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PHOTOVOLTAICS MATERIALS AND DEVICES
By
VIRESH DUTTA
PHOTOVOLTAIC LAB., CENTER FOR ENERGY STUDIES,
INDIAN INSTITUTE OF TECHNOLOGY , NEW DELHI 110016 INDIA
E-mail : [email protected]
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OUTLINE
PHOTOVOLTAIC EFFECT
PHOTOVOLTAIC DEVICES :
- FIRST GENERATION : CRYSTALLINE Si
- SECOND GENERATION : THIN FILMSDYE SENSITISED SOLAR CELLORGANIC SOLAR CELL
- THIRD GENERATION
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Solar Photovoltaic Generation Direct Conversion of Solar Energy
Optical Absorption – Electron Hole Pair Generation
Charge Separation by “Internal” Electric Field
Charge Transport to the Contacts Power Delivery to the Load
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Photovoltaic power generation achieved by exposing semiconductor devices called solar cells to solar radiation.
The semiconductor absorbs the incident photons .
Photo-absorption leads to electron-hole pair generation (EXCITON).
Photo-generated carriers are separated due to an electric field.
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This creates photo-voltage and photo-current through the external circuit - Power Generation.
Photovoltage maximum for open circuit – Voc
Photocurrent maximum for short circuit - Isc Power zero at both these operating points
Maximum Power Point- PMAX
Fill Factor- FF Efficiency- η
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All the physical processes internal to the device- no associated gas emission, no noise, no wear and tear ( except for slow degradation taking place )
Intermittent DC- can be converted to ac using power converting circuits, whole system pollution free and maintenance free
Battery storage for night applications –additional cost for equipment and operation & maintenance
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I-V CHARACTERISTICS
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ENERGY BAND DIAGRAM FOR P-N JUNCTION SOLAR CELL
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I-V CHARACTERISTICS OF A TYPICAL CELL
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EQUIVALENT CIRCUIT
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Solar cell is a Large Area device involving several physical processes :
Exciton (bound or unbound) creation Carrier Transport by Drift or DiffusionRecombination processes-( Band- to-Band, Intermediate state mediated, Auger, Grain Boundary)Charge carrier collection at Ohmic and Psuedo-ohmic contacts Interplay between these processes hides the complexity of the device!
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Optical Absorption
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Optimum Band Gap
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CRYSTALLINE SI
Available from Semiconductor Industry ( Euro 20-29 /Kg)
Specific Si Consumption by PV industry at present 14 Tonnes / MW ( to be reduced to 10-12 Tonnes / MW in near future)
For a PV growth rate of 27% the shortfall of economically prices Si ~ 22000 Tonnes in 2010
Dedicated Solar Grade Si production plants by all the major Si producers ( 1500-3000 Tonnes of additional Si at Euro 25 /Kg)
Lower prices can be achieved using fluidised-bed reactor or tube reactor
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Si Ingots from molten Si by crystal pulling
Czochralski Si or Directional Solidification ( Multi-crystalline Si) or Ribbon Growth.Float Zone Si – Purer and Expensive
Circular or Square Cross-section ( Psuedo-Square for Modules) : 150-200 mm Diameter or 150 mm x 150 mm
Wafering ( ~ 300 µm ) using wire saw. 180 µm wire and SiC abrasive gives kerf loss of ~ 250 µm . Wafer thickness of 180 µm ( ~ 100 µm ) with 90% yield ( currently 60%). Kerf loss reduction to 160 µm using thinner wires and smaller abrasive particles. Recycling of SiC Slurry
Reduction in loss in wafering ( ~ 50% ) by developing Ribbon technology ( EFG-Si)
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17%
7%
16%
60%
Cost of wafers Materials Processing Labour accounts Investment cost
Production cost of Si solar Cell
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TYPICAL VALUES ( Si Solar Cells):
JSC = 40 mA/cm2
VOC= 600 mVPMAX = 15 mW/cm2
FF = 0.8-0.9η = 14-16 %
