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
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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
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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
EE 666 Advanced Semiconductor Devices
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).
EE 666 Advanced Semiconductor Devices
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