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

0

20

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60

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100

120

140

160

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200

NREL ORNL NETL SNL ANL LBNL PNNL LANL INEEL LLNL BNL

Do

llar

s in

$M

FY02 Budget Authority

Renewable Energy Cost Trends

Levelized cents/kWh in constant $20001

Wind

1980 1990 2000 2010 2020

PV

CO

E c

en

ts/k

Wh

1980 1990 2000 2010 2020

40

30

20

10

0

100

80

60

40

20

0

BiomassGeothermal Solar thermal

1980 1990 2000 2010 2020 1980 1990 2000 2010 2020 1980 1990 2000 2010 2020

CO

E c

en

ts/k

Wh

10

8

6

4

2

0

70

60

50

40

30

2010

0

15

12

9

6

3

0

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

10

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50

60

70

80

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

Reaction

ΔGo (kJ mol-1)

n

ΔEo (eV)

λmax (nm)

_______________________________________________________________________________________________

H2O H2 + ½ O2 237 2 1.23 611

CO2 + H2O HCOOH + ½ O2 270 2 1.40 564

CO2 + H2O HCHO + O2 519 4 1.34 579

CO2 + 2H2O CH3OH + 3/2 O2 702 6 1.21 617

CO2 + 2H2O CH4 + 2O2 818 8 1.06 667

CO2 + H2O 1/6 C6H12O6(s) + O2 480 4 1.24 608

N2 + 3H2O 2NH3 + 3/2 O2 679 6 1.17 629

SOLAR HYDROGEN--PHOTOELECTROLYSIS

Outstanding Technological IssuesOutstanding Technological Issues

Discovery of “Holy Grail” of Photoelectrolysis:

Semiconductor with:Bandgap 1/6–2.0 eVAppropriate flatband potential

Catalytic surface for O2 (or H2) evolutionLong-term stability against photocorrosionConversion efficiency > 10%Low cost and environmentally benign

or

p-n combination of two different semiconductors in a tandem configuration with above properties, except bandgaps can be 1 eV.

Electrochemical Photovoltaic Cells

Band Diagram

Dye-Sensitized Nanocrystalline TiO2 Photochemical Solar Cell (Graetzel Cell)

B084717

Hot e- Relaxation

Main Process Limiting Conversion Efficiency

Detailed Balance Efficiency Calculation

The theoretical maximum efficiency of a solar cell is calculated usingthe Detailed Balance Model first introduced by Shockley and Queisser*.

VEFn

EFp

Load

J(V)

ASsUMPTIONSAbsorption of one photon produces one electron-holepair. Quantum Yield = 1.

Only photons with h > Eg are absorbed.

Radiative recombination is the only recombination mechanism present.

Hot carriers are relaxed to the band edges

The quasi-Fermi level separation is constant through-out the cell. infinite carrier mobility

Eg

)()()( ,VEJEJVJ gRgS

gE

BBgR dEVEqVEJ ,),(

BB = blackbody photon flux*Shockley and Queisser, J.Appl. Phys. 32, 510 (1961)

0

1

1

2

02

22

1

1

2

0

1

1

1

1

2

02

22

2

12

0

dheRR

cn

deRR

eeRR

cn

S

QAS

kT

h

E

S

kT

h

E

SkT

h

kT

h

E

S

Q

Net absorbed photon flux = solar flux + ambient flux – radiant emission flux

INET ABS (ν) = ∫{IS(ν) + IA (ν) – I(ν,μ,TQ,2π)}σ(ν, μ,TQ) dν dA

P = INET ABS (ν) μμ = chemical potential produced by lightηQ = power converison efficiency

ηQ = INET ABS (ν) μ / ∫ IS(ν) hν dν

For single threshold absorber, maximum efficiency = ηQ = .31

3rd Generation Photon Conversion

Valid Thermodynamic Approaches to Achieve Photon Conversion Efficiencies > 32%

(Exceeding the Shockley-Queisser Limit)

1. Tandem Cells (exceed S-Q limit but not new approach)

2. Hot Carrier Conversiona. Extract, collect, and utilize hot carriers b. Impact ionization/exciton multiplication

3. Intermediate Band Solar Cell

4. Thermophotonic Solar Cells

5. Down conversion and upconversion of incident photons (M. Green and P. Wuerfel)

See: M. Green, “Third Generation Photovoltaics”. Springer, 2003

A. Marti and A. Luque, “Next Generaton Photovoltaics”, Inst. Of Physics Series in Optics and Optoelectronics, 2003

Efficiency of Hot Carrier Photoconversion

Ross & Nozik, J. Appl. Phys. 53, 3813 (82)

