Energy and Nanotechnology Issues and Opportunities in Photovoltaics

Energy and Nanotechnology Issues and Opportunities in Photovoltaics S. Ismat Shah Physics and Astronomy Materials Science and Engineering Senior Policy Fellow Center for Energy and Environment Policy University of Delaware ICNMRE, Al Maghrib, 2010

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

Top Ten Global Concerns

Top Ten Global Concerns 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Population

Top Ten Global Concerns 1. 2. 3. 4. 5. 6. 7. 8. 9. Democracy 10. Population

Top Ten Global Concerns 1. 2. 3. 4. 5. 6. 7. 8. Education 9. Democracy 10. Population

Top Ten Global Concerns 1. 2. 3. 4. 5. 6. 7. Disease 8. Education 9. Democracy 10. Population

Top Ten Global Concerns 1. 2. 3. 4. 5. 6. Terrorism and War 7. Disease 8. Education 9. Democracy 10. Population

Top Ten Global Concerns 1. 2. 3. 4. 5. Poverty 6. Terrorism and War 7. Disease 8. Education 9. Democracy 10. Population

Top Ten Global Concerns 1. 2. 3. 4. Environment 5. Poverty 6. Terrorism and War 7. Disease 8. Education 9. Democracy 10. Population

Top Ten Global Concerns 1. 2. 3. Food 4. Environment 5. Poverty 6. Terrorism and War 7. Disease 8. Education 9. Democracy 10. Population

Top Ten Global Concerns 1. 2. Water 3. Food 4. Environment 5. Poverty 6. Terrorism and War 7. Disease 8. Education 9. Democracy 10. Population

Top Ten Global Concerns 1. Energy 2. Water 3. Food 4. Environment 5. Poverty 6. Terrorism and War 7. Disease 8. Education 9. Democracy 10. Population

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

The Global Picture Maroc Founded Fossil Fuel Derived Energy

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

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

Radiant Facts Diameter: About 100 times that of earth Mass: 99. 8% of the Solar System (Jupiter has most of the rest) Core Temperature: 15. 6 x 106 K Surface Temperature: 5800 K Energy Production: 386 billion megawatts Insolation: 1000 - 250 Watts per square meter Age: 4. 5 billion Years (5 billion years more to go)

Nanotechnology and Photovoltaics

PV Production

Case of Maroc IEA UNO

Total Consumption

Total Crude Oil Production

Proven Reserves

World Energy Production BP 2006 statistical review

Current PV Status • 2008: Global PV production 7 GW • 2008: Cumulative installed PV electricity generation capacity in the world was around 15 GW, with Europe accounting for more than 60% of this (9. 5 GW) • China as the new leading producer of solar cells, with an annual production of about 2. 4 GW, followed by Europe with 1. 9 GW, Japan with 1. 2 GW and Taiwan with 0. 8 GW. (Where is USA? )

Hashimoto Predictions

Hashimoto Solution

The Disconnect 1. Materials Issues 2. Device Issues 3. Not a chance!

Nate Lewis (CIT) calculations

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

How does the area required changes with high efficiency solar cells? • 20 TWatt model • With 10% cells, we needed 5 x 1011 square meters of solar cells. • With 50% cells, we will still need about 1011 square meters of solar cells. • We currently produce about 1 million sq. meters of solar panels. • We need to increase production by 5 orders of magnitude.

How much material do we need? • For 1 x 1011 m 2, we will need (1011 x 104 x 0. 01 cm 3)/(2. 33 g/cm 3) = 5 x 109 Kg of Silicon

How much material do we need? • For 1 x 1011 m 2, we will need (1011 x 104 x 0. 01 cm 3)/(2. 33 g/cm 3) = 5 x 109 kg of Silicon Each kg of Si requires 15 kg of carbon to produce electronic grade Si. To obtain a kg of refined grade of (poly)Si, we use up about 200 k. Wh of energy emitting 40 kg of CO 2, using 1000 gallons of water. (Availability, Toxicity)

Device Issues

Shockley-Queisser Limit Three types of losses are described: 1. Sub-band radiation 2. Radiative recombination 3. Thermalization

Sub-band Radiation hn < Eg Eg Non-absorbance of photons with energy below the bandgap energy

Radiative Recombination • Second Loss Mechanism: Radiative recombination, the inverse of photovoltaic electron-hole pair generation process. • It is a fundamental loss-mechanism that is always present at any non-zero cell temperature.

