Post on 09-Feb-2019
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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