Crystalline Silicon Solar Cell Technology
Rubin Sidhu, David E. Carlson BP Solar April 13, 2010 Agenda
• Global PV Market • Nomenclature • PV Technologies • Crystalline Silicon Solar Cell Value Chain • Solar Cell Physics (no equations!) • Why is solar cell efficiency only ~15%? • Reaching Grid Parity using BP Solar’s Crystalline Silicon PV Technology • The Solar America Initiative • Silicon Solar Technology Roadmap • Conclusion: Future of PV PV Experience Curve
Cost of Materials Limit (20% Modules)
PV module prices have followed an experience curve with a slope of ~ 80% 20% decrease in price with every doubling of cumulative production Cost of Solar Electricity Shipments of Photovoltaic Modules
Projection (Lux Research)
PV shipments actually increased about 20% 2009. PV Shipments & Countries of Origin
(Navigant Consulting)
Europe has been the largest consumer of PV in recent years (> 50% of all installations). China and Taiwan have become the largest producers. PV Shipments in 2009
European countries accounted for 5.60 GW, or 77% of world demand Germany, Italy and Czech Republic accounted for 4.07 GW
United States grew 36% to 485 MW
• World solar PV market installations reached a record high of 7.3 GW in 2009 growth of 20% over 2008 • $43 billion in global revenues in 2009, up 8% on the prior year • World solar cell production 9.34 GW in 2009, up from 6.85 GW in 2008 • Crystalline Si module price average for 2009 crashed 38% from previous year Grid Parity
GTM Research LCOE and kWh/kWp Levelized Cost Of Electricity (¢/kWh): LCOE is the minimum price at which energy must be sold for an energy project to break even
Typical Sunny Day
Including: 1200 7.5 Sun-Hour Day initial investment 24-Hour Equivalent 1000 operations and maint. cost of fuel 800 cost of capital 600 (W/m²)
400
200 7.5 kWh Spread over 24 Hours
0 06 6AM Noon 12 6 18PM 24 Insolation
kWh/kWp PV Facts
Using today’s PV Technology, an array field that is 300 miles on each side could produce the entire electrical energy used by the United States in a year. PV Facts
Over its lifetime, a typical PV module, in a sunny climate, will produce over twenty times the electricity initially used to manufacture it. Technology Overview: Nomenclature
A solar PV cell is the smallest semiconductor device that can convert sunlight into electrical energy
A module is an assembly of cells Multi (Poly) Cell Mono Cell in series or parallel to augment voltage and/or current A panel is an assembly of modules on a structure An Array is an assembly of panels at a site.
Large Roof-Top PV Array Conversion Efficiencies vs. Time (NREL)
There has been steady progress in the improvement of conversion efficiencies for a number of PV technologies over the last few decades. The Major Players
Crystalline Si a-Si/µc-Si CIGS CdTe Sharp United Solar Nanosolar First Solar SolarPower Kaneka Avancis Antec Solar Kyocera Fuji Electric Solar Frontier Abound Solar BP Solar Sharp Wurth Solar PrimeStar Solar Q-Cells Mitsubisihi Global Solar Calyxo Mitsubishi Schott Solar Honda Soltec SolarWorld SunTech Panasonic (Sanyo) EPV There are currently more than 300 Schott Solar PowerFilm companies developing or producing Isofoton AMAT licensees solar cells. Motech Orelikon licenses With prices continuing to decrease, Suntech and more companies entering the Evergreen Solar market, many small companies and JA Solar start-ups are likely to fail. Source: pvsociety Manufacturing Process for mc-Si PV Modules
CASTING SIZING WAFERING CLEANING DIFFUSION
% of % of % of % of Total Total Total Total 0.00% 0.00% 0.00% 0.00% 0.27% 2.03% 0.34% 0.85% 2.18% 13.23% 2.23% 2.53%
FINISH INTER- ANTI- TEST LAMINATION CONNECT METALLIZATION REFLECTIVE COATING
Most companies are manufacturing PV modules based on screen- printing contacts on multicrystalline silicon wafers CZ - Silicon for Solar
Silicon Feedstock Mono-crystalline wafer Drawbacks of CZ Solar – Cylindrical ingot (module packing) – Smaller throughput rate • CZ: 5.3 kg/h at 2mm/min • Cast: 12.9 kg/h at 0.3mm/min Mono – High oxygen content -> LID module – Higher energy use Mono – Feedstock limitations solar cell – Importance of skilled operators Multicrystalline Silicon Casting
Silicon Ingot
Crucible Liquid Si Solid Si
Silicon Feedstock
Drawbacks: • Crystal defects Heater Insulation • Iron from crucible • Inclusions from Silicon Bricks coating, furnace Casting Station cross-section • Not compatible with Multicrystalline pyramid texturing Solar Cell Multi- crystalline Wafer Multicrystalline Module The Typical Silicon Solar Cell
This device structure is used by most manufacturers today.
