Recent Development of
Thin Film Si Solar Cell Technologies for Commercialization
< Contents >
1. Introduction
2. Si Thin Film Solar Cell Technologies
3. Deposition Technologies
4. R&D Activities in LG Electronics
5. Conclusion
Heon-Min Lee
LG Electronics / Devices & Materials Lab.
1. Introduction
1) Market Trends
◆ Rapid increase of market size : governmental supports and policies
◆ Needs for low-cost solar cell development : Si feed-stock shortage
Market Perspectives
Market Driving Force
11.6 GW
24%
Governmental supports
Up
Installation
Feed-in
Tax
Subsidy
tariff
Refund
Module System
4.7
Install
(GW)
1.7
German
Japan (~’05)
2.6
USA
41.2
S. Korea
17%
23.7
24
16%
’08
’06
1.7 GW (55%)
Japan
(19%)
’10
Down
’15
Feed-stock shortage
Need
Italy(2%)
Spain(3%)
China(6%)
USA(7%)
Spain
Italy
14.9 13.4
Cost
11.2 8.2
(billon$) 6
’06
Germany
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Net Metering
S.Korea
(2%)
India(3%)
Italy(6%)
Spain
(8%)
China
(9%)
Supply (1,000ton)
Germany
’10
(35%)
4.7 GW
19
Japan
USA (11%)
(10%)
13
’06
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19 18
23 28
’07
’08
30
38
’09
38 41
’10
LG Electronics
1. Introduction
2) Customer & Product
Products
Product
(installation type)
Customer
Needs 1)
• Installation
Space
• Price ($/W)
• Weight
Roof
Usage
Installation
Place
Usage
Residential
• Residential house • Customer
• Installer
• Apartment
• Constructor
On-
• Business building
Grid Commercial
• Factory
• Price ($/W)
(needs to meet a
base efficiency)
• Price ($/m2)
• Appearance
Public building/ • Public building
• Power plant
Utility
• Public facility
(Windows &
building facings)
2)
• Contractor
Expectation
• Investment collect
through reduced
electric bill
• Ecological
business brand
image
• Profit from
power
generation
• Profit from power
generation
• Public awareness
Remote
• Remote area
• Costumer/
difficult to access
Installer
electric supply
Consumer
• Application • Auxiliary power
• Product imbedded Manufacturer supply
Off-
• Power per
Dimension
• Building
constructor
& designer
Investment • Idle & rental space • Investor
(roof/open space) • Installer
Ground
BIPV
Key decision
Maker
Grid
Miscellaneous
• Power supply
1) Interview data from SolarBuzz, Lahmeyer, domastic Installer, LGC BIPV TFT 2) Building Integrated Photovoltaic
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1. Introduction
3) Market Trends
◆ Residential house’s roof is one of the largest potential usages
◆ BIPV market is expected to grow rapidly
Market per usage
Market per product
< Unit :MW >
4,670
< Unit : MW >
CAGR
(’05~’10)
75
(3%)
26
398
(26%)
Usage
Product
Consumer
Etc
CAGR
4,670 (’05~’10)
385
(10%)
(38%)
480
(20%)
2,518 (30%)
BIPV
Commercial
Investment
22
125
Ground
1,460
646
Residential
1,728 (22%)
Roof
43
58
1,460
3,740 (21%)
164
1,197
666
’05
’05
’10
’10
1) Summary of Navigant, Lahmeyer Int’l, Solarbuzz
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1. Introduction
4) Bulk vs. Thin film
◆ Si bulk technology : Main stream in solar cell market
◆ Si thin film technology : High potential of efficiency improvement and low cost
Potential of Si thin film
Structures of bulk and thin film
Front contact
(Single)
Glass
n+ emitter
200~
250μm
0.5
Si Thin film
Eff. Improve
Glass
TCO
a-Si
Si Wafer
(p-type)
(Tandem)
TCO
(6%→10%)
High
efficiency
a-Si
0.3μm Back contact
2μm
μc-Si
Back contact
Si Bulk
• Mass production
• Easy achievement of
Advabase efficiency
ntage
• High efficiency
- 12 ~ 15% for module
Challenge
• High consumptions
of raw materials &
energy
Si Thin Film
