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SUPPLEMENTARY INFORMATIONARTICLE NUMBER: 16035 | DOI: 10.1038/NENERGY.2016.35
NATURE ENERGY | www.nature.com/natureenergy 1
Supplementary
The Role of Surface Passivation for Efficient and
Photostable PbS Quantum Dot Solar Cells
Yiming Cao1,+, Alexandros Stavrinadis1,+, Tania Lasanta1, David So1, Gerasimos
Konstantatos1,2★
1ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology,
08860 Castelldefels (Barcelona), Spain
2ICREA-Institució Catalana de Recerca i Estudis Avançats, Lluis Companys 23, 08010 Barcelona, Spain
+These authors contribute equally to this work.
★e-mail: [email protected].
The role of surface passivation for efficient and photostable PbS quantum dot solar cells
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Supplementary Figures
500 700 900 1,1000
20
40
60
80
100 150 nm 220 nm 300 nm
Abs
orpt
ion
(%)
Wavelength (nm)
Supplementary Figure 1. Optical simulation of absorption of PbS QD solar cells with different
thickness of TBAI-processed QD layers. The model solar cell structure is ITO (95nm)/ZnO
(120 nm)/ TBAI-PbS (130, 200, 280nm)/EDT-PbS (20nm)/MoO3 (10nm)/Ag (40 nm)/Au
(150 nm).
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500 700 900 1,1000
20
40
60
80
100
Abs
orpt
ion
(%)
Wavelength (nm)
non-annealed 80C-annealed
Supplementary Figure 2. Absorption of non- and 80 ºC-annealed TBAI-PbS /EDT-PbS layers
on ZnO/glass substrates.
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168 166 164 162 160 158
0.0
0.5
1.0
1.5
537 535 533 531 529 527
0
1
2PbS
unbound thiol
norm
aliz
ed in
tens
ity
Binding energy (eV)
bound thiolate
O1s(b)
COO, CO2Pb-O
Binding energy (eV)
Pb-OH
(a)S2p
Supplementary Figure 3. (a) S2p and (b) O1s spectra of 120 ºC-annealed PbS QD layer
processed with EDT. The fitting peak of S2p3/2 from PbS is normalized to unity. The fitting
peak from Pb-OH in O1s is normalized to that of non-annealed PbS QD layer processed with
EDT in Fig. 2a.
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636 632 628 624 620 616 636 632 628 624 620 616
I3d3/2
I3d5/2
I3d3/2
(b)
Inte
nsity
(a.u
.)
Binding energy (eV)
(a)80C-annealed
Binding energy (eV)
I3d5/2non-annealed
Supplementary Figure 4. I3d spectra of (a) non- and (b) 80 ºC-annealed PbS QD layer
processed with TBAI. The raw spectra can be best deconvoluted to three components which,
from higher to lower energy (those binding energies for the case of I3d5/2 are: 618.1 eV, 619.4
eV and 620.5 eV, all constrained to FWHM of <1.1 eV and within <0.2 eV deviation between
non- and 80 ºC annealed cases), should correspond to complexes with increasing charge
transfer to the iodideS1. Thus while the lower energy species correspond to iodide atoms that
are strongly bound to Pb atoms on the QD surface, the high energy component corresponds to
iodide ions loosely bound to organic cations or weakly attached on the QD surface. The
percentage of those three species before annealing (from higher to lower energy) are 6.3%,
89.5% and 4.2% and upon annealing they are 3.2%, 91.4% and 5.4%. The 80°C-annealing
results to a decrease of the relative intensity of the highest energy peak and a corresponding
increase of the lower energy peaks. The amount of increased iodide bound to Pb atoms, which
is calculated by 0.60x(91.4%+5.4%)-0.54x(89.5%+4.2%)=0.07, matches closely to the
amount of decreased Pb-OH in Supplementary Table 6. We propose that this effect is
attributed to an increase of iodide atoms that are strongly bound to the surface of the QDs
upon annealing replacing hydroxides.
