Efficency of Converting Solar Efficency of Converting Solar Irradiance into Electrical or Irradiance into Electrical or Chemical Free Energy Chemical Free Energy A.J. Nozik National Renewable Energy Laboratory and Department of Chemistry, Univ. Colorado, Boulder
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Efficency of Converting Solar Irradiance into Electrical or Chemical Free Energy
Efficency of Converting Solar Irradiance into Electrical or Chemical Free Energy. A.J. Nozik National Renewable Energy Laboratory and Department of Chemistry, Univ. Colorado, Boulder. The U.S. Department of Energy’s National Renewable Energy Laboratory. www.nrel.gov Golden, Colorado. - PowerPoint PPT Presentation
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Efficency of Converting Solar Irradiance Efficency of Converting Solar Irradiance into Electrical or Chemical Free Energyinto Electrical or Chemical Free Energy
A.J. Nozik
National Renewable Energy Laboratory
and
Department of Chemistry, Univ. Colorado, Boulder
The U.S. Department of Energy’s
National Renewable Energy Laboratory
www.nrel.govGolden, Colorado
FY02 EERE Funding at National LabsFY02 EERE Funding at National Labs
Source: NREL Energy Analysis Office1These graphs are reflections of historical cost trends NOT precise annual historical data.Updated: October 2002
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00
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0
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So
lar
Ph
oto
n F
lux
(mA
/cm
2 .eV
)
Energy (eV)
6000K BB integrated current
AM1.5G integrated current
6000K Blackbody Spectrum100 mW/cm2
(E) = AM1.5G Solar Spectrum
100 mW/cm2
Inte
gra
ted
ph
oto
n f
lux
(mA
/cm
2 )
Solar Spectrum and Available Photocurrent
Solar Electricity
● Solar Fuels
National Geographic, Sept., 2004
World Energy World Energy
Millions of Barrels per Day (Oil Equivalent)
300
200
100
01860 1900 1940 1980 2020 2060 2100
Source: John F. Bookout (President of Shell USA) ,“Two Centuries of Fossil Fuel Energy” International Geological Congress, Washington DC; July 10,1985. Episodes, vol 12, 257-262 (1989).
e-
usable photo-voltage (qV)
Energy
e-
n-typep-type
1 e- - h+ pair/photon
ηmax = 32%
heat loss
heat loss
hν
Conventional PV CellConventional PV Cell
C434703
Photoeffects in Semiconductor-Redox Electrolyte JunctionPhotoeffects in Semiconductor-Redox Electrolyte JunctionPhotoelectrochemistry (PEC)Photoelectrochemistry (PEC)
Absorption of light in depletion layer results in creation and separation of electron-hole pairs. For n-type semiconductors, holes move toward surface and electrons toward semiconductor bulk. For p-type semiconductors, reverse process occurs. Redox couples in electrolyte capture injected photogenerated carriers and reactions occur.
SOLAR PHOTOCHEMISTRY/PHOTOELECTROCHEMISTRY
Some Endergonic Fuel Generation ReactionsSome Endergonic Fuel Generation Reactions
Wavelength Contours for Efficiency of Water Splitting Utilizing Two Tandem Photosystems
High Efficiency Multijunction Solar CellsHigh Efficiency Multijunction Solar CellsHigh Efficiency Multijunction Solar CellsHigh Efficiency Multijunction Solar Cells
Want 1eV material lattice-matched to GaAs
Try GaInNAs
034016319
0 2 4 6 8 10 12 14 16 18 200
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20
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# junctions-> infinity
Conc = 46000X
Conc = 1X
Ma
xim
um
Eff
icie
ncy
(%
)
Number of junctions in tandem
Maximum Efficiency of Tandem Solar Cells
Calculated using a 6000K blackbody spectrum
Best Research-Cell EfficienciesBest Research-Cell Efficiencies
where: Eff = cell conversion efficiency x 1 Kw/m2 BOS = balance of systems (support structure,
installation,wiring, land, etc) $0.1 = power conditioner, AC – DC inverter
Also: 1$/Wp $0.05/kWh
Therefore, to achieve $0.02/kWh, need total cost of $0.40/ Wp
If BOS can be reduced to $75/ m2 (currently $250/m2), and module cost reduced to $50/ m2 (currently $300/ m2 ), then module efficiency needs to be 41% (and cell efficiency at least 50%).
