Organic Solar Cells

Samtel Centre for Display Technologies

S. Sundar Kumar Iyer

1 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

2 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

3 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) 4 World need: 350 km × 350 km History

1839 Photovoltaic effect discovered by Edmond Becquerel
1954 First Silicon Solar Cell Bell Lab by Chapin, Fuller and Pearson (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 ηηη~40.7% in Spectrolab
A big surge in solar cells research & development is underway

5 The Birth of Silicon Photovoltaics

Efficiency ηηη ≈ 6 %

1mm

Chapin et al . 1954 6 Space Applications

www.spacetoday.org

marsrovers.nasa.gov

Photovoltaics are the mainstay 7 Remote Locations

Photovoltaics are attractive

www.dacres.org

summitclimb.com web.worldbank.org 8 Consumer Electronics

9 Grid Supply

Need to make photovoltaics attractive in the marketplace

www.sun-consult.de

www.e2tac.org 10 Solar Energy Usage and Pricing

Solar markets Solar Price/Competing (average of last 5 years) 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

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

http://www.solarbuzz.com/StatsCosts.htm (2006 data; accessed 29.02.2008) 11 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) 12 Solar Energy Production and Price

R.M. Margolis 2003 13 Cost Breakdown of Silicon Photovoltaics

Module Cell Processing 35% 25%

Silicon Wafer 40%

Data from A. Rohatgi 14 Lowering Cost of Solar Cells

Thin Film Solar Cells ■ Multiple junction solar cells (a-Si :H, a-SiGe :H) ■ CdTe based cells ( CdTe , CdS ) CuInSe ■ 2 (CIS) Ternary & Multinary compound solar cells ■ Multicrystalline/Microcrystalline silicon solar cells ■ Thin film GaAs solar cells ■ Organic solar cells

S. Deb 2004 15 Efficiency of PV for Different Materials

Spectrolab 40.7%

Organics Photovoltaic Zweibel et al . 2004 16 Why Organic Solar Cells?

17 High-Throughput and Low-Cost Processing

Printing ■ Screen Pringing ■ Stamping
Spraying
Spin Coating
Vaporisation

18 Flexible Solar Cells

Flexible Surfaces
Conformal Surfaces

Example show is a Prof. Kippelen’s Group; Georgia Tech CIGS solar Cells 19 Eco-Friendly Technology

Appropriate Process

Biodegradable Molecule

20 Background

21 Efficiency of a Solar Cell

hννν I
Fill Factor FF is the ratio of p n area of maximum rectangle fitted in the 4 th quadrant I-V V RL I and the product of VOC and ISC Dark
Maximum Power Output Light × × Pmax = VOC ISC FF

VOC V
Efficiency Pmax (mA)

I η = Max Incident Optical Power Power Rectangle ISC

22 S.M.Sze 1991 V (V) Classic p-n Junction Photovoltaic Cell

Inorganic Semiconductor

hννν> Eg= Ec -Ev hννν

e- Ec Efn Ef φ E Efp bi h+ v n-type p-type

-ve Ebuilt-in +ve

• Incident photon immediately forms mobile electrons and holes

23 Organic Solar Cells Operation

A Heterojunction Organic Solar Cell Structure

Anode Hole Transport Electron Transport Cathode Layer Layer e- hν e-

Exciton by diffusion

eh-+ h+

Photon Absorption Exciton Formation Exciton Diffusion

Charge Transport & Collection EHP Formation Exciton Dissociation24 Photovoltaic Process In Organic Solar Cells  Coupling Absorption Creation Separation Collection Creation of sunlight of of of charges of charges of into incident ‘free’ by built-in at excitons Sunlight solar cell photons charges E field electrodes

Light Photons Excitons Charges Charges Reflected Not Recombine Recombine Recombine Away Absorbed     

25 Device Fabrication

- Metal Deposition Al - Ca Al Ca Active Layer Deposition Active Layer PEDOT:PSS

ITO PEDOT:PSS Coating Transparent Glass Substrate Contacts + + + + ITO Patterning

26 Highest Efficiency Reported OSC Till Date

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

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

27 Organic Solar Cell Work at IIT K

28 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 29 The Processing Laboratory

ISO 6, 220 m 2

30 Characterisation Facilities

31 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 32 Organic Solar Cell Model

V int D I 2 RS

R s, int. + D1 I V P Ddark RSH

Rshunt, int. -

New Model

RS I
IL is a function of voltage + V
I RSH Exciton generation IP is a constant L - 33 B. Mazhari 2006 Traditional Model Optical Efficiency ηηηO

 
Optical losses maybe due to ■ Reflection at the surface  n0=1 for air  n1, κκκ ■ Unabsorbed light leaking out Device
Solutions

Back electrode ■ Anti Reflection Coating (ARC) ■ Texturing the top surface ηηηO = 1-R where ■ Concentrators 2 2 (n1-n0) + κκκ R = 2 ■ Thickness of layers (n1+n0) + κκκ ni : refractive index of medium i κκκ: attenuation coefficient in device 34 Light Trapping by TiO 2 Nanoparticles

P3HT-PCBM-TiO2_40 100 100 TiO 2 particle is dispersed in the P3HT:PCBM blend 90 P3HT:PCBM 80 80

70 P3HT:PCBM + TiO 2 60 60

50   40

40Reflectance(%)

30

Reflectance (%) Device 20 20

10 Back electrode

0 0 300300 400 500 500 600 700 700 800 900 900 1000 λ (nm ) λλλ (nm) 35 Jyoti Singh 2008 Cathode Variation

Al Active Area Ca

) ITO Glass -2 Voltage (V)

Illumination: AM1.5D 100 mW cm -2 Current Density Current (mADensity cm

36 Nitin Sahai 2008 Effect of Post Process Anneal

P3HT: PCBM Blend Aluminium Cathode Heterostructure Polymer Blend PEDOT:PSS Glass ITO

37 Vinod Pagare 2007 Degradation Models

Degradation under Electrical & Optical Stress

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

38 Munish Jassi 2006 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 39 Let us make Organic Solar Cells Happen!

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