www.sciencemag.org/cgi/content/full/science.aai9081/DC1

Supplementary Materials for

Efficient and stable solution-processed planar perovskite solar cells via

contact passivation

Hairen Tan, Ankit Jain, Oleksandr Voznyy, Xinzheng Lan, F. Pelayo García de Arquer,

James Z. Fan, Rafael Quintero-Bermudez, Mingjian Yuan, Bo Zhang, Yicheng Zhao,

Fengjia Fan, Peicheng Li, Li Na Quan, Yongbiao Zhao, Zheng-Hong Lu, Zhenyu Yang,

Sjoerd Hoogland, Edward H. Sargent*

*Corresponding author. Email: ted.sargent@utoronto.ca

Published 2 February 2017 on Science First Release

DOI: 10.1126/science.aai9081

This PDF file includes:

Materials and Methods

Figs. S1 to S18

Tables S1 to S5

References

2

Materials and Methods

Density function theory calculation

The DFT calculations were performed using a Perdew-Burke-Ernzerhof generalized gradient

exchange correlational function [57]. All calculations were performed with mixed Gaussian and

plane-wave basis set, using the molecularly optimized MOLOPT double ζ-valence polarized basis

set as implemented in the quantum chemistry code CP2K [58]. The charge density cutoff was

fixed at 400 Ry along with a five-level multigrid and a relative energy cutoff of 60 Ry [58]. The

structural optimizations were performed using the Broyden-Fletcher-Goldfarb-Shanno algorithm

[59] until forces on all atoms were less than 10-4

Ha/Bohr. The TiO2/MAPbI3 interface was

modeled using the (110)-TiO2/(110)-MAPbI3 surface as discussed by De Angelis et al. [37] for

which interface lattice-mismatch is minimal. A 3×5×3 perovskite slab consisting of 45 MAPbI3

units with (110) exposed face was placed in contact with 5×3×2 slab of anatase TiO2 consisting of

120 TiO2 units with (110) exposed face. A vacuum of 15 Å was employed in the cross-plane

direction (perpendicular to the interface) along with the periodic boundary conditions in the in-

plane directions. The lattice parameters of TiO2 were fixed at the experimentally observed values

and the MAPbI3 slab was allowed to relax. Both MAX and PbX2 (X=I, Cl) terminated perovskite

surfaces were considered. The effect of Cl was taken into account by replacing all I atoms at the

interface by Cl atoms. Only charge-neutral defects are considered in our calculations. MAX

vacancy and PbX2 vacancy, Pb-X antisite defects were considered in the case of MAX and PbX2

terminated surfaces. For vacancy defects, one MAX or PbX2 was randomly removed from the

TiO2/MAPbX3 interface; and for antisite defects, the positions of neighboring Pb and X atoms

were flipped at the interface. The defect formation energies were calculated as: E^F

= E_product –

E_pure, where E^F

is the formation energy of defect, E_product is the energy of TiO2/MAPbX3 system

with a defect (sum of energies of TiO2/MAPbI3 system and one MAX/PbX2 unit, in the case of

MAX/PbX2 vacancy defect) and E_pure is the energy of TiO2/MAPbI3 in the absence of a defect.

3

Synthesis of TiO2 nanocrystals

The TiO2 nanocrystals were synthesized following modified non-hydrolytic sol-gel method with

all procedures in the ambient air. Firstly, 4 mL TiCl4 (99.9%, Sigma-Aldrich) was injected drop

by drop into 16 mL cold anhydrous ethanol (stored in the fridge before use) with strong stirring to

avoid local overheating of ethanol. After the solution cooled down to room temperature, 80 mL of

anhydrous benzyl alcohol was added to the previous solution after cooling and stirred for 10 min.