0

1

1

2

02

22

1

1

)(2

0

1

11

2

002

22

2

)(12

)(

dheR

R

cn

deR

Ree

R

R

cn

S

QAS

kT

h

E

S

kT

h

E

SkT

h

kT

h

E

S

Q

)(1)(

T

Th Q

683.0)( Q

Multiple Threshold Absorbers

For an infinite number of tandem of tandem absorbers:

2-PHOT0SYSTEM PEC CONVERSION

Multi-Layered/Multi-Photon Multi-Layered/Multi-Photon Photoelectrochemical Converters Photoelectrochemical Converters

(Photochemical Diode)(Photochemical Diode)

pp nn

H+/H2

e-

e-

h+ h1

h+

h2

O2

H2O/O2

h

H2

Transparent ohmic

contact

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

10

20

30

40

50

60

70

80

90

100

# 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

Effic

ienc

y (%

)

Universityof Maine

Boeing

Boeing

Boeing

BoeingARCO

NREL

Boeing

Euro-CIS

200019951990198519801975

NREL/Spectrolab

NRELNREL

JapanEnergy

Spire

No. CarolinaState University

Multijunction ConcentratorsThree-junction (2-terminal, monolithic)Two-junction (2-terminal, monolithic)

Crystalline Si CellsSingle crystalMulticrystalline

Thin Film TechnologiesCu(In,Ga)Se2

CdTeAmorphous Si:H (stabilized)

Emerging PVDye cellsOrganic cells(various technologies)

Varian

RCA

Solarex

UNSW

UNSW

ARCO

UNSWUNSW

UNSWSpire Stanford

Westing-house

UNSWGeorgia TechGeorgia Tech Sharp

NREL

Spectrolab

NREL

Masushita

MonosolarKodak

Kodak

AMETEK

Photon Energy

UniversitySo. Florida

NREL

NREL

NRELCu(In,Ga)Se2

14x concentration

NREL

United Solar

United Solar

RCA

RCARCA

RCA RCARCA

Spectrolab

Solarex12

8

4

0

16

20

24

28

32

36

University ofLausanne

University ofLausanne

Siemens

2005

Kodak UCSBCambridge

Groningen

University LinzBerkeley

Princeton

UniversityLinz

Technology Type MW %

Flat plates – Single crystal silicon 230.5 31.0

Cast polycrystalline silicon 443.8 59.6

Ribbon silicon 22.8 3.1

Thin film amorphous silicon 39.3 5.3

Thin film cadmium telluride 3.0 0.4

Thin film CIGS 4.0 0.5

Concentrators – Silicon 0.7 0.1

TOTALS 744.1 100

PV Module Production in 2003PV Module Production in 2003by Technology Type *by Technology Type *

* Source: PV News, March 2004

~94%

OO22 HH22

HH22OO HH++

p n p n p n

e- e-

Sunlight

Solid state solar cells

Dark electrolysis cell

Photovoltaic ElectrolysisPhotovoltaic Electrolysis

Two-Junction Cascade PV/PEC Device for Water Two-Junction Cascade PV/PEC Device for Water SplittingSplitting

pp nnnn pp

e-

h+

h2

e-

h1

h+

h

O2

H2O/O2

H+/H2

H2

Transparent ohmic

contact

Ohmic contact and metal cathode

Ohmic contact and metal anode

2

Multi-Layered/Multi-Photon Multi-Layered/Multi-Photon Photoelectrochemical Converters Photoelectrochemical Converters

(Photochemical Diode)(Photochemical Diode)

pp nn

H+/H2

e-

e-

h+ h1

h+

h2

O2

H2O/O2

h

H2

Transparent ohmic

contact

John Turner Cell - > 11% efficient water splitting

Projected Need for Carbon-Free Primary Power

Bottom Line: New “disruptive” energy technology is needed

UltimateThermodynamic

limit at 1 sun

min BOS

Shockley- Queisser limit

PV Power Costs as Function of Cell Efficiency and Module Cost From Martin Green

For PV or PEC to provide the level of C-free energy required for electricity and fuel—power cost needs to be 2-3 cents/kWh ($0.40 – $0.60/W)

$/peak watt = (module cost/Eff ) + (BOS cost/Eff) + 0.1

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)

1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 20010

100

200

300

400

Rest of worldEuropeJapanU.S.

