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

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

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

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

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

PVNews Reported US Production thru 2007 Source: PVNews

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

CIGS Device Structure CIGS 1-2.5 µm ZnO, ITO 2500 Å CdS 700 Å Mo 0.5-1 µm Glass, Metal Foil, Plastics 8

Best Research-Cell Efficiencies

Parameters of High Efficiency CIGS Solar Cells Sample Number Voc (V) Jsc (mA/cm2) Fill factor (%) Efficiency (%) M2992-11 0.690 35.55 81.2 19.9 (World Record) S2212-B1-4 0.704 34.33 79.48 19.2 S2232B1-3 0.713 33.38 79.54 18.9 S2232B1-2 0.717 33.58 79.41 19.1 S2229A1-3 0.720 32.86 80.27 19.0 S2229A1-5 0.724 32.68 80.37 S2229B1-2 0.731 31.84 80.33 18.7 S2213-A1-3 0.740 31.72 78.47 18.4 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%

*Third party confirmed “Champion” Modules Company Device Aperture Area (cm2) Efficiency* Power (W) Würth Solar CIGS 6500 13.0 84.6 Shell Solar GmbH CIGSS 4938 13.1 64.8 Showa Shell 3600 12.8 44.15 Shell Solar 7376 11.7 86.1* Global Solar 8390 10.2 88.9* First Solar CdTe 6623 67.5* *Third party confirmed

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

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

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.

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 Substrate preparation Base Electrode Absorber First Scribe Third Scribe Top Electrode Junction Layer Second Scribe External Contacts Encapsulation

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.

Challenges

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.

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

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

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

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

0.4 µm cell - Optical

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

SoloPower Advances 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 electrolyte anode V

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

C2318

Future Commercial Module Performance Based on today’s champion cell results and a module/cell-ratio of 80% Technology Future commercial performance Relative Performance (s.p. Si =1) Relative-cost/relative-performance (50% thin film cost advantage) Silicon (non-stand) 19.8% 1.18 0.85 (competitive) Silicon (standard) 17.0% 1.00 1.00 (reference) CIS 15.9% 0.94 0.53 (highly competitive) CdTe 13.2% 0.78 0.64 (highly competitive) a-Si (1-j) 8.0% 0.47 1.06 (about the same) a-Si (3-j) (or a-Si/nc-Si) 9.7% 0.57 0.88 (competitive) Source: Bolko Von Roedern, PVSC 2008, IEEE May 12,2008, San Diego

Best Production-Line PV Module Efficiency Values From Manufacturers’ Web Sites Compiled by Bolko von Roedern, September 2008

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

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 Zweibel Proceedings of the 4th WCPEC, May 7, 2006, Hawaii

The End

Solar Electricity cost 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

CIGS Manufacturing For high quality For low cost 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