High performance silicon solar cells Gabriela Bunea Ph
High performance silicon solar cells Gabriela Bunea Ph. D. Sun. Power Corporation 17 October 2003 1
Outline • • • Background Sun. Power brief history High efficiency solar cells High volume manufacturing Future directions 17 October 2003 2
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 1970 s. ” • “When will the big breakthrough come that will make solar cells practical? ” 17 October 2003 3
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 2003 4
Solar Cell Price Exhibits a Classic Experience Curve Behavior 2002 $3/W 17 October 2003 5
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 2003 6
Silicon Module Cost Components Higher efficiency leverages cost savings throughout the value chain Investing in high efficiency cell processing makes economic sense 17 October 2003 7
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: 1 MW → 100 MW Increased automation: none → some Improved manufacturing processes 17 October 2003 8
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 17 October 2003 9
The Future of Renewables Projected World Energy Production Source: Royal Dutch Shell Group 17 October 2003 10
Sun. Power company history • • • 1985: Record efficiency Silicon Solar Cell developed at Stanford Univ. 1988: Sun. Power formed to commercialize technology for concentrator applications 1993: Sun. Power supplies solar cells for Honda Dream, winner of World Solar Challenge 1994: Opto product line introduced 1996: Honda invests 1998: HP selects Sun. Power for Ir. DA detectors 1998: Pegasus product line introduced. 17 October 2003 11
Company History (cont. ) • • • 2000: Sun. Power ships 35 k. W to Aero. Vironment 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 2003 12
Solar cell operation I dark Voc V light Isc 17 October 2003 13
Solar cell parameters Fill Factor: 17 October 2003 Efficiency: 14
Solar spectrum 17 October 2003 15
Sun. Power solar cells • One-sun • Concentrator Building integrated 17 October 2003 Remote industrial Remote for habitat 16
Sun. Power one-sun Si solar cell A-300 5” semi-square 17 October 2003 17
Efficiency Losses in Silicon Detailed balance limit Silicon material intrinsic loss (Auger recombination, non-optimum bandgap) Conventional Cell 33% Practical Efficiency Limit 29% Silicon Limit Implementation loss Resulting efficiency 17 October 2003 18
Conventional Solar Cell Loss Mechanisms Reflection Loss I 2 R Loss 1. 8% 0. 4% 0. 3% 1. 54% 3. 8% Recombination Losses 2. 0% 1. 4% Back Light Absorption Limit Cell Efficiency Total Losses Generic Cell Efficiency 17 October 2003 2. 6% 29. 0% -14. 3% 14. 7% 19
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 17 October 2003 20
Impact of High Efficiency Processes 17 October 2003 21
High-Efficiency Back-Contact Loss Mechanisms 0. 5% 0. 8% 1. 0% 0. 2% 17 October 2003 0. 3% 0. 2% 1. 0% Limit Cell Efficiency 29. 0% Total Losses -4. 4% Enabled Cell Efficiency 24. 6% I 2 R Loss 0. 1% 22
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 2003 23
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 2003 24
Concentrators solar cells • Can achieve a higher efficiency because a higher carrier density increases output voltage NREL 17 October 2003 25
Concentrator Solar Cells HECO 17 October 2003 HEDA 26
One-sun Concentrator P+ 1/ 17 October 2003 FSF 1/ N+ P+ N+ N+ P+ FSF Si. O 2 n n 27
High efficiency Si Concentrators solar cells Cross section Texture + ARC Front Passivating Oxide Single Crystal Silicon P+ Back N+ P+ Gridlines N+ Localized Point Contacts Record efficiency=26. 8% at 25 W/cm 2 Irradiance 17 October 2003 28
Challenges in processing high efficiency Si solar cells • Process thin wafers • Anti-reflection coating • Low temperature passivation 17 October 2003 29
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 2003 30
Acknowledgments • Dr. Dick Swanson • Dr. Akira Terao • Dr. David Smith 17 October 2003 31
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