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16 12 8 4 00
1
2
3
16 12 8 4 00
1
2
3
18 17 16 150
1
2
3
18 17 16 150
1
2
3
2.5 2.0 1.5 1.0 0.50.00
0.04
0.08
0.12
2.5 2.0 1.5 1.0 0.50.00
0.04
0.08
0.12(c)
(b)
(a)In
tens
ity (x
105 c
ount
s)
Binding energy (eV)
non-annealed 80 C 120 C
Binding energy (eV)
Inte
nsity
(x10
5 cou
nts)
non-annealed 80 C 120 C
(d)
(f)
non-annealed 80 C 120 CIn
tens
ity (x
105 c
ount
s)
Binding energy (eV)
(e)
Inte
nsity
(x10
5 cou
nts)
Binding energy (eV)
non-annealed 80 C 120 C
non-annealed 80 C 120 C
Inte
nsity
(x10
5 cou
nts)
Binding energy (eV)
processed with TBAI
non-annealed 80 C 120 C
Inte
nsity
(x10
5 cou
nts)
Binding energy (eV)
processed with EDT
Supplementary Figure 5. UPS of PbS QD layer processed with EDT (a) and TBAI (d) with
varying annealing conditions. Detailed UPS with high binding energies of secondary electron
cutoff (b,e) and low binding energies of the onset regions (c,f), which are obtained by the
intercept of yellow lines, are presented. The ionization energy (valence band edge, Ev) was
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deduced by subtracting the energy of the incident beam with the difference in energy between
the secondary electron cutoff and onsetS2. The conduction band edge (Ec) was inferred by
taking the Ev together with the bandgap of the QD layers.
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
Ev5.09
4.92
3.62
5.06
120 C80 C
Ene
rgy
leve
ls (e
V)
non-annealed
3.76 3.80 Ec
Supplementary Figure 6. Band edge levels of EDT-processed PbS QD layers with varying
annealing derived from the UPS spectra of Supplementary Figure 5.
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
Ev
120C80C
5.20
3.91
5.48
4.18
5.59
Ener
gy le
vels
(eV) 4.29
non-annealedEc
Supplementary Figure 7. Band edge levels of TBAI-processed PbS QD layers with varying
annealing derived from the UPS spectra of Supplementary Figure 5.
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10-4
10-3
10-2
10-1 120C
80C
(c
m2 V
1s1
)
non annealed
Supplementary Figure 8. Carrier mobility in EDT-processed PbS QD layers with varying
annealing. The average (symbols) and stand deviation (error bars) were calculated from a
sample of four to seven devices.
10-4
120C
80C
(c
m2 V
1s1
)
non annealed
Supplementary Figure 9. Carrier mobility in TBAI-processed PbS QD layers with varying
annealing. The average (symbols) and stand deviation (error bars) were calculated from a
sample of four to seven devices.
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0.35 0.40 0.45 0.50 0.55
10-5
10-4
0.35 0.40 0.45 0.50 0.551021
1022
1023
non-annealed 80C-annealed
Life
time
(s)
Voc (V)
(a) (b) non-annealed 80C-annealed
R (c
m3
s1
)
Voc (V)
Supplementary Figure 10. (a) Carrier lifetime and (b) recombination rate (R) as a function of
the open-circuit potential for the non- and 80 ºC-annealed solar cell devices.
4 5 6 7 8 9 100
5
10
15
20
25
Cou
nt
PCE (%)
processed with TBAI EMII
Supplementary Figure 11. Histogram of PCE of 80 ºC-annealed PbS QD solar cells with Au
anode processed with TBAI and EMII.
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Supplementary Figure 12. Certificated PbS QD solar cell with first exciton peak at 930 nm of
colloidal QDs.
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0.0 0.2 0.4 0.6 0.8-5
0
5
10
15
20
25
400 500 600 700 800 900 1000 11000
20
40
60
80
100(c)
(b)
J (m
A cm
2)
V (V)
(a)
5 6 7 8 9 10 110
4
8
12
16
20
Cou
nt
PCE (%)
EQE
(%)
(nm)
Supplementary Figure 13. (a) J-V curves, (b) histogram of PCE and (c) EQE spectrum of
EMII-processed PbS QD solar cells fabricated by colloidal QDs with first exciton absorption
peak at 850 nm.
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Supplementary Figure 14. Certificated PbS QD solar cells with first exciton peak at 850 nm of
colloidal QDs.
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0.35 0.40 0.45 0.50 0.550.0
0.2
0.4
0.6
0.8
1.0processed with
TBAI EMII
Car
rier d
ensi
ty (x
1018
cm
3)
Voc (V)
Supplementary Figure 15. Voc-dependent carrier density of EMII and TBAI-processed PbS
QD solar cells annealed at 80 ºC.
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632 628 624 620 616
0.0
0.5
1.0
I3d3/2
I3d5/2processed with
TBAI EMII
Nom
aliz
ed in
tens
ity
Binding energy (eV)
Supplementary Figure 16. I3d spectra of 80 ºC-annealed PbS QD layer processed with TBAI
and EMII. The intensity is respectively normalized to the peak of Pb4f7/2.
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-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-5
0
5
10
15
20
25
30
initial PCE: 8.5% aged PCE: 8.3%
J (m
A cm
2)
V (V)
Supplementary Figure 17. Air stability of 80 ºC-annealed EMII-processed PbS QD solar cells
with Au anode aging for 30 days stored under dark in ambient condition.
