heterojunction silicon based solar cells
DESCRIPTION
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Heterojunction
silicon based solar cells
Miro Zeman
Photovoltaic Materials and Devices Laboratory, Delft University of Technology
Outline
Introduction to Si PV technologies
Motivation for developing HTJ Si solar cells
Achievements
Challenges
HET-Si project
Summary
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)
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)
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:
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
χe
absorber
EF
EC
EV
-qψ
Solar cell operating principles
Χe
electron affinity
1.
Solar energy → Chemical energy
of electron-hole pairs
-qψ
Solar cell operating principles
EFV
-μeh
EFCEC
EV
absorber
1.
Solar energy → Chemical energy
of electron-hole pairs
2.
Chemical energy
→ Electrical energy
-qψ
Solar cell operating principles
EFV
-μeh
EFCEC
EV
absorber
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
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
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
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
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
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
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:
• 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
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
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:
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
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:
• 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
• 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
•
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
•
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
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
Achievements
Sanyo HIT modules:
Achievements
Sanyo HIT Double Bifacial modules:
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
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
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/
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
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
• 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
• 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.
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)
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
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
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.
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.
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
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
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
00/00/200800/00/200800/00/2008
Project partnershipHETSI partnership
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