Transcript Lecture 22 (postgraduate)

1
Tandem and thin-film solar cells
LECTURE 22
•
Si sliver cells
•
tandem junction solar cells
•
CIGS as a promising solar absorber
•
CIGS solar cells
•
heterojunction basics
•
surface phenomena
2
Efficiency comparison: materials and modules
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Thin sc-Si solar cells
K.J. Weber et al., IEEE Photovoltaic Specialists Conf., 991-994, 2005.
10X reduction in Si
use claimed
4
Sec. 7.6.2
Multijunction cells: concept and practice
http://www.emcore.com/assets/photovoltaics/Emcore_Manuscript_Fatemi_3P-B5-03_WCPEC-3.pdf
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Matching the materials
J. M. Román, “State-of-theart of III-V Solar Cell
Fabrication Technologies,
Device Designs and
Applications,” Advanced
Photovoltaic Cell Design,
2004.
http://photochemistry
.epfl.ch/EDEY/NREL.
pdf
6
The world-record holder
7
Sec. 7.6.2
Cell mismatch
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Sec. 7.2
CIGS properties
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More CIGS properties
Electron minority carrier diffusion lengths: 0.5 – 2.5 micron
Photovoltaic Materials, Richard Bube
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10
Even more properties of CuIn1-xGaxSe2
• Chalcopyrite structure, tetragonal bonding
• Vacancy doping*
• Direct bandgap
• Eg(x) 1.04 - 1.7 eV
• High absorption coefficient
• Can be printed onto glass and metal
• Needs heteroface cell structure
• Google has invested $$$$$ in it.
*
VCu0  VCu  h 
:
Ea  0.03eV
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Preparation of CuIn1-xGaxSe2
www.tf.uni-kiel.de/...en/.../gerngross_reverey_paper_ws_08_1.pdf
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Sec. 7.6.1
CIGS cells: a lower cost alternative (?)
Noufi, Rommel; Ken Zweibel. HIGH-EFFICIENCY CDTE
AND CIGS THIN-FILM SOLAR CELLS: HIGHLIGHTS AND
CHALLENGES. National Renewable Energy Laboratory.
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Heterojunction advantages and problems
E
EC
Wide bandgap window, but
what happens at the
interfaces?
EV
And why is CdS needed?
x
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A proposed band diagram
www.tf.uni-kiel.de/...en/.../gerngross_reverey_paper_ws_08_1.pdf
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Real heterostructures
What are the effects on the electrical properties of some previously
unconsidered real surface effects?
• surface reconstruction
• dipole formation
• interruption of the periodicity of the semiconductor
• surface states