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Understanding the boron-oxygen defect: properties ... · The boron-oxygen defect Time B O Light...
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School of Photovoltaic & Renewable Energy Engineering
SPREE Seminar
University of New South Wales
Sydney, Australia
9 November 2017
Nitin Nampalli
Supervisors: Malcolm Abbott, Stuart Wenham,
Matthew Edwards
Faculty of Engineering \ School of Photovoltaic and Renewable Energy Engineering
Advanced Hydrogenation Group
Understanding the boron-oxygen defect: properties,
kinetics and deactivation mechanisms
Motivation
• p-type silicon dominant for forseeable future
• PERC cells seeing an increasing market share
• Surface passivation quality is improving
• “Reliable kWh” is increasingly important
Strong imperative to:
(a) Improve bulk quality
(b) Minimise cell degradation
International Technology Roadmap for Photovoltaics, 8th Edition, 2017.
Motivation
• p-type silicon dominant for forseeable future
• PERC cells seeing an increasing market share
• Surface passivation quality is improving
• “Reliable kWh” is increasingly important
Strong imperative to:
(a) Improve bulk quality
(b) Minimise cell degradation
International Technology Roadmap for Photovoltaics, 8th Edition, 2017.
Motivation
• p-type silicon dominant for forseeable future
• PERC cells seeing an increasing market share
• Surface passivation quality is improving
• “Reliable kWh” is increasingly important
Strong imperative to:
(a) Improve bulk quality
(b) Minimise cell degradation
International Technology Roadmap for Photovoltaics, 8th Edition, 2017.
Motivation
• p-type silicon dominant for forseeable future
• PERC cells seeing an increasing market share
• Surface passivation quality is improving
• “Reliable kWh” is increasingly important
Strong imperative to:
(a) Improve bulk quality
(b) Minimise cell degradation
International Technology Roadmap for Photovoltaics, 8th Edition, 2017.
Motivation
• Boron-oxygen defects are the most important
source of LID in commercial Cz solar cells
• Cell efficiency loss:
– PERC: 1-12%rel
– Al-BSF: 1-6%rel
– For a cell manufacturer producing 1,000 MWp/year:
• 60-120 MWp/year in lost production
• USD $12-24 million/year in lost savings
Mitigation of BO
defects is of vital
importance
The boron-oxygen defect
Time
B O Light Induced
Degradation
causes
but can be…
Temporarily
Deactivated
Permanently
Deactivated
“Dark Annealing”
(200 °C)
Illuminated Annealing
“Regeneration”
(~1 sun, >85 °C)
Deactivation is unstable!
Light Soaking
(1 sun, 25 °C)
Time
Op
en
Cir
cu
it V
olt
ag
e
Stable Deactivation
(Passivation)
Time
Light Soaking
(1 sun, 25 °C)
State A
State B
State C
Regeneration
Destabilisation Degradation
Annihilation
Direct stabilisation
Direct destabilisation
The boron-oxygen defect
• Behaviour described
by 3-state model
• Focus of this work:
1. Recombination
properties
2. Reaction kinetics
3. Deactivation
mechanisms
1
2
3
Recombination properties of
the boron-oxygen defect
𝜏𝑝0 =1
𝑁𝑡𝑟𝑎𝑝 ∙ 𝜎𝑝 ∙ 𝑣𝑡ℎ,ℎ
Recombination properties of BO
DA
LS BO 1
𝜏𝑒𝑓𝑓,𝐿𝑆=
1
𝜏𝑠𝑢𝑟𝑓+
1
𝜏𝑏𝑢𝑙𝑘,𝑛𝑜𝑛−𝐵𝑂+
1
𝜏𝑆𝑅𝐻,𝐵𝑂
Si wafer BO? 1
𝜏𝑒𝑓𝑓,𝐷𝐴=
1
𝜏𝑠𝑢𝑟𝑓+
1
𝜏𝑏𝑢𝑙𝑘,𝑛𝑜𝑛−𝐵𝑂
1
𝜏𝑆𝑅𝐻=
𝑛𝑝 − 𝑛𝑖2
∆𝑛 ∙ 𝝉𝒑𝟎 𝑛 + 𝒏𝟏 + 𝝉𝒏𝟎 𝑝 + 𝒑𝟏
𝑘 =𝜎𝑛𝜎𝑝
𝑝1 = 𝑛𝑖,𝑒𝑓𝑓 × 𝑒𝐸𝑖−𝐸𝑡𝑟𝑎𝑝
𝑘𝐵𝑇
𝜏𝑛0 =1
𝑁𝑡𝑟𝑎𝑝 ∙ 𝜎𝑛 ∙ 𝑣𝑡ℎ,𝑒
kBO = 14
IDLS
TDLS
Ntrap Ntrap
Recombination properties of BO
S. Rein and S. Glunz, Applied Physics Letters 82(7), pp.1054-1056, 2003.
X. Wang et al., Energy Procedia 55, pp.169-178, 2014.
Etrap
k
kBO = 9.3
Is the value of Etrap,BO correct?