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0 10 20 30 40
4
5
6
7
8
0 10 20 30 400.50
0.55
0.60
0 10 20 30 4018
20
22
24
26
0 10 20 30 400.3
0.4
0.5
0.6
non-annealed 80C-annealed
Time (hours)
PCE
(%)
Time (hours)
Voc
(V)
J sc (m
A cm
2)
Time (hours)
(d)(c)
(b)
FF
Time (hours)
(a)
Supplementary Figure 18. Stability of non- and 80 ºC-annealed EMII-processed PbS QD solar
cells with Au anode soaked under simulated AM1.5G 100 mW cm–2 illumination in ambient
condition. The average (symbols) and stand deviation (error bars) were calculated from a
sample of four to eight solar cells. The PbS solar cells are neither encapsulated nor equipped
with ultraviolet filter.
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0.0 2.0x10-4 4.0x10-40.43
0.44
0.45
0.46
0.0 1.0x10-4 2.0x10-40.04
0.05
0.06
0.07
0.0 1.0x10-4 2.0x10-40.470
0.475
0.480
0.485
0.0 5.0x10-5 1.0x10-4 1.5x10-40.11
0.12
0.13
0.14
0.0 5.0x10-5 1.0x10-4 1.5x10-40.506
0.508
0.510
0.512
0.0 5.0x10-5 1.0x10-4 1.5x10-40.27
0.28
0.29
0.30
0.0 4.0x10-5 8.0x10-50.549
0.550
0.0 5.0x10-5 1.0x10-4
1.20
1.21
1.22
Time (s)
V (V
)
Time (s)
I (m
A)
Time (s)
V (V
)
Time (s)I (
mA
)
Time (s)
V (V
)
Time (s)
I (m
A)
V (V
)
Time (s)
I (m
A)
Time (s)
Supplementary Figure 19. Transient photovoltage (left) and photocurrent (right) decay traces
of 80°C-annealed PbS QD solar cells processed with EMII ligand under variant white light
illumination intensity conditions as indicated by different data colour.
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-20 -10 0 10 20
0
5
10
15
20
-20 -10 0 10 20
0
10
20
Gate voltage (V)
processed with EDTD-S
cur
rent
(nA
)
Vds= 20V
processed with TBAID-S
cur
rent
(nA
)
Gate voltage (V)
Vds= 20V
Supplementary Figure 20. Representative traces of the FET characteristics of non-annealed
PbS QD FETs processed with EDT and TBAI ligands. Double traces represent forward and
reverse scans showing low hysteresis.
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Supplementary Tables
Supplementary Table 1. Impact of thermal annealing on photovoltaic parameters of TBAI-
processed QD solar cells under simulated AM1.5G 100 mW cm–2 illumination conditions.
anneal Voc (V) Jsc (mA cm–2) FF PCE (%)
non 0.46 17.37 0.44 3.5
60 ºC 0.49 21.08 0.52 5.4
80 ºC 0.51 21.88 0.57 6.4
100 ºC 0.47 21.28 0.57 5.7
120 ºC 0.40 23.10 0.50 4.6
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Supplementary Table 2. Atomic ratio of non- and 80 ºC-annealed PbS QD layer processed
with EDT.
anneal Pb S O C non 1.00 1.16 0.34 1.60
80 ºC 1.00 1.18 0.21 1.18
Supplementary Table 3. Fitting parameters and quantitative analysis of S2p spectra of non-
and 80°C-annealed PbS QD layer processed with EDT in Fig. 2a and 2d.
anneal component peak (eV) FWHM (eV) area (%) component ratioa
non PbS 160.6 0.9 52 0.60
161.8 0.9
bound thiolate 161.6 0.9 37 0.43
161.8 0.9
unbound thiol 163.6 0.9 11 0.13
164.8 0.9
80 ºC PbS 160.5 0.9 55 0.65
161.7 0.9
bound thiolate 161.5 0.9 45 0.53
162.7 0.9
aThe component ratio is calculated from the fitting area percentage of the corresponding
component multiplied by the atomic ratio of S to Pb in Supplementary Table 2. The amount
of the ratio of bound thiolate to Pb in the 80 ºC-annealed sample, being 0.53, is close to the
total amount of the ratio of bound thiolate and unbound thiol to Pb in the non-annealed
counterpart (0.43+0.13=0.56).
