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Supporting Information for

Fabrication of Planar Heterojunction Perovskite Solar Cells by

Controlled Low-Pressure Vapor Annealing

Yanbo Li,†, ‡ Jason K. Cooper, †, ‡ Raffaella Buonsanti, †, ‡ Cinzia Giannini,§ Yi Liu,ǁ Francesca

M. Toma,*, †,⊥ and Ian D. Sharp*,†,¶

†Joint Center for Artificial Photosynthesis, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California, 94720, United States

‡Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California, 94720, United States

§Institute of Crystallography, National Research Council, via Amendola 122/O, Bari 70126, Italy

ǁThe Molecular Foundry, Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States

⊥Chemical Sciences Division Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California, 94720, United States

¶Physical Biosciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California, 94720, United States AUTHOR INFORMATION

*Email: fmtoma@lbl.gov, idsharp@lbl.gov

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Experimental details

Synthesis of CH3NH3I: A 250 mL two-neck round bottom flask was charged with 24 mL of a

33% solution of methylamine (5.99 g, 193 mmol, Sigma-Aldrich) in absolute ethanol, 10 mL of

a 57% solution of hydrogen iodide (9.72 g, 76 mmol, Sigma-Aldrich) in water, and 100 mL of

ethanol under nitrogen atmosphere, and left stirring for 2 h at room temperature. The solvent was

then removed under reduced pressure at 50 °C, and a white precipitate formed. The product was

collected, thoroughly dried, and finally recrystallized from ethanol (Sigma-Aldrich ≥99.5%) and

diethylether (BDH 99.0%). The solid was then dried again at 60 °C for 24 h to yield CH3NH3I

(8.85 g, 56 mmol, 74% yield).

Solar Cell Fabrication and Characterization: Patterned FTO glass substrates (TFD, 7-10

Ω/sq) were sequentially cleaned with a 1% Alconox detergent diluted in deionized water,

deionized water, acetone, and isopropanol (IPA) in an ultrasonic bath and dried with a nitrogen

gun. The compact TiO2 layer, with a thickness of 75 nm, was deposited on the cleaned FTO

substrate by electron beam evaporation (Angstrom NEXDEP 006) at a base pressure of ~8 × 10-6

Torr, a substrate temperature of 350 °C, and a deposition rate of ~0.5 Å/s. A mixture of 0.8 M

PbI2 (Alfa Aesar, 99.9985%) and 0.2 M PbCl2 (Alfa Aesar, 99.999%) was dissolved in N,N-

dimethylformamide (DMF, Sigma-Aldrich, 99.9%) and filtered with a 0.45 µm syringe filter.

Spin coating of the mixed lead halide films was conducted in air at 2000 r.p.m. for 3 min and

dried on a hotplate at 110 °C for 15 min. The sample was then transferred to a test tube charged

with 0.1 g methylammonium iodide (CH3NH3I). The tube was evacuated with a rotary pump

before immersing into a silicone oil bath. The mixed lead halide film was annealed in CH3NH3I

vapor at 120 °C for 2 h under a pressure of ~0.3 Torr to form chlorine-doped methylammonium

lead iodide (CH3NH3PbI3-xClx). After the vapor annealing process, the sample was immediately

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removed from the test tube to avoid condensation of CH3NH3I on the perovskite film during

cooling. The perovskite film was washed with IPA and the hole transport layer (HTL) was

applied immediately after. The precursor solution for the hole transport layer was prepared by

dissolving 80 mg spiro-OMeTAD (Lumtec, 99.5%), 28.5 µL 4-tert-butylpyridine (Sigma-

Aldrich, 96%) and 17.5 µL lithium-bis(trifluoromethanesulfonyl)imide (Li-TFSI, Sigma-Aldrich,

99.95%) solution (520 mg Li-TFSI in 1 mL acetonitrile) in 1 mL chlorobenzene (Sigma-Aldrich,

99.8%). The HTL was deposited by spin coating at 3000 r.p.m. for 30 s in air. A 100-nm-thick

Au layer was deposited on top of the HTL through a metal shadow mask by electron beam

evaporation (Angstrom NEXDEP 006) at a base pressure of ~2 × 10-6 Torr and a deposition rate

of ~2 Å/s. The active area of the solar cell was defined as 0.062 cm2.