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Present status of efficiencies of Present status of efficiencies of bulk Si solar cellsbulk Si solar cells
FraunhoferFraunhofer, mc, mc--Si, 1cmSi, 1cm22Laser fired contactsLaser fired contacts20.320.3
FraunhoferFraunhofer, sc, sc--SiSiLaser fired contactsLaser fired contacts21.621.6SunpowerSunpower, Si PV, Si PV--FZ,149cmFZ,149cm22All back contactAll back contact21.521.5
Univ.NewUniv.New South WalesSouth WalesPERL cell, PERL cell, 4cm4cm2224.724.7
mcmc--Si, Uni. Si, Uni. KonstanzKonstanzBuried contactBuried contact17.617.6Kyocera, 232.5cmKyocera, 232.5cm22Manufacturing processManufacturing process17.7*17.7*
scsc--Si, BP SolarSi, BP SolarBuried contactBuried contact18.318.3
UNSW Solar Car TeamUNSW Solar Car TeamBuried contactBuried contact19.519.5
Sanyo , VSanyo , Vococ=725mV=725mVHIT cell, 100cmHIT cell, 100cm2221.5*21.5*
OrganizationStrucureEfficiency(%)
2010 Target: 20% cell, 16% module efficiencies2010 Target: 20% cell, 16% module efficiencies
* Production level* Production level
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High-Efficiency Bifacial Ga-doped Cz Si Solar Cells with a-Si Back Surface Passivation
100 Ω/ƀn+ emitter
SiNx
S≤100 cm/s
0.5-2.0 Ω-cm, Ga-doped, p-type Cz Si100-200 µm thickτ = 200 µs
p-type a-Siintrinsic a-Si
ITO
Ag contacts
Ag/Al contacts
η = 19%
0.5-2.0 Ω-cm, Ga-doped, p-type Cz Si300 µm thickτ = 200 µs
p+ Al-BSF
Ag contacts
Al contact
S>500 cm/sη = 17.5%
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p/i amorphous silicontransparent
electrode
n c-Si
HIT cell
i/n amorphous silicon
200 µm
- High efficiency by excellent surface passivation with a-Si layers- Less thermal stress through low temperature process (~ 200oC)- Advantage in high temperature performance
( HIT : Heterojunction with Intrinsic Thin-Layer )
Voltage (V)
0 0.2 0.4 0.6 0.80
1
2
3
4
Cur
rent
( A)
AM-1.5, 100mW/cm2, 25Cell size : 100.0cm2
5
Out
put (
W)
1.00
1
2
3
4
5
Measurement in Sanyo
717.0mV3.86 A
Eff. F.F. 77.0 %Isc
21.3 %Voc
Structure
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Approaching the 29% limit efficiency of SiliconApproaching the 29% limit efficiency of Silicon
solar cellssolar cellsR.M.SwansonR.M.Swanson, , SunPowerSunPower CorporationCorporation
Efficiency limit: 29%
HIT
PERL
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THIN FILM SOLAR CELLS Low cost alternative (!) to Si technology Integrated Module Production Flexible Substrates Use in Buildings with improved aesthetics-
homogeneous appearance Large scale production using Thin Film
Technologies
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SUBSTRATE
Substrate device structure: Metal or Metallic coating on Glass / Polymer
Superstrate device structure: Transparent Conducting Oxide
Flexible substrates for roll to roll deposition. High temperature deposition requires expensive and
rigid substrate, whereas low temperature process can use less expensive substrates.
Major Expense in the device
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CIGS solar cells based on superstratestructure inferior to substrate structure-Interdiffusion of CdS during high temperature CIGS growth.
Na diffusion from substrate improves the grain growth. ( use of NaF)
CdTe cells use superstrate structure for contacting to CdTe. CdS diffusion helps reducing the lattice mismatch.
High temperature deposition require borosilicate glass.
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Amorphous Si solar cells on Glass and Stainless Steel substrates – Roll to roll deposition and glass-in –module-out technologies.
P-I-N cells usually fabricated with glass substrate ( superstrate configuration).
N-I-P cells on metallic substrate ( substrate configuration).
Effect of plasma on TCO coating
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Transparent Conducting Oxide N-type degenerate semiconductors with good
electrical conductivity and high transparency in the visible region.
Contact as well light transmission Bi-layer structures using a highly conducting
layer for the low resistance contact and a much thinner high resistivity layer ( called HR layer by CdTe groups and buffer layer by CIGS groups) to minimize forward current through the pinholes in the window layer.
