energy and nanotechnology issues and opportunities in photovoltaics s. ismat shah physics and...

Energy and Nanotechnology Energy and Nanotechnology Issues and Opportunities in PhotovoltaicsIssues and Opportunities in Photovoltaics

S. Ismat ShahS. Ismat Shah

Physics and AstronomyPhysics and AstronomyMaterials Science and EngineeringMaterials Science and Engineering

Senior Policy FellowSenior Policy FellowCenter for Energy and Environment PolicyCenter for Energy and Environment Policy

University of DelawareUniversity of Delaware

ICNMRE, Al Maghrib, 2010ICNMRE, Al Maghrib, 2010

Richard Smalley Richard Smalley (1943 -2005) (1943 -2005) Nobel Prize in Chemistry (1996)Nobel Prize in Chemistry (1996)

Top Ten Global ConcernsTop Ten Global Concerns

10. Population10. Population

9. Democracy9. Democracy

8. Education8. Education

7. Disease7. Disease

6. Terrorism and War6. Terrorism and War

5. Poverty5. Poverty

4. Environment4. Environment

3. Food3. Food

2. Water2. Water

3. Food3. Food

1. Energy1. Energy

2. Water2. Water

3. Food3. Food

And we produce 90 million barrels of oil per day right now…….And we produce 90 million barrels of oil per day right now…….

The Global PictureThe Global Picture

MarocFounded

Fossil Fuel Derived EnergyFossil Fuel Derived Energy

There is no explicit solution!There is no explicit solution!-There is very little hope in new technologies -There is very little hope in new technologies but they have to be pursued because there but they have to be pursued because there is no other option. (look for regional is no other option. (look for regional solutions)solutions)-The only partial solution comes from -The only partial solution comes from increased efficiencies, new materials and increased efficiencies, new materials and designs, and most importantly,……..designs, and most importantly,……..

There is no explicit solution!There is no explicit solution!-There is very little hope in new technology -There is very little hope in new technology but they have to be pursued because there but they have to be pursued because there is no other option. (look for regional is no other option. (look for regional solutions)solutions)- The only partial solution comes from - The only partial solution comes from increased efficiencies, new materials and increased efficiencies, new materials and designs, and most importantly designs, and most importantly reduction in reduction in consumption.consumption.

Radiant FactsRadiant Facts

Diameter:Diameter: About 100 times that of earth About 100 times that of earthMass:Mass: 99.8% of the Solar System (Jupiter has most of the rest) 99.8% of the Solar System (Jupiter has most of the rest)Core Temperature:Core Temperature: 15.6 x 10 15.6 x 1066 K K Surface Temperature: 5800KSurface Temperature: 5800KEnergy Production:Energy Production: 386 billion billion megawatts 386 billion billion megawatts Insolation:Insolation: 1000 - 250 Watts per square meter 1000 - 250 Watts per square meter Age:Age: 4.5 billion Years (5 billion years more to go) 4.5 billion Years (5 billion years more to go)

Nanotechnology and PhotovoltaicsNanotechnology and Photovoltaics

PV ProductionPV Production

Case of MarocCase of Maroc

IEA UNOIEA UNO

Total ConsumptionTotal Consumption

Total Crude Oil ProductionTotal Crude Oil Production

Proven ReservesProven Reserves

World Energy ProductionWorld Energy Production

BP 2006 statistical review

Current PV StatusCurrent PV Status• 2008: Global PV production 7 GW2008: Global PV production 7 GW• 2008: Cumulative installed PV electricity 2008: Cumulative installed PV electricity

generation capacity in the world was around 15 generation capacity in the world was around 15 GW, with Europe accounting for more than 60% GW, with Europe accounting for more than 60% of this (9.5 GW)of this (9.5 GW)

• China as the new leading producer of solar cells, China as the new leading producer of solar cells, with an annual production of about 2.4 GW, with an annual production of about 2.4 GW, followed by Europe with 1.9 GW, Japan with 1.2 followed by Europe with 1.9 GW, Japan with 1.2 GW and Taiwan with 0.8 GW. GW and Taiwan with 0.8 GW. (Where is USA?)(Where is USA?)

