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Heterojunction
silicon based solar cells
Miro Zeman
Photovoltaic Materials and Devices Laboratory, Delft University of Technology
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Outline
Introduction to Si PV technologies
Motivation for developing HTJ Si solar cells
Achievements
Challenges
HET-Si project
Summary
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Introduction to Si PV technologies
Wafer-based crystalline silicon
½ century of manufacturing history, ~90% of 2008 markethighest performance of flat-plate technologiesgood track record and reliabilitycost reduction is main overall challengemodule efficiencies:
-
12 ~ 20% (now)-
18 ~ > 22% (long term)
Wim
Sinke
(ECN, Leader of WG 3 : Science, technology & applications of EU
PV Technology Platform)
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Introduction to Si PV technologies
Thin-film silicon
Wim
Sinke
(ECN, Leader of WG 3 : Science, technology & applications of EU
PV Technology Platform)
low-cost potential and new application possibilitiesapplication of micro-crystalline siliconefficiency enhancement is major challengestable module efficiencies:
- 6 ~ 9% (now)- 10 ~ 15% (longer term)
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http://us.sanyo.com/Dynamic/customPages/docs/solarPower_HIT_Solar_Power_10-15-07.pdf
Introduction to Si PV technologies
High performance Low-cost potentialHybrid technology HIT solar cell
Sanyo started R&D in 1990
HIT: Heterojunction
with Intrinsic Thin Layer
Most popular Si PV technologies:
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Motivation for HTJ solar cells
Solar cell operating principles:
Thermodynamic approach:
Conversion of energy of solar radiation into electrical energy
Two-step process:
1.
Solar energy → Chemical energy
of electron-hole pairs
2.
Chemical energy
→ Electrical energy
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χe
absorber
EF
EC
EV
-qψ
Solar cell operating principles
Χe
electron affinity
1.
Solar energy → Chemical energy
of electron-hole pairs
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-qψ
Solar cell operating principles
EFV
-μeh
EFCEC
EV
absorber
1.
Solar energy → Chemical energy
of electron-hole pairs
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2.
Chemical energy
→ Electrical energy
-qψ
Solar cell operating principles
EFV
-μeh
EFCEC
EV
absorber
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2.
Chemical energy
→ Electrical energy
Solar cell operating principles
absorber
EV
-qψ
EFC -qVOCEFV
Semi-
permeable membrane
for electrons
EC
Semi-
permeable membrane for holes
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2.
Chemical energy
→ Electrical energy
Solar cell operating principles
absorber
EV
-qψ
EFC -qVOCEFV
Semi-
permeable membrane
for electrons
EC
Semi-
permeable membrane for holes
n-typep-type
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2.
Chemical energy
→ Electrical energy
Solar cell operating principles
absorber
EV
-qψ
EFC -qVOCEFV
Semi-
permeable membrane
for electrons
EC
Semi-
permeable membrane for holes
n-typep-type
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2.
Chemical energy
→ Electrical energy
Solar cell operating principles
absorber
χeEC
EV
-qψ
EFC
χe
E
χe
EFV
Semi-
permeable membrane
for electrons
Semi-
permeable membrane for holes
-qVOC
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2.
Chemical energy
→ Electrical energy
Solar cell operating principles
absorber
EC
EV
-qψ
EFCE
EFV
Semi-
permeable membrane
for electrons
Semi-
permeable membrane for holes
-qVOC
n-typep-type
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2.
Chemical energy
→ Electrical energy
Solar cell operating principles
absorber
EC
EV
-qψ
EFCE
EFV
Semi-
permeable membrane
for electrons
Semi-
permeable membrane for holes
-qVOC
n-typep-type
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EF
Eg1
N c-Si P c-Si
Eg1
Silicon based solar cells
Eg1
N c-SiP a-Si
Eg2
EF
1. Tunneling2. Thermionic emission3. Trap-assisted tunneling
Homojunction Heterojunction
(band off-set)
Real world:
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• Between p and n-type materials there is an intrinsic a-Si:H layer.