The original yellow solution became reddish after the addition of benzyl alcohol. The mixed

solution was then transferred into a 200-mL vail, which was firmly sealed and stored without

stirring in an oven at 85 °C for 12 hours. After heating for one hour, the solution became yellow

again. The product TiO2 nanocrystals were then precipitated from the as-obtained solution by the

addition of diethyl ether and isolated by centrifugation at 5000 rpm for 2 min. The solid was

subsequently washed by adding anhydrous ethanol and diethyl ether, followed by a similar

centrifugation step (5000 rpm for 2 min). This washing procedure was repeated for twice. To

obtain the chlorine-capped TiO2 (TiO2-Cl) colloidal solution (~5 mg/mL), the washed TiO2

nanocrystals were dispersed into anhydrous chloroform and anhydrous methanol (1:1 volume

ratio). The solution is transparent and can be stable in air for at least 6 months without

precipitation. To get non-Cl capped TiO2 (TiO2) colloidal solution, the washed TiO2 nanocrystals

were dispersed in anhydrous ethanol (concentration around 6 mg/mL) by the addition of titanium

diisopropoxide bis(acetylacetonate) (15 µL/mL). The obtained solution are transparent and shows

slight yellow color.

Planar perovskite solar cell fabrication

The pre-patterned indium tin oxide (ITO, TFD Devices) coated glass was sequentially cleaned

using detergent, acetone, and isopropanol. The TiO2-Cl and TiO2 electron transport layers (ETLs)

were spin-coated on ITO substrates from the colloidal TiO2 nanocrystal solutions, and annealed

on a hot plate at the displayed temperature of 150 °C for 30 min in ambient air. The thicknesses

4

of TiO2-Cl and TiO2 ETLs are about 60 and 50 nm, respectively. After the substrates had cooled,

we transferred the TiO2-coated substrates immediately to a nitrogen-filled glovebox for the

deposition of perovskite films. The FA0.85MA0.15PbI2.55Br0.45 precursor solution (1.2 M) was

prepared in a mixed solvent of DMF and DMSO with a volume ratio of 4:1. The molar ratios for

PbI2/PbBr2 and FAI/MABr were both fixed at 0.85:0.15, and the molar ratio of

(FAI+MABr)/(PbI2+PbBr2) was fixed at 1:1. The Cs0.05FA0.81MA0.14PbI2.55Br0.45 precursor

solution (1.2 M) was prepared with molar ratios of PbI2/PbBr2 and FAI/MABr both fixed at

0.85:0.15, molar ratio of CsI/(FAI+MABr)=0.05:0.95, and the molar ratio of

(FAI+MABr+CsI)/(PbI2+PbBr2) was fixed at 1:1. The perovskite films were deposited onto the

TiO2 substrates with two-step spin coating procedures. The first step was 2000 rpm for 10 s with

an acceleration of 200 rpm/s. The second step was 4000 rpm for 20 s with a ramp-up of 1000

rpm/s. Chlorobenzene (100 µL) was dropped on the spinning substrate during the second spin-

coating step at 10 s before the end of the procedure. To form a thick but still smooth perovskite

film, chlorobenzene was slowly dropped on the precursor film within ~3 seconds to allow

sufficient extraction of extra DMSO through the entire precursor film. The substrate was then

immediately transferred on a hotplate and heated at 100 °C for 10 min. After cooling down to

room temperature, the hole-transport layer was subsequently deposited on top of the perovskite

film by spin coating at 4000 rpm for 30 s using a chlorobenzene solution which contained 65

mg/mL of Spiro-OMeTAD and 20 µL/mL of tert-butylpyridine, as well as 70 µL/mL of

bis(trifluoromethane)sulfonimide lithium salt (170 mg/mL in acetonitrile). Finally, 100 nm Au

contact was deposited on top of Spiro-OMeTAD by electron-beam evaporation in an Angstrom

Engineering deposition system.

Solar cell characterization

The current density-voltage (J-V) characteristics were measured using a Keithley 2400

sourcemeter under the illumination of the solar simulator (Newport, Class A) at the light intensity

of 100 mW cm−2

as checked with a calibrated reference solar cell (Newport). Unless otherwise

5

stated, the J-V curves were all measured in a nitrogen atmosphere with a scanning rate of 50 mV

s-1

(voltage step of 10 mV and delay time of 200 ms). The steady-state PCE, PCE(t), was

measured by setting the bias voltage to the VMPP and then tracing the current density. The VMPP at

maximum power point was determined from the J-V curve. The active area was determined by

the aperture shade mask (0.049 cm2 for small-area devices and 1.1 cm

2 for large-area devices)

placed in front of the solar cell to avoid overestimation of the photocurrent density. Spectral

mismatch factor of 1 was used for all J-V measurements. EQE measurements were performed

using an in-house built system with monochromatic light and white bias light (~0.1 Sun). The

photodiode used for the calibration of EQE measurements has been calibrated by Newport. The

dark long-term stability assessment of solar cells was carried out by repeating the J-V

characterizations over various times. The devices without encapsulation were stored in a cabinet

with dry air with relative humidity < 30%. The stability test at continuous MPP operation under 1