33.6 40.2 46.5 55.4 57.9 60.1 69.4 77.6

88.6125.8

154.9

201.3

287.7

390.5

2002

500

Source: PV News, March 2004

600 561.8

700

800744.1

2003

Two Ways to Utilize Photogenerated Two Ways to Utilize Photogenerated Hot eHot e- - for Useful Work and Increase for Useful Work and Increase

EfficiencyEfficiency

1. Higher photovoltage via hot e- transport, transfer, and conversion

2. Higher photocurrent via carrier multiplication through impact ionization (inverse Auger process)

Thermalized vs Hot Electron TransferThermalized vs Hot Electron Transfer

e-

Thermalized e-

Available Energy

Hot e-

e-

e-

Heat loss

Liquid Redox Electrolyteh+

p-type photoelectrode

Eg

Energy lostas heat

h

Nozik, et. al. ,J. Applied Physics 54, 6463 (1983)

Nozik &Turner, Appl. Phys. Lett., 41, 101 (1982)

Photocurrent Multiplication by Impact IonizationPhotocurrent Multiplication by Impact Ionization

1 photon yields 2 (or more) e- - h+ pairs

(I.I. previously observed in bulk Si, Ge, InSb)

h+

e e

hν 2 EgEg

e

h+ h+ h+h+

e e

Maximum Single Bandgap Efficiency at 1 Sun

A. De Vos, B. Desoete, Solar Energy Materials and Solar Cells 51 (1998) 413–424

Shockley-Queisser limit

ImpactIonization

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Ph

oto

n F

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(mA

/cm

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)

Energy (eV)

(E) = AM1.5G Solar Spectrum

Imp

act

Ion

iza

tion

Qu

an

tum

Yie

ld

ImpactIonization

DetailedBalance

Impact Ionization ProcessesImpact Ionization Processesin Bulk Semiconductorsin Bulk Semiconductors

Field

distance

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

(inverse Auger recombination)(inverse Auger recombination)

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

● R.D. Schaller and V.I. Klimov, Phys. Rev. Letts, 92, 186601 (May), 2004 (PbSe QDs)

● R.J. Ellingson, M. Beard, P. Yu, A.J. Nozik, NanoLetters 5, 865, 2005 (PbSe and PbS QDs; 300% QY in PbSe QDs at 4 times Eg)

Pump-probe transient absorptionPump-probe transient absorption

Pump h> nEg

IR Probe: ~5000nmHOMO-LUMO Probe: λ ~ 1300-1700 nm

α e-h pair (exciton) density; 1S bleach decay dynamics = f(multiexciton density); 1S bleach dynamics and induced exciton absorption determine carrier cooling rate and carrier multiplication rate

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

VB

CB

+

-

-

+

-

Transient AbsorptionTransient Absorption SpectroscopySpectroscopy SetupSetup

Clark CPA-2001 Amplified Ti:s

Sample CVI Digichrome 240 Monochrometer

IR - OPA

Signal

AgGaS2

775 nm

387 nm, 450 - 700 nm

DFG: 3 - 9 m

Delay

Probe

Pump

D

Vis - OPA775 nm

Experimental Parameters

Repetition Rate :0.989 kHzpump :387 nm, 450 – 700 nmprobe :~ 440 - 9000 nmPump Pulsewidth :~ 125 fsProbe Pulsewidth :~ 200 fs

125 fs

BBO

WL 440 - 950 nm

Idler

Al2O3

Pump

SRS Boxcar

PC

New Focus 3500

0.6

0.5

0.4

0.3

0.2

0.1

0.0

(n

orm

aliz

ed a

t ta

il)

4003002001000

Time delay (ps)

Eh/Eg = 5.00 Eh/Eg = 4.66 Eh/Eg = 4.25 Eh/Eg = 4.05 Eh/Eg = 3.60 Eh/Eg = 3.25 Eh/Eg = 1.90

(a)

300

250

200

150

100

Qu

an

tum

Yie

ld (

%)

5432

Eh/Eg

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