• Thin-layer: optimum thickness of the intrinsic a-Si:H is about 4 to 5 nm.
n-doped c-Si
p-doped a-Si:H
intrinsic a-Si:H
Heterojunction
Si solar cells
Sanyo HIT (Heterojunction with Intrinsic Thin Layer) solar cell:
http://us.sanyo.com/Dynamic/customPages/docs/solarPower_HIT_Solar_Power_10-15-07.pdf
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UNSW PERL c-Si solar cell Sanyo HIT solar cell
http://pvcdrom.pveducation.org/MANUFACT/LABCELLS.HTM
http://sanyo.com/news/2009/05/22-1.html
Efficiency record
25% 23%
Manufacturing
Complicated diffusion, oxidation Formation of pn junction, passivation, photomasking BSF are all completed by PECVD
Temperature High temperature processes Less than 200 ˚C requirement
(up to 1000˚C)
Heterojunction
Si solar cells
Comparison with homojunction
c-Si solar cell:
Jsc
, Voc
, FF, Area
42.7 mAcm-2, 0.705 V, 0.828, 4 cm2
39.5 mAcm-2, 0.729 V, 0.80, 100 cm2
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Good stability under light [1] and thermal exposure [2]
High efficiency (capability of reaching efficiency up to 25%)
• Negligible SWE due to very thin a-Si:H layer
• Favorable temperature dependence of the conversion efficiency
[1] T. Sawada, et al, Photovoltaic Energy Conversion, 2
(1994) 1219--1226
[2] Maruyama, E. et al, Photovoltaic Energy Conversion, 2
(2006) 1455--1460
Heterojunction
Si solar cells
Potential:
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1. Low thermal budget
2. Avoiding bowing of thin wafers. Route to use very thin wafers
3. Suppressing lifetime degradation of minority carriers; possible use low quality c-Si
Heterojunction
Si solar cells
Industrial benefits:
200
400
600
800
1000
Proc
ess
tem
pera
ture
[C°]
Time [min]
c-Si conventional technology
Junction diffusion
ARC
Contacts
Firing
30’
0,5’ 2’
0,3’
200
400
600
800
1000
Proc
ess
tem
pera
ture
[C°]
Plasma
3’
TCO
10’
Front/back contact
Firing
0,3’
a-Si/c-Si technology
Low Tem
perature
Rapid ProcessTime [min]
F. Roca, ENEA
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FZ/CZ Area Jsc Voc FF Efficiency
(cm2) (mA/cm
2) (mV) (%) (%)
Sanyo n CZ 100 39.5 729 80 23,0
AIST n CZ 0.2 35.6 656 75 17.5
Helmholtz
centre Berlin
n FZ 1 39.3 639 79 19.8
p FZ 1 36.8 634 79 18.5
IMT EPFL n FZ 0.2 34 682 82 19.1
p FZ 0.2 32 690 74 16.3
NREL p FZ 0.9 35.9 678 78.6 19.1
n FZ 0.9 35.3 664 74.5 17.2
Achievements
Laboratory solar cells:
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• The maximum efficiency was 12.3%
•
Low Voc and FF compared to c-Si homojunction
results from large interface state density.
n c-Si
p a-Si:H
TCO
metal
Achievements
Development of HIT solar cells at Sanyo:
M. Tanaka, et al, “Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)”, Appl. Phys., 31 (1992) 3518-3522
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• The maximum conversion efficiency is 14.8%
• Voc
is improved by 30 mV due toexcellent passivation
of a-Si:H
• FF is improved to 0.8
•
Thin intrinsic a-Si layer
introduced, better passivation
of silicon wafers
Achievements
Development of HIT solar cells at Sanyo:
ACJ-HIT
n c-Si
p a-Si:H
TCO
metal
i a-Si:H
M. Tanaka, et al, “Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)”, Appl. Phys., 31 (1992) 3518-3522
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•
Application of textured substrate
and back surface field
(BSF), the maximum conversion efficiencyincreases to 18.1% for 1cm2 area.
• Jsc
is improved by 20% to 37.9 mA/cm2
Achievements
Development of HIT solar cells at Sanyo:
TCO
p a-Si:H
i a-Si:H
n c-Si
metal
n a-Si:H
M. Tanaka, et al, “Development of New a-Si/c-Si Heterojunction Solar Cells: ACJ-HIT (Artificially Constructed Junction-Heterojunction with Intrinsic Thin-Layer)”, Appl. Phys., 31 (1992) 3518-3522
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•
The symmetrical structure
can suppress both thermal and mechanical stress.