Sun, AM 1.5G illumination was carried out in nitrogen by fixing the voltage at VMPP and then

tracking the current output. A 420-nm cutoff UV-filter was applied in front of the solar cells

during the MPP tracking tests. The cells were purged with nitrogen flow for 1 hour before MPP

tracking to get rid of residual moisture on the surface. We found that even residual moisture could

cause much faster degradation, especially under MPP operational conditions.

Other characterizations

High-resolution SEM images were obtained using the Hitachi S-5200 microscope with an

accelerating voltage of 1 kV. HRTEM samples were prepared by adding a drop of the solution of

TiO2 nanocrystals onto an ultrathin-carbon film on lacey-carbon support film (Ted Pella 01824)

and subsequently imaged using Hitachi HF3300 operating at 300 kV. XRD patterns were

collected using a Rigaku MiniFlex 600 diffractometer equipped with a NaI scintillation counter

and using monochromatized Copper Kα radiation (λ = 1.5406 Å). XPS analysis was carried out

using the Thermo Scientific K-Alpha XPS system, with a 300 μm spot size, 75 eV pass energy,

and energy steps of 0.05 eV. TiO2 thin films were prepared on ITO substrates and electron flood

6

gun was used for charge compensation to avoid peak shifting. All signals were normalized to Ti

for direct comparison between different samples. Optical absorption measurements were carried

out in a Lambda 950 UV/Vis spectrophotometer. Photoluminescence (PL) was measured using a

Horiba Fluorolog time correlated single-photon-counting system with photomultiplier tube

detectors. The light was illuminated from the perovskite film side. The excitation source is a laser

diode at a wavelength of 540 nm. Transient photocurrent/photovoltage decays were measured on

a home-made system. For the transient photovoltage decay measurements, a 480-nm light

emitting diode was used to modulate the Voc with a constant light bias. The pulse duration is set

to 1 µs and the repetition rate to 500 Hz. For the constant light bias, a continuous light source

from a Xe lamp was coupled through a fiber to collimate on the active area of the solar cell under

study. The intensity of the pulsed laser was set in a way that the modulated Voc was ~10 mV to

ensure a perturbation regime. The open circuit voltage transient, induced by the light perturbation

was measured with a digital oscilloscope set to an input impedance of 1 MΩ. The charge

recombination lifetime was fitted by single exponential decay.

7

Supplementary Figures and Tables

Fig. S1. DFT simulations of TiO2/perovskite interface. (A-B) The PbI2-terminated

(non-chlorinated) interface; (C-D) the PbCl2-terminated (chlorinated) interface. (A) and

(C) Zoom-in on the interface geometry as shown in Fig. 1. (B) and (D) Projected density

of states show the formation of trap states in the case of PbI2-termination and an absence

of trap states in the case of PbCl2-termination.

8

Fig. S2. Synthesis and stabilization of Cl-capped TiO2 (TiO2-Cl) colloidal

nanocrystals. (A) TiO2-Cl nanocrystals were obtained by dispersing the as-synthesized

NCs in the cosolvent system methanol and chloroform. The controls, TiO2 without Cl-

ligands, were dispersed in ethanol with titanium diisopropoxide bis(acetylacetonate)

(TiAcAc) as the stabilizer, where the Cl-ligands were exchanged by acetylacetonate

(AcAc) ligands. (B) HR-TEM images of TiO2-Cl nanocrystals. The nanocrystals are

crystalline and have a diameter of ~5 nm.

9

Fig. S3. Morphology and absorption of TiO2-Cl nanocrystal film on ITO-coated

glass substrate. (A) AFM height image of TiO2-Cl film deposited on ITO substrate

showing a surface roughness of ~4 nm. (B) Absorptance (1-reflection-transmission) of

the TiO2-Cl film (on glass) which exhibits high transparency and an optical bandgap of

3.4 eV.