Disruptive technology required.
World PV Cell/Module Production (MW)World PV Cell/Module Production (MW)
e- gain kinetic energy in a highelectric field, then scatter by II generating a secondary e-h pair.
Reverse biased p-i-n junction
h>2Eg
h
I
I
I
I
FF
F
F
Optically excited hot carriers
Electron initiated Hole initiated
I – initial states
F – final states
ETH>Eg
Impact Ionization along the (100) direction ( axis) of Si. Absorption of a photon h creates a first electron hole pair (e1/h1) at the point. The excess energy Ex = h - Eg of the electron suffices to generate a second electron hole pair (e2/h2) while the electron e1 relaxes towards the conduction-band minimum (e’1). Conservation of energy E and momentum hk/(2) is fulfilled if the two dash-dotted arrows add vectorially to zero.
QDs: Requirement for conservation of momentum is relaxed. Threshold should be lower.
Queisser, et al. 1994
Consequences of QuantizationConsequences of Quantization
Dramatic variation of optical and electronic properties
Large blue shift of absorption edge
Discrete energy levels/structured absorption and photoluminescence spectra
Enhanced photoredox properties for photogenerated electrons and holes
Greatly slowed relaxation and Greatly slowed relaxation and cooling of photogenerated hot cooling of photogenerated hot electrons and holeselectrons and holes
PL blinking in single QDsPL blinking in single QDs Enhanced impact ionization Enhanced impact ionization
Conversion of indirect semiconductors to direct semiconductors or vice versa
Greatly enhanced exciton absorption at 300 K
Greatly enhanced oscillator strength per unit volume (absorption coefficient)
Greatly enhanced non-linear optical properties
Greatly modified pressure dependence of phase changes and direct to indirect transitions
Efficient anti-Stokes luminescence
(slower thermalization rates)
Boudreaux, Williams and Nozik, JAP (1980)
Hot e- injection:APL (82) GaPJAP (82) InPJACS (90) INP
Quantized Depletion Layers (w ~ 50 to 200 Å)Quantized Depletion Layers (w ~ 50 to 200 Å)
Eg
E1
E3
E2
R/R-e-W
Hot eHot e-- Relaxation Pathways Relaxation Pathways
Quantum Films vs Quantum Dots
phonon bottleneck
Breaking the Phonon Bottleneck in Quantum Dots by an Auger-Breaking the Phonon Bottleneck in Quantum Dots by an Auger-like Process involving a Coulomb Interaction (Transfer of like Process involving a Coulomb Interaction (Transfer of
Electron Energy to Hole Followed by Fast Hole Relaxation) Electron Energy to Hole Followed by Fast Hole Relaxation) (Efros)(Efros)
Al. L. Efros et. al. Solid State Comm. 93, 281 (1995)
e-
e-
e-
h
O
h+
Oh+
Egap
One photon yields
two e-–h+ pairs
impact ionization
Enhanced Photovoltaic Efficiency in Quantum Dot Solar Enhanced Photovoltaic Efficiency in Quantum Dot Solar Cells by Inverse Auger Effect (Impact Ionization)Cells by Inverse Auger Effect (Impact Ionization)
A.J. Nozik, Physica E14,115, 2002; Ann. Rev. Phys. Chem. 52, 193, 2001;in “Next Generation Photovoltaics”, Marti& Luque, Eds, AIP, 2003; in Semiconductor Nanocrystals”, V. Klimov, Ed., Marcel-Dekker, 2004
Quantum Dot
Auger Ionization Process to Explain PL Blinking in QDs
Experimental Verification of Greatly Enhanced Experimental Verification of Greatly Enhanced Impact Ionization in Quantum DotsImpact Ionization in Quantum Dots
Determine the photogenerated carrier density (QY) and I.I. dynamics by: (a) measuring the free carrier absorption (IR probe) and exciton bleach (HOMO-LUMO probe); (b) measuring dynamics of multi-exciton decay vs single exciton decay, and the rise time of exciton bleaching and induced exciton absorption
Eg (Homo - Lumo) 0.72 eV 0.72 eV 0.72 eV 0.82 eV 0.91 eV 0.91 eV 0.91 eV 0.91 eV PbS - 0.85 eV
(b)
QY > 200% means 3 e-/photon QY > 200% means 3 e-/photon are created; QY = 300% means allare created; QY = 300% means all
dots have 3 e- !!dots have 3 e- !!
