photovoltaics: issues and opportunities · photovoltaics: issues and opportunities s. ismat shah...

Photovoltaics: Issues and Photovoltaics: Issues and OpportunitiesOpportunities

S. Ismat ShahS. Ismat ShahPhysics and AstronomyPhysics and Astronomy

Materials Science and EngineeringMaterials Science and EngineeringSenior Policy FellowSenior Policy Fellow

Institute of Energy and Environmental PolicyInstitute of Energy and Environmental PolicyUniversity of DelawareUniversity of Delaware

NCP March 2010

PV ProductionPV Production

US Share of World PV ProductionUS Share of World PV Production

StatusStatus

•• 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. (Where is USA?)GW and Taiwan with 0.8 GW. (Where is USA?)

World Energy ProductionWorld Energy Production

BP 2006 statistical review

Hashimoto PredictionsHashimoto Predictions

Hashimoto SolutionHashimoto Solution

The DisconnectThe Disconnect

1. Materials Issues1. Materials Issues

2. Device Issues2. Device Issues

3. 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?

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 We need to increase production by 5 orders of magnitude.of 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 10x 104 4 x 0.01 cmx 0.01 cm33)/(2.33 g/cm)/(2.33 g/cm33))

= = 5 x 105 x 1099 Kg of SiliconKg 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 10x 104 4 x 0.01 cmx 0.01 cm33)/(2.33 g/cm)/(2.33 g/cm33))

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

Each kg of Si requires 4 kg of carbon just in the Each kg of Si requires 4 kg of carbon just in the first step of the processfirst step of the process

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.

AvailabilityAvailability

•• Si Si ‐‐ also used for flat panel also used for flat panel •• Cost of Si gone up from $25/kg in 2004 to around Cost of Si gone up from $25/kg in 2004 to around $400/kg in 2008. It is supposed to go down with the $400/kg in 2008. It is supposed to go down with the new manufacturing plants coming on line.new manufacturing plants coming on line.

•• Most nonMost non‐‐Si based solar technology relies on a Si based solar technology relies on a transparent conducting electrode, Indium Tin Oxide transparent conducting electrode, Indium Tin Oxide (ITO). CIGS also uses Indium.(ITO). CIGS also uses Indium.

•• Indium is produced in limited quantities. Indium Indium is produced in limited quantities. Indium demand and prices soared: 100t in 2004 at $200/kg demand and prices soared: 100t in 2004 at $200/kg to 750t in 2008 at $1000/kg.to 750t in 2008 at $1000/kg.

Toxicity IssuesToxicity Issues•• Si solar cells based on thin films use Silane (SiHSi solar cells based on thin films use Silane (SiH44) for ) for manufacturing. Silane ismanufacturing. Silane is–– Pyrophoric: Dilute mixtures are even more explosive.Pyrophoric: Dilute mixtures are even more explosive.–– Toxic: Toxic: LC50LC50 is 0.96% (4 hours) is 0.96% (4 hours)

•• CuInGaSeCuInGaSe22 (CIGS):(CIGS):–– Tolerable upper intake level (UL) for selenium at 400 mcg.Tolerable upper intake level (UL) for selenium at 400 mcg.–– Causes cardiomyopthia, Liver damage, etc. Causes cardiomyopthia, Liver damage, etc.

•• Cadmium Sulfide:Cadmium Sulfide:–– CarcenogenicCarcenogenic–– Typical PEL 0.05 mg/mTypical PEL 0.05 mg/m33

•• PostPost‐‐use contamination issues.use contamination issues.

Device IssuesDevice Issues

ShockleyShockley‐‐Queisser LimitQueisser Limit

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

SubSub‐‐band Radiationband Radiation

Eghν < Eg

NonNon‐‐absorbance of photons with energy below the bandgap energyabsorbance 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 electronphotovoltaic electron‐‐hole pair hole pair generation process.generation process.

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

Radiative RecombinationRadiative Recombination

Recombination of electrons and holes generated by (a) optical absorption and (b) a forward‐biased p‐njunction.


•• 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 electronelectron‐‐hole pairs generated by photons with hole pairs generated by photons with energy above the bandenergy above the band‐‐gap (Eg) energy.gap (Eg) energy.

Loss MechanismsLoss Mechanisms

What is not included in SWhat is not included in S‐‐Q ModelQ Model

Some of the typically lossSome of the typically loss‐‐mechanisms observed mechanisms observed in practical PV cells that have been idealized in in practical PV cells that have been idealized in SQ model are:SQ model are:

•• nonnon‐‐radiative recombination (Phonons)radiative recombination (Phonons)•• contact resistancecontact resistance•• series resistance offered by semiconductorseries resistance offered by semiconductor

Also not considered:Also not considered:•• impact ionization or multiimpact ionization or multi‐‐photon processesphoton processes

•• The limiting efficiency (SQ limit) is then given byThe limiting efficiency (SQ limit) is then given by

•• where where PPsunsun is the solar power incident on the cell, is the solar power incident on the cell, nnabsabs is the number of photons absorbed per unit time is the number of photons absorbed per unit time nnrrrr(V ) the net number of photons re(V ) the net number of photons re‐‐radiated per unit radiated per unit timetime through radiative recombination which is applied through radiative recombination which is applied bias (V ) dependent at nonbias (V ) dependent at non‐‐zero cell temperatures T. zero cell temperatures T.