Microstructure and texture control for HAZE-scattering assisted light absorption in a-Si solar cells ( increased path length in thin cells)
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WINDOW LAYER
Heterojunction with the absorber layer No light absorption – no photcurrent generation For high optical throughput- large band gap Thin layer – to minimize series resistance Matched electron affinity- conduction band
spikes Lattice Mismatch- important for epitaxial or
oriented growth
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Chemical Bath deposited (CBD ) CdS is mainly used.
Thinner layer( < 50 nm) over a large area-less loss in the blue region
Cd free CIGS solar cells : InxSey, ZnO, ZnOSusing PVD for in-line process.
CdTe solar cells have intermixing to minimize the effect of lattice mismatch ( 9.7%).
Very thin (~10nm) n and p layers in a-Si to allow all light absorption in the i layer.
aSiC:H as window layer
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ABSORBER
Copper Indium Gallium Diselenide and related compounds:
CuInS2 with EG ~ 1.53 eV an ideal PV material-difficult material due to S
CuInSe2 with EG ~ 1 eV – optical absorptancecoefficient 3-6 x 105/cm)
Wide range of anion-to-cation off stiochiometry. N or P type doping by introduction of native
defects. Benign nature of structural defects- devices
using polycrystalline films. Alloying with Ga, Al or S to increase the band gap
, Voc and efficiency. Tandem solar cells using alloys(?)
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Substrate
Back contact
Absorber
Buffer
Window
AR layerGrid E-beam evaporation3 µm, Al / 50nm, Ni
ProcessThick, Materials
Cleaning2-3mm, SLG(Sus, Ti, Polymide)
DC sputtering1 µm, Mo
Co-evaporation(Sputtering + Selenization,
Sulphurization)
2-3 µm Cu(In,Al,Ga)(Se,S)(Wide bandgap, CZTS)
CBD50nm, CdS(Cd-free Zn(O,S,OH)x,In(OH)S)
RF sputtering(MOCVD)
500nm, n-AZO / 50nm, i-ZnO(BZO, GZO)
E-beam evaporation100nm, MgF2
CIGS solar cell structure and process
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MIASOLE CIGS PLANT
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Cadmium Telluride:
Ideal material due to its optical and chemical properties.
Direct Band gap of 1.4-1.5 eV – optimum of photovoltaic conversion.
Cd deficiency giving p-type films ( making junction with n-CdS)
Well passivated crystallites and high chemical and thermal stability.
Activation treatment using CdCl2 Difficulty in forming good stable ohmic contact Environmental problems due to Cd ( Cd
Sequestering, end of life module treatment to remove Cd and Te and reuse of recovered material).
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Cross section of CdTe solar cell
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First Solar one of the major CdTe module producer
Safe for people, animal life and the environment
No appreciable leaching of Cd in ground water if discarded into land fill
No release of Cd in a vapour form in fire
A safe method of using Cd by sequestering in a PV module than other uses.
‘Cradle to Grave’ Technology
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Emissions from use of conventional fuels for electricity generation
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Amorphous Si :
Low process temperature- module production on flexible and low cost substrates.
Low material requirements- inherent high absorption ( no k-selection rule)
Hydrogen incorporation to eliminate dangling bonds and allow de-pinning of Fermi level.
Poor charge transport properties- use of p-i-n junction Light induced defects – Staebler-Wronski effect Degradation of cell efficiency on light exposure – stabilized
efficiency Use of thinner layers to reduced this effect- tandem cells Use of a-Si alloys for I region in different cells (SiC,SiGe) Diffusely reflecting front and back contacts for optical
confinement Micromorph solar cells using microcrystalline Si – reduction
of SW effect Low rate of deposition- VHF, ECR PECVD, Hot Filament
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Single, Double and Triple Tandem Junctions.
