Polycrystalline silicon as carrier selective
contact for silicon solar cells
SPREE Seminar Talk
Udo Römer
21.07.2016
Outline
Process optimisation
poly-Si contacts
Theory / understanding
poly-Si contacts
Local overcompensation
via ion implantation
Combination of these
technologies
1
Motivation
• Removing front side metal shadowing
~2.5 mA/cm² gain in Jsc
• Voltage can be enhanced by reducing
recombination:
• SP-PERC cell has J0 of ~300 fA/cm²
• Poly-Si / c-Si-junction:
15 fA/cm² for p-type poly-Si1 and
20 fA/cm² for n-type poly-Si1
~70 mV gain in Voc
0
ln 1gen
OC
JkTV
q J
PERC solar cell
Poly-Si rear contact
solar cell
[1] J. Y. Gan and R. M. Swanson, IEEE Trans. Electron Devices, vol. ED-37, pp.245-250 (1990) 2
Passivation:
• Low Dit at wafer surface due to high
quality oxide
• Field effect passivation due to
highly doped poly-Si layer
Transport:
• Tunnel transport through the oxide1
or
• holes in the oxide2
Poly-Si contact: working principle
Band diagram of a n-typ poly-Si contact
(sketch)
[1] Steinkemper, Feldmann, Bivour, and Hermle, IEEE Journal of Photovoltaics, vol. 5, no. 5, pp. 1348 (2015)
[2] Peibst, Römer, Hofmann, Lim, Wietler, Krügener, Harder, and Brendel, IEEE Journal of Photovoltaics, vol. 4, no. 3, pp. 841 (2014)
electron transport
few interface
defects
interface oxide
many
defects
interface oxide:
larger tunnel barrier
for holes than for
electrons
Depth [nm]
Energ
y [
eV
]
3
Poly-Si contact: transport
RCA-oxide after 10 min at 950 °C RCA-oxide after 10 min at 1100 °C
• HR-TEM investigations show braking up of the oxide after high
temperature processing
4
Wolstenholme, Jorgensen, Ashburn, and Booker, J. Appl. Phys. 61, 225 (1987)
Process flow poly-Si test structures 1-step-process / 2-step-process
Thin oxidation
Deposition of doped poly-Si layers
Annealing / cracking of the oxide
5
Thin oxidation
Deposition of undoped poly-Si layers
Annealing / cracking of the oxide
Doping
Process flow poly-Si test structures 1-step-process / 2-step-process
5
Measurement of the recombination
characteristics
• J0-determination with photoconductance measurement and Kane &
Swanson method1
Flash lamp
Coil J0 = 5 fA/cm²
2.4 nm thermal oxide
30 min at 1050 °C
J0-test structure
6
[1] Kane and Swanson, Proc.of the 18th IEEE PVSC, pp. 578–583 (1985)
If oxide is perfectly insulating: R4PP ~ Rpoly
If oxide is perfectly conducting: R4PP ~ Rabs
Determination of the contact
resistance
If oxide is perfectly insulating: R4PP ~ Rpoly rel ~ 1
If oxide is perfectly conducting: R4PP ~ Rabs rel ~ 0
Rpoly
Rbulk
Rpoly
Rpoly
Rbulk
Rpoly
Rpoly
Rbulk
Rpoly
Rabs
4PP sheet resistance measurement Inductive sheet resistance
measurement
7
Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014.
Simulation of 4PP-measurements
Input parameters:
• Sample geometry
• Resistivities of wafer, poly-Si layer and oxide
Result:
• “Measured" sheet resistance
SENTAURUS-DEVICE
3D-simulation
8
Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014.
Simulation of 4PP-measurements
• Calculation of relative contact
resistance from simulated 4PP
resistance
• Plot vs. “real” (specified) contact
resistance
• Example: poly-Si layer with sheet
resistance of 280 Ω/:
rel < 0.05 corresponds to contact
resistance of < 0.5 Ωcm²
9
Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014.
Investigation of different oxides
• All oxides reach J0-values between 5 and 20 fA/cm²
• High temperatures needed for low contact resistance
• Boron-doped poly-Si contacts comparable to Phosphorus-doped 10
Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014.
Investigation of different oxides
• Passivation stable up to 60 min annealing at 1050 °C
• Contact resistance decreases with increasing annealing duration
• Good combination of low J0 and rel possible 11
Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014.
Influence of the metallization
• ILM measurements before and after metallization show comparable lifetime
level (apart from edge effects)
• Planar solar cell demonstrators show Voc of 714 mV1
• Series resistance of 0.6 Ωcm² not limited by poly-Si contact1
Lifetime distribution measured via Infrared
Lifetime Mapping (ILM)
Metallized pieces
Solar cell demonstrator
12
[1] Römer, Peibst, Ohrdes, Lim, Krügener, Bugiel, Wietler, and Brendel, Solar Energy Materials and Solar Cells, vol. 131, pp. 85-91, Dec. 2014.
