supplementary materials forthis includes: materials and methods figs. s1 to s16 table s1 references...
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science.sciencemag.org/cgi/content/full/science.aba3433/DC1
Supplementary Materials for
Efficient, stable silicon tandem cells enabled by anion-engineered wide-
bandgap perovskites Daehan Kim, Hee Joon Jung, Ik Jae Park, Bryon W. Larson, Sean P. Dunfield,
Chuanxiao Xiao, Jekyung Kim, Jinhui Tong, Passarut Boonmongkolras, Su Geun Ji, Fei
Zhang, Seong Ryul Pae, Minkyu Kim, Seok Beom Kang, Vinayak Dravid, Joseph J.
Berry, Jin Young Kim*, Kai Zhu*, Dong Hoe Kim*, Byungha Shin*
*Corresponding author. Email: [email protected] (J.Y.K.); [email protected] (K.Z.);
[email protected] (D.H.K.); [email protected] (B.S.)
Published 26 March 2020 on Science First Release
DOI: 10.1126/science.aba3433
This PDF file includes:
Materials and Methods
Figs. S1 to S16
Table S1
References
2
Materials and Methods
Materials
Formamidinium iodide (FAI), methylammonium bromide (MABr),
phenethylammonium iodide (PEAI), phenethylammonium thiocyanate (PEASCN) were
purchased from Greatcell Solar. Lead iodide (PbI2), lead bromide (PbBr2) were purchased
from TCI chemicals.
Cesium iodide (CsI), lead thiocyanate (Pb(SCN)2), poly(triaryl amine) (PTAA),
dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), toluene were purchased from
Sigma-Aldrich.
Fabrication of wide-bandgap perovskite solar cells
PTAA solution (5 mg/ml in toluene) was spin-coated on an ITO substrate at 6000 rpm
for 25 s, followed by annealing at 100C for 10 min. Wide-bandgap perovskite solutions
were prepared by dissolving FAI, MABr, CsI, PbI2, PbBr2, molar ratios of which were
adjusted to form stoichiometric (FA0.65MA0.20Cs0.15)Pb(I0.8Br0.2)3, in DMF and NMP
mixed solvent system (DMF:NMP = 4:1 volume ratio). To synthesize 2D additive
perovskite solution, 2 mol% Pb(SCN)2 and 2 mol% PEAX (X=I, SCN) precursor
chemicals were added to the 3D perovskite solution. The solution was spin-coated on the
PTAA film at 4000 rpm for 20 s. Subsequently, the spin-coated film, which was not fully
crystallized, was immersed in a bath consisting of diethyl ether (DE) for 30 s. After the
DE bathing, the color of the film changed to dark brown color indicating the formation of
crystalline perovskite. The film was then annealed at 100C for 10 min. Subsequent
layers (C60, bathocuproine (BCP), Ag electrode) following the perovskite absorber were
deposited using a thermal evaporator.
Fabrication of silicon cells
A floating-zone, double-polished, n-type phosphor-doped (3.0 Ω cm) Si wafers with
300 μm thickness were used for Si solar cells. The substrates were cleaned using the
RCA cleaning process before the deposition of hydrogen-terminated amorphous Si (a-
Si:H) thin films and the substrates were dipped in a HCl:H2O2 and H2SO4:H2O2 solution
to remove contaminants. A native oxide layer was removed by rinsing with deionized
(DI) water and dipping in the buffered oxide etching solution. Amorphous Si thin films
were deposited by a parallel-plate direct plasma-enhanced chemical vapor deposition
reactor operating at a radio frequency (RF13.56 MHz) power. Hydrogen-diluted PH3 and
B2H6 gases were used to dope the a-Si:H films. ITO films with a thickness of 20 nm were
deposited as the recombination layer using sputtering. 80-nm-thick ITO films were
deposited on the rear side of the Si cell, and a 300-nm-thick Ag electrode was deposited
using a thermal evaporator.
Fabrication of monolithic perovskite/Si tandem cells
On top of the silicon bottom cell with an ITO recombination layer,
PTAA/perovskite/C60 layers were sequentially deposited. PTAA/perovskite/C60 layers
were formed by the same procedure used to prepare a single-junction perovskite cell. A
0.2 wt% of polyethylenimine (PEIE, Sigma-Aldrich, 80% ethoxylated) solution in methyl
alcohol was spin-coated at 6000 rpm 30 s. ITO films were deposited on the C60/PEIE
layer using radiofrequency sputtering at room temperature (working pressure: 2x10-3
3
mTorr). A 150 nm-thick Ag metal grid was deposited using a thermal evaporator on the
ITO film.
Material characterizations
Structural analysis of perovskite films was done by an X-ray diffractometer (XRD, D-
Max 2200, Rigaku). The measurements were conducted at theta/2theta mode with an
anode operating at 40 kV and 200 mA. Morphology and microstructure of perovskite
films and cross-section of solar cells were characterized by a field-emission scanning
electron microscopy (FESEM, Nova 630 NanoSEM, FEI). Optical absorption
measurements were characterized using a UV-Vis spectrophotometer (Cary-6000i,
Agilent).
