PHOTOVOLTAICS MATERIALS AND DEVICES

By

VIRESH DUTTA

PHOTOVOLTAIC LAB., CENTER FOR ENERGY STUDIES,

INDIAN INSTITUTE OF TECHNOLOGY , NEW DELHI 110016 INDIA

E-mail : vdutta@ces.iitd.ac.in

OUTLINE

PHOTOVOLTAIC EFFECT

PHOTOVOLTAIC DEVICES :

- FIRST GENERATION : CRYSTALLINE Si

- SECOND GENERATION : THIN FILMSDYE SENSITISED SOLAR CELLORGANIC SOLAR CELL

- THIRD GENERATION

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

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.

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

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 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!

Optical Absorption

Optimum Band Gap

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

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)

17%

7%

16%

60%

Cost of wafers Materials Processing Labour accounts Investment cost

Production cost of Si solar Cell

TYPICAL VALUES ( Si Solar Cells):

JSC = 40 mA/cm2

VOC= 600 mVPMAX = 15 mW/cm2

FF = 0.8-0.9η = 14-16 %

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

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%

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

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

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

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

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.

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 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)

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

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

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(?)

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

MIASOLE CIGS PLANT

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

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

Emissions from use of conventional fuels for electricity generation

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

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

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

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

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.)

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.

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

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

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

Valence Bands

Conduction Bands

Bilayer

Mixed

CdTe

CdSe

Donor-acceptor inorganic nanocrystal solar cell

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

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

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

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

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

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

Photoelectrode Materials

TiO2ZnO

SnO2 Nb2O5 ZrO2

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)

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

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

200nm

SEM image of ZnO photoelectrode

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

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

Nanowire dye sensitized solar cell

Mat Law et al., Nature materials, May 2005

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

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

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

Inorganic 3D solar cell

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

Efficiency ~ 5%

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%

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

* 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

* 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

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

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)

Geometry for PN Heterojunction PV Cell

polymer IPN deviceIPCE~ 4 % at 550 nmHalls et al., Nature, 376, 498 (1995)

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+

Photoinduced Charge Transfer in organicsemiconductor

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.

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.

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.

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.