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Supplementary Table 4. Fitting parameters and quantitative analysis of O1s spectra of non-
and 80 ºC-annealed PbS QD layer processed with EDT in Fig. 2b and 2e.
anneal component peak (eV) FWHM (eV) area (%) component ratioa
non Pb-O 529.3 0.9 14 0.05
Pb-OH 531.0 1.1 70 0.24
COO, CO2 532.2 1.1 16 0.05
80 ºC Pb-O 529.3 0.9 5 0.01
Pb-OH 531.0 1.1 20 0.04
COO, CO2 532.2 1.1 33 0.07
OH 533.7 1.1 42 0.09
aThe component ratio is calculated from the fitting area percentage of the corresponding
component multiplied by the atomic ratio of O to Pb in Supplementary Table 2. Upon 80 ºC-
annealing, the amount of decreased ratio of Pb-OH (0.24-0.04=0.2), is comparable to that of
increased ratio of bound thiolate (0.53-0.43=0.1) in Supplementary Table 3, indicating that at
least half of the native hydroxide ligands being removed from the surface upon mild
annealing are replaced by originally unbound thiol moieties yielding OH containing by-
products, which show a new peak at 533.7 eV. That hypothesis is supported by the fact that
the component ratio from OH (0.09) is almost equal to the increase (by 0.1) of the component
ratio of the bound thiolate.
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Supplementary Table 5. Atomic ratio of non- and 80 ºC-annealed QD layer processed with
TBAI.
anneal Pb S O I C non 1.00 0.60 0.20 0.54 1.27
80 ºC 1.00 0.59 0.18 0.60 1.16
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Supplementary Table 6. Fitting parameters and quantitative analysis of O1s spectra of non-
and 80 ºC-annealed PbS QD layer processed with TBAI in Fig. 2c and 2f.
anneal component peak (eV)a FWHM (eV) area (%) component ratiob
non Pb-O 529.7 0.9 9 0.02
Pb-OH 531.4 1.1 67 0.14
COO, CO2 532.6 1.1 17 0.03
OH 533.8 1.1 6.5 0.01
80 ºC Pb-O 529.7 0.9 10 0.02
Pb-OH 531.4 1.1 47 0.08
COO, CO2 532.6 1.1 26 0.05
OH 533.8 1.1 17 0.03
aEven when correcting the energy axis of the XPS spectra by positioning the centre of the C1s
peak at 284.8eV, all other XPS peaks (O1s, S2p, Pb4f) of the TBAI-treated films appear to be
shifted by the same energy of 0.4 eV compared to the respective ones of the EDT samples
(e.g. compared to the respective O1s peak positions from Pb-OH shown in Supplementary
Table 4). We attribute this effect to the different surface dipole of the QD surface caused by
the different moieties (iodide and thiol ones).
bThe component ratio is calculated from the fitting area percentage of the corresponding
component multiplied by the atomic ratio of O to Pb in Supplementary Table 5. Although the
surface of TBAI-processed QD are passivated by iodide ligands as indicated by the ratio of I
to Pb being 0.54 in Supplementary Table 5, the native hydroxide ligands, of which the ratio is
0.14, could have profound effects on efficiency and stability of QD solar cells. After 80 ºC-
annealing, such ligands can be replaced by iodide ions from residual TBAI molecules found
in the QD layer. The amount of the ratio of Pb-OH to Pb reduces by 0.06 (0.14-0.08=0.06),
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which matches closely to that of increased ratio of iodide ions bound to Pb atoms in the
following Supplementary Figure 4.
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Supplementary Table 7. Light intensity-dependent photovoltaic parameters of 80 ºC-annealed
PbS QD solar cells with Au anode processed by EMII and TBAI.
processed with
irradiation
(mW cm−2)
Voc
(V)
Jsc
(mA cm−2)
FF PCE
(%)
EMII 100.00 0.56 24.27 0.66 9.0
86.89 0.56 21.21 0.65 8.9
53.08 0.54 12.71 0.66 8.5
10.59 0.46 2.52 0.68 7.4
1.98 0.40 0.47 0.65 6.2
TBAI 100.00 0.52 22.62 0.61 7.2
86.89 0.52 19.80 0.61 7.2
53.08 0.50 11.82 0.60 6.7
10.59 0.44 2.36 0.57 5.6
1.98 0.34 0.42 0.45 3.2
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Supplementary References
S1. Yang, C.-H., Yau, S.-L., Fan, L.-J. & Yang, Y.-W. Deposition of lead iodide films on
Rh(100) electrodes from colloidal solutions-the effect of an iodine adlayer. Surf. Sci. 540,
274–284 (2003).
S2. Schlaf, R., Parkinson, B. A., Lee, P. A., Nebesny, K. W. & Armstrong, N. R.
HOMO/LUMO alignment at PTCDA/ZnPc and PTCDA/ClInPc heterointerfaces determined
by combined UPS and XPS measurements. J. Phys. Chem. B 103, 2984–2992 (1999).