High performance silicon solar cells

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

2. Outline • Background • SunPower brief history • High efficiency solar cells • High volume manufacturing • Future directions

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. 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. Solar Cell Price Exhibits a Classic Experience Curve Behavior 2002 $3/W

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

8. Factors Driving PastCost 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. 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 Oil industry consensus: production will peak between 2004 and 2010 Source: C.J.Campbell “World Oil Resources” Dec 2000

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

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. 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. I dark Voc V Isc light Solar cell operation

14. Solar cell parameters Fill Factor: Efficiency:

15. Solar spectrum

16. SunPower solar cells • One-sun • Concentrator Remote industrial Remote for habitat Building integrated

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

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

19. Reflection Loss I2R Loss 1.8% 0.4% 0.4% 0.3% Recombination Losses 1.54% 3.8% 2.0% Back Light Absorption 1.4% 2.6% Conventional Solar Cell Loss Mechanisms

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

21. Impact of High Efficiency Processes

22. 0.8% 0.5% 1.0% 0.2% 0.2% 0.2% 0.3% 1.0% I2R Loss 0.1% High-Efficiency Back-Contact Loss Mechanisms

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. 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. Concentrators solar cells • Can achieve a higher efficiency because a higher carrier density increases output voltage NREL

26. Concentrator Solar Cells HECO HEDA

27. P+ P+ P+ N+ N+ N+ P+ One-sun Concentrator 1/ FSF 1/ FSF SiO2 SiO2 n n

28. Front Texture + ARC Single Crystal Silicon Passivating Oxide P+ P+ P+ N+ N+ N+ P+ Gridlines Back Localized Point Contacts High efficiency Si Concentrators solar cellsCross section Record efficiency=26.8% at 25W/cm2 Irradiance

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

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. Acknowledgments • Dr. Dick Swanson • Dr. Akira Terao • Dr. David Smith