1

S. Sundar Kumar Iyer

Samtel Centre for Display Technologies

Organic Solar Cells

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Outline

Motivation ■ Solar cells■ Organic solar cells

Background■ Working of organic solar cell■ Fabrication steps

Research at IIT K■ Molecule, device, circuit and system level

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Clean Energy Supply Needed for Quality of Life

Fossil and nuclear fuels are costly■ If we include the environmental cost

The sun shines on everyone ■ Ideal for distributed power generation and remote locations

Tap solar energy directly■ Ideal for distributed power generation■ More environmentally friendly

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Annual Mean Global Irradiance

On a horizontal plane at the surface of the earth W m-2 averaged over 24 h

With 10% efficient solar cell area of solar cell needed in 2004India 60 km × 60 km (0.12% area) World need: 350 km × 350 km

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History

1839 Photovoltaic effect discovered by Edmond Becquerel

1954 First Silicon Solar Cell Bell Lab by Chapin, Fuller and Pearson

(

1970s Surge in research to harness solar energy

1986 Heterojunction Organic Solar Cell by Tang of Eastman Kodak

2007 Highest efficiency solar cells with ~40.7% in Spectrolab

A big surge in solar cells research & development is underway

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1mm

Efficiency ≈ 6 %

Chapin et al. 1954

The Birth of Silicon Photovoltaics

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Space Applications

www.spacetoday.org

marsrovers.nasa.gov

Photovoltaics are the mainstay

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Remote Locations

www.dacres.org

web.worldbank.org summitclimb.com

Photovoltaics are attractive

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Consumer Electronics

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Grid Supply

www.e2tac.org

www.sun-consult.de

Need to make photovoltaicsattractive in the marketplace

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Solar Energy Usage and Pricing

Solar markets

(average of last 5 years)

Solar Price/Competing

Energy source

Remote Industrial 17% 0.1-0.5 times

Remote Habitation 22% 0.2-0.8 times

Grid Connected 59% 2-5 times

Consumer Indoor 2% n/a

http://www.solarbuzz.com/StatsCosts.htm (2006 data; accessed 29.02.2008)

Solar Energy: 30 c (Rs. 12) per kWhNeed to lower cost to 10c (Rs.4) per kWh and below

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Electricity Generation Cost

Energy Source Cost

Combined cycle gas turbine 3 ¢ -5 ¢ (Rs.1.20-Rs.2.00)

Wind 4 ¢ -7 ¢ (Rs.1.60-Rs.2.80)

Biomass gasification 7 ¢ -9 ¢ (Rs.2.80-Rs.3.60)

Remote diesel generation 20 ¢ -40 ¢ (Rs.8.00-Rs.16.00)

Solar PV central station 20 ¢ -30 ¢ (Rs.8.00-Rs.12.00)

Solar PV Distributed 20 ¢ -50 ¢ (Rs.8.00-Rs.20.00)

http://www.solarbuzz.com/StatsCosts.htm (2006 data; accessed 29.02.2008)

13R.M. Margolis 2003

Solar Energy Production and Price

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Silicon Wafer40%

Cell Processing25%

Module35%

Data from A. Rohatgi

Cost Breakdown of Silicon Photovoltaics

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Lowering Cost of Solar Cells

Thin Film Solar Cells■ Multiple junction solar cells (a-Si:H, a-SiGe:H)■ CdTe based cells (CdTe, CdS)■ CuInSe2 (CIS) Ternary & Multinary compound solar cells

■ Multicrystalline/Microcrystalline silicon solar cells■ Thin film GaAs solar cells■ Organic solar cells

S. Deb 2004

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Organics PhotovoltaicZweibel et al. 2004

Spectrolab 40.7%

Efficiency of PV for Different Materials

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Why Organic Solar Cells?

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Printing ■ Screen Pringing■ Stamping

Spraying Spin Coating Vaporisation

High-Throughput and Low-Cost Processing

Step 1 Step 3Step 2

Deposition Patterning Packaging

Raw MaterialsPlastic RollsRaw MaterialsPlastic Rolls

Finished Goods(Solar Cell)Finished Goods(Solar Cell)

A simplified overview

www.rolltronics.com

Roll-To-Roll

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Flexible Solar Cells

Flexible Surfaces

Conformal Surfaces

Prof. Kippelen’s Group; Georgia Tech

Example show is a CIGS solar Cells

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Eco-Friendly Technology

Appropriate Process

Biodegradable Molecule

Modifiedchromophore of Green Fluorescent Protein Molecule

Anode contacts

Cathode contacts

Solar cell device using the molecule

Ink Reservoir

Chamber

Nozzle

Print

Piezoelectric crystal

PulsedSignal

Epson

Example: Ink-jet technology uses material only where needed

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Background

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Efficiency of a Solar Cell

Fill Factor FF is the ratio of

area of maximum rectangle

fitted in the 4th quadrant I-V

and the product of VOC and ISC

Maximum Power Output

Pmax = VOC × ISC × FF

Efficiency

=

p n

V

I

I (m

A)

V (V)

VOC

ISC

I

V

Max Power

Rectangle

Dark

Light

S.M.Sze 1991

Pmax

Incident Optical Power

RL

h

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Classic p-n Junction Photovoltaic Cell

