Solar Cells --frontiers in materials and devices Ning Su EE 666 Advanced Semiconductor Devices April 14, 2005
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Transcript Solar Cells --frontiers in materials and devices Ning Su EE 666 Advanced Semiconductor Devices April 14, 2005
Solar Cells --frontiers in materials and devices
Ning Su
EE 666 Advanced Semiconductor Devices
April 14, 2005
Outline
Introduction
Market & technology comparison
Low cost solar cells
thin film solar cells (TFSC)
High efficiency solar cells
Advanced Si solar cells
Tandem cells
Thermophotovoltaic
other strategies
Conclusions
EE 666 Advanced Semiconductor Devices
April 14, 2005
Introduction
Why PV ?
Average power incident upon continental United states is ~ 500 times of
national energy consumption ( total, not just electricity)
Environmentally-friendly renewable
energy source
Quiet
Reliable
Applications
Residential
Cost-effective way to provide power
to remote area
Space applications
satellite, space stations
EE 666 Advanced Semiconductor Devices
April 14, 2005
Photovoltaic Cells, Modules and Systems
Solar cell is the basic building blocks of solar PV
Cells are connected together in series and encapsulated into models
Modules can be used singly, or connected in parallel and series into
an array with a larger current & voltage output
PV arrays integrated in systems with components for charge regulation
and storage
Cell
module
array
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system
April 14, 2005
Market for Solar PV
PV market grows at fast rate especially in recent years
Cumulatively, about 2GW of solar cells are being used in a variety of applications
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April 14, 2005
Comparison of PV Technology
World PV module production in 2003
main technologies available: single & multi- cystalline Si, a-Si, CuInSe2, CdTe….
Bulk cystalline Si remains dominant
Different technology comparison in efficiency & cost
EE 666 Advanced Semiconductor Devices
April 14, 2005
Low Cost vs. High Efficiency SC
Applications:
Demands: Low cost
Space
Terrestrial
High efficiency
High efficiency
Light weight
Radiation resistance
Technology:
Materials:
Thin film
Organic SC
Multicystalline Si
tandem
III-V
TPV
Single crystalline Si
a-Si ; CIS; CdTe
EE 666 Advanced Semiconductor Devices
April 14, 2005
Thin Film Solar Cells
“thin film” refers more to solar cell technologies with mass-production possibilities
Rather than the film thickness.
requirement for suitable materials: low cost, high absorption, doping, transport,
robust and stable
leading materials for TFSC: CdTe, CuInSe2, (CIS) ,a-SI…
advantages:
-- low material requirement
-- variety of processing methods
-- light weight modules
disadvantages:
-- low achieved efficiency
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April 14, 2005
CIS & CdTe TFSC
CIS, direct band gap with Eg~ 1eV, α>105 cm-1
high cell efficiency (19.2 %), model efficiency
(13.4%)
comparatively long lifetime
Current complicated and capital intensive
fabrication
CdTe, direct band gap with Eg~ 1.45eV, α>105
cm-1-- ideal suited for PV applications
Record cell efficiency 16.5 % (NREL)
Numerous promising processing techniques
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April 14, 2005
Solar Cell Efficiency
Ideal cell efficiency
E g bs ( E )dE
Eg
0
Ebs ( E )dE
Effect of bandgap on efficiency
GaAs, InP have Eg close to the optimum,
favored for high η cells
Si less favorable Eg but cheap & abundant
Effect of spectrum on efficiency
improving η by concentrating light
100 suns or more illumination
Parabolic reflector
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Fresnel lens
April 14, 2005
Minimize Losses in Real SC
Optical loss
Concentration of light
Trapping of light:
AR coatings
Mirrors ( metallization rear surface or growing
active layers on top of a Bragg stack)
Rear metal reflector
textured surface
Double path length in metallized cell
Photon recycling
reabsorption of photons emitted by radiative recombination inside the cell
Electrical loss
Surface passivation
Resistive loss
……
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April 14, 2005
Advanced Si Solar cells
Crystalline Si efficiency
PERL cell
large improvement in the last 15 years
1) textured surface & AR coating
2) Improved surface passivation
PERL cell ( 24% in 1994 )
Buried contact cell commercialized by BP Solarex
advantage: fine grid– reduced shading–Jsc
reduced contact recombination – Voc
series resistance – concentrator sc
Burried contact sc
•Martin A. Green etc.,” Very high efficiency silicon solar cells-science and technology,” IEEE Trans. Electron
Devices,vol. ED-46,pp1940-47,1999.