• The maximum conversion efficiency is 21.3% for 100 cm2.
TCO
p a-Si:H
i a-Si:H
n c-Si
n a-Si:H
i a-Si:H
metal
TCO
Achievements
Development of HIT solar cells at Sanyo:
M. Tanaka, et al, “Development of hit solar cells with more than 21% conversion efficiency and commercialization of highest performance hit modules”, Photovoltaic Energy Conversion, 1 (2003) 955--958
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Achievements
Development of HIT solar cells at Sanyo:
Y. Tsunomura, et al, “Twenty-two percent efficiency HIT solar cell”, Solar Energy Materials and Solar Cells, 93 (2009) 670--673
1. Improving the a-Si:H/c-Si heterojunction
Conversion efficiency 22.3% has been achieved in 2008 by further optimization:
2. Improving the grid electrode
3. Reducing the absorption in the a-Si:H and TCO
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Achievements
Sanyo HIT modules:
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Achievements
Sanyo HIT Double Bifacial modules:
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Achievements
Development of HIT solar cells at Sanyo:
Conversion efficiency 23,0% has been achieved in May 2009:
http://us.sanyo.com/News/SANYO-Develops-HIT-Solar-Cells-with-World-s-Highest-Energy-Conversion-Efficiency-of-23-0-
Voc(V) 0.729
Jsc(mA/cm2) 39.5
FF 0.8
Efficiency 23%
c-Si Thickness (µm) >200
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Achievements
Development of HIT solar cells at Sanyo:
Conversion efficiency 22.8% with 98 µm thick c-Si (EU-PVSEC Hamburg 2009):
http://techon.nikkeibp.co.jp/english/NEWS_EN/20090923/175532/
Highest Voc
for c-Si type solar cell, Voc
= 0.743V
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Achievements
Production development of HIT solar cells at Sanyo:
http://www.pv-tech.org/news/_a/sanyo_targets_600mw_hit_solar_cell_production_with_new_plant/
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Achievements
National Institute of
Advanced Industrial Science and Technology:
H. Fujiwara, et al, “Crystalline Si Heterojunction Solar Cells with the Double Heterostructure of Hydrogenated Amorphous Silicon Oxide”, Jpn. J. Appl. Phys., 48 (2009) 064506
Al
n c-Si
p a-SiO:HITO
i a-SiO:H
i a-SiO:Hn a-SiO:H
ITO
Ag
• a-SiO:H i layer can suppress epitaxial growth completely
• Efficiency decreases with decreasing thickness of c-Si
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Achievements
Institute of Microtechnology
(IMT) Neuchatel (EPFL):
Al or Ag
n c-Si
p a-Si:H/µc-Si:HITO
i a-Si:H
i a-Si:Hn a-Si:H/µc-Si:H
ITO
S.Olibet, PhD thesis, 2008
• a-Si:H/uc-Si:H layers fabricated by VHF-CVD
• Small area (0.2 cm2) cells without front metal contact
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• no intrinsic a-Si:H layer results in low Voc
Achievements
Helmholtz Center Berlin for Materials and Energy:
AZO
p a-Si:H
n c-Si
n a-Si:H
Al
M.Schmidt, et al, “Physical aspects of a-Si:H/c-Si hetero-junction solar cells”, Thin Solid Films, 515 (2007) 7475--7480
• reduction of optical loss due to thinner a-Si layer
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• a-Si:H layers fabricated by HW CVD
Achievements
National Renewable Energy laboratory (NREL):
n a-Si:H
p c-Si
p a-Si:H
i a-Si:H
metal
ITO
i a-Si:H
metal
Q. Wang, et al, “Crystal Silicon Heterojunction Solar cell by Hot-Wire CVD”, The 33rd IEEE Photovoltaic Specialists Conference, 2008.
