The role of surface passivation for efficient and …...The Role of Surface Passivation for...

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

http://dx.doi.org/10.1038/nenergy.2016.35

2 NATURE ENERGY | www.nature.com/natureenergy

SUPPLEMENTARY INFORMATION DOI: 10.1038/NENERGY.2016.35

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).



SUPPLEMENTARY INFORMATIONDOI: 10.1038/NENERGY.2016.35

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.




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.




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.




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)


(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)



Inte

nsity

(x10

5 cou

nts)

Binding energy (eV)

processed with TBAI


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




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.




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.




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


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.




Supplementary Figure 12. Certificated PbS QD solar cell with first exciton peak at 930 nm of

colloidal QDs.




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.




Supplementary Figure 14. Certificated PbS QD solar cells with first exciton peak at 850 nm of

colloidal QDs.




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.




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.




-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.




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


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.




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.




-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.




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




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).




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.




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




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),




which matches closely to that of increased ratio of iodide ions bound to Pb atoms in the

following Supplementary Figure 4.




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




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).


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