Microcrystlline Si, Micromorph Si
Hybrid cells of A-Si:H and microcrystalline providing ~ 75% of all thin film production
Cell stability with efficiencies ~ 10% or more Increased deposition rate
Design modification for better light harvesting
Amorphous Si
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Overv
iew
(KIE
R)
Solar Cells StructureSolar Cells Structure
GlassSnO2:F(AU) or textured ZnO:Al
p-type a-SiC:H (20nm)intrinsica-Si:H(200nm)n-type a(µc)-Si:HBuffer (ZnO)
p-type µc-Si:H (20nm)
Intrinsic µc-Si:H(2)
n-type a-Si:H (30nm)
ZnO
Ag
Glass
Textured ZnO:Al
p-type µc-Si:H (20nm)
Intrinsic µc-Si:H(2)
ZnO
Ag
n-type a-Si:H(30nm)
photon
µc-Si:H pin component cells a-Si:H/µc-Si:H pin tandem solar cells
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Stru
ctu
re &
Pro
cesses
nn--layerlayer
ZnO:AlZnO:Al TCO(front & back)TCO(front & back)Glass
Glass
Glass
µc-Si:H
Glass
µc-Si:H
GlassGlass
pp--layerlayerii--layerlayer
InIn--line line transfer transfer in a vacuumin a vacuum
PECVD
60MHz VHFCVD
PECVD
rf sputtering
Clean room process - e-beam & thermal evaporator- rf & dc sputter- Annealing furnace- Laser scriber
Deposition of tandem solar cells- 2 PECVDs, 1 VHFCVD, 1 HWCVD, 1 rf sputter
Reduced contamination, improved interface
Fabrication and Characterization Fabrication and Characterization ApparatusApparatus
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Stru
ctu
re &
Pro
cesses
ExperimentalExperimental
Textured front ZnO:AlDeposition : rf magnetron sputtering with 4” ZnO:Al2O3(2.5wt%) target (pressure, temperature)Chemical texture etching : 1% HCl + 99% DI water, 20 – 60sec
Solar cells (multi-chamber cluster system)p μc-Si:H : 13.56MHz PECVD, 250oC,SiH4(1sccm), H2(180sccm), 0.5Torr, 16W, 0.023nm/secp a-SiC:H : 13.56MHz PECVD, SiH4(6sccm), H2(5sccm), CH4(16sccm), B2H6(1sccm)0.2nm/seci a-Si:H : 60MHz VHFCVD, SiH4(7sccm), H2(60sccm), 8W, 0.17nm/seci μc-Si:H : 60MHz VHFCVD, SiH4(5sccm), H2(95sccm), 16W, 0.16nm/secn a-Si:H : 13.56MHz PECVD, SiH4(5sccm), H2(5sccm), PH3(5sccm), 5W, 0.1nm/sec
Back reflectorAg (thermal evaporation), ZnO/Ag and ZnO:Al/Ag
Intermediate layerZnO:Al
CharacterizationSolar cells area : 0.36cm2 (n a-Si:H and ZnO back reflector etched for cell isolation)I-V : dual light solar simulator (WACOM Inc.)Spectral response with filtered light bias (red & blue) (PV Measurement Inc.)
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BACK CONTACT
In CdTe and CIGS devices, contact to the p-type semiconductor.
Metal Work Function > Semiconductor Work Function
Mo for CIGS because of its relatively inert nature during the highly corrosive CIGS deposition- thin MoSe2 layer formation
No metals having work function > 4.5 eV for CdTe- Au, Ni, HgTe,ZnTe:Cu, Cu doped Graphite paste, Sb2Te3.
Psuedo-ohmic contact by creating Te rich layer by Br-Methanol etching.
In a-Si devices, contact to the n-type semiconductor – no such requirement – Ag, Al
Improved long wavelength response using ZnO / Ag or Al.
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INTERFACES TFSC comprise several layers of different semiconductors and
metal- large number of interfaces. Presence of grain boundaries in polycrystalline films – internal
interfaces Matched Lattice Constants, Electron Affinity/Work Function,
Thermal Expansion Coefficient Modifications in interface properties due to device processing
involving sequential deposition of multilayers at different deposition conditions.
Post Deposition treatments involving high-temperature annealing alter interface and intergrain properties.
Interfacial defect states , chemical and metallurgical changes affect optoelctronic and transport properties.
Manipulation of interfacial structure, chemistry and metallurgy provides a powerful tool to tailor / engineer the Fermi level, bandgap, electric field and their gradients to improve the device performance.
Use of a buffer layer at p/i interface in a-Si:H solar cells increases Voc.
Textured substrates causing interfacial roughness- improved photoresponse
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MANUFACTURING
Photovoltaic Modules involving the sequential deposition of different thin films over a large area substrate.
Substrate cleaning, TCO, Window Layer and Absorber layer formation
Laser or Mechanical scribing ( upto 3) to define , interconnect and isolate the cells.