• Low cost of raw materials
(1/3 of Si bulk)
• Power generation 10%↑
- weak effects from install
angle and temperature
• Good appearance (Color)
0.4
Power
Generation
Sun light
a-Si
0.3
μc-Si
0.2
0.1
0
Back contact
Type
Photo Current (A/W)
Si Bulk
400
600
Type
Si Bulk
Efficiency
14%
Actual
power generation
(per 1m2-year)
152kWh
Actual
power generation 1,086kWh
(per1kW-year)
• Relatively low efficiency
• High cost of production
system 1)
1) Investment costs : Bulk Cell($0.5/W)+Module($0.1/W), Si Tandem - $2.2/W, Compound semiconductor thin film - $3.0/W
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800
1000
Wave length (nm)
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Si Thin film
6%
(Single)
72kWh
1,203kWh
(Kaneka’s data)
LG Electronics
2. Si Thin Film Solar Cell Technologies
Back
Electrode
Interconnection
μc-Si
a-Si
Front
TCO
Glass Substrate
Front TCO Scribe Line
Silicon Scribe Line
Back Electrode Scribe Line
Ag
ZnO:Al
p (20nm)
SnO2:F or ZnO:Al
Reflector
Buffer layer ZnO deposition
by CVD or Sputtering
Back electrode
Cell
fabrication
μc-Si:H
Bottom-Cell
μc-Si:H Bottom-cell deposition
p,i,n-layer by 13.56MHz PECVD
Increases of gas pressure and power for i-layer
Laser Scribing
Back electrode / reflector formation
ZnO:Al, Ag by sputtering
Laser Scribing
a-Si:H
Top-Cell
TCO
Wiring
Module
fabrication
Glass
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Wet-Etching
Cleaning
n (30nm)
i (200nm)
TCO Sputtering
a-Si:H Top-cell deposition
p,i,n-layer by 13.56MHz PECVD
i (3um)
p (20nm)
Buffer
ZnO:Al (5nm)
Glass
Laser Scribing
n (30nm)
~5μm
Substrate
fabrication
1) Process Flow
Lamination
Flaming
Inspection (Use I-V Tester for efficiency
measurement)
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2. Si Thin Film Solar Cell Technologies
2) Generations
Tandem (double junction)
1st generation
(single junction)
2nd generation
3rd generation
Efficiency: 5~6 %
Efficiency: 6~7 %
Efficiency: 7~8 %
a-Si:H
Triple Junction
4th generation
5th generation
Efficiency: 11~12 % Efficiency: 13~14 %
a-Si:H
a-Si:H
a-Si:H
a-Si:H
a-Si:H
a-SiGe:H
μc-Si:H
a-SiGe:H
μc-Si:H
Mass Production Stage
- Japan: Kaneka(20MW), Sharp(15MW), Fuji Elec.,
Mitsubishi Heavy Ind.,
- US:
Uni-Solar, EPV
- EU:
Oerlikon, Shott Solar
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Pilot Stage
R&D Stage
Kaneka, Sharp
Kaneka
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2. Si Thin Film Solar Cell Technologies
3) Technology Issues
1. TCO
- Highly conductive & wide bandgap
hν
Top Cell
- i-layer=a-Si
- Eg=1.75 eV
Bottom Cell
- i-layer=uc-Si
- Eg=1.1 eV
2. p/i interface :
- low recombination reduction
3. a-Si i-layer
- low light induced degradation
4. nc-Si i-layer
- high deposition rate
5. metal contact
- highly conductive & reflective
6. top/intermediate/bottom TCO
- light trapping
EF
TCO
p-layer
i-layer
n-layer
metal contact
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2. Si Thin Film Solar Cell Technologies
Light Trapping
Surface Texturing
SnO2:F
ZnO:Al
9 Haze ratio (H)
H = Tdiffusion / (Tdiffusion + Tspecular)
H = 6~12% (Generally )
9 Enhancement factor (FE)
FE = 4n2 (Theoretical Max)
9 Reduced reflection
9 Increased travel length
9 Increased internal reflection
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4) TCO layer
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2. Si Thin Film Solar Cell Technologies
5) Nano Crystalline Si
Hydrogen Dilution
Passivation of dangling bonds by H atoms
nc-Si:H Solar Cell
nc-Si:H Is more stable under light exposure than a-Si:H.
