1. 17 October 2003 1 High performance silicon solar cells Gabriela Bunea Ph.D. SunPower Corporation

2. 17 October 2003 2 Outline Background SunPower brief history High efficiency solar cells High volume manufacturing Future directions

3. 17 October 2003 3 Questions often heard from the general public Why have solar cells never become a substantial source of energy?” “Too bad solar never made it, it seemed so promising back in the 1970s.” “When will the big breakthrough come that will make solar cells practical?”

4. 17 October 2003 4 Answers and fun facts Solar cell manufacturing is a vital and rapidly growing industry, enjoying over 30% annual growth over the last 10 years. In 2002, more square inches of silicon was used by the solar cell industry than the IC industry. There will be no big breakthrough that impacts the industry for at least 10 years, and probably 20 years. Instead, the existing technologies will evolve to where they will be cost effective in most distributed applications in 10 years, and will be competitive with fossil fuel generation in 20 years.

5. 17 October 2003 5 Solar Cell Price Exhibits a Classic Experience Curve Behavior 2002 $3/W

6. 17 October 2003 6 Solar Cell Rules of Thumb The annual production of solar modules increases ten-fold every decade The price of solar cell modules decreases by half every decade –2002: $3.00/W –2012: $1.50/W –2022: $0.75/W

7. 17 October 2003 7 Silicon Module Cost Components Higher efficiency leverages cost savings throughout the value chain Investing in high efficiency cell processing makes economic sense

8. 17 October 2003 8 Factors Driving Past Cost Reduction Poly silicon price: $300/kg → $30/kg Wire saws: now < $0.25/W Larger wafers: 3” → 6” Thinner wafers: 15 mil → 8 mil Improved efficiency: 10% → 16% Volume manufacturing: 1MW → 100MW Increased automation: none → some Improved manufacturing processes

9. 17 October 2003 9 The Renewable Energy Revolution Renewable energy will capture a meaningful share of the Global Energy Market in the next 25 years. Key drivers will be: –Falling costs for renewable energy –Declining fossil fuel production –Increasing energy demand worldwide –Environmental concerns Source: C.J.Campbell “World Oil Resources” Dec 2000 Oil industry consensus: production will peak between 2004 and 2010

10. 17 October 2003 10 The Future of Renewables Projected World Energy Production Source: Royal Dutch Shell Group

11. 17 October 2003 11 SunPower company history 1985: Record efficiency Silicon Solar Cell developed at Stanford Univ. 1988: SunPower formed to commercialize technology for concentrator applications 1993: SunPower supplies solar cells for Honda Dream, winner of World Solar Challenge 1994: Opto product line introduced 1996: Honda invests 1998: HP selects SunPower for IrDA detectors 1998: Pegasus product line introduced.

12. 17 October 2003 12 Company History (cont.) 2000: SunPower ships 35 kW to AeroVironment for Helios solar airplane. 2001: Helios flies to 96,500 ft. 2001: Low-cost, back-contact cell manufacturing process developed 2002: Cypress Semiconductor invests 2002: 21.1% efficiency one-sun in Austin, TX pilot line

13. 17 October 2003 13 Solar cell operation I V Isc Vocdark light

14. 17 October 2003 14 Solar cell parameters Fill Factor:Efficiency:

15. 17 October 2003 15 Solar spectrum

16. 17 October 2003 16 SunPower solar cells One-sun Concentrator Building integrated Remote industrialRemote for habitat

17. 17 October 2003 17 SunPower one-sun Si solar cell A-300 5” semi-square

18. 17 October 2003 18 Efficiency Losses in Silicon Practical Efficiency Limit Silicon material intrinsic loss (Auger recombination, non-optimum bandgap) Implementation loss Resulting efficiency Conventional Cell 29% Silicon Limit Detailed balance limit 33%

19. 17 October 2003 19 Conventional Solar Cell Loss Mechanisms 1.8% 0.4% 1.4% 1.54% 3.8% 2.6% 2.0% 0.4% 0.3% I 2 R Loss Reflection Loss Recombination Losses Back Light Absorption Limit Cell Efficiency29.0% Total Losses-14.3% Generic Cell Efficiency14.7%

20. 17 October 2003 20 Popular Efficiency-Enhancing Processes Aluminium or boron back-surface field (BSF)Aluminium or boron back-surface field (BSF) Silicon nitride ARCSilicon nitride ARC Laser buried grid metallization.Laser buried grid metallization. Selective emitterSelective emitter Oxide passivation with restricted metal contact openings.Oxide passivation with restricted metal contact openings. Rear surface reflector.Rear surface reflector. Higher lifetime silicon wafersHigher lifetime silicon wafers

21. 17 October 2003 21 Impact of High Efficiency Processes

22. 17 October 2003 22 High-Efficiency Back-Contact Loss Mechanisms Limit Cell Efficiency29.0% Total Losses-4.4% Enabled Cell Efficiency24.6% 0.5% 0.2% 0.8% 1.0% 0.2% 0.3% 0.2% I 2 R Loss 0.1%

23. 17 October 2003 23 Efficiency vs Lifetime A lower lifetime –reduces the collection of minority carriers, –increases bulk recombination. This effect is magnified in rear-contact solar cells. Conclusion: desire > 1 ms.

24. 17 October 2003 24 Efficiency vs Cell Thickness A thinner cell –increases the collection efficiency of minority carriers, –reduces bulk recombination. But thinner cells lose photogenerated current because not all photons absorbed. Over range 160–280 um efficiency is about constant. Simulated with t = 3 ms.

25. 17 October 2003 25 Concentrators solar cells Can achieve a higher efficiency because a higher carrier density increases output voltage NREL

26. 17 October 2003 26 Concentrator Solar Cells HECO HEDA

27. 17 October 2003 27 N+ P+ One-sunConcentrator n 1/  FSF n 1/  SiO 2

28. 17 October 2003 28 High efficiency Si Concentrators solar cells Cross section Record efficiency=26.8% at 25W/cm 2 Irradiance Single Crystal Silicon Front Back Localized Point Contacts Passivating Oxide Texture + ARC N+ P+ Gridlines

29. 17 October 2003 29 Challenges in processing high efficiency Si solar cells Process thin wafers Anti-reflection coating Low temperature passivation

30. 17 October 2003 30 Conclusions and future directions Solar generated energy will play a major role in energy generation One sun: high volume manufacturing of 20% efficiency solar cells Concentrators: –30% Si cell –6” wafers

31. 17 October 2003 31 Acknowledgments Dr. Dick Swanson Dr. Akira Terao Dr. David Smith