Hashimoto PredictionsHashimoto Predictions

Hashimoto SolutionHashimoto Solution

The DisconnectThe Disconnect

1. Materials Issues1. Materials Issues2. Device Issues2. Device Issues3. Not a chance!3. Not a chance!

Nate Lewis (CIT) calculationsNate Lewis (CIT) calculations

How does the area required changes with high How does the area required changes with high efficiency solar cells?efficiency solar cells?

• 20 TWatt model20 TWatt model• With 10% cells, we needed 5 x 10With 10% cells, we needed 5 x 101111 square square

meters of solar cells.meters of solar cells.• With 50% cells, we will still need about 10With 50% cells, we will still need about 101111

square meters of solar cells.square meters of solar cells.• We currently produce about 1 million sq. We currently produce about 1 million sq.

meters of solar panels.meters of solar panels.• We need to increase production by 5 orders of We need to increase production by 5 orders of

magnitude.magnitude.

How much material do we need?How much material do we need?

• For 1 x 10For 1 x 101111mm22, we will need , we will need (10(101111 x 10 x 104 4 x 0.01 cmx 0.01 cm33)/(2.33 g/cm)/(2.33 g/cm33))

= = 5 x 105 x 1099 Kg of Silicon Kg of Silicon

How much material do we need?How much material do we need?

• For 1 x 10For 1 x 101111mm22, we will need , we will need (10(101111 x 10 x 104 4 x 0.01 cmx 0.01 cm33)/(2.33 g/cm)/(2.33 g/cm33))

= = 5 x 105 x 1099 kg of Silicon kg of Silicon

Each kg of Si requires 15 kg of carbon to Each kg of Si requires 15 kg of carbon to produce electronic grade Si.produce electronic grade Si.

To obtain a kg of refined grade of (poly)Si, we To obtain a kg of refined grade of (poly)Si, we use up about 200 kWh of energy emitting 40 use up about 200 kWh of energy emitting 40 kg of COkg of CO22, using 1000 gallons of water., using 1000 gallons of water.

(Availability, Toxicity)

Device IssuesDevice Issues

Shockley-Queisser LimitShockley-Queisser Limit

Three types of losses are Three types of losses are described:described:1.1.Sub-band radiation Sub-band radiation 2.2.Radiative recombination Radiative recombination 3.3.Thermalization Thermalization

Sub-band RadiationSub-band Radiation

Egh < Eg

Non-absorbance of photons with energy below the bandgap energyNon-absorbance of photons with energy below the bandgap energy

Radiative RecombinationRadiative Recombination

• Second Loss Mechanism: Radiative Second Loss Mechanism: Radiative recombination, the inverse of recombination, the inverse of photovoltaic electron-hole pair photovoltaic electron-hole pair generation process.generation process.

• It is a It is a fundamental loss-mechanism fundamental loss-mechanism that that is always present at is always present at any non-zero cell any non-zero cell temperature.temperature.

Radiative RecombinationRadiative Recombination

Recombination of electrons and holes generated by (a) optical absorption and (b) a forward- biased p-n junction.

Shockley-Queisser LimitShockley-Queisser Limit

• The third mechanism for a PV cell using single The third mechanism for a PV cell using single semiconductor material is thermalization of semiconductor material is thermalization of electron-hole pairs generated by photons with electron-hole pairs generated by photons with energy above the band-gap (Eg) energy.energy above the band-gap (Eg) energy.

Loss MechanismsLoss Mechanisms

Breaking S-Q limitBreaking S-Q limit

Exceeding Shockley–Queisser limitExceeding Shockley–Queisser limit

1. Tandem cells (University of Delaware 1. Tandem cells (University of Delaware DARPA $57M ($147M) Project).DARPA $57M ($147M) Project).

