MaximumPC 2008 09

I

n one second, the nuclear fusion process

taking place inside the sun produces

enough energy to satisfy the needs of the

earth’s population for nearly 500,000 years.

Photovoltaic cells are capable of capturing

some of that energy and converting it into us-

able electricity; unfortunately, today’s technol-

ogy can’t do this very effi ciently.

French physicist Edmond Becquerel fi rst

described the photovoltaic eff ect in 1839. He

discovered that some materials were capable

of producing small amounts of electricity when

exposed to sunlight. The fi rst photovoltaic cell,

however, wasn’t created until 1883, and more

than 70 years passed before the next major sci-

entifi c advance took place, when researchers at

Bell Labs developed the fi rst crystalline silicon

photovoltaic cell in 1954.

Most modern photovoltaic cells are

still manufactured from silicon, the same

semiconductor material used to produce

GPUs, CPUs, and other integrated circuits.

The majority of commercial photovoltaic cells

are manufactured from crystalline silicon—

either single- or poly-crystal silicon. The

latter are less effi cient than the former, but

their lower manufacturing cost largely makes

up for the conversion shortfall.

The bulk of the progress that’s been made

since the 1950s stems from the effi ciency at

which absorbed light is converted into electric-

ity. The Bell Labs product was capable of just 4

percent effi ciency; today’s commercial products

are approaching 20 percent effi ciency.

THE PHOTOVOLTAIC PROCESS

A photovoltaic cell is created by sandwich-

ing two silicon wafers: an n-type layer and

a p-type layer. The n-type layer exhibits

a negative electrical charge and has an

excess of electrons, while the p-type layer

exhibits a positive electrical charge and has

a shortage of electrons. The two layers are

separated by an n-p junction.

The cell is then attached to a

backplane, a layer of metal used

to physically reinforce the cell

and provide an electrical contact

on its bottom. A second electrical

contact is placed on the top of the

cell to create an electrical circuit.

The cell is then treated with an

anti-reflective coating to compensate for

silicon’s otherwise shiny nature.

As photons—particles of light—hit the

photovoltaic cell, they pass through the n-type

layer and strike the p-type layer, where they

are either absorbed by the silicon atoms,

refl ected, or pass straight through the mate-

rial. Absorbed photons knock electrons loose

from the silicon atoms, leaving empty “holes,”

which are fi lled by electrons further back in

the circuit. The loose electrons fl ow through

the electrical contacts on the p-type layer to

the contacts on the n-type layer. This fl ow of

electrons produces an electric current that can

be drawn off and stored in a battery or used to

power an electrical device.

An array of cells is electrically connected

and mounted into a frame to form a photovol-

taic module. A narrow metal grid is applied to

the top of the module to transport electrical

energy, and a sheet of glass or plastic is placed

on top to protect the cells from the environ-

ment (everything from bad weather to bird

droppings and stray baseballs). A group of

interconnected modules is known as an array.

Photons contain varying amounts of en-

ergy, depending on their wavelength. Within

the visible spectrum, red light possesses the

least amount of energy while violet light

has the most. The same goes for the invisible

PHOTONS CONTAIN

VARYING AMOUNTS

OF ENERGY, DEPENDING ON

THEIR WAVELENGTHS.

64 |MAMAMAXIMXIMXIMXIMUUUUMMPPPCC|SEP 08 |www.maximumpc.com

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EXAMICICG TEC=COAOGN ACD PUTTICG IT TO USE

How these cells convert the sun’s energy into

electrical current — MICHAEL BROWN

Photovoltaic Cells

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Photovoltaic Cells

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A Photovoltaic Cell

HOW IT WORKS

Photons from the sun pass through the cell’s n-type layer to strike atoms in the p-type layer,

dislodging some of those atoms’ electrons in the process. The freed electrons move up toward the

n-layer, creating an electrical current that can be stored or service an electrical device.

NEGATIVE ELECTRICAL CONTACTS

NEGATIVE ELECTRICAL CONTACTS

N-TYPE LAYER

N-P JUNCTION

Current Flow

Electron Flow

P-TYPE LAYER

Electrons

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