1. Thin Film CIGS Photovoltaics
Rommel Noufi SoloPower, Inc.
5981 Optical Court, San Jose, CA •

2. 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

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

9. Best Research-Cell Efficiencies

10. Parameters of High Efficiency CIGS Solar Cells
Sample Number
Voc (V)
Jsc (mA/cm2)
Fill factor (%)
Efficiency (%) M
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) to 0.82
Ga/(In+Ga) to 0.31
Yields device efficiency of 17.5% to 19.5%

11. *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

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

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

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 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

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%
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

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

30. Best Production-Line PV 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 Zweibel Proceedings of the 4th WCPEC, May 7, 2006, Hawaii

32. The End

34. 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

35. 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