2. Hot carrier solar cells2. Hot carrier solar cells4. Multiband and impurity solar cells 4. Multiband and impurity solar cells 5. Thermophotovoltaic/thermophotonic cells5. Thermophotovoltaic/thermophotonic cells3. Solar cells producing multiple electron- 3. Solar cells producing multiple electron-

hole hole pairs per photon through impact ionizationpairs per photon through impact ionization6. Nanocomposite solar cells6. Nanocomposite solar cells

Approaches to High EfficiencyApproaches to High EfficiencyAssumption in Assumption in Shockley-QueisserShockley-Queisser

Approach which circumvents assumptionApproach which circumvents assumption ExamplesExamples

Input is solar Input is solar spectrumspectrum

Multiple spectrum solar cellsMultiple spectrum solar cells: transform the : transform the input spectrum to one with same energy but input spectrum to one with same energy but narrower wavelength rangenarrower wavelength range

Up/down conversionUp/down conversionThermophotonicsThermophotonics

One photon = one One photon = one electron-hole pairelectron-hole pair

Multiple absorption path solar cellsMultiple absorption path solar cells: any : any absorption path in which one photon absorption path in which one photon one- one-electron hole pairelectron hole pair

Impact ionizationImpact ionizationTwo-photon absorptionTwo-photon absorption

One quasi-Fermi level One quasi-Fermi level separationseparation

Multiple energy level solar cellsMultiple energy level solar cells: Existence of : Existence of multiple meta-stable light-generated carrier multiple meta-stable light-generated carrier populations within a single devicepopulations within a single device

Intermediate bandIntermediate bandQuantum well solar Quantum well solar cellscells

Constant temperature Constant temperature = cell temperature = = cell temperature = carrier temperaturecarrier temperature

Multiple temperature solar cellsMultiple temperature solar cells. Any device in . Any device in which energy is extracted from a difference in which energy is extracted from a difference in carrier or lattice temperaturescarrier or lattice temperatures

Hot carrier solar cellsHot carrier solar cells

Steady state Steady state (( equilibrium) equilibrium)

AC solar cellsAC solar cells: Rectification of electromagnetic : Rectification of electromagnetic wave.wave.

Rectenna solar cellsRectenna solar cells

Multiple Junction (Tandem) Multiple Junction (Tandem) Solar CellsSolar Cells

Tandem Solar CellsTandem Solar Cells

Dimroth and Kurtz MRS Bulletin

Multiple Junction (Tandem) Multiple Junction (Tandem) SolarSolar Cells Cells• Multiple junction (tandems) are Multiple junction (tandems) are

first class of approaches to first class of approaches to exceed single junction efficiency.exceed single junction efficiency.

• To reach >50% efficiency, need To reach >50% efficiency, need ideal Eideal Egg 6-stack tandem or 6-stack tandem or

equivalent, can reach ~75% of equivalent, can reach ~75% of detailed balance limit.detailed balance limit.

• Key issue in tandem is to identify Key issue in tandem is to identify materials which can be used to materials which can be used to implement ideal tandem stack.implement ideal tandem stack.

# junctions in solar cell

1 junction

2 junction

3 junction

junction 68.2%

Max con.

n Values of Band Gap (eV) %

4 0.60, 1.11, 1.69, 2.48 62.0

5 0.53, 0.95, 1.40, 1.93, 2.68 65.0

6 0.47, 0.84, 1.24, 1.66, 2.18, 2.93 67.3

7 0.47, 0.82, 1.19, 1.56, 2.0, 2.5, 3.21 68.9

8 0.44, 0.78, 1.09, 1.4, 1.74, 2.14, 2.65, 3.35 70.2

UD - DARPA: Very High Efficiency Solar CellUD - DARPA: Very High Efficiency Solar Cell

• Goal 50% Efficient Solar ModuleGoal 50% Efficient Solar Module– Prototype: 0.5W 10 cmPrototype: 0.5W 10 cm22

– Reduce weight of batteries carried by Reduce weight of batteries carried by soldiersoldier

– Initial application: charge batteries Initial application: charge batteries for flashlightfor flashlight

– Less sensitive to spectral variationLess sensitive to spectral variation– Need for tracking reducedNeed for tracking reduced

Best efficiency 42.7 % (individual cells: ~ 20 suns)

Multiple Spectrum Solar CellsMultiple Spectrum Solar Cells

Multiple Spectrum Solar CellsMultiple Spectrum Solar CellsMultiple spectrum devices: take the input solar spectrum, and change it to a new Multiple spectrum devices: take the input solar spectrum, and change it to a new spectrum with the same power densityspectrum with the same power density

Does not need to be incorporated into solar cell – can use existing solar cells, and Does not need to be incorporated into solar cell – can use existing solar cells, and add additional optical coatingsadd additional optical coatings

Does not require electrical Does not require electrical transport of generated transport of generated carriers – no contacts, carriers – no contacts, collection, resistivity, collection, resistivity, mobility issues.mobility issues.

