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Commercial In Confidence Copyright Dyesol 2012 1
Long term stability of dye solar cells –
meeting IEC 61646 requirements
Hans Desilvestro, Nancy Jiang,
Martin Berry and Paul Murray
Presenter: Ben Wilkinson (Dyesol UK)
4 April 2012
Outline
� Motivation
� The challenge
� Disentangling degradation mechanisms through DSC
accelerated testing
Commercial In Confidence Copyright Dyesol 2012 2
− Hydrophilic vs hydrophobic dye
− IV data
− IPCE (Incident Photon-to-Electron Conversion Efficiency)
− EIS (Electrochemical Impedance Spectroscopy)
� Towards larger devices
� Summary
Motivation
� Develop integrated understanding of all materials
and design aspects of dye solar cells (DSC)
‒ To best serve and advise our commercial partners
‒Optimum focus on most promising materials and
technologies
Commercial In Confidence Copyright Dyesol 2012 3
‒
‒
technologies
� Achieve optimised LCOE for any given application
‒ Performance
‒ Stability
‒ Cost
Stability
Performance
1/Cost Stability
Performance
1/Cost
The challenge
� To achieve grid parity under a moving target of
declining PV prices
� Performance – Stability - Cost
PerformanceHero cells
Performance
Industrial design today
Commercial In Confidence Copyright Dyesol 2012 4
� 20+ years life time in building applications
� Standards: PV-specific + Building Standards
Stability1/Cost Stability1/Cost
20-year product life goal
System’s requirements
At the molecular level
>100 million turnovers ���� at least for certain dyes/electrolytes
~No dye desorption ���� at least for certain dyes/electrolytes
Loss of NCS-, substitution by ?
Lund et al vs Falaras et al. Low 80-
Commercial In Confidence Copyright Dyesol 2012 5
Loss of NCS-, substitution by I-, nitriles, imidazoles, etc.
?Lund et al vs Falaras et al. Low 80-85oC stability with nitrile solvents
Isomerisation, e.g. N- to S-bound SCN -
?Not significant according to micro-Raman (Falaras et al)
Decomposition of electrolyte components
?Little is known, more in situ spectroscopic work required
Stability of electrocatalyst? ����At least for Pt and certain electrolytes
At the cell level (Dyesol up to 250 mm length): Seals
20-year product life goal
System’s requirements
Suppress ingress of O2, H2O ���� Dyesol, Fujikura, Fraunhofer ISE
Suppress egress of solvent ���� Dyesol, Fujikura, Fraunhofer ISE
Performance stability, 85oC/1,000h; -40oC/+85oC
at least for certain dyes/electrolytes, 85oC/85% r.h.: requires glass-based
Commercial In Confidence Copyright Dyesol 2012 6
85oC/1,000h; -40oC/+85oC thermal cycling; 85oC/85% r.h./1,000h
����85oC/85% r.h.: requires glass-based encapsulation (i.e. similar to CIGS or CdTe), best assessed at the panel level
� Excellent DSC durability under light soaking conditions (~60oC). E.g. Dyesol >25,000 h quasi-continuous illumination ���� 25-40 years life time extrapolated, depending on locationR. Harikisun, H. Desilvestro, Sol. Energy, 85, 1179 (2011), “Long-term stability of dye solar cells”
� IEC 61646 85oC/1,000h and thermal cycling tests remain challenging for DSC
At the module level
- Cell-to-cell interconnects
- Corrosion protection of current collectors
- Potential shunt paths
20-year product life goal
System’s requirements
Commercial In Confidence Copyright Dyesol 2012 7
- Potential shunt paths
- Sealing
At the system’s level
- Environmental, temperature extremes, hail, etc.