•• It can be shown that nIt can be shown that nrrrr(V ) can be written as(V ) can be written as

here h is the Planck's constant, K is the Boltzmann here h is the Planck's constant, K is the Boltzmann constant and constant and HHoutout is the etendue of the solid angle in is the etendue of the solid angle in which rewhich re‐‐radiated radiation escapes.radiated radiation escapes.

•• The usual SQ limit assumes that The usual SQ limit assumes that rere‐‐radiated radiation radiated radiation escapes isotropically escapes isotropically so Hso Houtout = n= n22pA with A being the pA with A being the area of the cell. area of the cell.


•• nnabsabs can be estimated by integrating the AM1.5 global can be estimated by integrating the AM1.5 global spectrum or, for simplicity, the black body radiation spectrum or, for simplicity, the black body radiation spectrum at Tspectrum at Tsunsun = 6000K which is= 6000K which is

where Hwhere Hinin = n= n22ππA (full concentration) or sinA (full concentration) or sin22((θθsunsun))ππA A (no concentration) is the etendue of the solid angle of (no concentration) is the etendue of the solid angle of the incoming photons with the incoming photons with θθsunsun = 0.267 being the half = 0.267 being the half angle that solar disc subtends on the cell.angle that solar disc subtends on the cell.


•• If we approximate the input spectrum by If we approximate the input spectrum by black body radiation thenblack body radiation then

•••••• where Hwhere Hsunsun = sin= sin22(q(qsunsun))ππA and sA and ssbsb is the Stefanis the Stefan‐‐Boltzmann constant.Boltzmann constant.


•• For the cell illuminated by a black body source For the cell illuminated by a black body source of photons at 6000K and assuming the cell of photons at 6000K and assuming the cell temperature to be 300K, the upper limit temperature to be 300K, the upper limit reachable with a single semiconductorreachable with a single semiconductor

•• solar cell is:solar cell is:

Breaking SBreaking S‐‐Q limitQ limit

Efficiency EnhancementEfficiency Enhancement

•• FirstFirst‐‐generation cells: generation cells: Based on expensive silicon Based on expensive silicon wafers. wafers.

•• SecondSecond‐‐generation cells:generation cells:Based onBased on thin films of less thin films of less expensive materials.expensive materials.

•• ThirdThird‐‐generation cells: generation cells: Research goals: may use Research goals: may use carrier multiplication, hot carrier multiplication, hot electron extraction, electron extraction, multiple junctions, sunlight multiple junctions, sunlight concentration, or new concentration, or new materials.materials.

George W. Crabtree and Nathan S. LewisPhysics Today, March 2007, page 37Physics Today, March 2007, page 37

Exceeding ShockleyExceeding Shockley––Queisser limitQueisser 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 electron3. 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 Efficiency

Assumption in Assumption in ShockleyShockley--QueisserQueisser

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 electronelectron--hole pairhole pair

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

Impact ionizationImpact ionizationTwoTwo--photon absorptionphoton absorption

One quasiOne quasi--Fermi level Fermi level separationseparation

Multiple energy level solar cellsMultiple energy level solar cells: Existence of : Existence of multiple metamultiple meta--stable lightstable light--generated carrier 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

Tandem Solar CellsTandem Solar Cells

Dimroth and Kurtz MRS Bulletin

Multiple Junction (Tandem) Multiple Junction (Tandem) SolarSolar CellsCells•• 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 66‐‐stack tandem or 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

1 sun η

30.8%

42.9%

49.3%

∞ junction 68.2%

Max con. η

40.8%

55.7%

63.8%

86.8%

n Values of Band Gap (eV) η % 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

DARPA: Very High Efficiency Solar CellDARPA: 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)

Inverted Metamorphic Solar CellsInverted Metamorphic Solar Cells

J.F. Geisz, et. al. NREL 2008

Inverted Metamorphic Solar CellsInverted Metamorphic Solar Cells

Multiple Spectrum Solar CellsMultiple Spectrum Solar Cells

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

Does not need to be incorporated into solar cell Does not need to be incorporated into solar cell –– can use existing solar cells, and 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 carriers –– no contacts, 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 thermallyThermophotonics: Use thermally‐‐excited LED to generate a narrow solar spectrum. excited LED to generate a narrow solar spectrum.

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

Requires demonstration of efficient thermallyRequires demonstration of efficient thermally‐‐excited LED and cooling from excited LED and cooling from light emissionlight emission

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 electronone electron‐‐hole pairhole pair

Mechanisms include:Mechanisms include:

TwoTwo‐‐photon absorption 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 viaReverse Auger Reverse Auger Process.Process.