Efficient optical processesEfficient optical processesdesired for applicationsdesired for applicationsother than solar – other than solar – development effort is development effort is shared.shared.

Requires efficient opticalRequires efficient opticalconversion over broadconversion over broadspectrum.spectrum.

Multiple Spectrum Solar CellsMultiple Spectrum Solar CellsApproaches for multiple spectrum solar cells.Approaches for multiple spectrum solar cells.

Thermophotonics: Use thermally-excited LED to generate a narrow solar spectrum. Thermophotonics: Use thermally-excited LED to generate a narrow solar spectrum.

Assuming efficient spectrum conversion and max concentration, efficiency can Assuming efficient spectrum conversion and max concentration, efficiency can be >80%be >80%

Requires demonstration of efficient thermally-excited LED and cooling from light Requires demonstration of efficient thermally-excited LED and cooling from light emissionemission

Using known materials and biases, efficiency is 50%.Using known materials and biases, efficiency is 50%.

Biased

Multiple Absorption Path Multiple Absorption Path (Impact Ionization) Solar Cells(Impact Ionization) Solar Cells

Change absorption mechanisms such Change absorption mechanisms such that one photon that one photon one electron-hole one electron-hole pairpair

Mechanisms include:Mechanisms include:

Two-photon absorption Two-photon absorption

Impact ionization/Auger generationImpact ionization/Auger generation

Absorption process have beenAbsorption process have beenobserved in bulk materials, but observed in bulk materials, but absorption coefficient is very small – absorption coefficient is very small – e.g., quantum efficiency > 80% in silicon e.g., quantum efficiency > 80% in silicon solar cells.solar cells.

Materials with quantum confinement Materials with quantum confinement allow increases in alternate absorption allow increases in alternate absorption processes.processes.

Multiple Absorption Path Solar CellsMultiple Absorption Path Solar Cells

Multiple Exciton GenerationMultiple Exciton Generation

Hot electron cooling Hot electron cooling generates multiple generates multiple excitations viaexcitations via Reverse Auger Reverse Auger Process.Process.

Higher voltage:Higher voltage: Extracting hot-Extracting hot-electrons before electrons before they cool down.they cool down.

Higher Current:Higher Current:Reverse Auger Reverse Auger process is faster process is faster than the hot than the hot electron cooling.electron cooling.

MRS BULLETIN • VOLUME 32 • MARCH 2007

Impact ionization or multiple exciton generation demonstrated efficient absorption Impact ionization or multiple exciton generation demonstrated efficient absorption processes in PbS and PbSe colloidal processes in PbS and PbSe colloidal quantum dotsquantum dots..

Efficiency depends on number of excitons generated (measured by quantum Efficiency depends on number of excitons generated (measured by quantum efficiency) and threshold energy (Eth). For a photon with energy efficiency) and threshold energy (Eth). For a photon with energy mmEg, Eg, should should generate generate mm electron-hole pairs. electron-hole pairs.

Efficiency for demonstrated processes is similar to three junction tandem.Efficiency for demonstrated processes is similar to three junction tandem.

R.J. Ellingson, M.C. Beard, J.C. Johnson, P.Yu, O.I. Micic, A.J. Nozik, A. Shabaev, and A.L. Efros “Highly Efficient Multiple Exciton Generation in Colloidal PbSe and PbS Quantum Dots” Nano Letters Vol. 5, No. 5 p. 865-871 (2005)

Multiple Absorption Path Solar CellsMultiple Absorption Path Solar Cells

Multiple Energy Level /Multiple Energy Level /Quantum Dot Solar Cells Quantum Dot Solar Cells

Quantum Dot Solar CellsQuantum Dot Solar Cells

• An ordered array of QD allows a multiple energy level solar cell via formation of mini-bands (also called intermediate band or hot carrier solar cells).