10

Fig. S4. J-V curves of perovskite solar cells with TiO2-Cl films with various post-

annealing temperatures. The devices showed optimal performance at the annealing

temperature of 150°C.

11

Fig. S5. XPS spectra depicting the Cl 2p peak of the TiO2-Cl film without any

treatment, washed with DMSO solvent, and washed with DMSO solvent after

perovskite film deposition. It clearly shows that the perovskite precursor solvent, e.g.

DMSO, could not wash away the Cl-ligands on TiO2 surface. Once the interfacial Cl

atoms are incorporated into the perovskite crystal at TiO2-Cl/perovskite interface, the

interfacial Cl atoms in the perovskite can be dissolved in DMSO solvent. The weak Cl 2p

signal could be from the underlying Cl-ligands (away from the TiO2/perovskite interface,

no contact with perovskite film) in the bulk of TiO2-Cl NC film.

12

Fig. S6. Top-view SEM images of perovskite films and cross-sectional SEM images

of planar PSCs on TiO2-Cl (A-B) and TiO2 (C-D). The perovskite films exhibit similar

characteristics on both TiO2-Cl and TiO2.

13

Fig. S7. Absorbance and PL spectra of perovskite film formed on glass substrate.

14

Fig. S8. Current-voltage traces and trap density of perovskite films on TiO2 and

TiO2-Cl as determined by the space-charge-limited current (SCLC) method. The

trap density Ntrap is determined by the equation: VTFL=eNtrapL2/(2ɛɛ0), where VTFL is trap-

filled limit voltage, L is the thickness of perovskite film, ɛ is the relative dielectric

constant of perovskite, and ɛ0 is the vacuum permittivity.

15

Fig. S9. Band alignments of TiO2 and TiO2-Cl with the perovskite films grown on

top as determined from the UPS and absorption measurements.

16

1.0

1.1

1.2

18

20

22

24

0.6

0.7

0.8

12

14

16

18

20

TiO2-Cl

Vo

c (

V)

TiO2

Jsc (

mA

cm

-2)

FF

PC

E (

%)

Fig. S10. Comparison of the photovoltaic performance of PSCs with TiO2 and TiO2-

Cl ESLs. Devices were measured at reverse scan with a delay time of 200 ms and voltage

step of 10 mV. Statistics of 40 devices for each ESL are shown. The devices were

fabricated at 8 otherwise-identical runs.

17

Fig. S11. Stabilized maximum power output and the photocurrent density at

maximum power point as a function of time for the best performing PSC as shown

in Figure 4a recorded under simulated one-sun AM1.5G illumination.

18

400 500 600 700 8000

20

40

60

80

100

EQ

E (

%)

Wavelength (nm)

TiO2, Jsc,cal = 21.1 mA cm-2

TiO2-Cl, Jsc,cal = 22.6 mA cm-2

Fig. S12. EQE spectra of PSCs with TiO2 and TiO2-Cl ESLs.

19

1 10 1000.85

0.90

0.95

1.00

1.05

1.10

1.15

1.73 kT/q

Vo

c (

V)

Light intensity (mW cm-2)

TiO2

TiO2-Cl

1.25 kT/q

Fig. S13. Voc vs. light intensity for planar PSCs with TiO2 and TiO2-Cl ESLs.

20

Fig. S14. The certified result of the small-area CsMAFA perovskite solar cell. The

device has an active area of 0.049 cm2 and a PCE of 20.1% (Voc=1.17 V, Jsc=21.7 mA

cm-2

, and FF=79.4%) with negligible hysteresis.

21

Fig. S15. The certified result of the large-area CsMAFA perovskite solar cell. The

device has an active area of 1.1 cm2 and a PCE of 19.5% (Voc=1.195 V, Jsc=21.5 mA

cm-2

, and FF=75.7%) with negligible hysteresis.

22

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

EQ

E (

%)

Wavelength (nm)

small-area PSC

large-area PSC

Fig. S16. Normalized EQE spectra of the certified small-area and large-area CsMAFA

perovskite solar cells as shown in Figs. S14 and S15.

