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Photovoltaic principles
指導老師：林克默 老師
學
生：陽顯奕
報告日期：2010.06.18
From： Martin A. Green ，Physica E 14 (2002) 11 –17.
Special Research Centre for Third Generation Photovoltaics, University of New
South Wales, Sydney 2052, Australia
Outline
•
•
•
•
1. Introduction
2. p–n Junction devices
3. Dye-sensitised cells
4. Conclusion
Introduction
• Bequerel [2] is usually credited
as being the first to
demonstrate the photovoltaic
effect in 1839 by
illuminating.Pt electrodes
coated with AgCl or AgBr
inserted into acidic solution, as
in Fig. 1 [3].
Fig. 1. Diagram of apparatus described by
Becquerel [2].
Fig. 2. Sample geometry used by Adams and
Day [4] for the investigation of the photovoltaic
effect in selenium.
• The next significant
development arose when
Adams and Day [4] in
1876 were investigating
the photoconductive
effect in Se. They noted
anomalies when Pt
contacts were pushed
into an Se bar (Fig.
2).This led them to
demonstrate that it was
possible to start a
current in selenium
merely by the action of
light.
• This led to the first
thin-film Se solar cells
being fabricated by
Fritts in 1883 [5]. Up
until the 1940s,the
most efficient
photovoltaic devices
used either Se,Cu2O or
T l2S as the absorbing
layer with a rectifying
metal contact as in Fig.
3 [3], a structure very
similar to that earlier
demonstrated by Fritts.
Fig. 3. Structure of the most efficient photovoltaic
devices developed during the 1930s.
Fig. 4. Silicon solar cell reported in 1941
relying on ‘grown-in’ junctions formed by
impurity segregation in recrystallised silicon
melts.
• The first
semiconductor p–n
junction solar cells
were described in
1941 by Russel Ohl
of Bell
Laboratories[6].
These junctions
formed naturally in
slowly solidified
melts of silicon (Fig.
4).
• One new feature,
Fig. 5. Standard silicon solar cell structure developed
in the 1970s.
introduced in 1974,
was the use of
crystallographic
texturing on the top
surface, to reduce
refection loss. Other
features of the
fabrication
technology largely
have been adapted
from the standard or
hybrid
microelectronics
2. p–n Junction devices
Fig. 7. A p–n junction formed by bringing the isolated p- and n-type
regions together. Also shown is the corresponding energy-band
diagram at thermal equilibrium.
Fig. 8. Effciency loss processes in a
p–n junction solar cell:
(1)thermalisation loss;
(2) junction loss
(3) contact loss
(4)recombination loss.
3. Dye-sensitised cells
• In a p–n junction cell, the light
absorption and photogenerated
carrier transport processes
occur side by side throughout
the device volume. Dyesensitised cells [12] exhibit a
different mode of operation.
• Here,absorption occurs at a
very specific location at the
dye molecules attached to a
porous titania medium
Fig. 9. Nanocrystalline TiO2 dye-sensitised solar
cell.
• Light absorption prospects
are increased by depositing
the titania in a porous form
that gives a large surface
area to be coated with the
dye. The liquid solution
can also penetrate the
pores allowing even the
most inaccessible dye
molecules to be recharged.
Fig. 10. Energy relationships in a
nanocrystalline dye cell in region
where the titania semiconductor is coated
by the dye.
Conclusion
•
Photovoltaics requires the excitation by light of
an electron from a ground to an excited state, with
some way provided both for selectively extracting
excited electrons and for replacing those in the
ground state.Reverse radiative recombination
processes from the excited to the ground state
provide the limits on the conversion efficiency on
such a process, although at least double the
efficiency of the standard process is possible in
principle by extensions to it.
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