Characterization: The SEM images of the samples were acquired using a FEI QUANTA FEG

250. XRD spectra of the samples were measured with a Rigaku SmartLab X-ray diffractometer

using Cu Kα radiation at 40 kV and 40 mA. UV-VIS spectra of the perovskite films were

measured with a Shimadzu SolidSpec-3700 spectrometer. Spectroscopic ellipsometry data were

collected on a M-2000 ellipsometer with extended NIR range by J.A.Wollam Co., Inc (Lincoln,

NE, USA). Data fitting was conducted with CompleteEASE software to extract absorption

coefficient. Photoluminescence measurements were conducted at room temperature with

excitation by a 488 nm CW laser (300 uW fluence). X-ray photoelectron spectroscopy (XPS),

including valence band spectroscopy, was performed using a monochromatized Al Kα source (hν

= 1486.6 eV), operated at 225 W, on a Kratos Axis Ultra DLD system at a takeoff angle of 0º

relative to the surface normal, and pass energy of 20 eV. Transient absorption pump-probe

spectroscopy was performed using a Coherent Libra (Coherent, CA, USA) laser with pulse width

of 100 fs and repetition rate of 1 kHz. The pump beam was generated using a Coherent OPerA

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Solo optical parametric amplifier (OPA), with an output wavelength of 350 nm, pulse energy of

700 nJ, and beam diameter of 0.3 mm at the sample. The transient absorption system was

produced by Ultrafast Systems (Sarasota, FL, USA) which is equipped with a probe laser

produced by Leukos (Leukos Systems, France) with a pulse width < 1 ns and broad band

emission which was detected by a fiber coupled grating spectrometer with Si CMOS detector

array for analysis of the ~315-800 nm spectral range. The differential absorption spectrum was

collected as a function of pump probe delay through accumulated random sampling of

electronically triggered delay times. The emitted photons were collected 90 degrees to the

excitation light, filtered through a 488 nm long-pass filter, separated by and Andor spectrometer

equipped with a 600 lines/mm 500 nm blaze grating, and detected by an Andor iDus 420 CCD

operated at -90°C. Wavelength calibration of the PL spectra was done with a Hg calibration lamp

and the CCD and grating efficiency was corrected for using a NIST traceable quartz tungsten

halogen 45W light source with a known color temperature. J-V characteristics of the solar cells

were measured in air using a solar simulator (Newport, 91192) equipped with a 150-W Xe lamp

and an AM 1.5G filter as light source and a Keithley 2400 source meter. Light intensity was

calibrated with an NREL-calibrated Si solar cell with a KG-5 filter to 1 sun (100 mW/cm2). The

IPCE spectrum was measured in AC mode under chopped (10 Hz) monochromatic light obtained

using a 300 W Xenon lamp and a monochromator. The incident beam was focused within the

active area of the device. The signal was measured with a DSP lock-in amplifier (Stanford

Research System, SR810).

XRD Rietveld Refinement: The XRD patterns were analyzed by using a whole-profile Rietveld-

based fitting program (FULLPROF) [Refinement of powder (Rietveld) and single-crystal

diffraction data; http://www-llb.cea.fr/fullweb.], as it follows: the instrumental resolution

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function (IRF) was evaluated by fitting the XRD pattern of a LaB6 NIST standard [National

Institute of Standards and Technology; http://www.nist.gov/] recorded under the same

experimental conditions of the films; the IRF data file was provided separately to the program in

order to allow subsequent refinement of the XRD patterns of the films. In this second step, the

crystal structure models of the majority phases: tetragonal CH3NH3PbI3 beta-Methylammonium

Lead Tri-iodide (space group I 4 c m; cell parameters: a=b= 8.849 Å and c = 12.642 Å;

α=β=γ=90°) and fluorine-doped tin oxide (space group P 42/m n m; cell parameters: a=b=

4.765758 Å and c=3.203804 Å; α=β=γ=90°) were provided to the software.