What is the real value of kBO?
Statistical uncertainty?
Is kBO temperature-dependent?
Recombination properties of BO
Etrap
Ntrap
k
At low injection (Δn = 0.1×NA):
𝑁𝑡𝑟𝑎𝑝 ∝1
𝜏𝑆𝑅𝐻,𝐵𝑂
∝1
𝜏𝐿𝑆−
1
𝜏𝐷𝐴= 𝑁𝐷𝐷
𝑁𝑡𝑟𝑎𝑝 =1
𝜏𝑛0 ∙ 𝜎𝑛 ∙ 𝑣𝑡ℎ,𝑒
(Normalised Defect Density)
NDDmax NDD(t)
ºº
Recombination properties of BO
K. Bothe et al., Solid State Phenomena 95-96, p. 223-228 (2004)
D. Walter et al., Solar Energy Materials and Solar Cells 131, p. 51-57 (2014)
Etrap
Ntrap
k
Firing What is the impact of firing on NDD?
Could k also be affected by firing?
Recombination properties of BO
• SRH properties of the BO defect ( k , Etrap )
• Impact of firing on k
• Impact of firing on defect density
Recombination properties of BO
• SRH properties of the BO defect ( k , Etrap )
• Impact of firing on k
• Impact of firing on defect density
SRH Recombination Properties
Methods for determining recombination properties: – IDLS: Injection-Dependent Lifetime Spectroscopy
• A common characterization method
• Good sensitivity to k (if Etrap is known)
• Low sensitivity to Etrap (for mid-gap defects)
– TDLS: Temperature-Dependent Lifetime Spectroscopy
• Good sensitivity to Etrap, k
• Analysis at single injection level (Δn)
– TIDLS: Temperature- and Injection-Dependent Lifetime Spectroscopy
• Best sensitivity to Etrap, k
• Analysis over full range of Δn
GOF
QDA = 1.60
QLS = 2.93
Qglobal = 2.22
k1 = 12.98
J0e,DA = J0e,LS = 84.7 fA/cm2
1013 1014 1015 1016 1017
5
10
15
20
25
30
GOF
QDA = 1.60
QLS = 1.02
Qglobal = 1.32
k1 = 11.74
J0e,LS = 80.8 fA/cm2
J0e,DA = 84.7 fA/cm2
1013 1014 1015 1016 1017
Inve
rse L
ifeti
me (
ms
-1)
Excess Carrier Density, n (cm-3)
Method B
(Constrained)
Method D
(Free J0e)
GOF
QDA = 1.60
QLS = 2.93
Qglobal = 2.22
k1 = 12.98
J0e,DA = J0e,LS = 84.7 fA/cm2
1013 1014 1015 1016 1017
5
10
15
20
25
30
GOF
QDA = 1.60
QLS = 1.02
Qglobal = 1.32
k1 = 11.74
J0e,LS = 80.8 fA/cm2
J0e,DA = 84.7 fA/cm2
1013 1014 1015 1016 1017
Inve
rse L
ifeti
me (
ms
-1)
Excess Carrier Density, n (cm-3)
Method B
(Constrained)
Method D
(Free J0e)
GOF
QDA = 1.60
QLS = 2.93
Qglobal = 2.22
k1 = 12.98
J0e,DA = J0e,LS = 84.7 fA/cm2
1013 1014 1015 1016 1017
5
10
15
20
25
30
GOF
QDA = 1.60
QLS = 1.02
Qglobal = 1.32
k1 = 11.74
J0e,LS = 80.8 fA/cm2
J0e,DA = 84.7 fA/cm2
1013 1014 1015 1016 1017
Inve
rse L
ifeti
me (
ms
-1)
Excess Carrier Density, n (cm-3)
Method B
(Constrained)
Method D
(Free J0e)
GOF
QDA = 1.60
QLS = 2.93
Qglobal = 2.22
k1 = 12.98
J0e,DA = J0e,LS = 84.7 fA/cm2
1013 1014 1015 1016 1017
5
10
15
20
25
30
GOF
QDA = 1.60
QLS = 1.02
Qglobal = 1.32
k1 = 11.74
J0e,LS = 80.8 fA/cm2
J0e,DA = 84.7 fA/cm2
1013 1014 1015 1016 1017
Inve
rse L
ifeti
me (
ms
-1)