• Incident photon immediately forms mobile electrons and holes

e-

h+Ev

Ec

hEg= Ec - Ev

bi

n-type p-type

Ebuilt-in

Efn

Efp

h

Inorganic Semiconductor

Ef

-ve +ve

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hν

h+

Exciton

e-

h+

e-

Hole Transport Layer

Electron Transport Layer

CathodeAnode

Photon Absorption Exciton FormationExciton DiffusionExciton DissociationCharge Transport & Collection

Photon Absorption Exciton Formation

Exciton Dissociation

Exciton Diffusion

Charge Transport & Collection

e-

EHP Formation

Organic Solar Cells Operation

A Heterojunction Organic Solar Cell Structure

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Photovoltaic Process In Organic Solar Cells

LightReflected

Away

PhotonsNot

Absorbed

Couplingof sunlight

into solar cell

Absorptionof

incidentphotons

Creationof ‘free’ charges

Separationof chargesby built-inE field

ChargesRecombine

ChargesRecombine

Collectionof charges

atelectrodes

Creationof

excitons

ExcitonsRecombine

Su

nli

gh

t

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ITO Patterning

PEDOT:PSS Coating

Active Layer Deposition

Metal Deposition

Device Fabrication

PEDOT:PSSActive Layer

Ca

Ca

ITO

Al

Al

Contacts+ + + +

-

-

Transparent Glass Substrate

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www.sciencemag.org SCIENCE VOL 317 13 JULY 2007 pp. 223-225

Tandem Cell: Jsc = 7.8 mA cm-2, Voc = 1.24 V, FF = 0.67 and = 6.5%

Highest Efficiency Reported OSC Till Date

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Organic Solar Cell Work at IIT K

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The Team

Prof. Satyendra Kumar (Physics)

Dr. Ashish Garg (MME)

Prof. Baquer Mazhari (EE)

Prof. R. Gurunath (Chemistry)

Dr. S.P. Das (EE)

Dr. P.S. Sensarma (EE)

Dr. R.S. Anand (EE)

Dr. Vibha Tripathi (EE)

Prof. Y.N. Mohapatra, Prof. Deepak Gupta, Prof. Monica Katiyar, Dr. Siddhartha Panda, Dr. Narain, …

S. Sundar Kumar Iyer

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The Processing Laboratory

ISO 6, 220 m2

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Characterisation Facilities

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Three Pronged Approach

Increasing efficiency of device■ Physics and circuit model of organic solar cells■ Choice of Material■ Structure – Blend, Bilayer, Tandem …■ Process Optimisation

Reliability and Stability■ Choice of Material■ Mechanism of Degradation■ Encapsulation Techniques

New & emerging technology issues■ Novel methods of fabrication■ System level issues

CuPC

PCBM

R = Hexyl group

P3HT

Stable MoleculeFrom P3HT FamilyAVPV Indium Tin

Oxide LinesPEDOT:PSSActive Layer

Contact to Cathode

Encapsulation

ITOContact to AnodeGlass

Printed Ink

EngravedCells

Substrate

+

-

V

I

RSH

RS

IL

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Organic Solar Cell Model

IL is a function of voltage

Exciton generation IP is a constant

B. Mazhari 2006

IP

Rshunt, int.

D1

D2

Vint

Rs, int.

Ddark

+

-

V

I

RSH

RS

New Model

+

-

V

I

RSH

RS

IL

Traditional Model

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Optical Efficiency O

Optical losses maybe due to

■ Reflection at the surface

■ Unabsorbed light leaking out

Solutions

■ Anti Reflection Coating (ARC)

■ Texturing the top surface

■ Concentrators

■ Thickness of layersDevice

Back electrode

O = 1-R where

R =(n1-n0)2 + 2

(n1+n0) + 2

ni : refractive index of medium i: attenuation coefficient in device

n0=1 for air

n1,

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Light Trapping by TiO2 Nanoparticles

Device

Back electrode

300 500 700 900 (nm)

100

80

60

40

20

0

Ref

lect

ance

(%

) P3HT:PCBM + TiO2

P3HT:PCBM

Jyoti Singh 2008

TiO2 particle is dispersed in the P3HT:PCBM blend

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Cathode Variation

Illumination: AM1.5D 100 mW cm-2

Nitin Sahai 2008

Glass

Active Area

AlCa

ITO

Cu

rren

t D

ensi

ty (

mA

cm

-2)

Voltage (V)

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Effect of Post Process Anneal

P3HT: PCBM BlendHeterostructure

Vinod Pagare 2007

Aluminium Cathode

Polymer Blend PEDOT:PSS

ITO Glass

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Degradation Models

Munish Jassi 2006

Degradation under Electrical & Optical Stress

• Statistically arrive at parameters that matter most• Identify the physics of degradation• Use learning to increase device lifetime

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Summary

Organic solar cells offers unique opportunities in future■ Low-cost high volume production■ Distributed production■ Environmentally benign devices

Work at IIT Kanpur■ Molecule and material level■ Process■ Device level■ Circuit level■ System level

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Let us make Organic Solar Cells Happen!