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17 October 2003 1
High performance silicon solar cells
Gabriela Bunea Ph.D.
SunPower Corporation
17 October 2003 2
Outline
• Background • SunPower brief history• High efficiency solar cells• High volume manufacturing• Future directions
17 October 2003 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?”
17 October 2003 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.
17 October 2003 5
Solar Cell Price Exhibits a Classic Experience Curve Behavior
1
10
100
1 10 100 1000 10000 100000
Cum ulative Production (MW)
Mo
du
le P
rice
($/
W)
($20
02)
Historical
Projected1980
$21.83/W
1985
$11.20/W1990
$6.07/W 1995
$4.90/W2000
$3.89/W2005
$2.70/W2010
$1.82/W2013
$1.44/W
2002$3/W
17 October 2003 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
17 October 2003 7
Silicon Module Cost Components
Ingot Growth
30%
Wafering20%
Module Assembly
30%
Cell Processing
20%
Higher efficiency leverages cost Higher efficiency leverages cost savings throughout the value savings throughout the value chainchain
Investing in high efficiency cell Investing in high efficiency cell processing makes economic processing makes economic sensesense
17 October 2003 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
17 October 2003 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
Source: C.J.Campbell “World Oil Resources” Dec 2000
Oil industry consensus: production will peak between 2004 and 2010
17 October 2003 10
The Future of Renewables Projected World Energy Production
0
100
200
300
Co
al Oil
Gas
Nu
clea
r
Bio
mas
s
Hyd
ro
Win
d
So
lar
Geo
19992020
20402060
Exa
jou
les
Source: Royal Dutch Shell Group
17 October 2003 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 introduced1996: Honda invests1998: HP selects SunPower for IrDA detectors1998: Pegasus product line introduced.
17 October 2003 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
17 October 2003 13
Solar cell operation
I
V
Isc
Vocdark
light
LInkT
qVII
1)exp(0
17 October 2003 14
Solar cell parameters
SCOC
MPMP
IV
IVFF
IN
MPMP
P
IVFill Factor: Efficiency:
17 October 2003 15
Solar spectrum
17 October 2003 16
SunPower solar cells• One-sun• Concentrator
Building integrated Remote industrial Remote for habitat
17 October 2003 17
SunPower one-sun Si solar cellA-300
5” semi-square
17 October 2003 18
Efficiency Losses in SiliconPractic
al Efficiency Limit
14.7%
24.6%
14.3%
4.4%
4.0% 4.0%
Silicon material intrinsic loss(Auger recombination, non-optimum bandgap)
Implementation loss
Resulting efficiency
Conventional Cell
29%SiliconLimit
Detailed balance limit 33%
17 October 2003 19
Conventional Solar Cell Loss Mechanisms
1.8%
0.4%
1.4%
1.54% 3.8%
2.6%
2.0%
0.4%
0.3%
I2R LossReflection Loss
RecombinationLosses
Back LightAbsorption
Limit Cell Efficiency 29.0%
Total Losses -14.3%
Generic Cell Efficiency 14.7%
17 October 2003 20
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 Oxide passivation with restricted metal contact openings.openings.•Rear surface reflector.Rear surface reflector.•Higher lifetime silicon wafersHigher lifetime silicon wafers
17 October 2003 21
Impact of High Efficiency Processes
14.7%14.8%
15.6%16.5% 16.8%
17.1%
18.3%
19.7%
21.2%
10.0%
12.0%
14.0%
16.0%
18.0%
20.0%
22.0%C
ell
Eff
icie
ncy
Conve
ntion
al Silic
on C
ell
High L
ifetim
e Bas
e
Back S
urfa
ce F
ield
(BSF)
Rear L
ocal
Conta
cts (
RLC)
Passiv
ated
Em
itter (
PE)
Selecti
ve E
mitt
er (S
E)
BSF+ PE
SE + R
CL
High L
ifetim
e +
SE + R
CL
17 October 2003 22
High-Efficiency Back-Contact Loss Mechanisms
Limit Cell Efficiency 29.0%
Total Losses -4.4%
Enabled Cell Efficiency 24.6%
0.5%
0.2%
0.8%
1.0%
1.0%
0.2%
0.3%0.2%
I2R Loss0.1%
17 October 2003 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.
0
5
10
15
20
0.01 0.1 1 10
Minority-carrier lifetime (ms)C
ell e
ffic
ienc
y
(%)
17 October 2003 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.
16
18
20
22
160 200 240 280 320
Cell thickness (um)
Cel
l eff
icie
ncy
(%
)
17 October 2003 25
Concentrators solar cells• Can achieve a higher efficiency because a higher carrier
density increases output voltage
Heda312 with cover glass Efficiency vs. Irradiance
10.00
15.00
20.00
25.00
30.00
0 5 10 15 20 25 30 35 40 45 50
Irradiance (W/cm2)
Eff
icie
ncy
(%
)
NREL
17 October 2003 26
Concentrator Solar Cells
HECO HEDA
17 October 2003 27
N+ N+ N+P+ P+ P+P+
One-sun Concentrator
n
1/ FSF FSF
n
1/
SiO2SiO2
17 October 2003 28
High efficiency Si Concentrators solar cellsCross section
Record efficiency=26.8% at 25W/cm2 Irradiance
Single Crystal Silicon
Front
Back Localized PointContacts
PassivatingOxide
Texture + ARC
N+ N+ N+P+ P+ P+P+
Gridlines
17 October 2003 29
Challenges in processing high efficiency Si solar cells
• Process thin wafers
• Anti-reflection coating
• Low temperature passivation
17 October 2003 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
17 October 2003 31
Acknowledgments
• Dr. Dick Swanson
• Dr. Akira Terao
• Dr. David Smith