Excess Carrier Density, n (cm-3)
Method B
(Constrained)
Method D
(Free J0e)
GOF
QDA = 1.60
QLS = 2.93
Qglobal = 2.22
k1 = 12.98
J0e,DA = J0e,LS = 84.7 fA/cm2
1013 1014 1015 1016 1017
5
10
15
20
25
30
GOF
QDA = 1.60
QLS = 1.02
Qglobal = 1.32
k1 = 11.74
J0e,LS = 80.8 fA/cm2
J0e,DA = 84.7 fA/cm2
1013 1014 1015 1016 1017
Inve
rse L
ifeti
me (
ms
-1)
Excess Carrier Density, n (cm-3)
Method B
(Constrained)
Method D
(Free J0e)
GOF
QDA = 1.60
QLS = 2.93
Qglobal = 2.22
k1 = 12.98
J0e,DA = J0e,LS = 84.7 fA/cm2
1013 1014 1015 1016 1017
5
10
15
20
25
30
GOF
QDA = 1.60
QLS = 1.02
Qglobal = 1.32
k1 = 11.74
J0e,LS = 80.8 fA/cm2
J0e,DA = 84.7 fA/cm2
1013 1014 1015 1016 1017
Inve
rse L
ifeti
me (
ms
-1)
Excess Carrier Density, n (cm-3)
Method B
(Constrained)
Method D
(Free J0e)
SRH Recombination Properties
IDLS analysis
N. Nampalli et al., Frontiers in Energy 11(1), 2017.
𝟏
𝝉𝑫𝑨=
1
𝜏𝑠𝑢𝑟𝑓+
1
𝜏𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑+
1
𝜏𝑆𝑅𝐻,𝑛𝑜𝑛−𝐵𝑂
𝟏
𝝉𝑳𝑺=
1
𝜏𝑠𝑢𝑟𝑓+
1
𝜏𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑+
1
𝜏𝑆𝑅𝐻,𝑛𝑜𝑛−𝐵𝑂+
1
𝜏𝑆𝑅𝐻,𝐵𝑂
SRH Recombination Properties
IDLS analysis
N. Nampalli et al., Frontiers in Energy 11(1), 2017.
𝟏
𝝉𝑫𝑨=
1
𝜏𝑠𝑢𝑟𝑓+
1
𝜏𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑+
1
𝜏𝑆𝑅𝐻,𝑛𝑜𝑛−𝐵𝑂
𝟏
𝝉𝑳𝑺=
1
𝜏𝑠𝑢𝑟𝑓+
1
𝜏𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑+
1
𝜏𝑆𝑅𝐻,𝑛𝑜𝑛−𝐵𝑂+
1
𝜏𝑆𝑅𝐻,𝐵𝑂
A B C D E F G H I J
Constrained Partially unconstrained Free
B C E F G H J
11.89 11.96 11.94 11.96 12.24 12.12 13.15 12.38 14.55 14.72
0.68 0.74 0.66 0.55 0.98 1.51 1.01 0.85 1.20 1.51
A D I
4
6
8
10
12
14
16
18
20
22
24
26
Be
st
Fit
Ca
ptu
re C
ros
s S
ec
tio
n R
ati
o, k
1
klit= 9.3
MAD
Median
kBO > 9.3
Not all fitting methods
are accurate kBO > 9.3
kBO = 11.9 ± 0.45
SRH Recombination Properties
TIDLS analysis
1015 1016 1017
0
5
10
15
20
25
Elevated T (K)
298
323
344
364
374
395
420
444
464
Inve
rse lif
eti
me a
t ele
vate
d T
, D
A(T
)-1 (
s-1
)
Excess Carrier Density, n (cm-3)
Increasing T
1014 1015 1016 1017
0
5
10
15
20
25
30 Elevated T (K)
298
326
344
364
373
394
420
445
463
Inve
rse lif
eti
me a
t ele
vate
d T
, L
S(T
)-1 (
s-1
)
Excess Carrier Density, n (cm-3)
Increasing T
SRH Recombination Properties
TIDLS analysis
– Determine Etrap , k
EC – Etrap = 0.41 ± 0.10
kBO = 11.5 ± 1.00
SRH Recombination Properties
TIDLS analysis
– Determine Etrap , k
– T-dependence of τSRH,BO
EC – Etrap = 0.41 ± 0.10
kBO = 11.5 ± 1.00
𝜎𝑛 𝑇 = 𝜎𝑛 300𝐾 ∙𝑇
300
𝜶𝒏
Assumption: αn = αp = αBO
𝜏𝑛0 𝑇 = 𝑁𝑡𝑟𝑎𝑝 ∙ 𝝈𝒏 𝑻 ∙ 𝑣𝑡ℎ,𝑒 𝑇−1
αBO = –2.3
𝝈𝒏/𝒑 𝑻 ∝ 𝑻−𝟐.𝟑
Recombination properties of BO
• SRH properties of the BO defect ( k , Etrap )
• Impact of firing on k
• Impact of firing on defect density
Measure Normalised Defect Density (NDD)