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April 14, 2005
Tandem Cells – beyond efficiency limit
Concept
Intrinsic efficiency limit using single
semiconductor material is 31%
Stack different band gap junctions in
series larger band gap topmost
efficiency of 86.8% calculated for an
infinite stack of independently operated
cells *
* A. Marti, G. L. Araujo, Sol. Energy Mater. Sol. Cells 43 (1996) 203.
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April 14, 2005
Tandem Cells -- Practical approaches
Advantages : high efficiency
Cover wider range of solar spectrum
reduce thermerlisation loss
(absorbed photon with energy just little
higher than Eg)
Practical approaches
individual cells grown separately and mechanically stacked
monolithically grown with a tunnel-junction interconnect
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April 14, 2005
GaInP/GaAs/Ge Dual- and triple-junction SC
Dual-junction (DJ)
GaInP/GaAs cells on Ge (average AM0 η 21.4 %) *
small-area lab cells
large-scale manufacturing
approach megawatt level **
Triple-junction (TJ)
efficiency of 27.0% under AM0 illumination at 28 0C *
* N. H. Karam etc. Solar Energy Materials & Siolar cells 66 (2001) 453-466.
**N. H. Karam etc. Trans. Electron Dev. 46 (10) 1999 pp.2116.
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April 14, 2005
Multiple Junction Cells
Four-junction cells under development
addition of 1-eV GaInNAs subcells
under GaAs to form 4 junctions
InGaN – potential material
for MJ cells
Direct energy gap of InGaN cover
most of the solar spectrum*
MJ solar cells based on this single
ternary could be very efficient
* LBNL/Conell work: J. Wu et al. APL 80, 3967 (2002).
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April 14, 2005
Thermophotovoltaic (TPV)
TPV
solar energy conversion
Photovoltaic conversion with the addition of an intermediate thermal
absorber/emitter is known as thermophotovoltaic (TPV) energy conversion.
Solar radiation is used to heat absorber/emitter to temperature of 1200-2500 K
emitter radiates photons
PV cell converts the energy of radiation
into electrical power.
Advantage
By matching the spectrum of the emitter to the PV cells, efficiency improved.
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April 14, 2005
TPV Configuration
Components
of a TPV system
All TPV systems include: 1) heat source 2) radiator 3) PV converter
4) means of recovering unusable photons
Selective emitter matched to PV cells
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April 14, 2005
Other Strategies – for high efficiency
Intermediate band solar cells
A.Luque and A. Marti,”Increasing the effiency of ideal solar cells by photon
Induced transitions at intermediate levels”, Phys. Rev. Lett. 78, 5014 (1997)
Low-dimentional strucutrues, QWs, QDs
Impact ionization solar cells
P. Wueerfel, “Radiative efficiency limit of terrrestrial solar-cells with internal
carrier multiplication”, Appl. Phys. Letts. 67, 1028 (1995).
Hot carrier solar cells
P. Wueerfel, “Radiative efficiency limit of terrrestrial solar-cells with internal
carrier multiplication”, Appl. Phys. Letts. 67, 1028 (1995).
……
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April 14, 2005
Conclusions
Remarkable progress made in synthesis, processing and characterization
leads to major improvement in PV efficiency and reduction in cost
Silicon continues to dominate the PV industry
Thin film and organic solar cells offer promising options for substantially
reducing the cost, competitive for terrestrial applications
Very high efficiency achieved in multiple junction III-V semiconductors
presently commercialized for space applications
New device concept for high efficiency facing challenges and prospects
EE 666 Advanced Semiconductor Devices
April 14, 2005