Photo Voltaic Materials And Devices

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]

OUTLINE ●

PHOTOVOLTAIC EFFECT

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PHOTOVOLTAIC DEVICES : - FIRST GENERATION : CRYSTALLINE Si - SECOND GENERATION : THIN FILMS DYE SENSITISED SOLAR CELL ORGANIC SOLAR CELL - THIRD GENERATION

Solar Photovoltaic Generation ●

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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

I-V CHARACTERISTICS

ENERGY BAND DIAGRAM FOR P-N JUNCTION SOLAR CELL

I-V CHARACTERISTICS OF A TYPICAL CELL

EQUIVALENT CIRCUIT

Solar cell is a Large Area device involving several physical processes : Exciton (bound or unbound) creation Carrier Transport by Drift or Diffusion Recombination 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!

Optical Absorption

Optimum Band Gap

CRYSTALLINE SI ●

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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 ( Multicrystalline Si) or Ribbon Growth. Float Zone Si – Purer and Expensive

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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)

16% 7%

17% 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 mV PMAX = 15 mW/cm2 FF = 0.8-0.9 η = 14-16 %

Present status of efficiencies of bulk Si solar cells 2010 Target: 20% cell, 16% module efficiencies Efficiency(%)

Strucure

Organization

24.7

PERL cell, 4cm2

Univ.New South Wales

21.5*

HIT cell, 100cm2

Sanyo , Voc=725mV

21.6

Laser fired contacts

Fraunhofer, sc-Si

21.5

All back contact

Sunpower, Si PV-FZ,149cm2

20.3

Laser fired contacts

Fraunhofer, mc-Si, 1cm2

19.5

Buried contact

UNSW Solar Car Team

18.3

Buried contact

sc-Si, BP Solar

17.6

Buried contact

mc-Si, Uni. Konstanz

17.7*

Manufacturing process

Kyocera, 232.5cm2

* Production level

High-Efficiency Bifacial Ga-doped Cz Si Solar Cells with a-Si Back Surface Passivation Ag contacts 0.5-2.0 Ω-cm, Ga-doped, p-type Cz Si 300 µm thick τ = 200 µs

SiNx 100 Ω/ƀ n+ emitter

p+ Al-BSF Al contact

Ag contacts 0.5-2.0 Ω-cm, Ga-doped, p-type Cz Si 100-200 µm thick τ = 200 µs intrinsic a-Si p-type a-Si ITO Ag/Al contacts

S≤100 cm/s S>500 cm/s

η = 17.5%

η = 19%

HIT cell (

HIT : Heterojunction with Intrinsic Thin-Layer

)

- High efficiency by excellent surface passivation with a-Si layers

- Less thermal stress through low temperature process (~ 200oC) - Advantage in high temperature performance 3.86 A

F.F.

77.0 %

Voc

717.0mV

Eff.

21.3 %

5

AM-1.5, 100mW/cm2, 25℃ Cell size : 100.0cm2

200 µm

n c-Si i/n amorphous silicon

Structure

Current (A)

4

Measurement in Sanyo

5 4

3

3

2

2

1

1

0 0

0.2

0.4

0.6

Voltage (V)

0.8

0 1.0

Output (W)

transparent electrode

p/i amorphous silicon

Isc

Approaching the 29% limit efficiency of Silicon solar cells R.M.Swanson, SunPower Corporation

Efficiency limit: 29%

PERL HIT

THIN FILM SOLAR CELLS ● ● ● ●

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Low cost alternative (!) to Si technology Integrated Module Production Flexible Substrates Use in Buildings with improved aestheticshomogeneous appearance Large scale production using Thin Film Technologies

SUBSTRATE ●

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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 superstrate structure inferior to substrate structureInterdiffusion 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

Transparent Conducting Oxide ●

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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 HAZEscattering assisted light absorption in a-Si solar cells ( increased path length in thin cells)

WINDOW LAYER ● ● ● ● ●

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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, ZnOS using 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

ABSORBER ●

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Copper Indium Gallium Diselenide and related compounds: CuInS2 with EG ~ 1.53 eV an ideal PV materialdifficult material due to S CuInSe2 with EG ~ 1 eV – optical absorptance coefficient 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(?)

CIGS solar cell structure and process

Grid AR layer Window Buffer Absorber Back contact

Thick, Materials

Process

3 µm, Al / 50nm, Ni

E-beam evaporation

100nm, MgF2

E-beam evaporation

500nm, n-AZO / 50nm, i-ZnO (BZO, GZO)

RF sputtering (MOCVD)

50nm, CdS (Cd-free Zn(O,S,OH)x,In(OH)S)

CBD

2-3 µm Cu(In,Al,Ga)(Se,S) (Wide bandgap, CZTS)

Co-evaporation (Sputtering + Selenization, Sulphurization)

1 µm, Mo

DC sputtering

2-3mm, SLG (Sus, Ti, Polymide)

Cleaning

Substrate

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).

Cross section of CdTe solar cell

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First Solar one of the major CdTe module producer

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Safe for people, animal life and the environment

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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

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 kselection 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

Amorphous Si ●

Single, Double and Triple Tandem Junctions.