Dark Anneal: 200 oC, 15 min
Light Soak: 35 oC 0.77 suns, 48 hr
Impact of firing – Experiment details
PECVD SiNx:H
NDD Meas. 1
Impact of firing – Experiment details
PECVD SiNx:H
NDD Meas. 1
Fired
Not Fired
NDD Meas. 2
Relative change
due to firing
Varying Tpeak
0 10 20 30 40 50 60 700
100
200
300
400
500
600
700
800 Tpeak
= 827 C
Tpeak
= 790 C
Tpeak
= 735 C
Tpeak
= 686 C
Tpeak
= 632 C
Tpeak
= 580 C
Tpeak
= 518 C
Tpeak
= 463 C
Tem
pera
ture
,
(C
)
Time, t (s)
350
Nfire = 1, 2, 3
0 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
0 463 518 580 632 686 735 790 827
4
8
12
16
20
24
28
32
Bes
t F
it C
ap
ture
Cro
ss S
ecti
on
Rati
o, k
1
Tpeak
(C)
Nfire
Impact of firing on kBO
kBO
Before Firing
After Firing
New, non-BO
CID defect
Impact of firing on NDDBO
0 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
0 463 518 580 632 686 735 790 827
10
100
Norm
alis
ed D
efe
ct D
ensity, N
DD
aft
er (m
s-1)
Tpeak
(C)
Nfire
Before Firing After Firing
New, non-BO
CID defect
Summary – Properties of the BO defect
• SRH properties of BO: – Determined kBO = 11.9 (> 9.3)
– Confirmed that Etrap = EC – 0.41 eV
– Determined that σn/p(T) ∝ T–2.3
• Impact of firing on BO properties: – Confirmed that kBO is not affected by firing
– Demonstrated that firing reduces NDDBO
– Firing can induce other (non-BO) CID defects in Cz silicon
Transition kinetics of the BO
system
0 50 100 150 200 250 30010-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
De
fec
t tr
an
sit
ion
ra
te c
on
sta
nt,
ij (
s-1
)
Temperature, T ( C)
AB
BA
BC
CB
0 50 100 150 200 250 30010-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
De
fec
t tr
an
sit
ion
ra
te c
on
sta
nt,
ij (
s-1
)
Temperature, T ( C)
AB
BA
BC
CB
0 50 100 150 200 250 30010-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
De
fec
t tr
an
sit
ion
ra
te c
on
sta
nt,
ij (
s-1
)
Temperature, T ( C)
AB
BA
BC
CB
Transition kinetics of the BO system
κBA
κAB
κBC
κCB
Dependence of transition rates
on T and illumination (Δn) are
of vital importance!
• Degradation:
– 𝜅𝐴𝐵 appears to be independent
of illumination intensity (>0.1
suns)
– Other studies show 𝜅𝑑𝑒𝑔 ∝
𝑝02
Transition kinetics of the BO system
J. Schmidt and K. Bothe, Physical Review B 69, p.024107, 2004.
K. Bothe and J. Schmidt, Journal of Applied Physics 99, p. 013701, 2006.
Which is true?
𝜿𝑨𝑩 ∝ 𝒑𝟎𝟐
𝜿𝑨𝑩 ∝ 𝒑 ∙ 𝒑𝟎
𝜿𝑨𝑩 ∝ 𝒑 𝟐
• Annealing:
– Known to occur in dark (no
carrier dependence
assumed)
– But…one study showed
𝜅𝐵𝐴 ∝1
𝑝0 for compensated Si
Transition kinetics of the BO system
B. Lim et al., Applied Physics Letters 98, p.162104, 2011.
Does 𝜿𝑩𝑨 have a
carrier dependence?
Issue with reaction rate studies
• Process T ≠ Measurement T !
