* Tel.: #61-2-9385-4018; fax: #61-2-9662-4240.
E-mail address: m.green@unsw.edu.au (M.A. Green).
Energy Policy 28 (2000) 989}998
Photovoltaics: technology overview
M.A. Green*
Centre for Photovoltaic Engineering, University of New South Wales, Sydney, NSW 2052, Australia
Received 24 May 2000
Abstract
Solar electricity produced by photovoltaic solar cells is one of the most promising options yet identi"ed for sustainably providing
the world's future energy requirements. Although the technology has, in the past, been based on the same silicon wafers as used in
microelectronics, a transition is in progress to a second generation of a potentially much lower-cost thin-"lm technology. Cost
reductions from both increased manufacturing volume and such improved technology are expected to continue to drive down cell
prices over the coming two decades to a level where the cells can provide competitively priced electricity on a large scale. The
subsidised, urban residential rooftop application of photovoltaics is expected to provide the major application of the coming
decade and to provide the market growth needed to reduce prices. Large centralised solar photovoltaic power stations able to provide
low-cost electricity on a large scale would become increasingly attractive approaching 2020. 2000 Elsevier Science Ltd. All rights
reserved.
1. Introduction
Photovoltaics involves the direct conversion of sun-
light into electricity in thin layers of material known as
semiconductors with properties intermediate between those
of metals and insulators. Silicon, the material of microelec-
tronics and the information age, is the most common
semiconductor. In the latter half of the 20th century, silicon
photovoltaic solar cells started to be used mainly to gener-
ate small amounts of electricity in remote areas where there
was no conventional source of electricity. In the 21st cen-
tury, photovoltaics will grow to maturity. Almost everyone
will be aware of photovoltaics since photovoltaic solar cells
will be on the roof of their home or that of their neighbours
* be they in one of the growing megacities across the globe
or in a remote rural village (Green, 2000).
This paper reviews the current state of solar cell tech-
nology, outlines developments expected over the coming
few decades and considers implications for energy policy.
2. Brief history
Solar cells have their origins from some of the most
important scienti"c developments of the 20th century,
combining the Nobel prize winning work of several of the
most important scientists of that century. The German
scientist, Max Planck, began the century engrossed in the
problem of trying to explain the nature of light emitted
by hot bodies, such as the sun. He had to make assump-
tions about energy being restricted to discrete levels to
match theory and observations. This stimulated Albert
Einstein, in his `miraculous yeara of 1905 (Stachel, 1998),
to postulate that light was made of small `particlesa, later
called photons, each with a tiny amount of energy that
depends on the photon's colour. Blue photons have
about twice the energy of red photons. Infrared photons,
invisible to the eye have even less energy. Ultraviolet
photons, the cause of sunburn and skin cancer, are also
invisible but carry even more energy than the blue ones,
accounting for the damage they can do.
Einstein's radical suggestion led to the formulation
and development of quantum mechanics, culminating in
1926 in Edwin SchroK dinger's wave equation. Wilson sol-
ved this equation for material in solid form in 1930. This
allowed him to explain the di!erence between metals,
good conductors of electricity and insulators; also the
properties of semiconductors with their intermediate
electrical properties. Electrons, the carriers of electrical
charge, are free to move around in metals, allowing
electrical currents to #ow readily. In insulators, electrons
are locked into the bonds holding the atoms of the
insulator together. They need a jolt of energy to free them
0301-4215/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.
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