• The front contact is usually formed by POCl3 diffusion • The rear contact is formed by firing screen-printed Al to form a back-surface field The cell efficiencies for screen-printed multicrystalline silicon cells are typically in the range of 14 – 17%. Operation of a Solar Cell
The theoretical limit for a crystalline silicon solar cell is ~ 29%. Typical module construction
J-box Cable Backsheet EVA encapsulant
Solar cells
EVA encapsulant Glass Frame w/adhesives
BP Solar now uses a polyester backsheet to protect the rear of the module Also a new encapsulant 5% lower temperature 2 – 3% gain in energy production PV Module Costs
Si Wafer PV Module Electricity Major Energy Production Cost 10% Consuming $2.10/W Silicon 23% Steps Equipment Feedstock 3% 15% Ingot 3% Wafering 2% Other Materials Labor 25% 27% Cell 2%
About half of the total module cost is associated with the cost of materials Silicon prices have been dropping rapidly over the last 18 months Cost Reduction
Source: George Nemet, U Wisc.
Cost of Materials Limit (20% Modules) Solar America Initiative
This program is a BP Solar cost shared Technology Pathways Partnership under the DOE Solar Energy Program. The program addresses all aspects of crystalline silicon PV manufacturing and systems deployment. Overall Program Objective
Accelerated development of crystalline silicon technology using thin Mono2 TM wafers as the platform. Module designed for use in residential and commercial markets with products designed specifically for these applications. System components designed to add value to electricity produced. PV Product Value Chain
Semi- Metallurgical Crystal Cell Module conductor Wafering Silicon Growth Fabrication Assembly Silicon Mono2 TM Silicon Casting
Silicon Ingot
Crucible Liquid Si Solid Si
Silicon Feedstock
Heater Insulation
Silicon Bricks Casting Station cross-section
Mono2 2 Mono Mono2 Module Solar Cell Wafer Mono2 TM Lifetime Uniformity
Multi Mono2 TM
Material Lifetime Jsc Voc Median % mA/cm2 mV Std. Dev. Simulation Multi 75 s 15.46 33.15 610 results 80% Mono2 TM 75 s 15.87 33.55 618 40% Mono2 TM, 75 s 16.41 34.71 619 pyramid etched 40%
PV Product Value Chain
Semi- Metallurgical Crystal Cell Module conductor Wafering Silicon Growth Fabrication Assembly Silicon c-Si Solar Cells: Loss Mechanisms
Thermodynamic limit for
0.7% crystalline Si solar cells ~29% Sources of loss relative to 2.4% maximum achievable efficiency – Front surface reflection
6.3% – Emitter contact shadowing – Poor rear reflectance for light trapping 2.2% – Surface recombination (i.e., Reflection Shadowing surface cleanliness) Other (Optical) – Bulk recombination (i.e., 2.5% Ohmic material quality) Recombination – Resistive losses at the cell and module level Cell Process
Texture Etch – ICT o Preferred option for Mono2 and multi o Cell efficiency improvements demonstrated o Improvements lost when cells are encapsulated – Alkaline texture etch o Good for Mono2 but not multi o Increased cell and module efficiencies by ~ 0.5% absolute. Cell Process
Back Print Optimization – Maximize area covered with Al paste for improved performance – Minimize area of Ag pads to reduce cost – Implemented new pattern in production with 0.6% gain in cell efficiency and 1% increase in module efficiency. Advanced Metallization – Looking at new approaches to increase metallization thickness, to reduce series resistance while also reducing finger width and therefore shadowing. – Looking at aerosol jet printing, ink jet printing and offset printing. – Successfully printed 80 to 90 µm wide and ~ 35 µm thick c-Si Solar Cells: Advances
Source: pvsociety The Selective Emitter
A number of organizations are developing solar cells with selective emitters in order to use thinner emitters and improve the short- wavelength spectral response. Passivated Rear Point Contacts
Fraunhofer ISE has used laser firing of aluminum to fabricate high efficiency (21.7%) solar cells with passivated rear point contacts. The PERL Solar Cell
+ p
n + n oxide p-silicon + + p p rear contact p + oxide
PERL Cell Structure
The PERL solar cell has a passivated emitter with a rear locally diffused base contact, and efficiencies as high as 25% have been obtained with this structure. SunPower Back Contact Solar Cell