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

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

Loss Mechanisms

Breaking S-Q limit

Exceeding Shockley–Queisser limit 1. Tandem cells (University of Delaware DARPA $57 M ($147 M) Project). 2. Hot carrier solar cells 4. Multiband impurity solar cells 5. Thermophotovoltaic/thermophotonic cells 3. Solar cells producing multiple electron- hole pairs per photon through impact ionization 6. Nanocomposite solar cells

Approaches to High Efficiency Assumption in Shockley-Queisser Approach which circumvents assumption Examples Input is solar spectrum Multiple spectrum solar cells: transform the input spectrum to one with same energy but narrower wavelength range Up/down conversion Thermophotonics One photon = one electron-hole pair Multiple absorption path solar cells: any absorption path in which one photon oneelectron hole pair Impact ionization Two-photon absorption One quasi-Fermi level separation Multiple energy level solar cells: Existence of multiple meta-stable light-generated carrier populations within a single device Intermediate band Quantum well solar cells Constant temperature = cell temperature = carrier temperature Multiple temperature solar cells. Any device in which energy is extracted from a difference in carrier or lattice temperatures Hot carrier solar cells Steady state ( equilibrium) AC solar cells: Rectification of electromagnetic wave. Rectenna solar cells

Multiple Junction (Tandem) Solar Cells

Tandem Solar Cells

Dimroth and Kurtz MRS Bulletin Tandem Solar Cells

Multiple Junction (Tandem) Solar Cells • Multiple junction (tandems) are first class of approaches to exceed single junction efficiency. • To reach >50% efficiency, need ideal Eg 6 -stack tandem or equivalent, can reach ~75% of detailed balance limit. • Key issue in tandem is to identify materials which can be used to implement ideal tandem stack. # junctions in solar cell 1 sun h Max con. h 1 junction 30. 8% 40. 8% 2 junction 42. 9% 55. 7% 3 junction 49. 3% 63. 8% junction 68. 2% 86. 8%

UD - DARPA: Very High Efficiency Solar Cell • Goal 50% Efficient Solar Module – Prototype: 0. 5 W 10 cm 2 – Reduce weight of batteries carried by soldier – Initial application: charge batteries for flashlight – Less sensitive to spectral variation – Need for tracking reduced Best efficiency 42. 7 % (individual cells: ~ 20 suns)

Multiple Spectrum Solar Cells

Multiple Spectrum Solar Cells Multiple spectrum devices: take the input solar spectrum, and change it to a new spectrum with the same power density Does not need to be incorporated into solar cell – can use existing solar cells, and additional optical coatings Does not require electrical transport of generated carriers – no contacts, collection, resistivity, mobility issues. Efficient optical processes desired for applications other than solar – development effort is shared. Requires efficient optical conversion over broad spectrum.

Multiple Spectrum Solar Cells Approaches for multiple spectrum solar cells. Thermophotonics: Use thermally-excited LED to generate a narrow solar spectrum. Assuming efficient spectrum conversion and max concentration, efficiency can be >80% Requires demonstration of efficient thermally-excited LED and cooling from light emission Using known materials and biases, efficiency is 50%. Biased

Multiple Absorption Path (Impact Ionization) Solar Cells

Multiple Absorption Path Solar Cells Change absorption mechanisms such that one photon one electron-hole pair Mechanisms include: Two-photon absorption Impact ionization/Auger generation Absorption process have been observed in bulk materials, but absorption coefficient is very small – e. g. , quantum efficiency > 80% in silicon solar cells. Materials with quantum confinement allow increases in alternate absorption processes.

