Thin Film CIGS Photovoltaics

1. Thin Film CIGS Photovoltaics Rommel NoufiSoloPower, Inc. 5981 Optical Court, San Jose, CA 95138www.solopower.com • email: rnoufi@solopower.com

2. Acknowledgements: Bulent Basol SoloPower, Inc., California Robert Birkmire Institute of Energy Conversion, Delaware Bolko von Roedern, Michael Kempe, and Joel Del Cueto National Renewable Energy Laboratory, Colorado

3. Outline Status of the Technology – Laboratory cells – Modules Challenges Ahead

4. Status of PV • 3700 MW produced world wide • 266 MW produced in the US • Thin Film Market Share: 10% world wide, 65% in the US Source: PV News, Photon International, Navigant Consultants

5. Status of Thin Film PV • Currently, FIRST SOLAR [ CdTe ] is the largest Thin Film manufacturing company in the US • 277 MW in 2007 • 910 MW expected in 2009 • Demonstrated the viability of Thin Film PV • High Throughput • Large Scale • Low Cost per Watt Source: First Solar.com

6. PVNews Reported US Production thru 2007 Source: PVNews

7. CIS PV Companies Production of CIGS modules has also been demonstrated by: Würth Solar, Showa Shell, Honda, and Global Solar Energy(<20 MW manufactured) Ascent, CO DayStar Technologies, NY/CA Energy Photovoltaics, NJ Global Solar Energy, AZ HelioVolt, TX ISET, CA MiaSole, CA NanoSolar Inc., CA SoloPower, CA Solyndra, CA Stion, CA Aleo Solar, Germany AVANCIS, Germany CIS Solartechnik, Germany CISEL, France Filsom, Switzerland Honda, Japan Johanna Solar Tech, Germany Odersun, Germany PVflex, Germany Scheuten Solar, Holland Showa Shell, Japan Solarion, Germany Solibro, Sweden SULFURCELL, Germany Würth Solar, Germany

8. ZnO, ITO2500 Å CdS700 Å CIGS1-2.5 µm Mo0.5-1 µm Glass,Metal Foil,Plastics CIGS Device Structure

9. Best Research-Cell Efficiencies

10. Parameters of High Efficiency CIGS Solar Cells Tolerance to wide range of molecularity Cu/(In+Ga) 0.95 to 0.82 Ga/(In+Ga) 0.26 to 0.31 Yields device efficiency of 17.5% to 19.5%

11. “Champion” Modules *Third party confirmed

12. theoretical High efficiency range Absorber band gap (eV) Optical Band-Gap/Composition/Efficiency

13. Closing the Gap between Laboratory Cells and Modules Primary Focus: Utilizing Lab Technology base totranslate results to manufacturing

14. CIGS Modules are Fabricated On: I. Soda lime glass as the substrate; cells are monolithically integrated using laser/mechanical scribing. Courtesy of Dale Tarrant, Shell Solar Monolithic integration of TF solar cells can lead to significant manufacturing cost reduction; e.g., fewer processing steps, easier automation, lower consumption of materials.

15. Substratepreparation Base Electrode FirstScribe Absorber ThirdScribe JunctionLayer TopElectrode SecondScribe ExternalContacts Encapsulation CIGS Modules are Fabricated On: (cont.) The number of steps needed to make thin film modules are roughly half of that needed for Si modules. This is a significant advantage. CIGS Modules Process Sequence

16. CIGS Modules are Fabricated On: (cont.) II. Metallic web using roll-to-roll deposition; individual cells are cut from the web; assembled into modules. III. Plastic web using roll-to-roll deposition; monolithic integration of cells.

17. Challenges

18. Long-Term Stability (Durability) • Improved module package allowed CIGS to pass damp heat test (measured at 85°C/85% relative humidity). • CIGS modules have shown long-term stability. However, performance degradation has also been observed. • CIGS devices are sensitive to water vapor; e.g., change in properties of ZnO. - Thin Film Barrier to Water Vapor - New encapsulants and less aggressive application process • Stability of thin film modules are acceptable if the right encapsulation process is used. • Need for better understanding degradation mechanisms at the prototype module level.

