Environmental Engineering Reference
In-Depth Information
Semiconductors are a class of materials whose capacity for conducting
electrical charges falls between those of true conductors (such as copper
and other metals) and those of insulators (such as rubber or glass).There are
a number of semiconductors that can be used for photovoltaics (the conver-
sion of light to electricity), but Guha's concern for societal and environ-
mental problems led him to focus on amorphous silicon, an element found
in sand that can be applied as a thin film to produce a photovoltaic material.
Other semiconductors that can be used for thin-film photovoltaics require
the use of certain toxins in their production, but amorphous silicon presents
no hazards to humans or to the environment in its manufacture or use.
In an amorphous material, the atoms are not placed in a regular orderly
manner, as they are, for example, in a crystal. In the early 1970s, the physics
of amorphous materials was not well understood. In the United States,
Stanford Ovshinsky and his collaborators had begun pioneering work in
this area of physics, generating a great deal of interest. New theories were
being formulated for amorphous solids, and new devices were being built
to take advantage of their inherent properties. The possibility of doing
cutting-edge research in an area that had practical applications appealed
strongly to Guha.
In 1974-75, Guha did a year of postdoctoral training at the University
of Sheffield in England, pursuing his interest in amorphous silicon. Return-
ing to the Tata Institute, he began to explore a way of producing amor-
phous silicon that had not been tried before. His research led him to believe
that by adding hydrogen in the production process, a more useful form of
amorphous silicon could be created. He explains the reasons for his exper-
iments as follows:
In the late seventies, it was recognized that amorphous silicon has potential as an
inexpensive solar cell material.Till that time, solar cells were made only with single
crystals.They were reliable but expensive, since you have to take a great deal of care
in preparing these materials in which the atoms must be arranged in an ordered
structure. The first amorphous semiconductor solar cells were made using amor-
phous silicon prepared by breaking up a gas called silane (silicon hydride) in an
electric field. The quality of the material, however, became poorer when exposed
to light. Solar cells, of course, work only in light, and this caused serious doubts
among the physicists whether these materials could ever be useful for making effi-
cient solar cells.
Rather than using only silane as the starting material, I diluted the gas with
hydrogen.The reason at that time was twofold: the dilute mixture is less costly and
