Improving Silicon Solar Cells Efficiency with Aluminum Nitride (AlN) thin films
J.Dulanto
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Outline • Our Material Science Group – Physics • What is photovoltaics? • What is passivation? • State of the art: Silicon Solar Cells • Research on future materials (AlN)
• Our research in cooperation with the Fraunhofer Institute for Solar Energy Systems (ISE) and Techniche Universitat Ilmenau
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Group of Materials Science (Physics, PUCP) Total: 18 3 Cientists/PhD Roland Weingärtner
Amaru Töfflinger
Andrés Guerra
3 Phd students
10 Master students
1 Bachelor student
1 Assistant
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Group of Materials Science (Physics, PUCP) Lines of investigation
Latest publications ▪ J. A. Guerra, et al., J. Phys. D: Appl. Phys. 121, 173104. (2016)
▪ Wide band-gap semiconductors doped with rare earths for solar cells and light emitters
▪ N. Preissler, J. A. Töfflinger, et al., Prog. Photovolt., DOI: 10.1002/pip.2852 (2016)
▪ Passivation materials for silicon solar cells
▪ J. A. Guerra, et al., J. Phys. D: Appl. Phys. 49, 375104. (2016)
▪ Characterization of a-SiC:H for the photoelectrochemical production of Hydrogen Deposition
▪ N. Preissler, J. A. Töfflinger, et al., Phys. Status Solidi A 213 (7), 1697 (2016)
Characterization
Magnetron sputtering
Optical properties 1
3
D2
1
H4
G4
5
TEM SiC:H Other: AlN:H, SiN:H, ITO...
CL Intensities [a.u.]
Structural properties
3
H6
Tm3+
7
D4
SiC:Tb3+
Application In cooperation with
Rare earth luminescence
F5
4
Tb3+ 3+
H13/2
Dy
7
Eu3+
6
F9/2 5
D0
F1,2
4
G5/2
6
H7/2
4
200
400
600
LPC-Si
line-shaped laser or ebeam IL (≈ 200 nm)
3+
H9/2Sm
G5/2
6
2
2
F5/2
Thin-film solar cell
3+ F7/2 Yb
800
Wavelength [nm]
1000
liquid Si
a-Si (10 – 20 µm)
glass (1 – 3 mm)
Electrical properties
4
Potential of solar energy
5
•State of the art - commertial solar modules
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©Fraunhofer ISE: Photovoltaics Report, updated: 6 June 2016 6
Semiconductors
https://www.edx.org/course/solar-energy-delftx-et3034x-0
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Intrinsic and Extrinsic semiconductors
N-doped Si
P-doped Si
Extrinsic http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/dope.html 8
The Photovoltaic effect Standard mc-Si (p) Solar cell efficiency η ≈ 16%
ARC + Front passivation (standard SiNx)
(n+)
+
3.
1. 1. Absorption of light and generation of charge carriers
2. mc-Si (p)
Al-BSF
(p++)
> 200 µm
+
2. Separation of charge carriers (p/n-junction: E-field)
3. Extract photo-voltage and -current -> electric power
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Diffusion
https://www.edx.org/course/solar-energy-delftx-et3034x-0
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Chemical Passivation
-
+
Passivation layer: SiO2 http://www.iue.tuwien.ac.at
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Field Effect Passivation E
Isolator
+ e + e + e + e + + e e + e + + e e + + + e
h h
N-doped Si
h
Fixed charges
12
What is Passivation? Reduction of Recombination rate (carriers) Chemical Passivation
Field Effect
13
Samples from ISE Freiburg
-2
Charge Qiso,eff (cm )
CV Characterization
12
1x10
0
12
-1x10
0
100
200
300
400
500
Anneal T (°C)
Capacitance-Voltage Curve MIS capacitor (metal-insulator-semiconductor)
Dit (1/eVcm2)
Qeff (fixed charge density)
1013
1012
1011
1010 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
PHIs (eV)
Dit (interface trap density) 14
The Photovoltaic effect (II)
Standard mc-Si (p) Solar cell efficiency η ≈ 16%
ARC + Front passivation (standard SiNx)
1. Absorption of light and generation of charge carriers
1 .+ +
mc-Si (p)
2. Al-BSF (p++)
> 200 µm
-
3.
(n+)
2. Separation of charge carriers (p/n-junction: E-field) 3. Extract photo-voltage and -current -> electric power Solar cell efficiency η: η
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Trend in the silicon wafer based solar cells “Standard” solar cell η ≈ 16%
FS contact (n+)
+
Passivated Emitter and Rear Cell (PERC) η > 18%
ARC +
Absorber
(p++)
(n + )
c-Si (CZ, mc) (p)
Passivation stack
< 150 µm
> 200 µm
Emitter
c-Si (CZ, mc) (p)
Al
RS contact
Heterojunction solar cell (HIT) η > 20% +)
( n
+
c-Si (CZ, mc) (p)
( n +)
TCO a-Si:H (n) a-Si:H (i)
Key technology: high level passivation!!!