Metallization for interconnection Lamination External leads Monolithic Integration of the cells in the module
manufacturing process with minimum area loss. Device uniformity over a large area- bad area can
destroy the entire module performance
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Source-Photon 04/2006
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Different Materials: Module Costs
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Nanotechnology: Application to solar photovoltaics
Quantum dot Solar cells Nanorod-Branched nanocrystal based solar cells Nanocrystal-Nanocrystal combinations Dye Sensitised Solar cells(DSSC) Dye Sensitized solar cells using TiO2 nanotubes ZnO nanowire solar cells Quantum dots as sensitizers for DSSC Nanocomposite or 3D solid state solar cells
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DSSC using TiO2 nanotube arrays
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External Quantum efficiency of tetrapods and rods
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All-Inorganic nanocrystal solar cells
Valence Bands
Conduction Bands
Bilayer
Mixed
CdTe
CdSe
Donor-acceptor inorganic nanocrystal solar cell
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Spherical particles and sub micron rodsSpherical particlesHexagonalHexagonalHgS
Nanotubes with bamboo structure
Nanotubes with bamboo structureHexagonalHexagonalHgS: Iodine
Spherical particles and submicron rodsSpherical particlesCubicCubicHgSe
NanotubesNanotubesCubicCubicHgSe: Iodine
Spherical particles and submicron rodsSpherical particlesCubicCubicHgTe
NanotubesNanotubesCubicCubicHgTe: Iodine
Nanofibers and NanorodsNanofibersHexagonalHexagonalCdTe: Iodine
NanorodsSpherical particlesHexagonalCubicCdTe
Nanofibers and NanorodsNanofibersHexagonalHexagonalCdSe: Iodine
NanorodsSpherical particlesHexagonalHexagonalCdSe
700VWithout voltage700VWithout voltage
MorphologyStructureFilm
Effect of iodine on nanocrystalline II-VI semiconductor thin film morphology
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Dye Sensitized Solar Cells(DSSC) : Mimicking Photosynthesis
Photosynthesis – Conversion of Solar Energy into Chemical Energy
Two Stage Process – Light Reactions + Dark Reactions
Light Reactions use Photon Energy to create “Energy Carrier Molecules” Chlorophyll
Dark Reactions using these molecules creates carbohydrates Carbon Fixation
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Photovoltaic Effect in DSSC Light Reactions use Photon Energy to create “Energy
Carrier Molecules” – Photon Absorber with electron excitation from lower energy state to higher energy stateOrganic Dyes or Inorganic Semiconductors
Dark Reactions using these molecules to separate the electrons and holes using electron and hole transporting mediumsTiO2 as electron transporting and electrolyte containing a REDOX couple for hole transporting
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Internal Processes inside Dye sensitized solar cell
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DSSC vs P-N Junction Solar Cells
Separation of Light Harvesting and Charge Transportation processes in DSSC vs Semiconductor layers involved in both these processes
- Semiconductor Properties have strong influence on the device characteristics
- Purer materials causing enhanced material and production costs
- Majority carrier transport in DSSC
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Dye sensitized solar cells (DSSC) promises to be an inexpensive method for solar to electrical energy conversion
Utilizes the electrical potential difference between the photo-absorber electrode and the electrolyte to separate the photo-generated carriers and generates electrical work externally.
The costly diffusion process to form p-n junction is avoided
Less sensitive to the grain boundaries etc. in the material compared to p-n junction solar cells
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DSSC Design
Electron and holes separated by the sensitizer layer preventing recombination.
