Heterojunction silicon based solar cells

Heterojunction silicon based solar cells
Miro Zeman
Photovoltaic Materials and Devices Laboratory, Delft University of Technology

Outline
ƒ
Introduction to Si PV technologies
ƒ
Motivation for developing HTJ Si solar cells
ƒ
Achievements
ƒ
Challenges
ƒ
HET-Si project
ƒ
Summary

Wafer-based crystalline silicon
ƒ
½century of manufacturing history, ~90% of 2008 market
ƒ
highest performanceof flat-plate technologies
ƒ
good track record and reliability
ƒ
cost reduction is main overall challenge
ƒ
module efficiencies:
- 12 ~ 20% (now)
- 18 ~ > 22% (long term)
Wim Sinke (ECN, Leader of WG 3 : Science, technology & applications of EU PV Technology Platform)

Thin-film silicon
Wim Sinke (ECN, Leader of WG 3 : Science, technology & applications of EU PV Technology Platform)
ƒ
low-cost potentialand new application possibilities
ƒ
application of micro-crystalline silicon
ƒ
efficiency enhancement is major challenge
ƒ
stable module efficiencies:
-
6 ~ 9% (now)
-
10 ~ 15% (longer term)

http://us.sanyo.com/Dynamic/customPages/docs/solarPower_HIT_Solar_Power_10-15-07.pdf
High performance
Low-cost potential
Hybrid 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.

-qψ
EFV
-μeh
EFC
EC
EV
absorber
1.

2.
-qψ
EFV
-μeh
EFC
EC
EV
absorber

2.
absorber
EV
-qψ
EFC
-qVOC
EFV
Semi- permeable membrane for electrons
EC
Semi- permeable membrane for holes

2.
absorber
EV
-qψ
EFC
-qVOC
EFV
EC
n-type
p-type

2.
absorber
χe
EC
EV
-qψ
EFC
χe
E
χe
EFV
-qVOC

2.
absorber
EC
EV
-qψ
EFC
E
EFV
-qVOC
n-type
p-type

EF
Eg1
N c-Si
P c-Si
Eg1
Silicon based solar cells
Eg1
N c-Si
P a-Si
Eg2
EF
1. Tunneling
2. Thermionic emission
3. 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)
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

ƒ
Good stability under light [1]and thermal exposure [2]
ƒ
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
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
Industrial benefits:
200
400
600
800
1000
Process temperature [C°]
Time [min]
c-Si conventional technology
Junction diffusion
ARC
Contacts
Firing
30’
0,5’
2’
0,3’
200
400
600
800
1000
Process temperature [C°]
Plasma
3’
TCO
10’
Front/back contact
Firing
0,3’
a-Si/c-Si technology
Low Temperature
Rapid Process
Time [min]
F. Roca, ENEA

FZ/CZ
Area
Jsc
Voc
FF
Efficiency
(cm2)
(mA/cm2)
(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 to
excellent passivation of a-Si:H
•
FF is improved to 0.8
•
Thin intrinsic a-Si layer introduced, better passivation of silicon wafers
Achievements
ACJ-HIT
n c-Si
p a-Si:H
TCO
metal
i a-Si:H

•
Application of textured substrate and
back surface field (BSF),
the maximum conversion efficiency
increases to 18.1% for 1cm2 area.
•
Jsc is improved by 20% to 37.9 mA/cm2
Achievements
TCO
p a-Si:H
i a-Si:H
n c-Si
metal
n a-Si:H

•
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
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
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
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
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:H
ITO
i a-SiO:H
i a-SiO:H
n 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:H
ITO
i a-Si:H
i a-Si:H
n 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 surface
2. Low absorption of TCO and a-Si
3. High aspect ratio of grid electrode
Recombination losses:
1. cleaning
2. Hydrogen termination of wafer surface
3. High quality a-Si:H
Resistance losses:
1. High conductivity TCO
2. Good ohmic contact between different layers
n c-Si
a-Si:H (i/n)
TCO
a-Si:H (p/i)
TCO
Grid electrode
reflection
absorption
shading
Optical losses (Jsc)
+
-
Recombination losses (Voc)
Resistance losses (FF)

Challenges
1. Wafer cleaning
ƒ
Partial passivationby H2or HF solution to saturate dangling bonds
ƒ
Remove particles and metallic contaminants from the surface
SC1 + SC2 (RCA Cleaning)
NaOH : H2O
HNO3 : HF
HF : H2O
HCl:HF
CH3OH:HF
CH3CH(OH)CH3:HF (or HI)
HF:H2O2:H2O
CF4/O2 (8% Mix)
NF3
H2
N2
O2
Ar
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
ƒ
Optimum growth temperature and rfpower density
ƒ
Suppression of the epitaxial growth

Challenges
3. Controlling layer thickness
ƒ
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.

ƒ
Optical loss in short wavelength region is caused by the absorption of a-Si.
ƒ
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:H
2. High-quality TCO with high carrier mobility and
relatively low carrier density.

ƒ
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

ƒ
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 paste
2. 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

00/00/2008
Project concept and objectives
Hetorojunction concepts for high efficiency solar cells
Short-term target: demonstrate the industrial feasibility of heterojunction solar cells in Europe
Medium term target: demonstrate the concept of ultra- high efficiency rear-contact cells based on a-Si/c-Si heterojunction

00/00/2008
Project partnership
HETSI 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

Heterojunction silicon based solar cells

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