Multiple Exciton Generation Higher voltage: Extracting hotelectrons before they cool down. Hot electron cooling generates multiple excitations via Reverse Auger Process. Higher Current: Reverse Auger process is faster than the hot electron cooling. MRS BULLETIN • VOLUME 32 • MARCH 2007

Multiple Absorption Path Solar Cells Impact ionization or multiple exciton generation demonstrated efficient absorption processes in Pb. S and Pb. Se colloidal quantum dots. Efficiency depends on number of excitons generated (measured by quantum efficiency) and threshold energy (Eth). For a photon with energy m Eg, should generate m electron-hole pairs. 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 Pb. Se and Pb. S Quantum Dots” Nano Letters Vol. 5, No. 5 p. 865 -871 (2005)

Multiple Energy Level / Quantum Dot Solar Cells

Quantum 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) Threelevel band structure; (2) Fermi-level at intermediate band. • Need to determine material system to implement QD MEL solar cell. intrinsic with quantum dots p-type n-type

Multiple Energy Level Solar Cells 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 minibands. Lower Voc. Can use quantum dots, quantum wires, quantum wells.

Nanocomposite Solar Cells

Basic Solar Cell Layout • Energy from light frees electron-hole pairs • Electrical field sends electron to n-side and hole to p-side • Power created (I * V) – Current (I) due to electron flow – Voltage (V) due to electric field

Nature’s way • Photosynthesis: Light harvesting complex embedded in folded membrane (Chloroplast) • Multiple interfaces high optical depth

Blended Molecular Materials • Blend hole accepting with electron accepting material • Length scale of blend ~ exciton diffusion length • Charge separation at D-A interface • Continuous paths for electron and hole percolation

Dye Sensitized Solar Cell Electrolytes: Room Temperature Ionic liquids (RTILs) (Redox Couple in a solvent. Dyes: N 3: cis-(NCS)2 bis(4, 4’dicarboxy-2, 2’bipyridine)-ruthenium(II). Black Dye:

Quantum 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 Cu. Cl (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 Size Bohr Radius of Si = 4. 6 nm at 300 K, Band Gap of Bulk Si = 1. 1. e. V Bohr radius of Ge = 24 nm at 300 K, Band Gap of bulk Ge = 0. 66 e. V

Nanocomposite Cell Schematics Electron • Ge-Metal junction • • • Ti. O 2 -TCO junction ° ° ° Hole Schematic of Desired Solar cell Energy Band Diagram of Ti. O 2 -Ge Nanocomposite Bohr radius of Ge = 24 nm at 300 K, Band Gap of bulk Ge = 0. 66 e. V

Why Ti. O 2 -Ge? • A very simple fabrication process can be used. • An initial amorphous composite of Ti. O 2 -Ge can be deposited as a thin films. • The electronegativity of Ti is lower than that of Ge • The thermodynamics and relative stabilities of the Ge. O 2 and Ti. O 2 can be exploited by a controlled deposition and annealing procedures to obtain the right size and size distribution of the Ge nanodots.

Why Ti. O 2 -Ge? • All layers (including active and non-active) can be fabricated in a single multi-target sputtering system. • Without any multi-junction configuration, and only by the introduction of different sizes Ge nanodots in Ti. O 2 matrix, it is possible to absorb a wide range of solar radiation with energies in UV to VIS to IR. • All this is accomplished in a single active layer. • Bohr radius of Ge is relatively large, 24 nm, therefore, it is easy to make size gradient of Ge nanodots in the Ti. O 2 matrix. • Ti. O 2 -Ge is cost effective and environmentally stable and the processes involved have very small, if any, environmental footprints.

HRTEM (Planar)

Band gap shifts due to change in Ge concentration and particle size are related Ge Particle size

I-V Curve of the Solar Cells

World Energy Production BP 2006 statistical review

Hashimoto Predictions

Conclusions and Path Forward • ALL technological pathways to acquire renewable energy are, by definition, unsustainable. • It is too late to address the question of sustainability. • There are many technological and non-technological formulae for the achievement of surviving with nature including: - consumption reduction - increase in efficiency of power consumption - life style alteration - renewable energies, etc.

Current Paradigm Maroc • Energy and the Environment Robert Ristinen Jack Kraushaar

Invert the Paradigm

Inverted Paradigm

Inverted Paradigm • Maroc

Inverted Paradigm • Maroc

Inverted Paradigm • Maroc

Nanomaterials and Thin Films Group Not present: Bakhtyar Ali Inci Bahtyar

NSF ACT NSF NIRT