S. Sundar Kumar Iyer

1. Organic Solar Cells S. Sundar Kumar Iyer Samtel Centre for Display Technologies

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 (h~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 h~40.7% in Spectrolab • A big surge in solar cells research & development is underway

6. The Birth of Silicon Photovoltaics 1mm Efficiency h≈ 6 % Chapin et al. 1954

7. Space Applications www.spacetoday.org marsrovers.nasa.gov Photovoltaics are the mainstay

8. Remote Locations www.dacres.org Photovoltaics are attractive summitclimb.com web.worldbank.org

9. Consumer Electronics

10. Grid Supply www.e2tac.org Need to make photovoltaics attractive in the marketplace www.sun-consult.de

11. Solar Energy Usage and Pricing 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)

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

13. Solar Energy Production and Price R.M. Margolis 2003

14. Cost Breakdown of Silicon Photovoltaics Cell Processing 25% Module 35% Silicon Wafer 40% Data from A. Rohatgi

15. 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. Efficiency of PV for Different Materials Spectrolab 40.7% Organics Photovoltaic Zweibel et al. 2004

17. Why Organic Solar Cells?

18. High-Throughput and Low-Cost Processing Roll-To-Roll A simplified overview Raw Materials Plastic Rolls Finished Goods (Solar Cell) Step 3 Step 1 Step 2 Deposition Patterning Packaging www.rolltronics.com • Printing • Screen Pringing • Stamping • Spraying • Spin Coating • Vaporisation

19. Flexible Solar Cells Prof. Kippelen’s Group; Georgia Tech • Flexible Surfaces • Conformal Surfaces Example show is a CIGS solar Cells

20. Eco-Friendly Technology Ink Reservoir Piezoelectric crystal Pulsed Signal Chamber Nozzle Epson Print • Appropriate Process • Biodegradable Molecule Anode contacts Cathode contacts Solar cell device using the molecule Modified chromophore of Green Fluorescent Protein Molecule Example: Ink-jet technology uses material only where needed

21. Background

22. Fill Factor FF is the ratio of area of maximum rectangle fitted in the 4th quadrant I-V and the product of VOC andISC Maximum Power Output Pmax = VOC× ISC × FF Efficiency h = Efficiency of a Solar Cell I Dark Light I (mA) VOC V Pmax Max Power Rectangle Incident Optical Power ISC V (V) S.M.Sze 1991 hn I n p V RL

23. Classic p-n Junction Photovoltaic Cell Ef Efn Efp n-type p-type Ebuilt-in Inorganic Semiconductor hn > Eg= Ec - Ev hn e- Ec fbi Ev h+ +ve -ve • Incident photon immediately forms mobile electrons and holes

24. Organic Solar Cells Operation A Heterojunction Organic Solar Cell Structure Hole Transport Layer Electron Transport Layer Anode Cathode e- hν e- Exciton e- h+ h+ Photon Absorption Charge Transport & Collection Exciton Formation Exciton Diffusion Exciton Dissociation Photon Absorption Exciton Formation Exciton Diffusion Charge Transport & Collection EHP Formation Exciton Dissociation

25. Photovoltaic Process In Organic Solar Cells Creation of excitons Excitons Recombine   Coupling of sunlight into solar cell Absorption of incident photons Creation of ‘free’ charges Separation of charges by built-in E field Collection of charges at electrodes Sunlight Light Reflected Away  Photons Not Absorbed  Charges Recombine  Charges Recombine 

26. Device Fabrication - Al - Ca Al Active Layer Ca PEDOT:PSS ITO Transparent Glass Substrate Contacts + + + + Metal Deposition Active Layer Deposition PEDOT:PSS Coating ITO Patterning

27. Highest Efficiency Reported OSC Till Date 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 h = 6.5%

28. Organic Solar Cell Work at IIT K

29. 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

30. The Processing Laboratory ISO 6, 220 m2

31. Characterisation Facilities

32. Three Pronged Approach Printed Ink RS I CuPC + V RSH IL Substrate - Engraved Cells R = Hexyl group Stable Molecule From P3HT Family P3HT PCBM AVPV Contact to Cathode Encapsulation Active Layer Indium Tin Oxide Lines PEDOT:PSS ITO Contact to Anode Glass • 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

33. Organic Solar Cell Model Vint I D2 RS Rs, int. + D1 V IP Ddark RSH Rshunt, int. - RS I + V RSH IL - Traditional Model • IL is a function of voltage • Exciton generation IP is a constant New Model B. Mazhari 2006

34. Optical Efficiency hO (n1-n0)2 + k2 (n1+n0) + k2 • 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   n0=1 for air n1, k Device Back electrode hO = 1-R where R = ni : refractive index of medium i k: attenuation coefficient in device

35. Light Trapping by TiO2 Nanoparticles 100 80 60 Reflectance (%)   40 Device 20 Back electrode 0 300 500 700 900 l (nm) TiO2 particle is dispersed in the P3HT:PCBM blend P3HT:PCBM P3HT:PCBM + TiO2 Jyoti Singh 2008

36. Cathode Variation Al Ca Active Area ITO Glass Voltage (V) Current Density (mA cm-2) Illumination: AM1.5D 100 mW cm-2 Nitin Sahai 2008

37. Effect of Post Process Anneal Aluminium Cathode Polymer Blend PEDOT:PSS ITO Glass P3HT: PCBM Blend Heterostructure Vinod Pagare 2007

38. 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 Munish Jassi 2006

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!