Too thick a layer may prevent electron and hole injection
Flat electrode with monolayer of dye will have poorer light absorption and hence efficiency
Use of nanocrystalline TiO2 to provide a larger area with dye coverage increasing both light absorption and electron injection
Mesoporous layer to further increase the light harvesting
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Photoelectrode Materials
TiO2ZnO
SnO2 Nb2O5 ZrO2
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Sensitizer
Ru-Polypyridine Family Soaking the mesoporous layer in the dye to
create the required monolayer coverage over a large area with good adhesion to TiO2surface
High Incident Photon to Current Conversion efficiency (IPCE) = Light Harvesting Efficiency (Dye Spectral & PhotophysicalProperties) * Charge Injection Yield (Excited State Redox Potential & Lifetime) * Charge Collection Efficiency (Structure & Morphology of TiO2 layer)
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Electrolyte & Counter Electrode Organic Electrolyte containing Redox
couple (Iodide I- / Tri-iodide I3-) : Liquid Electrolyte
Volatile organic liquid replaced by Gel, Polymer electrolyte, Ionic Liquid
Counter Electrode coated with a catalyst (Pt- 5 to 10 µg /cm2) for cathodicreduction of triiodide to iodide : anodic corrosion
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SEM image of the photo-electrode prepared by spray deposition method
200nm
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SEM image of ZnO photoelectrode
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0.0 0.2 0.4 0.6 0.80
2
4
6
8
10
curre
nt d
ensi
ty (m
A/c
m2 )
Voltage (V)
S1
S2
S3S4 Sample Voc Isc
S1 0.486 6.657S2 0.525 7.381S3 0.558 9.123S4 0.581 9.733
TiO2 based dye sensitized solar cell characteristics
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0.0 0.2 0.4 0.6 0.80
1
2
3
4
5
6
7
8
9
10
Cur
rent
den
sity
(mA
/cm
2 )
voltage (V)
Zn1
Zn2
Zn3
Zn4
Zn1 0.461 5.589 Zn2 0.527 7.373Zn3 0.558 8.386Zn4 0.560 9.123
Sample Voc Isc
ZnO based dye sensitized solar cell characteristics designed in our lab
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Nanowire dye sensitized solar cell
Mat Law et al., Nature materials, May 2005
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1.8€/Wp1.4€/Wp1.1€/Wp0.8€/Wp11%(EPFL <1 cm2)
2.0€/Wp1.6€/Wp1.2€/Wp0.9€/Wp10%(EPFL>1 cm2)
2.5€/Wp2.0€/Wp1.5€/Wp1.2€/Wp8%(EPFL) using robust electrolyte
2.7€/Wp2.1€/Wp1.6€/Wp1.2€/Wp7.5%(ECN masterplate)
150€/m2120€/m290€/m270€/m2
Active area Efficiency
Costs/m2
Based on the forecasted future material costs
Based on the present materialcosts
Manufacturing costs
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12.96%
12.96%
13.89%14.81%
37.04% 8.33%
Running Costs
TCO glass
Investments
Dye
Screen printable pastes
Various materials16.35%
13.46%21.15%
23.08%
20.19%5.77%
Screen printable pastes
Various materials
InvestmentsRunning Costs
TCO glass
Dye
Analysis A Analysis B1 MWpeak/year 4 MWpeak/year
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Present problems
Low efficiency compared to p-n junction solar cells
Only a very limited number of dyes give high photocurrent quantum yields and the stability of the dye against photo-degradation is a major problem.
Amount of dye adsorbed on the photo-electrode is limited. The low coverage of the semiconductor surface by the dye molecules, typically a monolayer
Due to the usage of liquid electrolyte sealing of the cells is a major problem
Interpenetrating network between the oxide material and the dye is not easily possible
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Inorganic 3D solar cell
Other possible materialsCISe, CdX(X= S, Se and Te), HgX, HgCdTe, PbS, InP, CuS.
Efficiency ~ 5%
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Extremely thin absorber layer (ETA) solar cell
Structure of ETA solar cell
Other possible materialsNanocrystalline CuInS2, CdX(X= S, Se and Te), HgCdTe, PbS quantum dots, InP, CuS.
Efficiency 2.1%
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ORGANIC PHOTOVOLTAICS-Low Cost
-Disposable
-Flexible
- Variety of shapes
- Thin films
-Processible from solution
-Tunable in conductivity
-Metallic Vs semiconducting
-Light Weight
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Organic photovoltaic material differ
from Inorganic semiconductor in the
following respects*Photogenerated excitations (excitons) are bounded and do not spontaneously dissociate into charge pairs.
*Charge transport proceed by hopping between localized states, rather than transport with a band, and mobilities are low.
[CdSe (at 300 K) : 1050 cm2 V-1 s-1, conjugated polymers below 1 cm2
V-1 s-1]
*The spectral range of optical absorption is relatively narrow compared to the solar spectrum.
*Absorption coefficients are high so that high optical density can be achieved, at peak wavelength, with films less than 100 nm thick.
*Many materials are susceptible to degradation in the presence of oxygen or water.
