1

S. Sundar Kumar Iyer

Samtel Centre for Display Technologies

Organic Solar Cells

2

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

3

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

4

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 2004

India 60 km × 60 km (0.12% area) World need: 350 km × 350 km

5

History

� 1839 Photovoltaic effect discovered by Edmond Becquerel

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

(η∼6%)η∼6%)η∼6%)η∼6%)

� 1970s Surge in research to harness solar energy

� 1986 Heterojunction Organic Solar Cell by Tang of Eastman Kodak

� 2007 Highest efficiency solar cells with ηηηη~40.7% in Spectrolab

� A big surge in solar cells research & development is underway

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1mm

Efficiency ηηηη ≈ 6 %

Chapin et al. 1954

The Birth of Silicon Photovoltaics

7

Space Applications

www.spacetoday.org

marsrovers.nasa.gov

Photovoltaics are the mainstay

8

Remote Locations

www.dacres.org

web.worldbank.orgsummitclimb.com

Photovoltaicsare attractive

9

Consumer Electronics

10

Grid Supply

www.e2tac.org

www.sun-consult.de

Need to make photovoltaicsattractive in the marketplace

11

Solar Energy Usage and Pricing

n/a2%Consumer Indoor

2-5 times59%Grid Connected

0.2-0.8 times22%Remote Habitation

0.1-0.5 times17%Remote Industrial

Solar Price/Competing

Energy source

Solar markets

(average of last 5 years)

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

12

Electricity Generation Cost

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

20 ¢ -30 ¢ (Rs.8.00-Rs.12.00)

20 ¢ -40 ¢ (Rs.8.00-Rs.16.00)

7 ¢ -9 ¢ (Rs.2.80-Rs.3.60)

4 ¢ -7 ¢ (Rs.1.60-Rs.2.80)

3 ¢ -5 ¢ (Rs.1.20-Rs.2.00)

Cost

Combined cycle gas turbine

Wind

Biomass gasification

Remote diesel generation

Solar PV central station

Energy Source

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

13R.M. Margolis 2003

Solar Energy Production and Price

14

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

16

Organics Photovoltaic

Zweibel 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

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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

21

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

η η η η =

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

hνννν

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Classic p-n Junction Photovoltaic Cell

• Incident photon immediately forms mobile electrons and holes

e-

h+Ev

Ec

hνννν > > > > Eg= Ec - Ev

φbi

n-type p-type

Ebuilt-in

Efn

Efp

hνννν

Inorganic Semiconductor

Ef

-ve +ve

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hν

h+

Exciton

e-

h+

e-

Hole Transport

Layer

Electron Transport

LayerCathodeAnode

Photon Absorption Exciton Formation

Exciton Dissociation

Exciton Diffusion

Charge Transport & Collection

e-

EHP Formation

Organic Solar Cells Operation

A Heterojunction Organic Solar Cell Structure

by diffusion

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Photovoltaic Process In Organic Solar Cells

�

Light

ReflectedAway

����

Photons

NotAbsorbed

����

Coupling

of sunlightinto

solar cell

Absorption

of incidentphotons

Creation

of ‘free’charges

Separation

of chargesby built-inE field

ChargesRecombine

����

ChargesRecombine

����

Collection

of chargesat

electrodes

Creationof

excitons

ExcitonsRecombine

����

Su

nlig

ht

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ITO Patterning

PEDOT:PSS Coating

Active Layer

Deposition

Metal Deposition

Device Fabrication

PEDOT:PSS

Active 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 η = 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

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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 ηηηηO

�� � 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 layers

Device

Back electrode

ηηηηO = 1-R where

R =(n1-n0)

2 + κκκκ2

(n1+n0) + κκκκ2

ni : refractive index of medium i

κκκκ: attenuation coefficient in device

n0=1 for air

n1, κκκκ

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Light Trapping by TiO2 Nanoparticles

300 400 500 600 700 800 900 1000

0

10

20

30

40

50

60

70

80

90

100

Re

fle

cta

nce

(%)

λ (nm)

P3HT-PCBM-TiO2_40

��������

Device

Back electrode

300 500 700 900λλλλ (nm)

100

80

60

40

20

0

Refl

ecta

nc

e (

%)

P3HT:PCBM + TiO2

P3HT:PCBM

Jyoti Singh 2008

TiO2 particle is dispersed in the P3HT:PCBM blend

36

Cathode Variation

Illumination:

AM1.5D 100 mW cm-2

Nitin Sahai 2008

Glass

Active Area

Al

Ca

ITO

Cu

rren

t D

en

sit

y (

mA

cm

-2)

Voltage (V)

37

Effect of Post Process Anneal

P3HT: PCBM BlendHeterostructure

Vinod Pagare 2007

Aluminium Cathode

Polymer Blend

PEDOT:PSS

ITO Glass

38

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

39

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

40

Let us make Organic Solar Cells Happen!