Si-Absorber a-Si:H (i) a-Si:H (p) TCO
16
+
Surface passivation of c-Si Passivated Emitter and Rear Cell (PERC) η > 18%
+
Qf
(n + )
+
-
Dit
+
c-Si (CZ, mc) (p)
-
c-Si
-
< 150 µm
SiN:H
+
+
Surface passivation =
Reduction of the recombination of photogenerated charge carriers at surfaces Main recombination parameters: Interface defect state density Dit Fixed charge density Qf
Chemical passivation Field-effect passivation
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Surface passivation of c-Si
SiNx
Inversion
EF
DB E
c-Si (p)
Si N
qS0
x1
Under illumination c-Si (p) SiO2 or SiNx
x1 (µm)
2. Recombination ↗ in c-Si bulk: Jsc ↘, Voc ↘ Positive charges in SiNx limit p-type c-Si solar cell efficiency
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•Passivation properties of AlOx:
AlOx (or Al2O3) for passivation of p-type c-Si [1]
n-type c-Si: positive charges in SiNx p-type c-Si: negative charges in AlOx [1] Dingemans and Kessels, J. of Vac. Sc. & Techn. A 30 (4) (2012) [2] Hoex, et al., Appl. Phys. Letters 89 (11) (2006)
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•Requirements on the passivation system
•
Surface passivation
•
Optical match Minimal Reflection at the front side
Reduction of recombination centers at surfaces and interfaces Partial reflection
Incident light
(n+)
Maximal back Reflection at rear side
•
Volume passivation
•
Good insulating properties
(p) c-Si (CZ, mc)
+
(n+)
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•State of the art of commercial Silicon solar cells
2016:
90% market share for crystalline Si
Silicon material:
30-40% of solar module cost
Passivated emitter and rear cell (PERC)
Low efficiency η = 15-16 % Low cost €
High efficiency η > 20 % Higher cost $
> 200 µm
+
ARC & Front passivation SiNx:H
(n+) c-Si (p)
Al-BSF
(p++)
Backside passivation + reflector: AlOx
+
(n+) c-Si (p) (p++) Al
< 150 µm
Commercial solar cell
n-type c-Si: positive charges in SiNx p-type c-Si: negative charges in AlOx 21
•AlN for passivation of PERC solar cells
Passivated emitter and rear cell (PERC), efficiency η > 20%
SiNx
+
(n+)
Replace two standard passivation materials with one: AlN:H[1]
c-Si (p) Samples from ISE Freiburg (p++)
AlN is suitable for passivation of n- and p-type c-Si
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-2
Al
Charge Qiso,eff (cm )
AlOx
1x10
0
12
-1x10
0
100
200
300
400
500
Anneal T (°C)
[1] G. Krugel, et. al., Energy Procedia 55, 797–804 (2014) [2] L. Montañez, J. A. Töfflinger, et al., Proceedings of 31st EU-PVSEC, 969 (2015) [3] L. Montañez, J. A. Töfflinger, et al., submitted to Materials Today Proceedings EDS2016
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•AlN for passivation of PERC solar cells
AlN (as-deposited) compared to AlOx (fired) and SiO2 (FGA): Chemical passivation Interface: c-Si/AlOx (PECVD)
11
10
47 cm/s
c-Si/SiO2 (thermal)
AlN
SiO2
sputtering) (thermal)
-1 -2 -3 -4 -5
10
10
0 -2
Valence Band
-1
-2
Dit (eV cm )
12
10
c-Si/AlN (sputtering)
12
13
10
1
Qfix (10 cm )
9 cm/s
(c)
14
10
Conduction Band
(b)
Field-effect passivation
0.0
0.2
0.4
0.6
E-EV (eV)
0.8
1.0
1.2
AlOx
AlN
SiO2
(PECVD) (sputtering) (thermal)
Next steps: Optimize AlN passivation properties through thermal treatments [1] G. Krugel, et. al., Energy Procedia 55, 797–804 (2014) [2] L. Montañez, J. A. Töfflinger, et al., Proceedings of 31st EU-PVSEC, 969 (2015) [3] L. Montañez, J. A. Töfflinger, et al., submitted to Materials Today Proceedings EDS2016 23
AlN samples (ISE)
Negative fixed charges
? ? ? ? ------------------------------------------------------------xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Dit (defects)
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AlN: C-V characterization 50nm thin film of sputtered AlN over p-type silicon wafer give 4 samples: • Sample A9: as deposited
• Sample A10: treated thermally at 200℃ • Sample A11: treated thermally at 300℃ • Sample A8: treated thermally at 500℃ 25
Results’ summary: Sample + thermal treatment
Dit @ mid gap
Qiso,eff @ mid gap
A9:
N/A (leakage)
Fixed (0.5x1012cm-2)
A10: 200°C
Low Dit (1.4x1011 eV-1cm-2)
Unstable
A11: 300°C
Low Dit (1.5x1011 eV-1cm-2)
Unstable
A8 : 500°C
Low Dit (1.4x1011 eV-1cm-2)
Fixed (-1.0x1012cm-2) Negative charge!!
as-deposited
Fixed charge
? ?
??
----------------xxxxxxxxxxxxxxxxxxxxxxxxxxx
Dit (defects)
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Summary • AlN is a promising material as a passivation layer: • Excellent chemical passivator • Good field effect passivator for n-type and p-type silicon • Optically, it serves as a anti-reflection coating layer. • All in a one material, reduces production cost!
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References: • [1] S.M. Sze, Physics of Semiconductor Devices. Third edition, Chapter 4 (2007). • [2] G. Krugel, et al., Study of hydrogenated AlN as an antireflective coating and for the effective surface passivation of silicon, Phys. Status Solidi RRL 7, No. 7, 457–460 (2013). • [3] G. Krugel, et al., Passivation of solar cell emitters using aluminum nitride, Proc. 39th IEEE PV Specialists Conf., Tampa, Florida, USA, 2013. • [4] G. Krugel, et al., Investigations on the Passivation Mechanism of AlN:H and AlN:H-SiN:H Stacks, Energy Procedia 55, 797–804 (2014). • [5] J. A. Toefflinger, Interface investigations of passivating oxides and functional materials on crystalline silicon, PhD Thesis, Technischen Universität Berlin (2014) 28
Acknowledgements
Materials Science group, Sección Física Roland Weingärtner, Amaru Töfflinger
Rolf Grieseler PNICP contract No 274-PNICP-BRI-2015
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