Photovoltaic Materials

Fundamentals of Photovoltaic Materials
Olivia Mah NSPRI
_____________________________________________________________________________________________
Fundamentals
of
Photovoltaic Materials
National Solar Power Reasearch Institute, Inc.
12/21/98
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12/21/98


Fundamentals of Photovoltaic Materials
Olivia Mah NSPRI_____________________________________________________________________________________________
Introduction
Photovoltaics (PV) comprises the technology to convert sunlight directly into electricity. The term
“photo” means light and “voltaic,” electricity. A photovoltaic (PV) cell, also known as “solar cell,”
is a semiconductor device that generates electricity when light falls on it . Although photovoltaic
effect was observed in 1839 by the Frenchscientist Edmund Becquerel, it was not fully
comprehensible until the development of quantum theory of light and solid state physics in early to
middle 1900s. Since its first commercial use in powering orbital satellites of the US space
programs in the 1950s, PV has made significant progress with total U.S. photovoltaic module and
cell shipments reaching $131 million dollars in 1996.
While mostPV cells in use today are silicon-based, cells made of other semiconductor materials are
expected to surpass silicon PV cells in performance and cost and become viable competitors in the
PV marketplace.
This paper surveys the major types of PV cell materials including silicon- and non-silicon-based
materials, providing an overview of the advantages and limitations of each type of materials.Photovoltaics and Photovoltaic Cells
When sunlight strikes a PV cell, the photons of the absorbed sunlight dislodge the electrons from
the atoms of the cell. The free electrons then move through the cell, creating and filling in holes in
the cell. It is this movement of electrons and holes that generates electricity. The physical process
in which a PV cell converts sunlight into electricityis known as the photovoltaic effect.
One single PV cell produces up to 2 watts of power, too small even for powering pocket
calculators or wristwatches. To increase power output, many PV cells are connected together to
form modules, which are further assembled into larger units called arrays. This modular nature of
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12/21/98


Fundamentals of Photovoltaic Materials
Olivia MahNSPRI
_____________________________________________________________________________________________
PV enables designers to build PV systems with various power output for different types of
applications.
A complete PV system consists not only of PV modules, but also the “balance of system” or BOS -
- the support structures, wiring, storage, conversion devices, etc. i.e. everything else in a PVsystem except the PV modules. Two major types of PV systems are available in the marketplace
today: flat plate and concentrators.
As the most prevalent type of PV systems, flat plate systems build the PV modules on a rigid and
flat surface to capture sunlight. Concentrator systems use lenses to concentrate sunlight on the PV
cells and increase the cell power output. Comparing the twosystems, flat plate systems are
typically less complicated but employ a larger number of cells while the concentrator systems use
smaller areas of cells but require more sophisticated and expensive tracking systems. Unable to
focus diffuse sunlight, concentrator systems do not work under cloudy conditions.
Types of PV cell materials
PV cells are made of semiconductor materials. The major typesof materials are crystalline and thin
films, which vary from each other in terms of light absorption efficiency, energy conversion
efficiency, manufacturing technology and cost of production. The rest of the paper discusses the
characteristics, advantages and limitations of these two major types of cell materials.
1 . Crystalline Materials
1 . 1 Single-crystal silicon
Single-crystal...