23

Fig. S17. Operation stability of unsealed MAFA perovskite solar cells under

continuous one-sun full light illumination (AM 1.5G; 100 mW cm-2

) without UV-

filter. The cells were operating at maximal power point continuously under nitrogen.

0 20 40 60 80 100 120 140 160 180

6

9

12

15

18

21

24 Continuous MPP tracking

under AM 1.5G full spectrum

without UV-filter

TiO2

TiO2-Cl

PC

E (

%)

Time (min)

MAFA

24

Fig. S18. XRD pattern of CsMAFA device after 500 hour MPP operation measured

from the Au electrode. Neither yellow-phase nor PbI2 phase was observed.

25

Table S1. Formation energies at the TiO2/perovskite interface: interface binding,

vacancy and antisite defects.

Formation

Energy

Chlorinated interface Non-chlorinated interface

MACl-terminated PbCl2-terminated MAI-terminated PbI2-terminated

Interface

(eV/nm2)

-4.1 -6.0 -3.2 -4.5

Vacancy (eV) 1.3 0.9 0.7 1.1

Antisite (eV) - 3.2 - 1.5

26

Table S2. Photovoltaic parameters of perovskite solar cells with TiO2-Cl annealed at

various temperatures.

Temperature

(°C)

Voc

(V)

Jsc

(mA cm-2

)

FF

(%)

PCE

(%)

Rs

(Ω cm-2

)

RT 1.140 21.0 71.6 17.2 6.1

100 1.151 22.1 76.5 19.5 4.9

150 1.144 22.2 78.4 19.9 3.9

250 1.145 22.1 78.4 19.8 4.4

27

Table S3. PL decay lifetimes of perovskite films on bare glass, TiO2 and TiO2-Cl

coated ITO substrates. τ1 and τ2 correspond to the fast and slow decay components,

respectively.

substrate τ1

(ns)

τ2

(ns)

Bare glass 19.9 468

glass /ITO/TiO2 0.9 112

glass /ITO/TiO2-Cl 1.2 99

28

Table S4. Photovoltaic performance of perovskites on various types of TiO2 based

ESLs with respect to reverse and forward scans. TiO2 and TiO2-Cl represent the low-

temperature processed TiO2 nanocrystal films without and with Cl-ligands, respectively.

Compact TiO2 represents the TiO2 ESL processed at 500oC which is commonly used as

the compact layer in mesoporous TiO2 based perovskite solar cells. TiO2/TiO2-Cl

represents one layer of TiO2-Cl on another layer of Cl-free TiO2, where TiO2-Cl contacts

with perovskite film in the solar cell. Recycled TiO2-Cl presents the TiO2-Cl film washed

with DMSO after perovskite film deposition; it equals to untreated TiO2-Cl film without

Cl-ligands on the film surface but still retaining Cl-ligands in the bulk film.

ESL Scan

direction

Voc

(V)

Jsc

(mA/cm2)

FF

(-)

PCE

(%)

TiO2

Reverse 1.129 21.6 0.760 18.5

Forward 1.105 21.5 0.590 14.0

TiO2-Cl

Reverse 1.151 23.0 0.789 20.9

Forward 1.152 23.0 0.788 20.9

500oC compact

TiO2

Reverse 1.062 21.7 0.694 16.4

Forward 1.077 21.5 0.607 14.1

TiO2/TiO2-Cl

Reverse 1.131 22.9 0.803 20.8

Forward 1.130 22.7 0.801 20.6

Recycled TiO2-

Cl

Reverse 1.115 21.7 0.748 18.1

Forward 1.112 21.7 0.511 12.3

29

Table S5. Summary of certified performance of high-efficiency large-area (≥ 1 cm2)

perovskite solar cells. The best-reported, uncertified performance of low-temperature

planar perovskite solar cells is shown for reference as well.

Device

structure

ETL (HTL)

Processing

temperature (oC)

Voc

(V)

Jsc

(mA/cm2)

FF

(%) PCE (%) Ref.

planar 150 1.195 21.51 75.7 19.5 this

work

planar 150 1.057 19.3 71.6 14.5

(uncertified) [25]

planar

(inverted) 500 1.081 21.48 78.4 18.2 [24]

mesoporous 500 1.143 22.59 75.7 19.6 [23]

mesoporous 500 1.104 24.67 72.3 19.7 [48]

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