19. Processing Improvements: I.Uniform Deposition over large area: (a) significant for monolithic integration (b) somewhat relaxed for modules made from individual cells II.Process speed and yield: some fabrication approaches have advantage over others III.Controls and diagnostics based on material properties and film growth: benefits throughput and yield, reliability and reproducibility of the process, and higher performance

20. Processing Improvements: (cont.) IV.Approaches to the thin film CIGS Deposition 1. Multi-source evaporation of the elements - Produces the highest efficiency - Requires high source temperatures, e.g., Cu source operates at 1400°-1600°C - Inherent non-uniformity in in-line processing - Fast growth rates my become diffusion limited - Complexity of the hardware with controls and diagnostic - One of a kind hardware design and construction - Expensive - Throughput, and material utilization need improvement

21. Processing Improvements: (cont.) IV.Approaches to the thin film CIGS Deposition (cont.) 2. Reaction of precursors in Se and/or S (Selenization)to form thin film CIGS: two stage process - Variety of materials delivery approaches: (a) sputtering of the elements (b) electroplating of metals or binaries (c) Printing of metal (or binaries) particles on substrate - Reaction time to form high quality CIGS films is limited by reaction/diffusion - Modules on glass are processed in batch mode in order to deal with long reaction time - Flexible roll-to-roll requires good control of Se vapor and reaction speed - Ga concentration thru the film is inhomogeneous limiting performance

22. Processing Improvements: (cont.) V.Reduction of the thickness of the CIGS film • Reduces manufacturing costs: higher throughput and less materials usage • More sensitive to yield, e.g. threshold thickness non-uniformity, pin-holes • Challenge is to reduce thickness and maintain performance Thin Cells Summary

23. 0.4 µm cell - Optical

24. Toward Low Cost • Module performance is a significant determining factor of cost • Cell processing affects performance • The benefits of each process and how it is handled in manufacturing need to be assessed • To date, relatively high cost methods adapted for manufacturing

25. SoloPower has developed a low cost electro-deposition process to manufacture CIGS solar cells and modules A conversion efficiency approaching 14% has been confirmed at NREL Modules have been manufactured demonstrating process flow V V electrolyte anode SoloPower Advances

26. The Electrodeposition Process • Hardware is low cost • Can be high throughput once the hardware is tuned to the specifics of the process • Near 100% material utilization • Pre-formed expensive materials are not required, e.g. sputtering targets, nano-particles • Crystallographically oriented CIGS films with good morphology and density have been demonstrated • Thickness and composition control of the deposited films are integral part of the process • Readily scalable

27. C2318

28. Future Commercial Module Performance Based on today’s champion cell results and a module/cell-ratio of 80% Source: Bolko Von Roedern, PVSC 2008, IEEE May 12,2008, San Diego

29. Best Production-LinePV Module Efficiency Values From Manufacturers’ Web Sites Compiled by Bolko von Roedern, September 2008

30. Best Production-LinePV Module Efficiency Values (cont.) From Manufacturers’ Web Sites Compiled by Bolko von Roedern, September 2008

31. Further Reading Sources “Accelerated UV Test Methods for Encapsulants of Photovoltaic Modules” “Stress Induced Degradation Modes in CIGS Mini-Modules” Michael D. Kempe et al, Proceedings of the 33rd IEEE,PVSC, May 11, 2008, San Diego “Modeling of Rates of Moisture Ingress into Photovoltaic Modules”Michael D. Kempe, Solar Energy Materials & Solar Cells, 90 (2006) 2720–2738 “Stability of CIS/CIGS Modules at the Outdoor Test Facility Over Two Decades”J.A. del Cueto, S. Rummel, B. Kroposki, C. Osterwald, A. Anderberg,Proceedings of the 33rd IEEE,PVSC , May 11, 2008, San Diego “Pathways to Improved Performance and Processing of CdTe & CuInSe2 Based Modules”Robert W. Birkmire, Proceedings of the 33rd IEEE,PVSC, May 11, 2008, San Diego “The Role of Polycrystalline Thin-Film PV Technologies in Competitive PV Module Markets”Bolko von Roedern and Harin S. Ullal, Proceedings of the 33rd IEEE,PVSC , May 11, 2008, San Diego “High Efficiency CdTe and CIGS Thin Film Solar Cells: Highlights and Challenges”Rommel Noufi and Ken ZweibelProceedings of the 4th WCPEC, May 7, 2006, Hawaii

32. The End

34. PV Energy Cost DOE, Solar America Initiative Projections and Goals • Costs are constant 2005 dollars • Residential and commercial are cost to customer • Utility is cost of generation Solar Electricity cost

35. CIGS Manufacturing Requirements for a CIGS absorber film growth technique for high efficiency devices include: • For high quality • Stoichiometric control [Cu/(Ga+In), Ga/(Ga+In), S/(S+Se)] • Good microstructure • Bandgap control • For low cost • Low cost equipment • High materials utilization