𝑮 𝑻 =∆𝑛 𝑇
𝝉𝒆𝒇𝒇 𝑻, ∆𝒏
p(T) = p0(T) + Δp(T) p(300K) = p0(300K) + Δp(300K)
𝑮 𝟑𝟎𝟎𝑲 =∆𝑛 300𝐾
𝝉𝒆𝒇𝒇 𝟑𝟎𝟎𝑲, ∆𝒏
Need a method to determine ∆𝒏 𝑻
from 𝝉𝒆𝒇𝒇 𝟑𝟎𝟎𝑲 and 𝑮 𝟑𝟎𝟎𝑲
Reaction kinetics of the BO system
• Model to obtain τeff(T) from τeff(300 K)
• Temporary deactivation (annealing) kinetics
• Degradation kinetics
Temperature dependence of lifetime
280 300 320 340 360 380 400 420 440 460 480 500
0.98
1.00
1.02
1.04
1.06 Data
Fit
Ge
ne
rati
on
ra
te c
orr
ecti
on
fac
tor,
fc
orr(T
)
Measurement temperature, T (K)
𝐺 𝑇 =∆𝑛 𝑇
𝜏𝑒𝑓𝑓 𝑇, ∆𝑛
Temperature dependence of lifetime
𝟏
𝝉𝑫𝑨 𝑻, 𝜟𝒏=
1
𝜏𝑠𝑢𝑟𝑓 𝑇, 𝛥𝑛+
1
𝜏𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑 𝑇+
1
𝜏𝑆𝑅𝐻,𝑛𝑜𝑛−𝐵𝑂 𝑇
𝟏
𝝉𝑳𝑺 𝑻, 𝜟𝒏=
1
𝜏𝑠𝑢𝑟𝑓 𝑇, 𝛥𝑛+
1
𝜏𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑 𝑇+
1
𝜏𝑆𝑅𝐻,𝑛𝑜𝑛−𝐵𝑂 𝑇+
1
𝜏𝑆𝑅𝐻,𝐵𝑂 𝑇, 𝛥𝑛
𝝉𝑫𝑨 𝑻, 𝜟𝒏 𝝉𝑳𝑺 𝑻, 𝜟𝒏
𝐺 𝑇 =∆𝑛 𝑇
𝜏𝑒𝑓𝑓 𝑇, ∆𝑛
𝑷𝒂𝒓𝒂𝒎 𝑻 = 𝑷𝒂𝒓𝒂𝒎 𝟑𝟎𝟎𝑲 ×𝑻
𝟑𝟎𝟎
𝜶𝒑𝒂𝒓𝒂𝒎
Temperature dependence of lifetime
𝜏𝑛0,𝐵𝑂 𝑇 = 𝝉𝒏𝟎,𝑩𝑶 𝟑𝟎𝟎 𝑲 ∙𝑻
𝟑𝟎𝟎
𝜶𝒏𝟎,𝑩𝑶
𝜏𝑛0,𝑛𝑜𝑛−𝐵𝑂 𝑇 = 𝝉𝒏𝟎,𝒏𝒐𝒏−𝑩𝑶 𝟑𝟎𝟎 𝑲 ∙𝑻
𝟑𝟎𝟎
𝜶𝒏𝟎,𝒏𝒐𝒏−𝑩𝑶
1
𝜏𝑠𝑢𝑟𝑓 𝑇, 𝛥𝑛= 2 ∙
𝑺𝒆𝒇𝒇(𝟑𝟎𝟎 𝑲
𝑊∙
𝑻
𝟑𝟎𝟎
𝜶𝑺𝒆𝒇𝒇
∙ 1 +𝛥𝑛
𝑝0
𝟏
𝝉𝑫𝑨 𝑻, 𝜟𝒏=
1
𝜏𝑠𝑢𝑟𝑓 𝑇, 𝛥𝑛+
1
𝜏𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑 𝑇+
1
𝜏𝑆𝑅𝐻,𝑛𝑜𝑛−𝐵𝑂 𝑇
𝟏
𝝉𝑳𝑺 𝑻, 𝜟𝒏=
1
𝜏𝑠𝑢𝑟𝑓 𝑇, 𝛥𝑛+
1
𝜏𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑 𝑇+
1
𝜏𝑆𝑅𝐻,𝑛𝑜𝑛−𝐵𝑂 𝑇+
1
𝜏𝑆𝑅𝐻,𝐵𝑂 𝑇, 𝛥𝑛
𝜏𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑 𝑇 = 𝝉𝒃𝒖𝒍𝒌,𝒇𝒊𝒙𝒆𝒅 𝟑𝟎𝟎 𝑲 ∙𝑻
𝟑𝟎𝟎
𝜶𝑏𝑢𝑙𝑘,𝑓𝑖𝑥𝑒𝑑
1
𝜏𝑠𝑢𝑟𝑓 𝑇, 𝛥𝑛=
2 ∙ 𝑝0 + 𝛥𝑛
𝑞 ∙ 𝑊∙ 𝑱𝟎𝒆 𝟑𝟎𝟎𝑲 ∙
𝑻
𝟑𝟎𝟎
𝜶𝑱𝟎𝒆
𝐺 𝑇 =∆𝑛 𝑇
𝜏𝑒𝑓𝑓 𝑇, ∆𝑛
Temperature dependence of lifetime
Parameter Relevant lifetime
component
Exponent of temperature
dependence Value for αparam
τbulk,fixed* τbulk,fixed(T) αb 2.880 ± 0.032
Seff τsurf(T) αSeff -1.395 ± 0.030
J0e τsurf(T) αJ0e 41.449 ± 0.044