• Bands formed by overlap of energy levels in QD array.• Band structure of an intermediate band solar cell requires: (1) Three-

level band structure; (2) Fermi-level at intermediate band.• Need to determine material system to implement QD MEL solar cell.

p-type

n-type

intrinsic with quantum dots

Introduce more than a single quasi-Fermi level separation by introducing additional energy levels or bands, such that extracted energy of photon energy of band gap and

The energy levels must all simultaneously be radiatively coupled.

Energy levels can be spatially localized (energy levels) or interacting to form mini-bands.

Lower Voc.

Can use quantum dots, quantum wires, quantum wells.

Multiple Energy Level Solar CellsMultiple Energy Level Solar Cells

Nanocomposite Solar CellsNanocomposite Solar Cells

• Energy from light frees Energy from light frees electron-hole pairselectron-hole pairs

• Electrical field sends Electrical field sends electron to n-side and hole electron to n-side and hole to p-sideto p-side

• Power created (I * V)Power created (I * V)– Current (I) due to electron Current (I) due to electron

flowflow– Voltage (V) due to electric Voltage (V) due to electric

fieldfield

Basic Solar Cell Layout Basic Solar Cell Layout

Nature’s wayNature’s way

• Photosynthesis: Light harvesting complex embedded Photosynthesis: Light harvesting complex embedded in folded membrane (Chloroplast)in folded membrane (Chloroplast)

• Multiple interfaces Multiple interfaces high optical depth high optical depth

Blended Molecular MaterialsBlended Molecular Materials

• Blend hole accepting with Blend hole accepting with electron accepting materialelectron accepting material

• Length scale of blend ~ exciton Length scale of blend ~ exciton diffusion lengthdiffusion length

• Charge separation at D-A Charge separation at D-A interfaceinterface

• Continuous paths for electron Continuous paths for electron and hole percolationand hole percolation

Electric field

photon

electron transport

hole transport

anode cathode

anode cathode hole acceptor

electron acceptor

Dye Sensitized Solar CellDye Sensitized Solar Cell

Electrolytes:Electrolytes:Room Temperature IonicRoom Temperature Ionic liquids (RTILs) (Redox Couple in a liquids (RTILs) (Redox Couple in a solvent.solvent.

Dyes:Dyes:N3: cis-(NCS)2bis(4,4’-N3: cis-(NCS)2bis(4,4’-dicarboxy-2,2’bipyridine)-ruthenium(II).dicarboxy-2,2’bipyridine)-ruthenium(II).Black Dye:

Ionic Liquids Viscosity(mPa s)

(mA cm2 )Voc

(mV)FF η(%)

EMImTFSI 39 9.4 550 0.45 2.4

EMImBF4 43 9.9 602 0.55 3.3BMImPF6 352 4.3 576 0.62 1.6BPTFSI 72 6.3 577 0.56 2.0EMImDCA 21 7.8 703 0.66 3.8

Quantum Confinement EffectQuantum Confinement Effect• Efros and Efros (1982 Sov. Phys.

Semicond.) first proposed the quantum confinement effect based on the experimental findings by Ekimov and Onushchenko (1981 JETP Lett.) of the size effect on the blue shift in the main exciton absorption of CuCl (30 Å) nanocrystallite.