- Building code requirements
- Maximum power point tracking independent of
any ageing phenomena
Cell chemistry
NN
HO C
NN
HO2C
RuN
CS
NC
S
CO2TBA Z907N719
Commercial In Confidence Copyright Dyesol 2012 8
� N719 (hydrophilic) vs Z907 (hydrophobic)
� High-boiling solvent, non-nitrile
� Pt catalyst based on Dyesol Platinum Paste PT1
� 8××××11 or 8××××168 mm active area
NHO2C
CO2TBA
S
Cell Efficiency
Effectively meeting IEC 61646 for most practical light levels
Loss mainly due to loss of Jsc - Why?0
1
2
3
4
5
6
7
8
0 200 400 600 800 1,000 1,200
Eff
icie
nc
y (
%)
Time (hours)
1,000h, 85oC
1/3 sun, Z907
1/3 sun N719
1 sun, N719
1 sun, Z907
-10%-10%
-18%-18%
Commercial In Confidence Copyright Dyesol 2012 9
• Virtually no loss of Voc for
either dye (≤≤≤≤3.5%)
• At 1 sun N719 loses less
Jsc, but some ff (-2%)
while Z907 loses more Jscand gains some ff (+3%)
0
2
4
6
8
10
12
14
0 200 400 600 800 1,000 1,200
Jsc
(mA
/cm
2)
Time (hours)
1,000h, 85oC
1/3 sun, Z907
1/3 sun N719
1 sun, N719
1 sun, Z907
-13%
-19%
-7%-8%
30%
40%
50%
60%E
QE
(%
) N719, initial N719, 300h 85oC N719, 1000h 85oC Z907, initial Z907, 300h 85oC Z907, 1000h 85oC
IPCE (after 85oC storage)Incident Photon-to-Electron Conversion Efficiency
Commercial In Confidence Copyright Dyesol 2012 10
0%
10%
20%
30%
400 500 600 700 800 900
EQ
E (
%)
Wavelength (nm)
Some loss of IPCE over large part of spectrum over the first 300 h at 85oC, then
almost complete recovery in the 400 to ~570 nm region (due to decreasing
conduction band level?)
IPCE
� No major dye desorption or decomposition such as ligand exchange
� Loss of IPCE under low intensity monochromatic light is only partly responsible for loss of Jsc under higher intensity light, particularly
• after 1,000 h 85oC storage
Commercial In Confidence Copyright Dyesol 2012 11
• after 1,000 h 85 C storage
• at the higher light levels of 1 sun
• for Z907
∆∆∆∆(IPCE) *) ∆∆∆∆Jsc (0.33 sun) ∆∆∆∆Jsc (1 sun) ∆∆∆∆(IPCE) *) ∆∆∆∆Jsc (0.33 sun) ∆∆∆∆Jsc (1 sun)
N719 -6.4% -3.2% -7.4% -3.7% -6.6% -13.1%
Z907 -7.3% -5.8% -14.6% -3.9% -8.0% -18.9%
300 h 1,000 h
Jsc as a function of light level
and storage time at 85oC
10
15(m
A/c
m2)
N719, initial
N719, 300h
N719, 1000h
Commercial In Confidence Copyright Dyesol 2012 12
Serious current limitation at the higher light levels as a
result of thermal stress testing, particularly for Z907
0
5
0.0 0.2 0.4 0.6 0.8 1.0
Jsc
(mA
/cm
Sun level
Z907, initial
Z907, 300h
Z907, 1000h
100
200
-Zi(O
hm
cm
2)
Initial400h 85oC1000h 85oC
Z907
0.1Hz
100
200
(Oh
m c
m2)
Initial400h 85oC1000h 85oC
N719
0.1Hz
EIS Electrochemical Impedance Spectroscopy, 0.4V, 0.33 sun
Commercial In Confidence Copyright Dyesol 2012 13
00 100 200 300
Zr (Ohm cm2)
00 100 200 300
-Zi(O
hm
cm
Zr (Ohm cm2)
• Significant decrease of Rbr with prolonged
85oC storage, mainly from 400 to 1,000h
• Significant increase of Rd with prolonged
85oC storage from 400 to 1,000h
• Significant increase of CE Rct at 0.4 V, vs
decrease at 0.7 or 0.8 V
• Initial increase of Rbr then decrease from
400 to 1,000h
• More significant increase of Rd with
prolonged 85oC storage compared to N719
• Significant increase of CE Rct at 0.4 V, vs
decrease at 0.7 or 0.8 V
EIS (AC impedance)Transmission line model
rbr rbr rbr
Transport resistance, TiO2 resistance
Commercial In Confidence Copyright Dyesol 2012 14
Electron collection efficiency = Rbr/(Rbr+Rt)
Electron diffusion constant De = d2 / (RtCc)
Electron diffusion length Ln = d (Rbr/Rt)1/2 d = TiO2 layer thickness
a) F. Fabregat-Santiago, et al Solar Ener. Mat. and Solar Cells 87, (2005) 117-131. b) Hoshikawa, et al. J. Electroan. Chem., 588 (2006) 59
Cc: chemical capacitance = dQ/dVRbr: electron backtransfer resistance
4
5
N719, initial
Conclusions from EIS
1) 85oC: Rbr ����, Rt: no major change →→→→ ηηηηcoll ���� →→→→ Jsc ����
- to a larger extent for Z907 than N719 (various batches)