The SunPower cell has all its electrical contacts on the rear surface of the cell. Production cells ~ 22.4% efficiency; new prototypes at 23.4%. Diffusion lengths > 3 x cell thickness (using 145 m thick CZ-Si at end of 2008). Sanyo HIT Solar Cell
The HIT cell utilizes amorphous Si intrinsic layers (~ 5 nm) as passivation layers. The cell is symmetric except for the a-Si p+ emitter layer (~ 10 nm) on the front and the a-Si n+ contact layer (~ 15 nm) on the rear. Best lab efficiency = 22.3% (open-circuit voltages as high as 739 mV). PV Product Value Chain
Semi- Metallurgical Crystal Cell Module conductor Wafering Silicon Growth Fabrication Assembly Silicon
Module Assembly defines product reliability and longevity Material choice is key to meet life time goal Good choice of materials allows life time to exceed warranty period Balance of performance and cost Module Packaging Materials
Encapsulant: Glue for the whole package: Key component Back sheet: Protect cells against the environment and provide electrical isolation Superstrate glass: Cell protection and light capture Frames and framing adhesives: Mounting and load bearing J-box/Cables: Electric isolation and longevity Tabbing ribbons: For cell interconnection Module Development
AR Coated Glass – Qualified and implemented Gen 1 AGC Flat Glass NA AR coated glass with 1.5 to 2% increase in STC power and 4% increase in overall energy collection – Gen 2 now under development with improved cosmetic uniformity and ~ 2.5% STC power gain. EVA cure level determination – Standard approach is to use chemical extraction of un-cross linked EVA (gel content) – Developed differential scanning calorimetry (DSC) as a fast and reliable way of measuring cross-link density. ARC vs. standard module
Standard modules AR coated modules
9%
7%
5%
3%
1% Percentage kWh/kWp difference difference kWh/kWp kWh/kWp Percentage Percentage
6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 kWh/kWp difference between AR coated & Uncoated glass glass & Uncoated & Uncoated coated coated AR AR between between difference difference kWh/kWp kWh/kWp -1% TimeTime ofof Day Day
Normalized energy generation of a day – 6% to 7% more energy at early and late periods of the day BP Solar testing beyond industry standards
BP Solar ensures the long-term dependability and reliability of our products by testing beyond industry standards to mimic 25 years of exposure and real world conditions.
BP Testing Implications
Thermal cycles stress and crack module interconnects; failures Thermal cycling at 2.5 times IEC Standards of interconnects lead to significant loss of power
Moisture and heat can lead to corrosion leading to significant Damp heat exposure at 25% more than IEC Standards power loss or module failure
Modules experience significant flexing due to wind pressure that Addition of wind simulation to IEC humidity freeze cycles can cause cell cracks and lead to reduced module output or failure
Modules experience significant flexing due to snow loads that Load testing as much as twice the IEC Standard can cause cell cracks and lead to reduced module output or failure Residential Building-Integrated PV
Building-integrated PV may become ubiquitous in the next few decades. Large Grid-Connected PV Arrays
The levelized cost of electricity should fall to ~ 6 ¢/kWh by 2015 for large grid-connected arrays Solar Energy – the Long-Term Solution?
Source: German Advisory Council on Global Change
Some forecasts predict that solar will provide most of our energy needs in the latter half of this century. Projections for the Future of PV
The levelized cost of PV electricity could fall to ~ 6 ¢/kWh by 2015 Disruptive technologies with theoretical limits of > 60% may emerge in the next few decades Assuming a CAGR of 35% (average over the last few decades), the cumulative PV production would be ~ 3.5 TWp by 2026. 3 TWp of solar electricity will reduce carbon emissions by about 1 Gton per year (7 Gtons of carbon were emitted as CO2 in 2000) Thus, by about 2030 PV could be producing about 10% of the world’s electricity and start to play a major role in reducing CO2 emissions Questions?