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Microcrystlline Si, Micromorph Si

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Hybrid cells of A-Si:H and microcrystalline providing ~ 75% of all thin film production

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Cell stability with efficiencies ~ 10% or more Increased deposition rate

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Design modification for better light harvesting

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Glass Glass Textured ZnO:Al

photon

Overview (KIER)

Solar Cells Structure

p-type µc-Si:H (20nm)

SnO2:F(AU) or textured ZnO:Al

p-type a-SiC:H (20nm) intrinsic a-Si:H(200nm) n-type a(µc)-Si:H Buffer (ZnO)

Intrinsic µc-Si:H(2㎛)

p-type µc-Si:H (20nm) Intrinsic µc-Si:H(2㎛)

n-type a-Si:H(30nm) n-type a-Si:H (30nm) ZnO

Ag µc-Si:H pin component cells

ZnO

Ag a-Si:H/µc-Si:H pin tandem solar cells

Structure & Processes

Fabrication and Characterization Apparatus Deposition of tandem solar cells

rf sputtering Glass ZnO:Al TCO(front & back)

- 2 PECVDs, 1 VHFCVD, 1 HWCVD, 1 rf sputter Reduced contamination, improved interface

n-layer

PECVD

In-line transfer in a vacuum

µc-Si:H

Glass i-layer

p-layer µc-Si:H Glass Glass

Clean room process - e-beam & thermal evaporator - rf & dc sputter - Annealing furnace - Laser scriber

Glass 60MHz VHFCVD

PECVD

Structure & Processes

Experimental Textured front ZnO:Al Deposition : 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/sec p a-SiC:H : 13.56MHz PECVD, SiH4(6sccm), H2(5sccm), CH4(16sccm), B2H6(1sccm)0.2nm/sec i a-Si:H : 60MHz VHFCVD, SiH4(7sccm), H2(60sccm), 8W, 0.17nm/sec i μc-Si:H : 60MHz VHFCVD, SiH4(5sccm), H2(95sccm), 16W, 0.16nm/sec n a-Si:H : 13.56MHz PECVD, SiH4(5sccm), H2(5sccm), PH3(5sccm), 5W, 0.1nm/sec Back reflector Ag (thermal evaporation), ZnO/Ag and ZnO:Al/Ag Intermediate layer ZnO:Al Characterization Solar 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.)

BACK CONTACT ●

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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.

INTERFACES

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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

MANUFACTURING ●

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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

Source-Photon 04/2006

Different Materials: Module Costs

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

DSSC using TiO2 nanotube arrays

External Quantum efficiency of tetrapods and rods

All-Inorganic nanocrystal solar cells Bilayer Mixed CdTe CdSe

Valence Bands

Conduction Bands

Donor-acceptor inorganic nanocrystal solar cell

Effect of iodine on nanocrystalline II-VI semiconductor thin film morphology Structure Film

Morphology

Without voltage

700V

Without voltage

700V

HgS

Hexagonal

Hexagonal

Spherical particles

Spherical particles and sub micron rods

HgS: Iodine

Hexagonal

Hexagonal

Nanotubes with bamboo structure

Nanotubes with bamboo structure

HgSe

Cubic

Cubic

Spherical particles

Spherical particles and submicron rods

HgSe: Iodine

Cubic

Cubic

Nanotubes

Nanotubes

HgTe

Cubic

Cubic

Spherical particles

Spherical particles and submicron rods

HgTe: Iodine

Cubic

Cubic

Nanotubes

Nanotubes

CdSe

Hexagonal

Hexagonal

Spherical particles

Nanorods

CdSe: Iodine

Hexagonal

Hexagonal

Nanofibers

Nanofibers and Nanorods

Cubic

Hexagonal

Spherical particles

Nanorods

Hexagonal

Hexagonal

Nanofibers

Nanofibers and Nanorods

CdTe CdTe: Iodine

Dye Sensitized Solar Cells (DSSC) : Mimicking Photosynthesis ●

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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}

Photovoltaic Effect in DSSC ●

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Light Reactions use Photon Energy to create “Energy Carrier Molecules” – Photon Absorber with electron excitation from lower energy state to higher energy state {Organic Dyes or Inorganic Semiconductors} Dark Reactions using these molecules to separate the electrons and holes using electron and hole transporting mediums {TiO2 as electron transporting and electrolyte containing a REDOX couple for hole transporting}

Internal Processes inside Dye sensitized solar cell

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

¾ Dye sensitized solar cells (DSSC) promises to be an

inexpensive method for solar to electrical energy conversion

¾ Utilizes the electrical potential difference between the photo-absorber electrode and the electrolyte to separate the photo-generated carriers and generates electrical work externally. ¾ The costly diffusion process to form p-n junction is avoided ¾ Less sensitive to the grain boundaries etc. in the material compared to p-n junction solar cells

DSSC Design ●

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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