• The confinement effect on the band gap, EG, of a nanosolid of radius R was expressed as:

Band Gap Variation with Particle SizeBand Gap Variation with Particle Size

Bohr Radius of Si = 4.6 nm at 300K, Band Gap

of Bulk Si = 1.1. eV

Bohr radius of Ge = 24 nm at 300K, Band Gap

of bulk Ge = 0.66 eV

Nanocomposite Cell SchematicsNanocomposite Cell Schematics

Schematic of Desired Solar cellSchematic of Desired Solar cell

Bohr radius of Ge = 24 nm at 300K, Band Gap of bulk Ge = 0.66 eVBohr radius of Ge = 24 nm at 300K, Band Gap of bulk Ge = 0.66 eV

Ge-Metal junction

Electron

TiO2-TCO junction

Energy Band Diagram ofEnergy Band Diagram ofTiOTiO22-Ge Nanocomposite-Ge Nanocomposite

• ••

•• •

Why TiOWhy TiO22-Ge?-Ge?• A very simple fabrication process can be used. A very simple fabrication process can be used. • An initial amorphous composite of TiOAn initial amorphous composite of TiO22-Ge can be -Ge can be

deposited as a thin films. deposited as a thin films. • The electronegativity of Ti is lower than that of Ge The electronegativity of Ti is lower than that of Ge • The thermodynamics and relative stabilities of the The thermodynamics and relative stabilities of the

GeOGeO22 and TiO and TiO22 can be exploited by a controlled can be exploited by a controlled deposition and annealing procedures to obtain the deposition and annealing procedures to obtain the right size and size distribution of the Ge nanodots.right size and size distribution of the Ge nanodots.

• All layers (including active and non-active) can be All layers (including active and non-active) can be fabricated in a single multi-target sputtering system.fabricated in a single multi-target sputtering system.

• Without any multi-junction configuration, and only by Without any multi-junction configuration, and only by the introduction of different sizes Ge nanodots in TiOthe introduction of different sizes Ge nanodots in TiO22 matrix, it is possible to absorb a wide range of solar matrix, it is possible to absorb a wide range of solar radiation with energies in UV to VIS to IR. radiation with energies in UV to VIS to IR.

• All this is accomplished in a single active layer. All this is accomplished in a single active layer. • Bohr radius of Ge is relatively large, 24 nm, therefore, it Bohr radius of Ge is relatively large, 24 nm, therefore, it

is easy to make size gradient of Ge nanodots in the TiOis easy to make size gradient of Ge nanodots in the TiO22 matrix. matrix.

• TiOTiO22-Ge is cost effective and environmentally stable and -Ge is cost effective and environmentally stable and the processes involved have very small, if any, the processes involved have very small, if any, environmental footprints.environmental footprints.

Why TiOWhy TiO22-Ge?-Ge?

HRTEM (Planar)HRTEM (Planar)

Band gap shifts due to change in Ge Band gap shifts due to change in Ge concentration concentration

0 1 2 3 4 5 6 7 80

7x102 A6 (200W, 6000C)

C6 (200W, 6000C)

B6 (200W, 6000C)

cm-1 e

E= h (eV)

Ge Particle size

Ge concentration and particle size are related

I-V Curve of the Solar CellsI-V Curve of the Solar Cells

World Energy ProductionWorld Energy Production

BP 2006 statistical review

Hashimoto PredictionsHashimoto Predictions

Conclusions and Path ForwardConclusions and Path Forward

• ALL technological pathways to acquire renewable ALL technological pathways to acquire renewable energy are, by definition, unsustainable. energy are, by definition, unsustainable.

• It is too late to address the question of It is too late to address the question of sustainability. sustainability.

• There are many technological and non-There are many technological and non-technological formulae for the achievement of technological formulae for the achievement of surviving with nature including:surviving with nature including:- consumption reduction- consumption reduction- increase in efficiency of power consumption- increase in efficiency of power consumption- life style alteration- life style alteration- - renewable energies, etc.renewable energies, etc.

Current ParadigmCurrent Paradigm

Energy and the Environment Robert Ristinen

Jack Kraushaar

MarocMaroc

Invert the ParadigmInvert the Paradigm

Inverted ParadigmInverted Paradigm

• MarocMaroc

Nanomaterials and Thin Films GroupNanomaterials and Thin Films Group

Not present:Not present:Bakhtyar AliBakhtyar AliInci BahtyarInci Bahtyar

NSF ACT NSF ACT NSF NIRTNSF NIRT

� شكرًا�شكرًا

energy and nanotechnology issues and opportunities in photovoltaics s. ismat shah physics and...

global concerns1

global concernstop

global pv production

new materials

partial solution

explicit solution

new technology

new technologies

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