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Challenges
Losses in HIT solar cell:
Optical losses:1. Textured surface2. Low absorption of TCO and a-Si3. High aspect ratio of grid electrode
Recombination losses:1. cleaning2. Hydrogen termination of wafer surface3. High quality a-Si:H
Resistance losses:1. High conductivity TCO2. Good ohmic
contact between different layers
n c-Si
a-Si:H (i/n)
TCOa-Si:H (p/i)
TCO
Grid electrode
reflection absorption shading
Optical losses (Jsc)
+-
Recombination losses (Voc)
Res
ista
nce
loss
es (
FF)
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Challenges
1. Wafer cleaning
Partial passivation by H2 or HF solution to saturate dangling bonds
Remove particles and metallic contaminants from the surface
SC1 + SC2 (RCA Cleaning) NaOH : H2OHNO3 : HFHF : H2OHCl:HFCH3OH:HFCH3CH(OH)CH3:HF (or HI)HF:H2O2:H2OCF4/O2 (8% Mix)NF3H2N2O2Ar
wet
Chemicals
dry
PVMD/DIMES
results:
F. Roca, ENEA
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Challenges
2. Epitaxial growth at the heterojunction
interface
H. Fujiwara, et al, “Impact of epitaxial growth at the heterointerface of a-Si:H/c-Si solar cell”, Appl. Phys. Lett., 90 (2007) 013503--3
Optimum growth temperature and rf power density
Suppression of the epitaxial growth
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Challenges
3. Controlling layer thickness
Efficiency is highly related to the thickness of the intrinsic and doped layers
T. Sawada, et al, “High efficiency a-Si/c-Si heterojuction solar cell”, IEEE Photovoltaic Specialists Conference, Vol. 2 (1994) 1219—1226
•
Thicker intrinsic a-Si:H
layers lead to rapid reduction in Jsc
and FF
•
Jsc
is sensitive to thickness of p-type a-Si:H
layer.
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Optical loss in short wavelength region is caused by the absorption of a-Si.
Optical loss in long wavelength region is caused by the free carrier absorption of TCO.
Challenges
4. Reducing absorption loss in a-Si and TCO
E.Maruyama, et al, “Sanyo's Challenges to the Development of High-efficiency HIT Solar Cells and the Expansion of HIT Business”, Photovoltaic Energy Conversion, 2 (2006) 1455--1460
Solutions:1. High-quality wide gap alloys such as a-SiC:H2. High-quality TCO with high carrier mobility and
relatively low carrier density.
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Surface-textured substrates are used due to optical confinement effect
Challenges
5. Surface-textured wafer surface
M. Tucci, et al, “CF4/O2 dry etching of textured crystalline silicon surface in a-Si:H/c-Si heterojunction for photovoltaic applications”, Solar energy materials and solar cells, 69 (2001) 175-185
Problems:
1. Fabrication of an uniform a-Si layer on the textured c-Si
2. Insufficient cleaning of c-Si surfaces before a-Si film growth
Solutions:1. Optimization of deposition condition
2. Clean c-Si surface with hydrogen plasma treatment
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Finer width (W) and no spreading area of grid electrode reduce shade losses
Challenges
6. Improvement of grid electrode
Solutions:
1. Optimize viscosity and rheology of silver paste2. Optimize process parameters in screen printing
Y.Tsunomura, et al, “Twenty-two percent efficiency HIT solar cell”, Solar Energy Materials and Solar Cells, 93 (2009) 670--673
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00/00/200800/00/200800/00/2008
Project concept and objectivesHetorojunction
concepts for high
efficiency
solar
cells
Short-term target:
demonstrate the industrial feasibility
of heterojunction
solar cells
in EuropeMedium term target:
demonstrate the concept of ultra-
high efficiency rear-contact cells
based on a-Si/c-Si heterojunction
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00/00/200800/00/200800/00/2008
Project partnershipHETSI partnership
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1.
HTJ Si solar cells offer promising potential to conventional c-Si solar cells-
lower production cost-
better thermal stability-
higher electrical yield
Summary
2. HIT Si solar cells contain a-Si/c-Si heterojunction
and use intrinsic a-Si:H
for high-quality passivation
3. The efficiency record of HIT solar cells is 23.0%
4.
Challenges to fabricate high-efficiency HTJ Si solar cells-
clean and textured c-Si surfaces-
abrupt heterojunctions
with low interface-defect densities-
optimum a-Si :H deposition conditions and layer thickness- TCO