NanoLetts 5, 865 (2005)
2Pe
1Se
1Sh
2Ph
2Pe
1Se
1Sh
2Ph
2Pe
1Se
1Sh
2Ph
2Pe
1Se
1Sh
2Ph
NEW MODEL FOR MEG Coherent Superposition of Multi-Excitonic States in PbSe QDs
NanoLetts 5, 865 (2005)
SUMMARY/CONCLUSIONSSUMMARY/CONCLUSIONS
● The ultimate thermodynamic efficiency for converting solar irradiance into chemical or electrical free energy is 32% for a single thereshold absorber, and 68% for a system that does not permit thermal degradation of the solar photons. With full solar concentration (46,000X) the latter efficiency is 86%.
● Ultra-high conversion efficiency (>50%) together with very low system cost (< $150/m2) is required to produce solar power (fuels or electricity) at costs comparable to current fossil fuels cost (few cents/kWh), to avoid great economic and environmental disruption in the future. “Disruptive technology” is probably required.
● Size quantization in semiconductors may greatly affect the relaxation dynamics of photoinduced carriers. These include:
- slowed hot electron relaxation (partial phonon bottleneck)- enhanced impact ionization (inverse Auger process)
● The theoretical and measured energy threshold for impact ionization in bulk semiconductors (e.g. Si, InAs, GaAs) is 4-5 times the band gap. Much lower thresholds are predicted for QDs because of the relaxation of the need to conserve momentum. The rate of impact ionization is also expected to be much faster in QDs (Auger processes α 1/d6 )
● Very efficient exciton multiplication has been experimentally observed in PbSe and PbS QDs; the threshold photon energy is 2Eg. Up to 3 electrons per photon (300% QY) have been observed at sufficiently high photon energies ( 4Eg ). A new model based on coherent superposition of multiexcitonic states is introduced to explain these results.
● For QDs with m*e << m*h (InP) slowed electron cooling (by about 1 order of magnitude) may be achieved by either fast hole trapping at the surface or by electron injection in the dark, which blocks hot electron cooling via the Auger process(results consistent with earlier results on CdSe QDs by Guyot-Sionnest and Klimov). If m*e ~ m*h (PbSe and PbS) phonon bottleneck and slowed cooling is apparent.
Summary/ConclusionsSummary/Conclusions
Summary/Conclusions - ContinuedSummary/Conclusions - Continued
Three configurations of Quantum Dot Solar Cells are suggested:
1. Nanocrystalline TiO2 sensitized with QDs2. QD arrays exhibiting 3-D miniband formation3. QDs embedded in a polymeric blend of electron- and
hole-conducting polymers.These configurations may be expected to produce enhanced photovoltages via hot carrier transport and transfer or enhanced photocurrents via multiple exciton generation.
● THE DYNAMICS OF HOT ELECTRON COOLING, FORWARD AND INVERSE AUGER RECOMBINATION (MEG), AND ELECTRON TRANSFER CAN BE MODIFIED IN QD SYSTEMS TO POTENTIALLY ALLOW VERY EFFICIENT SOLAR PHOTON CONVERSION VIA EFFICIENT MULTIPLE EXCITON GENERATION