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

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

17 October 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?”

17 October 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.

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

17 October 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

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

17 October 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

17 October 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

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

17 October 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.

17 October 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

17 October Solar cell operation I V Isc Vocdark light

17 October Solar cell parameters Fill Factor:Efficiency:

17 October Solar spectrum

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

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

17 October 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%

17 October 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%

17 October 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

17 October Impact of High Efficiency Processes

17 October 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%

17 October 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.

17 October 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.

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

17 October Concentrator Solar Cells HECO HEDA

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

17 October 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

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

17 October 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

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