Process optimisation
poly-Si contacts
Theory / understanding
poly-Si contacts
Local overcompensation
via ion implantation
13
Local counterdoping with in-situ
patterned ion implantation
• Counterdoping: Overcompensating one polarity
of dopants with dopants of the other polarity
• With in-situ patterned counterdoping local doping
possible without structured dielectric layers
• Counterdoping with in-situ masked ion
implantation enables elegant process flow for
back contacted solar cells
• Process results in formation of lateral pn-junction
with heavily doped p- and n-regions
- Risk for band to band tunneling
- Risk for trap-assisted tunneling
14
Full area counterdoping
• Processing:
- 1.5 x 1015 cm-2 B implantation
- 3 x 1015 cm-2 P implantation
- Annealing at 1050 °C
• SIMS measurement:
- Phosphorus profile covers boron
profile over whole depth
Counterdoping works fine!
Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, Wütherich, et al., Proc. 39th ,IEEE PVSC, pp. 1280 (2013) 15
Local counterdoping
• Simulations featuring lateral pn-junction
• Variation of the lifetime in the “implanted” area
• Measurements on test structures and comparison with simulations
Simulations of the diode
characteristics
Measurements on test structures
p-type silicon
Full area boron
implantation
Local phosphorus
implantation
Aluminum contacts
Passivation layer
Position x [µm]
Po
sit
ion
z [
µm
]
Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, Wütherich, et al., Proc. 39th ,IEEE PVSC, pp. 1280 (2013) 16
Local counterdoping
• Simulations show no detrimental influence of highly doped pn-junction:
n = 1 as long as implant damage is well annealed
• Otherwise strong recombination in space charge region with n = 2
• Measurements on test structures show n = 1 Everything is fine!
Simulations of the diode
characteristics
Measurements on test
structures
Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, Wütherich, et al., Proc. 39th ,IEEE PVSC, pp. 1280 (2013) 17
Local counterdoping
• Further investigations (incl. influence of the
lateral doping profile & characteristics in
reverse direction) published1
Everything is fine!
• Large area (156 mm x 156 mm psq.) ion
implanted IBC solar cells featuring local
counterdoping reach efficiencies of 22.1 %2
[1] Römer, Peibst, Ohrdes, Larionova, Harder, Brendel, Grohe, Stichtenoth, et al., Proc. 39th ,IEEE PVSC, pp. 1280 (2013)
[2] Bosch Solar Energy, ISFH, press release, Aug. 15th, 2013 18
Process optimisation
poly-Si contacts
Theory / understanding
poly-Si contacts
Local overcompensation
via ion implantation
Combination of these
technologies
19
Process flow for ion implanted
poly-Si with counterdoping
Thermal
oxidation
LPCVD poly-Si
deposition
Boron
implantation
Annealing/
oxide break-up
Boron implanted
Test structure
Phosphorus
implanted test
structure
20
Process flow for ion implanted
poly-Si with counterdoping
Annealing/
oxide break-up
Thermal
oxidation
LPCVD poly-Si
deposition
Boron
implantation
Masked
phosphorus
implantation
Boron implanted
Test structure
Phosphorus
implanted test
structure
Test structure
full area
counterdoping
20
Process flow for ion implanted
poly-Si with counterdoping
Annealing/
oxide break-up
Thermal
oxidation
LPCVD poly-Si
deposition
Boron
implantation
Masked
phosphorus
implantation
Boron implanted
Test structure
Phosphorus
implanted test
structure
Test structure
local
counterdoping
Test structure
full area
counterdoping
20
Ion implantation in poly-Si
• Decreasing J0 with increasing dose
• Very low values of 1.1 fA/cm² (phosphorus) and 4.4 fA/cm² (boron)
• Increase at too high doses, especially for boron doping
Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015) 21
Ion implantation in poly-Si
• Doping concentration constant inside poly-Si layer
• Oxide acts as diffusion barrier, in particular for phosphorus
• For high doses strong diffusion of boron into wafer 22
Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015)
Recombination characteristics of
boron-implanted test structure
• Implantation dose: 1x1015cm-2 B
• Highest Voc,impl. value reported so far for
p+ doped poly-Si junctions
Best value F-ISE1: 694 mV
• High pFFimpl.-value of 84.6 %
(ideal value for the given Voc is 85%)
Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015)
[1] Feldmann, Müller, Reichel, and Hermle, Phys. Stat. Sol. RRL, pp. 1 (2014) 23
Recombination characteristics of
phosphorus-implanted test structure
• Implantation dose: 5 x 1015 cm-2 P
• Very high Voc,impl. of 742 mV
• Due to J0,poly of only 1.1 fA/cm² and very
high bulk lifetime, recombination
characteristics at MPP dominated by
Auger recombination
nAuger ≈ 2/3 results in very high pFFimpl.
Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015) 24
Counterdoping in poly-Si
• J0-values comparable to values without counterdoping
• Contact resistance of some samples very high
• For others comparable to samples without counterdoping
Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015) 25
Counterdoping in poly-Si
• For some samples diffusion of boron into the wafer
No contact between n+ poly-Si and n-type wafer
• High phosphorus doses prevent in-diffusion of boron
Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015) 26
Recombination characteristics of
counterdoped test structure
• Implantation doses: 1x1015cm-2 B
5x1015cm-2 P
• Very high Voc,impl.-value
Best value F-ISE: 682 mV1
• Despite J0,poly-value of 0.9 fA/cm²
pFFimpl.-value of “only” 84.7%
27
[1] C. Reichel, F. Feldmann, R. Müller, A. Moldovan, M. Hermle, and S. W. Glunz, 29th EUPVSEC (2014)
Recombination characteristics of
masked counterdoped test structure
• Implantation doses: 1x1015cm-2 B
5x1015cm-2 P
• Curve only fitable by adding a further
recombination path with n>2
• Standard SRH-Theory: 1<n<2 in SCR
Non-standard behavior e.g. coupled
defects1
e
h
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h
h h
e
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h h e e
h
h h e e e
h h
e
Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015)
[1] Steingrube, Breitenstein, Ramspeck, Glunz, Schenk, and Altermatt, Journal of Applied Physics, vol. 110, no. 1 (2011) 28
Recombination characteristics of
masked counterdoped test structure
JL
Jcell
Jpara Rpara
Rs
e
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h h
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h h e e
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h h e e e
h h
e
Römer, Peibst, Ohrdes, Lim, Krügener, Wietler, and Brendel, IEEE Journal of Photovoltaics, vol. 5, no. 2, pp. 507 (2015) 29
Reduction of pn-junction
recombination
[1] Peibst, Römer, Patent application
[2] Rienäcker, Merkle, Römer, Kohlenberg, Krügener, Brendel, and Peibst, 6th SiliconPV Conference 2016
• Lowering of poly-Si thickness
- Possibly problems with metallisation
• Adaption of implantation parameters
- Lower dose at pn-junction
- Not very helpful (see thesis)
• Removal of lateral pn-junction
- Oxidisation1
- Wet chemical etching
η = 23.9 %2
Voc = 722 mV2
FF = 78.7 %2
e
h
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h h
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h h e e
h
h h e e e
h h
e
JL
Jcell
Jpara Rpara
Rs
30
Summary
Fast and non-destructing method for the
estimation of the contact resistance between
different layers
Poly-Si contacts with J0-values of 0.66 fA/cm²
and 4.4 fA/cm² for phosphorus and boron
doping developed
Full poly-Si contacted solar cell with Voc of
714 mV and low contact resistance fabricated
31
Summary
Local overcompensation via masked ion
implantation investigated
Overcompensated poly-Si contacts with
J0-values of 0.9 fA/cm² fabricated
Anomalous recombination behaivour in
locally overcompensated poly-Si contacts
investigated
32
Many thanks to
• You for your attention
• Many colleagues at ISFH and LUH for their help:
Tobias Wietler, Robby Peibst, Susanne Mau, Heike Kohlenberg, Miriam
Berger, Tobias Ohrdes, Michael Häberle, Jan Krügener, Agnes Merkle,
Bianca Lim, Yevgeniya Larionova, Sarah Spätlich, David Sylla, …
…and everyone else for the always nice atmosphere
• The Laboratory for Nano- and Quantum Engineering Hanover
• The BMWi for funding
Back-up slides
Influence of process sequence
Solar cell results
• Best process used for production of proof of principle solar cells
• High implied Voc measured; implied PFF nearly ideal (85.0 %)
impl. Voc = 716 mV
impl. PFF = 84.5 % full area wafer
Solar cell results
impl. Voc = 716 mV
impl. PFF = 84.5 % full area wafer
impl. Voc = 714 mV
impl. PFF = 72.7 %
Voc = 705 mV
PFF = 73.1 %
• Voc reduced further due to the edge effects (measured through 2 x 2 cm² mask)
Measurement without mask yields Voc of 714 mV
• Large area solar cells would not suffer from this effect
laser-cut into a
2.5 x 2.5 cm² piece
Solar cell results
• Flat surface of the solar cell and rather thick poly-Si layer (> 200 nm thick)
Low Jsc
• Edge recombination
Low PFF
• Excellent passivation quality of the poly-Si layer
high Voc
• Good transport through the poly-Si / c-Si junction
low Rs
Area
[cm²]
Voc
[mV]
Voc [mV]
(full area
illumination)
PFF
[%]
FF
[%]
Rs,FF
[Ωcm²]
Jsc
[mA/cm²]
η
[%]
4.25 705 714 73.1 71.2 0.6 28.8 14.5