τn0,BO τSRH, BO(T) αn0,BO 1.870 ± 0.003
τn0,non-BO* τSRH,non-BO(T) αn0,non-BO -1.420 ± 0.006
* may be specific to wafers used in this study
1 1.1 1.2 1.3 1.4 1.5 1.6
0.1
1
10
J0e
bulk,fixed
n0,BO
n0,non-BO
Seff
T / 300 K
Para
me
ter
valu
e r
ela
tive t
o 3
00
K
10-9
10-7
10-5
10-3
10-1
101
103
105
107
109
J0e r
ela
tive t
o 3
00
K
25 50 75 100 125 150 175 200
Temperature, T ( C)
Reaction kinetics of the BO system
• Model to obtain τeff(T) from τeff(300 K)
• Temporary deactivation (annealing) kinetics
• Degradation kinetics
Annealing kinetics
0 1 10 100 1000
8
9
10
11
12
13
14
15
16 T = 478 K (205 C)
T = 448 K (175 C)
T = 438 K (165 C)
T = 433 K (160 C)
T = 428 K (155 C)
Inve
rse lif
eti
me
, T
(t,T
=3
00
K)
(ms
-1)
Anneal time, t (s)
Defect dissociation
(B A) 𝜅BA ∝1
𝑝(𝑇)
DA LS BO
Annealing kinetics
κBA κBA (p/NV)-1
𝜅BA = 𝜈𝐵𝐴′ ∙ 𝑒
−𝐸𝑎,𝐵𝐴𝑘𝐵𝑇 𝜅BA = 𝜈0,𝐵𝐴 ∙
𝑁𝐶(𝑇
𝑝(𝑇)∙ 𝑒
−𝐸𝑎,𝐵𝐴𝑘𝐵𝑇
2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7
-14
-12
-10
-8
-6
-4
-2
0
Ea,BA = 1.26 0.14 eV
BA' = 1012.6 1.6 s-1
This work
Rein et al. (2001)
Schmidt et al. (2004)
Schmidt et al. (2002)
Lim et al. (2011)
Fit line to data in this work
ln(
BA [
s-1
] )
1000/T (K-1)
220 200 180 160 140 120 100
Dark anneal temperature (C)
Reaction kinetics of the BO system
• Model to obtain τeff(T) from τeff(300 K)
• Temporary deactivation (annealing) kinetics
• Degradation kinetics
Degradation kinetics
J. Schmidt and K. Bothe, Physical Review B 69, p.024107, 2004.
𝜅AB ∝ 𝑝 𝑇 2
DA LS BO
Degradation kinetics
K. Bothe and J. Schmidt, Journal of Applied Physics 99, p. 013701, 2006.
κAB κAB (p/NC)2
(This work)
𝜅AB = 𝜈𝐴𝐵′ ∙ 𝑒
−𝐸𝑎,𝐴𝐵𝑘𝐵𝑇 𝜅AB = 𝜈0,𝐴𝐵 ∙
𝑝(𝑇)
𝑁𝐶(𝑇)
2
∙ 𝑒−𝐸𝑎,𝐴𝐵𝑘𝐵𝑇
Implications of degradation kinetics
• Fast initial degradation (“FRC”)