Stability
behavior of an
all nc-Si:H p-i-n
solar cell under
long term light
exposure.
Franz et al, Thin Solid Films 383 (2001) 11-14.
Meier J et al, Appl. Phys. Lett. 65 (1994) 7.
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2. Si Thin Film Solar Cell Technologies
a-Si:H
(1.7eV)
a-Si:H
nc-Si:H
nc-Si:H
(1.1eV)
5) Tandem Cell Development
9 Neuchatel pioneered in 1994
« micromorph » tandem
9 Needs of a fast deposition technique
for high-quality nc-Si:H
Photo Current (A/W)
0.5
0.4
Sun
light
a-Si
0.3
μc-Si
0.2
0.1
0
400
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600
9 In a micromorph tandem, the solar
spectrum is ideally shared between
top (a-Si:H) and bottom (nc-Si:H) cell
800
1000
Wave length (nm)
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3. Deposition Technologies
1) Categorization of PECVD Systems
◆ SS/MC type PECVD system is suitable for R&D
◆ MS/MC type PECVD system is required for mass production
Single
Substrate
Single
Multi
SS/SC Type
MS/SC Type
Adv. : Low cost of production facility
Disadv. : Cross contamination
between processes
Chamber
Manufacturer : EPV (USA)- for R&D
Multi
SS/MC Type
Adv. : High productivity in mass
production
Disadv. : Cross contamination
Manufacturer : EPV [USA],
Oerlikon [Swiss]
MS/MC Type
Adv. : High stability of each process
Low cost of raw material
Disadv. : Low productivity in mass
production
Manufacturer : MV Systems (USA)
▶ Suitable for R&D
Adv. : High stability of each process
Disadv. : Low perfection of production
system development
High cost of raw material
Manufacturer : IHI (Japan),
Centrotherm (Germany)
SS: Single Substrate, SC: Single Chamber, MS: Multi Substrate, MC: Multi Chamber
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3. Deposition Technologies
2) Cluster System
PrePre-heating Chamber
Basic Principle
Process
Chamber
2
Process
Chamber
3
Process
Chamber
1
Process
Chamber
4
Consideration
9 Pre-heating chamber
9 Sputter system for BR-layer
L/D IN & OUT
(Multi(Multi-slot or
Cassette)
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9 Capacitively-coupled parallel plate
plasma reactor
9 p-, i-, n-layer at each chamber
9 Multiple i-chamber
9 Plasma Cleaning (NF3 or SF6)
9 VHF for nc-Si:H film
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3. Deposition Technologies
3) Batch Systems
Process
Chamber
1
Process
Chamber
2
X10
X10
System specification
9 2 Process chambers :
Transfer
Chamber
• 40MHz VHF-PECVD
• 10 reactors / 1 chamber
9 Substrate size :
X20
X20
L/D OUT
• 520x410, 1250x1100 mm2
L/D IN
Features
9 High throughput equipment
9Cross contamination problem between
processes
L/D
IN/OUT
Transfer
Chamber
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Process
Chamber 1
9Difficulty of maintenance
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3. Deposition Technologies
4) High pressure & High Power (HpHP) Process
◆ Requirements for nc-SI:H in tandem junction thin film solar cells
- High hydrogen dilution and denser films
- High material quality at high deposition rate
Low plasma damage
VHF-PECVD (40~120MHz)
High pressure & High Power
Efficiency > 12 %
nc-Si DR > 3 nm/s
from J. K. Rath, Solar Energy Materials & Solar
Cells 76, 431, 2003
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Kyocera, World Conf. 2003
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3. Deposition Technologies
5) VHF PECVD
Phase Shift of Incidence Waves
Specific Electrode Design
U shape : IHI
Δθ=-90o
Δθ=0o
Δθ=90o
Δθ=180o
Wave 1
Wave 2
Electrode
plasma
Glass
Ladder shape : MHI
Divided electrode
: Sharp Inc.