Photoelectrode Materials 9 9 ● ● ●

TiO2 ZnO SnO2 Nb2O5 ZrO2

Sensitizer ● ●

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Ru-Polypyridine Family Soaking the mesoporous layer in the dye to create the required monolayer coverage over a large area with good adhesion to TiO2 surface High Incident Photon to Current Conversion efficiency (IPCE) = Light Harvesting Efficiency (Dye Spectral & Photophysical Properties) * Charge Injection Yield (Excited State Redox Potential & Lifetime) * Charge Collection Efficiency (Structure & Morphology of TiO2 layer)

Electrolyte & Counter Electrode ●

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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 cathodic reduction of triiodide to iodide : anodic corrosion

200nm SEM image of the photo-electrode prepared by spray deposition method

SEM image of ZnO photoelectrode

TiO2 based dye sensitized solar cell characteristics

S4 S3

10

S2

2

current density (mA/cm )

8

Sample

Voc

Isc

S1 S2 S3 S4

0.486 0.525 0.558 0.581

6.657 7.381 9.123 9.733

S1

6

4

2

0 0.0

0.2

0.4

Voltage (V)

0.6

0.8

ZnO based dye sensitized solar cell characteristics designed in our lab 10

Zn4

9

Zn3

2

Current density (mA/cm )

8

Sample Zn1 Zn2 Zn3 Zn4

Zn2

7 6

Zn1

Voc 0.461 0.527 0.558 0.560

5 4 3 2 1 0 0.0

0.2

0.4

voltage (V)

0.6

0.8

Isc 5.589 7.373 8.386 9.123

Nanowire dye sensitized solar cell

Mat Law et al., Nature materials, May 2005

Manufacturing costs Active area Efficiency

Costs/m2 Based on the forecasted future material costs

Based on the present material costs

70€/m2

90€/m2

120€/m2

150€/m2

7.5%(ECN masterplate)

1.2€/Wp

1.6€/Wp

2.1€/Wp

2.7€/Wp

8%(EPFL) using robust electrolyte

1.2€/Wp

1.5€/Wp

2.0€/Wp

2.5€/Wp

10%(EPFL>1 cm2)

0.9€/Wp

1.2€/Wp

1.6€/Wp

2.0€/Wp

11%(EPFL <1 cm2)

0.8€/Wp

1.1€/Wp

1.4€/Wp

1.8€/Wp

TCO glass 14.81%

Dye 21.15%

Dye 13.89% Screen printable pastes 12.96%

Various materials 12.96% Running Costs 37.04%

8.33% Investments

1 MWpeak/year Analysis A

Screen printable pastes 13.46%

Various materials 16.35%

TCO glass 23.08%

20.19% Running Costs

4 MWpeak/year Analysis B

5.77% Investments

Present problems

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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 photodegradation 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

Inorganic 3D solar cell

Efficiency ~ 5%

Other possible materials CISe, CdX(X= S, Se and Te), HgX, HgCdTe, PbS, InP, CuS.

Extremely thin absorber layer (ETA) solar cell

Other possible materials Nanocrystalline CuInS2, CdX(X= S, Se and Te), HgCdTe, PbS quantum dots, InP, CuS. Efficiency 2.1%

Structure of ETA solar cell

ORGANIC PHOTOVOLTAICS -Low Cost -Disposable -Flexible - Variety of shapes - Thin films -Processible from solution -Tunable in conductivity -Metallic Vs semiconducting -Light Weight

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.

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)

Advantages & Disadvantages of Polymer based Photovoltaic Devices Advantages * 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

Disadvantages * 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

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 Components Size of the Individual Components (bulk / nm) Interface between the Two Components

Different Approaches in Polymer Solar Cell First organic solar cell

C.W. Tang, Appl. Phys. Lett., 48,183 (1986)

Polymer-Polymer Layer Devices Laminated film 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)

Polymer Layer Device

Polymer-Inorganic Blend Device

Power conversion efficiency ~ 1% (by Tang [Kodak] in 1986)

CdSe nanorod/poly-3(hexylthiophene) blend film Power conversion efficiency = 1.7 % Power conversion efficiency ~ 1.2 % Jenekhe et al., Appl. Phys. Lett., 77, 2635 (2000)

Huynh et al., Science, 295, 2425 (2002)

Geometry for PN Heterojunction PV Cell polymer IPN device IPCE~ 4 % at 550 nm Halls et al., Nature, 376, 498 (1995)

Our Approach for Hybrid Solar Cell

Al

{X= Te, Se and S} CdX, and TiO2 Polyanilinene ,MEH-PPV, P3HT. Acc.

Don.

hν

ITO Glass

eh+ Al

ITO

Photoinduced Charge Transfer in organic semiconductor

Photocurrent generation

THIRD GENERATION PHOTOVOLTAIC DEVICES

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 Konarka Technologies, Inc., talked of developing organic PV products based on a lightactivated 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 3terminal 2-junction GaInP/GaAs solar cells for spectrum-splitting PV module · ~8% efficiency on InP-based GaInPAs/GaInAs 2-junction cells with a GaAsSb/GaInAs tunneljunction. Successful demonstration of a GaInP/GaAs 2junction cell on wafer-bonded Ge/Si epitaxial templates.