- in contrast to light soaking (LS) where ηcoll hardly decreases
Commercial In Confidence Copyright Dyesol 2012 15
0
1
2
3
4
0.6 0.7 0.8 0.9 1.0
j sc
(m
A/c
m2)
ηηηη coll: electron collection efficiency at 0.4V
Z907, initial
N719, LS
Z907, LS
N719, 85oC
Z907, 85oC
Conclusions from EIS
1) 85oC: Rbr ����, Rt: no major change →→→→ ηηηηcoll ���� →→→→ Jsc ����
- to a larger extent for Z907 than N719
- in contrast to light soaking (LS) where ηcoll hardly decreases
2) 85oC: evidence of conduction band downward shift by ~50-100
mV. Possibly reason for increased IPCE over large part of
Commercial In Confidence Copyright Dyesol 2012 16
spectrum. In contrast to light soaking with no significant shift of
Vcb over 1,000h
3) 85oC: Rd ����, due to diffusion polarisation under photogeneration,
particularly for Z907 →→→→ Jsc ����, diffusion limitation →→→→ Vmpp ���� (Vocis much less affected, no diffusion polarisation)
I3- (CE) ���� →→→→ Rct(photogeneration) ����
while Rct(0.7 or 0.8V) ���� due to ‘standard’ Pt activation
Conclusions from EISI3
- diffusion polarisation
-4
-2
0
2
4I3
- diffusion resistance @ 0.33sun
Initial
400h 85oC
1000h 85oC
400h light soaking
1000h light soaking
j (m
A c
m-2
)N719
Commercial In Confidence Copyright Dyesol 2012 17
-6
0 20 40 60 80 100 120 140 160 180
1000h light soaking
-8
-6
-4
-2
0
2
4
0 20 40 60 80 100 120 140 160 180Rd (Ohm)
Initial
400h 85oC
1000h 85oC
400h light soaking
1000h light soaking
j (m
A c
m-2
)
Z907
Conclusions from EISI3
- diffusion polarisation
� Mainly occurs under increasing photocurrents, due to I3
- concentration polarisation
� Much less diffusion polarisation under net negative currents, due to high [I-]/[I3
-] concentration ratio
− most notable with Z907 as a result of 1,000 h at 85oC
� Much more pronounced for Z907:
Commercial In Confidence Copyright Dyesol 2012 18
� Much more pronounced for Z907:
− significant increase of Rd as a result of light soaking
− significant increase of Rd as a result of 400 h at 85oC
− very significant increase of Rd as a result of 1,000 h at 85oC
� Much less pronounced for N719:
− almost no increase of Rd as a result of light soaking
− some increase of Rd as a result of 400 h at 85oC
− very significant increase of Rd as a result of 1,000 h at 85oC
Thermal cycling – IEC 61646
Commercial In Confidence Copyright Dyesol 2012 19
60%
80%
100%R
ela
tive
eff
icie
ncy (
1 s
un
)
Aft
er
30 m
in i
llu
min
ati
on
at
1 s
un
Thermal cycling – IEC 616468×168 mm, MPN-based electrolyte
Commercial In Confidence Copyright Dyesol 2012 20
0%
20%
40%
0 50 100 150 200 250
Rela
tive
eff
icie
ncy (
1 s
un
)
# Cycles
Aft
er
30 m
in i
llu
min
ati
on
at
1 s
un
� No visual changes
� No electrolyte leaks
� After 200 thermal cycles with 168mm long cells
− temporary loss of 20% efficiency, probably due to
nitrile-based solvent
Thermal cycling – IEC 61646Results / towards larger devices
Commercial In Confidence Copyright Dyesol 2012 21
nitrile-based solvent
− largely recoverable after 30 min illumination (@ 1 sun)
� Similar tests for larger multi-cell devices and with
electrolytes offering improved high temperature stability
� Very promising chemical and mechanical stability achievable under IEC 61646 85oC storage and thermal cycling
� Loss of Jsc (rather than Voc or ff) under 85oC storage with the
specific cell chemistry is due to
− decreasing electron collection efficiency ηcoll− increasing I - diffusion resistance
Conclusions
Commercial In Confidence Copyright Dyesol 2012 22
− increasing I3- diffusion resistance
� Chemical reasons for increased diffusion resistance upon high temperature exposure are presently not known
− more in situ spectroscopic and electrochemical work
required
� Strategies in place to further improve DSC high temperature stability
Commercial In Confidence Copyright Dyesol 2012 23
Thank you for your attention!
www.dyesol.com