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Organic Material for solar cell application:PPP- Poly (Para phenylene)
PPV- Poly (Para Phenylene vinylene
CN-PPV- Cyano-subsituted PPV
MEH- 2-methoxy, 5- (2-ethyl-hexyloxy)-PPV
MCP- CN substituted MEH-PPV
PANI- Poly (aniline)
Pc - Phthalocyanine
PEDOT- Poly (ethylene dioythiophene)
Per - Perylene diimide derivative
PIF- Poly (indenofluorene)
PT - Poly (thiophene) , PVK- Poly (vinyl Carbazole)
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Advantages & Disadvantages of Polymer based Photovoltaic Devices
* Increased quantum efficiency by increased mobility under applied bias
* Possess flexibility
* Adjustability of the electronic bandgap through molecular tailoring
* Easy processability
* Possible to fabricate devices using coating or printing technique at room tem
* Low cost device fabrication
* Large area device formation
* Possess low specific weight
Advantages
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* A strong driving force is required to break up the photogenerated excitons.
* Low charge carrier mobilities limit the useful thickness of devices.
* Limited light absorption across the solar spectrum limits the photocurrents.
* Very thin devices mean interference effects can be important
* Photocurrent is sensitive to temperature through hopping transport.
* Current efficiencies < 3-5%
* Long term stability
Disadvantages
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Hybrid absorber for photovoltaic :
-Improving light harvesting
- Improving photocurrent generation
- Improving charge transport
- Stability
- Understanding device function Properties of Hybrid Materials Depends on :Individual Organic and Inorganic ComponentsSize of the Individual Components (bulk / nm)Interface between the Two Components
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Different Approaches in Polymer Solar CellFirst organic solar cell
Power conversion efficiency ~ 1% (by Tang [Kodak] in 1986)
C.W. Tang, Appl. Phys. Lett., 48,183 (1986)
Polymer-Polymer Layer Devices
Au/PEDOT/POPT:MEH-CN-PPV (19:1)/MEH-CN-PPV:POPT (19:1)/ca
Power conversion efficiency ~ 1.9 %Friend’s group in Cambridge, Nature, 395, 257 (1998)
Laminated film
Polymer Layer Device
Power conversion efficiency ~ 1.2 %
Jenekhe et al., Appl. Phys. Lett., 77, 2635 (2000)
Polymer-Inorganic Blend DeviceCdSe nanorod/poly-3(hexylthiophene) blend filmPower conversion efficiency = 1.7 %
Huynh et al., Science, 295, 2425 (2002)
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Geometry for PN Heterojunction PV Cell
polymer IPN deviceIPCE~ 4 % at 550 nmHalls et al., Nature, 376, 498 (1995)
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Our Approach for Hybrid Solar Cell
Al
CdX, and TiO2Polyanilinene,MEH-PPV, P3HT.
ITO
Glass
X= Te, Se and S
Al
hν
ITO
Acc. Don.
e-
h+
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Photoinduced Charge Transfer in organicsemiconductor
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Photocurrent generation
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THIRD GENERATION PHOTOVOLTAIC DEVICES
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Tim Coutts’s report on 33rd
IEEE PV Specialist Conference Building on the many years of investment
in research and development, the PV industry is now the fastest growing industry in the world. Given this rapid translation of research to the market place, this year’s keynote addresses focused on experiences of industry and the investments being made by private and government entities.
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Howard Berke, senior advisor to Good Energies and Founder of KonarkaTechnologies, Inc., talked of developing organic PV products based on a light-activated conductive polymer active layer, having over 6% efficiency.
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A very interesting talk was given by Dave Eaglesham of First Solar entitled “The Pathway to Grid Parity” that is the drive to cost parity with electricity from the fossil-fuel grid. First Solar is the current benchmark for low-cost PV module manufacturing, with a cost that is well below c-Si PV and a proven production cost all-in of $1.14/W. The company is growing quickly and is on the verge of being the first Giga Watt producer. This talk outlined the current status, the issues around managing rapid growth and rapid technology change, and the pathway to further reductions in cost.
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Impressive results were presented on the component sub-cells and process technologies for advanced (4-6 junction) multijunction concepts.
These results include: ·31% conversion efficiency at 13x for a 3-terminal 2-junction GaInP/GaAs solar cells for spectrum-splitting PV module ·~8% efficiency on InP-based GaInPAs/GaInAs2-junction cells with a GaAsSb/GaInAs tunnel-junction. Successful demonstration of a GaInP/GaAs 2-junction cell on wafer-bonded Ge/Si epitaxialtemplates.
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