– May be partially related to p2 dependence
K. Bothe and J. Schmidt, Journal of Applied Physics 99, p. 013701, 2006.
κAB is not constant
(due to p2 dependence)
Fast initial degradation
occurs in same time
scale as change in κAB
Implications of degradation kinetics
• Regeneration rates
– Regen (B C) occurs only after degradation (A B)
– κBC will be limited by κAB
A. Herguth et al., Progress in Photovoltaics: Research and applications 16, pp.135-140, 2008
P. Hamer et al., Solar Energy Materials and Solar Cells 145, pp.440-446, 2016
A: ~100%
B: ?% (<100%)
C: ~100%
Starting State: B
(fast recovery)
Starting State: A
(slow recovery)
B: 100%
C: ~100% κBC must be determined
starting from State B (LS),
not State A (DA)
Summary – Reaction Kinetics
• Developed parameterization to obtain
τeff(T) from τeff(300 K)
• Carrier dependence confirmed for
annealing (A B), degradation (B A)
• Carrier dependence explains other
observed kinetics phenomena
1 1.1 1.2 1.3 1.4 1.5 1.6
0.1
1
10
J0e
bulk,fixed
n0,BO
n0,non-BO
Seff
T / 300 K
Pa
ram
ete
r v
alu
e r
ela
tiv
e t
o 3
00
K
10-9
10-7
10-5
10-3
10-1
101
103
105
107
109
J0e r
ela
tiv
e t
o 3
00
K
25 50 75 100 125 150 175 200
Temperature, T ( C)
A,
B:
~0
%
C:
~1
00
%
Mechanisms for permanent
deactivation of BO defects
Permanent deactivation
D. Walter et al., Solar Energy Materials and Solar Cells 131, p. 51-57 (2014)
K. Bothe et al., Solid State Phenomena 95-96, p. 223-228 (2004)
V. Voronkov and R. Falster, Physica Status Solidi C 13(10-12), p. 712-717 (2016)
State A
State B
State C
Regeneration
Destabilisation Degradation
Annihilation
Direct stabilisation
Direct destabilisation
Defect precursors
Thermal formation
Thermal deactivation
BO defect
Firing
1
2
Why does permanent
deactivation occur?
Deactivation mechanisms
• Why does regeneration occur?
• Why does thermal deactivation occur?
• Are they related?
Measure Normalised Defect Density (NDD)
Dark Anneal: 200 oC, 15 min
Light Soak: 35 oC 0.77 suns, 48 hr
Experimental Details
PECVD SiNx:H
Bare
Thermal SiO2
NDD Meas. 1
Regeneration
PECVD SiNx:H (hydrogen)
Bare (no hydrogen)
Thermal SiO2
(no hydrogen)
NDD Meas. 1
Fired
Not Fired
NDD Meas. 2 NDD Meas. 3
Regeneration
Illuminated Anneal: 172 oC, 0.66 suns, 2 hr
Relative change
due to regeneration
Relative change
due to firing
Regeneration
D. Walter and J. Schmidt, Solar Energy Materials and Solar Cells 158, p.91-97 (2016)
Fired Non-fired Fired Non-fired Fired Non-fired
No dielectric SiNx:H SiO
2
0.00
0.25
0.50
0.75
1.00
1.25
Hydrogen-related reduction
in FDD due to regeneration
FD
D a
fter
regen.
(re
l. to
ND
Daft
er)
Rel.
ch
an
ge in
ND
D d
ue t
o r
eg
en
.
Some improvement in hydrogen-lean wafers
Regeneration observed in
hydrogen-lean samples
(very long time scales)
Deactivation mechanisms
• Why does regeneration occur?
• Why does thermal deactivation occur?
• Are they related?
Thermal deactivation
PECVD SiNx:H (hydrogen)
Bare (no hydrogen)
Thermal SiO2
(no hydrogen)
NDD Meas. 1
Fired
Not Fired
NDD Meas. 2 NDD Meas. 3
Regeneration
Illuminated Anneal: 172 oC, 0.66 suns, 2 hr
Relative change
due to firing
Varying Tpeak
0 10 20 30 40 50 60 700
100
200
300
400
500
600
700
800 Tpeak
= 827 C
Tpeak
= 790 C
Tpeak
= 735 C
Tpeak
= 686 C
Tpeak
= 632 C
Tpeak
= 580 C
Tpeak
= 518 C
Tpeak
= 463 C
Tem
pera
ture
,
(C
)
Time, t (s)
350
Nfire = 1, 2, 3
Thermal deactivation
0 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
0 463 518 580 632 686 735 790 827
10
100
Norm
alis
ed D
efe
ct D
ensity, N
DD
aft
er (m
s-1)
Tpeak
(C)
Nfire
Before Firing After Firing
→
New, non-BO
CID defect Wafers with SiNx:H dielectric
→
0 450 500 550 600 650 700 7500.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0Simple Arrhenius
model
Rela
tive N
DD
after
firing
Peak firing temperature, Tpeak
(C)
Data Model
Nfire
= 1
Nfire
= 2
Nfire
= 3
Increasing Nfire
Thermal deactivation
V. Voronkov and R. Falster, Physica Status Solidi C 13(10-12), p. 712-717 (2016)
0.9 1.0 1.1 1.2 1.3 1.4 1.5-4.0
-3.0
-2.0
-1.0
0.0
1.0
Nfire
= 1
Nfire
= 2
Nfire
= 3
Linear fit (Nfire
= 1 only)
Linear fit (all Nfire
)
ln [
-ln
(
ND
D%
,BO)
]
1000/T (K-1)
800 750 700 650 600 550 500 450 400
Peak Firing Temperature, Tpeak
(C)
Reaction: BO X
All Nfire
Ea,f
= 0.79 0.08 eV
(ft
step) = 10
4.19 ± 0.5
Nfire
= 1
Ea,f
= 0.77 0.1 eV
(ft
step) = 10
4.15 ± 0.6
Thermal deactivation
V. Voronkov and R. Falster, Physica Status Solidi C 13(10-12), p. 712-717 (2016)
0 450 500 550 600 650 700 750
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Rela
tive N
DD
after
firing
Peak firing temperature, Tpeak
(C)
Data
Model - Freeze-in
Model - Freeze-in (adj.)
