supporting information for fabrication of planar heterojunction
TRANSCRIPT
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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: [email protected], [email protected]
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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.