiting 2 adjacent nucleii,

while conduction electrons, being less closely

bound to the nucleii, are free to move in response

to an applied voltage or electric field. The fewer

conduction electrons there are, the higher the

electrical resistivity of the material. In

semiconductors, the materials from which solar

sells are made, the energy gap Eg is fairly small.

Because of this, electrons in the valence band can

easily be made to jump to the conduction band by

the injection of energy, either in the form of heat or

light [Book 4]. This explains why the high

resistivity of semiconductors decreases as the

temperature is raised or the material illuminated.

The excitation of valence electrons to the

conduction band is best accomplished when the

semiconductor is in the crystalline state, i.e. when

the atoms are arranged in a precise geometrical

formation or "lattice". At room temperature and

low illumination, pure or so-called "intrinsic"

semiconductors have a high resistivity. But the

resistivity can be greatly reduced by "doping", i.e.

introducing a very small amount of impurity, of the

order of one in a million atoms. There are 2 kinds

of dopant. Those which have more valence

electrons that the semiconductor itself are called

"donors" and those which have fewer are termed

"acceptors" [Book 2]. In a silicon crystal, each

atom has 4 valence electrons, which are shared

with a neighbouring atom to form a stable

tetrahedral structure. Phosphorus, which has 5

valence electrons, is a donor and causes extra

electrons to appear in the conduction band. Silicon

so doped is called "n-type" [Book 5]. On the

other hand, boron, with a valence of 3, is an

acceptor, leaving so-called "holes" in the lattice,

which act like positive charges and render the

silicon "p-type"[Book 5]. The drawings in Figure

1.2 are 2-dimensional representations of n- and

p-type silicon crystals, in which the atomic nucleii

in the lattice are indicated by circles and the

bonding valence electrons are shown as lines

between the atoms. Holes, like electrons, will

remove under the influence of an applied voltage

but, as the mechanism of their movement is

valence electron substitution from atom to atom,

they are less mobile than the free conduction

electrons [Book 2]. In a n-on-p crystalline silicon

solar cell, a shadow junction is formed by diffusing

phosphorus into a boron-based base. At the

junction, conduction electrons from donor atoms

in the n-region diffuse into the p-region and

combine with holes in acceptor atoms, producing

a layer of negatively-charged impurity atoms. The

opposite action also takes place, holes from

acceptor atoms in the p-region crossing into the

n-region, combining with electrons and producing

positively-charged impurity atoms [Book 4]. The

net result of these movements is the disappearance

of conduction electrons and holes from the vicinity

of the junction and the establishment there of a

reverse electric field, which is positive on the

n-side and negative on the p-side. This reverse

field plays a vital part in the functioning of the

device. The area in which it is set up is called the

"depletion area" or "barrier layer"[Book 4]. When

light falls on the front surface, photons with energy

in excess of the energy gap (1.1 eV in crystalline

silicon) interact with valence electrons and lift them

to the conduction band. This movement leaves

behind holes, so each photon is said to generate

an "electron-hole pair" [Book 2]. In the crystalline

silicon, electron-hole generation takes place

throughout the thickness of the cell, in

concentrations depending on the irradiance and

the spectral composition of the light. Photon

energy is inversely proportional to wavelength.

The highly energetic photons in the ultra-violet and

blue part of the spectrum are absorbed very near

the surface, while the less energetic longer wave

photons in the red and infrared are absorbed

deeper in the crystal and further from the junction

[Book 4]. Most are absorbed within a thickness

of 100 æm. The electrons and holes diffuse

through the crystal in an effort to produce an even

distribution. Some recombine after a lifetime of the

order of one millisecond, neutralizing their charges

and giving up energy in the form of heat. Others

reach the junction before their lifetime has expired.

There they are separated by the reverse field, the

electrons being accelerated towards the negative

contact and the holes towards the positive [Book

5]. If the cell is connected to a load, electrons will

be pushed from the negative contact through the

load to the positive contact, where they will

recombine with holes. This constitutes an electric

current. In crystalline silicon cells, the current

generated by radiation of a particular spectral

composition is directly proportional to the

irradiance [Book 2]. Some types of solar cell,

however, do not exhibit this linear relationship.

The silicon solar cell has many advantages such as

high reliability, photovoltaic power plants can be

put up easily and quickly, photovoltaic power

plants are quite modular and can respond to

sudden changes in solar input which occur when

clouds pass by. However there are still some

major problems with them. They still cost too

much for mass use and are relatively inefficient

with conversion efficiencies of 20% to 30%. With

time, both of these problems will be solved

through mass production and new technological

advances in semiconductors. Bibliography 1)

Green, Martin Solar Cells, Operating Principles,

Technology and System Applications. New

Jersey, Prentice-Hall, 1989. pg 104-106 2)

Hovel, Howard Solar Cells, Semiconductors and

Semimetals. New York, Academic Press, 1990.

pg 334-339 3) Newham, Michael ,"Photovoltaics,

The Sunrise Industry", Solar Energy, October 1,

1989, pp 253-256 4) Pulfrey, Donald

Photovoltaic Power Generation. Oxford, Van

Norstrand Co., 1988. pg 56-61 5) Treble,

Fredrick Generating Electricity from the Sun. New

York, Pergamon Press, 1991. pg 192-195

31d