Preferred orientations (POs) were also introduced in the structural model, mainly along the

(110) and (002) crystallographic directions, using the March-Dollase function [March (1932). Z.

Kristallogr. 81, 285-297; Dollase (1986). J. Appl. Cryst. 19, 267-272;]:

( ) ( ) ( ) ( )2/3

1

,,2

2,,122,,22,,

sincos11

−

+−+=−+∝

GGGGWGGPOD lkh

lkhlkhlkh

ααα

where G1 and G2 and are refinable parameters describing the habit (G1) of the crystallite/grains,

and the fraction of not-textured sample (G2). The March parameter, 0 < G1 <1, determines the

shape of the function ( )lkhW ,,α and, respectively, the strength of the preferred orientation: at G1 =

1 (random powder), ( )lkhW ,,α = 1 and does not depend on lkh ,,α ; at G1 = 0 (perfect uniaxial

preferred orientation), ( )lkhW ,,α transforms to a delta function, and there is total preferred

orientation. Here, the POs have been described with different G1 refinable parameters.

Other refinable parameters were the unit cell parameters (a, c). The background was linearly

interpolated and unrefined. The quality of the obtained fits was checked by means of a goodness-

of-fit statistical indicator (GoF). GoF values of <8 were found.

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The Rietveld refinement for different samples that are of interest are plotted in Figure S3 and

the refined parameters are listed below:

Sample G1 a (Å) c (Å)

CH3NH3PbI3 annealed at 120 °C 0.6883 8.876 12.670

CH3NH3PbI3-xClx annealed at 120 °C 0.9630 8.873 12.602

CH3NH3PbI3-xClx annealed at 150 °C 0.6550 8.876 12.578

From the refined G1 values, it can be seen that almost no PO were found for the CH3NH3PbI3-

xClx sample annealed at 120 °C, whereas a texture effect was revealed on the CH3NH3PbI3

sample annealed at 120 °C and the CH3NH3PbI3-xClx sample annealed at 150 °C.

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Figure S1. Low-magnification SEM image of the CH3NH3PbI3-xClx perovskite film deposited by

low-pressure vapor annealing. The film is homogeneous and pin-hole free over a large scale.

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Figure S2. SEM images of (a, b) PbI2 film, and (c, d) CH3NH3PbI3 perovskite film. The

morphology of the CH3NH3PbI3 film is similar to that of CH3NH3PbI3-xClx film.

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Figure S3. Rietveld refinement for (a) FTO substrate, (b) CH3NH3PbI3 annealed at 120 ˚C, (c)

CH3NH3PbI3-xClx annealed at 120 ˚C, and (d) CH3NH3PbI3-xClx annealed at 150 ˚C. Inset is a

zoom of the (004) and (220) reflections.

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Figure S4. X-ray diffraction (XRD) patterns of PbICl (black curve), mixed lead halide on FTO

(PbI2/PbCl2, red curve), lead iodide on FTO (PbI2, blue curve).

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Figure S5. Tauc plot for CH3NH3PbI3 and CH3NH3PbI3-xClx films. A direct optical band gap of

~1.6 eV is revealed for both compositions.

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Figure S6. Statistics of the (a) JSC, (b) VOC, (c) FF, and (d) PCE for the CH3NH3PbI3-xClx based

solar cells.

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Figure S7. J-V curves of solar cells based on perovskite films annealed at different temperature.

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Figure S8. Dark J-V curves for the solar cells shown in Figures 2, 3, and S6 plotted in (a) linear

and (b) semi-log scale.

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Figure S9. J-V hysteresis behavior of the CH3NH3PbI3-xClx based solar cell over repeated

measurements.

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Figure S10. Forward and forward scan of dark J-V curves for a typical CH3NH3PbI3-xClx device.

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Figure S11. Reverse (filled square) and forward (empty square) scan J-V curves of a typical

CH3NH3PbI3 device.