Photovoltaic Power: Science and Technology Fundamentals

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1 Photovoltaic Power: Science and Technology Fundamentals Bob Clark-Phelps, Ph.D. Evergreen Solar, Inc. Renewable Energy Seminar, Nov. 2, 2006

2 Photovoltaic Principle Energy Conduction Band electron Energy Gap hole Valence Band Semiconductor band model Light in, electrons out High performance requires Minimize light reflection Maximize light absorption Maximize electron lifetime (don t let it fall back to VB) Get electrons out of cell with minimal resistance

Minimize light reflection Maximize light absorption Maximize electron lifetime

3 Basic Parameters of the Solar Cell Efficiency: η = P E m A = I m V E A m P m is the maximum power output of the cell E is the irradiance on the cell under standard conditions (1000W/m 2 ) A is the area of the cell I m I IV Characteristic I m is the current at maximum power V m is the voltage at maximum power V m V

standard conditions (1000W/m 2 ) A is the area of the cell I m I IV

4 Effect of material quality on electron lifetime Energy Conduction Band Material imperfections cause electrons to fall back to VB before they are collected and delivered to external circuit Efficiency is reduced Energy Gap Defect site (impurity, grain boundary, surface) Valence Band

are collected and delivered to external circuit Efficiency is reduced

5 Fabrication of a solar cell Source: Professor A. Rohatgi, Georgia Tech Ag front contacts SiN x ARC n type Si p type Si base e - e - e - e - e - h + e - h + e - h + e - h µm Al back contact

6 Common Solar Materials I Crystalline Silicon Monocrystalline (single crystal): c-si 32% m.s. Similar to wafers used for computer chips Expensive to manufacture but provides highest efficiency Manufacturers (SunPower, Sanyo, some BP) Multicrystalline (polycrystalline): mc-si 45% m.s. Many smaller crystals separated by grain boundaries instead of one large crystal Lower in manufacturing cost and lower efficiency Cast ingots sawn into wafers (Sharp, Q-Cells, Kyocera, BP) Ribbons eliminates sawing waste (Evergreen, EverQ, Schott)

7 Recent US Cell and Module Prices Source: Energy Information Administration, Form EIA-63B, "Annual Photovoltaic Module/Cell Manufacturers Survey, 2005

8 Cost components for Bulk Si Modules Si Wafer, 45% Processing and Assembly, 20% Module Materials, 35%

9 Common Solar Materials II Thin Film 24% m.s. All seek to reduce material cost by depositing thin films of highly absorbing material on inexpensive substrates such as glass or polymers Generally lower efficiency than bulk Si, but potentially also lower manufacturing cost Some technologies exhibit greater long-term degradation Some substrates are flexible; potentially better compatibility with Building Integrated PV (BIPV) Amorphous Silicon (Uni-Solar) Crystalline Silicon on Glass (CSG-Solar) CIGS- Copper Indium Gallium Diselenide (Daystar, Nanosolar)

All seek to reduce material cost by depositing thin films of highly absorbing material on inexpensive substrates such as glass or polymers

10 Bulk Si vs. Thin Film US Market Share Bulk Si (c-si, mc-si) Thin Film Source: Energy Information Administration, Form EIA-63B, "Annual Photovoltaic Module/Cell Manufacturers Survey, 2005

11 Module Fabrication Module Efficiency Defined and measured the same as cell efficiency Typically 1.5-2% (abs.) lower than cell efficiency (cells at 14.5% yield module at 13%)

12 Flatroof Installation Cost breakdown Modules $3/Wp Balance of systems $3-4/Wp Higher efficiency modules reduce BOS costs

13 Projecting Future Module Price Trends $30/W 1980 $21.83/W Historical Projected Module Price ($/W ) ($2002) $11.20/W 1990 $6.07/W 1995 $4.90/W 2000 $3.89/W $3/W $2.70/W 2010 $1.82/W 2013 Distributed generation price point $1.44/W Cum ulative Production (MW) Source: Dr. Richard Swanson s presentation to Austin Clean Energy Seminar, 2003

90/W 2000 $3.89/W 2005 2002 $3/W $2.70/W 2010 $1.82/W 2013 Distributed generation price point $1.

14 Summary Solar photovoltaic power offers a source of renewable energy, reducing dependence on fossil fuels Cost reduction is the key to growth of the industry Experience curve based on historical data provide reason to expect significant further cost reduction Photovoltaic power is a key part of an overall energy strategy including conservation and renewable generation

historical data provide reason to expect significant further cost reduction Photovoltaic

15 Back Up Slides

16 Yachiyo High School, Japan

17 Evergreen modules installed at Japanese site

18 String Ribbon Growth

19 Evergreen Crystal Growth

20 String Ribbon

21 Spruce Module

Thin Is In, But Not Too Thin!

Thin Is In, But Not Too Thin! K.V. Ravi Crystal Solar, Inc. Abstract The trade-off between thick (~170 microns) silicon-based PV and thin (a few microns) film non-silicon and amorphous silicon PV is addressed