Control of standing-wave position
by changing the phase difference
of two waves with time
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4. R&D Activities in LG Electronics
1) a-Si based Solar Cell
P-layer CH4 Flow Dependences1)
Cell Structure & Properties
Cross sectional View
0.8
0.76
0.72
2
Jsc [mA/cm ]
0
FF
100
150
Light
Glass
22
p/i/n
TCO
ZnO/Al
20
18
16
14
0
50
100
J-V Characteristics
150
0.7
0.68
0.66
0.64
0.62
0.6
20
0
Efficiency [%]
50
50
100
J [mA/cm2]
Voc [V]
0.84
150
11
10
9
15
Voc: 0.829V
Jsc: 19.52mA/cm2
FF: 0.662
Eff: 10.71%
10
5
8
0
7
0
50
100
150
0
CH4 [sccm]
0.2
0.4
0.6
0.8
1
V [V]
1) H2:SiH4:CH4:B2H6 = 100:50:x:3, 20W, 0.2T
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4. R&D Activities in LG Electronics
2) a-Si based Solar Module
Module Design & Measured Results
Cell No.: 17
Period: 9.75mm
Length: 182mm
Area: 17.74cm2
Effective width: 9.35mm
Scribing pattern
TCO
PV
Metal
0.4 mm
0.30
Ideal Module
TCO
Metal
TCO
Current (A)
Metal
0.25
PV
Fab. Module
0.20
0.15
Voc: 14.3V
Isc: 0.278A
FF: 0.603
Eff: 7.4%
0.10
0.05
0.00
0
Burr formation
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Sn detection (FE Auger analysis )
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2
4
6
8
10
Voltage (V)
12
14
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4. R&D Activities in LG Electronics
i a-Si:H optimization
3) Tandem Cell
H2/SiH4 effects
Glass
TCO (SnO2)
p + buffer
a-Si:H
Crystallity : 60 ~ 70 %
n
p + buffer
nc-Si:H
n
TCO
Metal (Al)
Amorphous
R&D subjects
• Deposition Rate
• Crystallite and structure
• Incubation time control
• Low defect density
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4. R&D Activities in LG Electronics
4) Nano structure control
Hydrogen Annealed Multi-layer
Hydrogen annealing (tens sec)
PECVD Deposition
i a-Si (~7nm)
i a-Si (~7nm)
Glass
Glass
i a-Si
▪
▪
▪
i a-Si
i a-Si
Multi
layer
Glass
i a-Si
Glass
Expectations
• Low Defect density
: High Initial Efficiency
• Low Photo-degradation
- s Dark: 1.30E-12, s Photo: 1.76E-5
- In investigation :
Bonding structure & Defect density
: High Stabilized Efficiency
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5. Conclusion
‰ High potentials in thin film Si tandem solar cells
Î Eventual raw material cost reduction (current problem in Si feed-stock)
Î Expectation of a high efficiency (more than 12%)
via tandem cell technologies
‰ For commercial success, high demands for the development
of nano-crystalline Si thin films deposition technologies
with a very high deposition rate
Î Process-condition optimization technology
: Batch type PECVD and HpHP Process
Î VHF-PECVD technology : various electrode designs
‰ LG Electronics is focusing
on the key technology development of thin film Si solar cell
and on its commercialization
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