april 14, 2005 ee 666 advanced semiconductor devices solar cells --- frontiers in materials and...
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April 14, 2005EE 666 Advanced Semiconductor Devices
Solar Cells ---frontiers in materials and devices
Ning Su
April 14, 2005EE 666 Advanced Semiconductor Devices
OutlineIntroductionMarket & 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
April 14, 2005EE 666 Advanced Semiconductor Devices
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
April 14, 2005EE 666 Advanced Semiconductor Devices
Photovoltaic Cells, Modules and SystemsSolar 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 system
April 14, 2005EE 666 Advanced Semiconductor Devices
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
April 14, 2005EE 666 Advanced Semiconductor Devices
Comparison of PV Technology
main technologies available: single & multi- cystalline Si, a-Si, CuInSe2, CdTe…. Bulk cystalline Si remains dominant Different technology comparison in efficiency & cost
World PV module production in 2003
April 14, 2005EE 666 Advanced Semiconductor Devices
Low cost High efficiency
Thin film Organic SC tandem
Terrestrial Space
TPV
Light weight
Radiation resistance
High efficiency
Applications:
Demands:
Technology:
Materials: Multicystalline Si III-V
a-Si ; CIS; CdTe
Single crystalline Si
Low Cost vs. High Efficiency SC
April 14, 2005EE 666 Advanced Semiconductor Devices
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
April 14, 2005EE 666 Advanced Semiconductor Devices
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
April 14, 2005EE 666 Advanced Semiconductor Devices
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
Solar Cell Efficiency
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Ideal cell efficiency
Effect of spectrum on efficiency improving η by concentrating light 100 suns or more illumination
Parabolic reflector Fresnel lens
April 14, 2005EE 666 Advanced Semiconductor Devices
Minimize Losses in Real SCOptical loss
Electrical loss
Concentration of light
Trapping of light:
AR coatings Mirrors ( metallization rear surface or growing active layers on top of a Bragg stack) textured surface
Photon recycling reabsorption of photons emitted by radiative recombination inside the cell
Rear metal reflector
Double path length in metallized cell
Surface passivationResistive loss ……
April 14, 2005EE 666 Advanced Semiconductor Devices
Advanced Si Solar cells
•Martin A. Green etc.,” Very high efficiency silicon solar cells-science and technology,” IEEE Trans. Electron Devices,vol. ED-46,pp1940-47,1999.
PERL cell
Burried contact sc
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
Crystalline Si efficiency
April 14, 2005EE 666 Advanced Semiconductor Devices
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.
April 14, 2005EE 666 Advanced Semiconductor Devices
Advantages : high efficiency
Practical approaches
Cover wider range of solar spectrum
reduce thermerlisation loss (absorbed photon with energy just little higher than Eg)
individual cells grown separately and mechanically stacked
monolithically grown with a tunnel-junction interconnect
Tandem Cells -- Practical approaches
April 14, 2005EE 666 Advanced Semiconductor Devices
GaInP/GaAs/Ge Dual- and triple-junction SC
Dual-junction (DJ)
* 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.
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 *
April 14, 2005EE 666 Advanced Semiconductor Devices
Multiple Junction CellsFour-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).
April 14, 2005EE 666 Advanced Semiconductor Devices
Thermophotovoltaic (TPV)
TPV solar energy conversion
Photovoltaic conversion with the addition of an intermediate thermalabsorber/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 radiationinto electrical power.
Advantage
By matching the spectrum of the emitter to the PV cells, efficiency improved.
April 14, 2005EE 666 Advanced Semiconductor Devices
All TPV systems include: 1) heat source 2) radiator 3) PV converter 4) means of recovering unusable photons
TPV Configuration
Components of a TPV system
Selective emitter matched to PV cells
April 14, 2005EE 666 Advanced Semiconductor Devices
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).
……
April 14, 2005EE 666 Advanced Semiconductor Devices
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