Higher voltage:Higher voltage:Extracting hotExtracting 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 efImpact ionization or multiple exciton generation demonstrated efficient absorption ficient 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 Efficiency depends on number of excitons generated (measured by quantum quantum efficiency) and threshold energy (Eth). For a photon with energyefficiency) and threshold energy (Eth). For a photon with energy mm××Eg, Eg, should should generate generate mm electronelectron‐‐hole pairs.hole pairs.

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

Basic Solar Cell Layout Basic Solar Cell Layout

•• Energy from light frees Energy from light frees electronelectron‐‐hole pairshole pairs

•• Electrical field sends Electrical field sends electron to nelectron to n‐‐side and hole side and hole to pto p‐‐sideside

•• 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

•• Energy from light frees Energy from light frees electronelectron‐‐hole pairshole pairs

•• Electrical field sends Electrical field sends electron to nelectron to n‐‐side and hole side and hole to pto p‐‐sideside

•• 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

NatureNature’’s ways 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 depthhigh 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 DCharge separation at D‐‐A 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

LUMO

HOMO

LUMO

HOMO

electron acceptor

1 2e-

h+

e-

Dye Sensitized Solar CellDye Sensitized Solar Cell

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

Dyes:Dyes:N3: cisN3: cis‐‐(NCS)2bis(4,4(NCS)2bis(4,4’’‐‐dicarboxydicarboxy‐‐2,22,2’’bipyridine)bipyridine)‐‐ruthenium(II).ruthenium(II).Black Dye:

Ionic Liquids Viscosity(mPa s)

Jsc(mA cm2 )

Voc(mV)

FF η(%)

EMImTFSI 39 9.4 550 0.45 2.4EMImBF4 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-Metaljunction

Electron

°

TiO2-TCO junction

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

Hole

• ••

•• •

° °

°

°°

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 TiOand 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 nonAll layers (including active and non‐‐active) can be active) can be fabricated in a single multifabricated in a single multi‐‐target sputtering system.target sputtering system.

•• Without any multiWithout any multi‐‐junction configuration, and only by junction configuration, and only by the introduction of different sizes Ge nanodots in TiOthe introduction of different sizes Ge nanodots in TiO22matrix, 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 TiO22matrix. 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

1x102

2x102

3x102

4x102

5x102

6x102

7x102 A6 (200W, 6000C) C6 (200W, 6000C) B6 (200W, 6000C)

(α

hν)1/

2 (cm

-1 e

V)1/

2

E= hν (eV)

Ge Particle size

Ge concentration and particle size are related

II‐‐V Curve of the Solar CellsV Curve of the Solar Cells

Hashimoto PredictionsHashimoto Predictions

World Energy ProductionWorld Energy Production

BP 2006 statistical review

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 nonThere are many technological and non‐‐technological formulae for the achievement living technological formulae for the achievement living within the within the ““Tolerance MarginTolerance Margin”” of nature including:of nature including:‐‐ consumption reductionconsumption reduction‐‐ increase in efficiency of power consumptionincrease in efficiency of power consumption‐‐ life style alterationlife style alteration‐‐ renewable energiesrenewable energies, etc., etc.

Thin Films and Nanostructures GroupThin Films and Nanostructures GroupMSEGMSEGPHY

SPHY

S

Inci BakhtyarMatt OlsekaAndrew HackendornRoy Murray, N. Rujisamphan

Solar Module Retail Price IndexSolar Module Retail Price Index

Absorption limitsAbsorption limits•• The SQ limit assumes unit absorptivity for all photons The SQ limit assumes unit absorptivity for all photons above the bandgap. Hence, the above the bandgap. Hence, the SQ limit is independent SQ limit is independent of the thickness of the absorbing layer as well as of the thickness of the absorbing layer as well as independent of the absorption properties independent of the absorption properties of this layerof this layer(apart from its bandgap)(apart from its bandgap)

•• Real materials absorb very weakly near the bandgap Real materials absorb very weakly near the bandgap and absorption gradually increases for shorter and absorption gradually increases for shorter wavelengths. wavelengths.

•• Light trapping designs lead to a higher probability of Light trapping designs lead to a higher probability of absorption in a smaller volume of active materials absorption in a smaller volume of active materials which can lead to a decrease in the cost of PV which can lead to a decrease in the cost of PV technologies.technologies.

Light Trapping DesignsLight Trapping Designs

http://pvcdrom.pveducation.org/DESIGN/LITETRAP.HTM

Solar Cell Efficiencies: Solar Cell Efficiencies: ShockleyShockley‐‐Queisser LimitQueisser Limit

• Efficiency depends on optical concentration, details of input spectrum (i.e. if spectrum is black body, measured spectrum, etc)

• Trade‐off between voltage and current: high band gap gives large voltage, but absorbs small fraction of solar spectrum and so gives a small current.

• Maximum efficiency for band gap ~1.3 eV for space radiation, 1.1 eV and 1.34 eV for terrestrial radiation.

• Maximum single junction efficiency = 32.8% under one sun and 40.8% under max concentration (called Shockley‐Queisser limit).

photovoltaics: issues and opportunities · photovoltaics: issues and opportunities s. ismat shah...

Documents