Model - Arrhenius
→
Deactivation mechanisms
• Why does regeneration occur?
• Why does thermal deactivation occur?
• Are they related?
0 450 500 550 600 650 700 750 8000
2
4
6
8
10N
orm
alis
ed D
efe
ct D
ensity (
ms
-1)
Peak firing temperature, Tpeak
(C)
SiNx:H, NDD
before
SiNx:H, NDD
after
SiO2, NDD
after
Thermal reduction in NDD
due to firing
Fired Non-fired Fired Non-fired
SiNx:H SiO
2
0.00
0.25
0.50
0.75
1.00
1.25F
DD
aft
er
firin
g (
rel. to
ND
Db
efo
re)
Thermal reduction in FDD
due to firing
Rel.
ch
an
ge in
ND
D d
ue t
o f
irin
g
Thermal deactivation
N
Regeneration
Fired Non-fired Fired Non-fired Fired Non-fired
No dielectric SiNx:H SiO
2
0.00
0.25
0.50
0.75
1.00
1.25
Hydrogen-related reduction
in FDD due to regeneration
FD
D a
fter
regen.
(re
l. to
ND
Daft
er)
Rel.
ch
an
ge in
ND
D d
ue t
o r
eg
en
.
Revision to 3 state model?
4 state model
State D
High temperature
Thermal effects
Hydrogenation
+ other minor effects
Summary – Permanent deactivation
• Effective regeneration requires sufficient
quantities of hydrogen. Regeneration could
involve other mechanisms (slower).
• Thermal deactivation occurs independent of
regeneration. Likely not hydrogen-related.
• Thermal deactivation is likely to be defect
dissociation
• 4-state model proposed to account for thermal
deactivation
Fired Non-fired Fired Non-fired
SiNx:H SiO
2
0.00
0.25
0.50
0.75
1.00
1.25
FD
D a
fte
r firin
g (
rel. to
ND
Dbefo
re)
Thermal reduction in FDD
due to firing
Implications of this work
Implications
1. Improved understanding of the BO defect
– Recombination properties
– Mechanism of permanent deactivation
– 4-state model
Fired Non-fired Fired Non-fired
SiNx:H SiO
2
0.00
0.25
0.50
0.75
1.00
1.25
FD
D a
fte
r firin
g (
rel. to
ND
Db
efo
re)
Thermal reduction in FDD
due to firing
Impact:
- Easier identification of the BO defect
- Multiple, tailored solutions to mitigate BO defect
Implications
2. Accurate reaction kinetics modelling for BO
– 4-state model
– Conversion from τeff(300K) to τeff(T)
– Carrier dependence of reactions
1 1.1 1.2 1.3 1.4 1.5 1.6
0.1
1
10
J0e
bulk,fixed
n0,BO
n0,non-BO
Seff
T / 300 K
Para
me
ter
valu
e r
ela
tive t
o 3
00 K
10-9
10-7
10-5
10-3
10-1
101
103
105
107
109
J0e r
ela
tive t
o 3
00
K
25 50 75 100 125 150 175 200
Temperature, T ( C)
Impact:
- Better estimate of regeneration time-scales for
industrial wafers / solar cells
Acknowledgements
• Funding sources – Australian Renewable Energy Agency (ARENA)
– Australian Center for Advanced Photovoltaics (ACAP)
– UK Institution of Engineering and Technology (IET) / A.F. Harvey Engineering Prize.
– Commercial partners (ARENA 1-A060)
Acknowledgements
• Supervisors: Malcolm Abbott, Stuart Wenham, Matt Edwards
• Hydrogenation group
• Laboratory: – MAiA processing team
– LDOT team
– Others
• Admin staff: – TETB
– SPREE
• Friends and family
Motivation
• p-type silicon dominant for forseeable future
• PERC cells seeing an increasing market share
• Surface passivation quality is improving
• “Reliable kWh” is increasingly important
Strong imperative to:
(a) Improve bulk quality
(b) Minimise cell degradation
International Technology Roadmap for Photovoltaics, 8th Edition, 2017.