Time-resolved microwave conductivity (TRMC) measurements
For TRMC measurements, perovskite thin films were prepared in an identical fashion as
device films onto pre-cleaned quartz substrates (1 cm x 2.5 cm x 0.1 cm). Our microwave
conductivity cavity and instrumentation has been described many times elsewhere in the
literature, including recently with detailed discussions on the quantification of TRMC
data.(26) In short, the sample is placed at the electric field maximum of an X-band waveguide
cavity probed at ca. 9 GHz, and pumped optically through a microwave reflective optical
window. The excitation source used was a 5 ns pulse width 640 nm beam operating at 10 Hz
(Continuum Panther OPO pumped by a Continuum Nd:YAG 355 nm beam). Transient
absorption of microwaves in the cavity upon and following optical excitation was monitored
over 500 ns as a change in ΔP which relates to photoconductivity (∆G) through ΔP/P =
−KΔG where K is an empirically determined calibration factor for the microwave cavity used
in this experiment. The photoconductivity is proportional to the quantum yield of photo-
generated charges and their mobility. It can be expressed as ΔG = eβFAI0(ϕ∑μ) where e is
the elementary charge, β = 2.2 is the geometric factor for the X-band waveguide used, I0 is the
incident photon flux (measured for each sample), FA the fraction of light absorbed at the
excitation wavelength, ϕ is the quantum efficiency of free carrier generation per photon
absorbed and ∑μ = µe + µh the sum of the mobilities of electrons and holes. Bi-exponential
fits of the photoconductivity decay transients were weighted to calculate the average carrier
lifetime using the equation: τavg = (A0τ0 + A1τ1)/(A0 + A1). For high-quality perovskite thin
films, a charge-carrier yield of = 1 can be assumed, meaning the combined charge
carrier mobility at t = 0 can be extracted from the sum of the pre-exponential factors
(∑A) of the fits.
Conductive atomic force microscopy (C-AFM)
Conductive atomic force microscopy (C-AFM) results were acquired by a D5000
Bruker AFM system in an Ar-filled glovebox with H2O and O2 concentrations of 0.1
ppm. Current mapping was acquired in contact mode using a nanosensor PPP-EFM tip
(Pt-Ir-coated). Samples were electrically connected to the AFM stage biased at 1 V, and
the tip was virtually grounded. The scan area was 2 × 2 μm2 with 1,024 points on the fast
axis and 256 lines on the slow axis with a scan rate of 0.2 Hz. All results were scanned by
the same tip, and at least two locations from each sample were examined to assure
reliable results. The reproducibility of the data was confirmed by remeasuring the same
sample after scanning of other samples. For example, three samples (A, B, C) were
4
measured following the order of A→B→C→A→B→C, and results from the same
sample were very similar.
Device characterization
Current density–voltage (J–V) curves were acquired under a simulated AM 1.5G
illumination (100 mW cm-2, Oriel Sol3A Class AAA Solar Simulator, Newport) using a
Keithley 2400 source meter in an N2-filled glove box. The AM 1.5G illumination was
calibrated using a standard Si cell (Oriel, VLSI standards) with a KG2 filter. Stabilized
power output (SPO) of perovskite solar cells was also measured using the same
instrument. All J-V characteristics were measured with a metal aperture (0.059 and 0.188
cm2). The scan rate of J-V is 100 ~ 200 mV/s for single-junction cells and 2T
perovskite/Si tandem cells. The structure that consists of
glass/PTAA/perovskite/C60/PEIE/ITO was used to measure the J-V of a filtered Si bottom
cell. External quantum efficiency (EQE) spectra of devices were measured using a
quantum-efficiency measurement system (PV measurements).
Long-term stability test under illumination
Solar cells without any encapsulation were loaded into a home-built degradation
testing setup, dubbed the Stability Parameter Analyzer (SPA). The setup consists of a
flow chamber to control the environment of the cells, cooling tubes to keep the housing at
room temperature, electrical housing, and electronics that switch between devices,
measure J-V curves, and hold the devices under resistive load, and a light source to
provide constant illumination. In this study, the devices were kept in a nitrogen
environment underneath a sulfur plasma lamp at ~0.8 suns and held under a resistive load
of 510 Ohms (placing the cells near maximum power point). Every 30 minutes, the
system removes the resistive load and takes a J-V scan using a Keithley 2450 source-
measure unit. J-V curves are then analyzed to extract relevant parameters.
TEM imaging and sample preparation
HAADF and ABF images were recorded at 200 kV using a probe Cs-corrected JEM
ARM200CF (JEOL Ltd.) under spherical aberration (C3) of 0.5~1 μm resulting in the
measured phase of 27~28 mrad. The convergence semi-angles for imaging is 21 mrad
using a 30-μm condenser aperture, and the collection semi-angle for HAADF & ABF are
90~370 & 10~23 mrad, respectively. Micrographs were acquired at electron probe sizes
of 8C & 9C (JEOL defined), which are measured to be 1.28 Å & 1.2 Å, respectively, and
a pixel dwell time of 10~15 μs with 2048 x 2048 or 1024 x 1024 pixel area. Emission
current of 7 μA results in a probe current range of 2.7-5 pA with a 30-μm condenser
aperture. The 30-μm aperture results in a beam convergence semi-angle of α=21 mrad.
The electron dose introduced per image varied in around 50~100 e/Å2 depending on the
magnification and dwell time. ABF images are obtained with a BF aperture of 3 mm with
a center beam stop. EDS map was performed using two SDD detectors (Thermo Fisher
Scientific) with a 5C probe size and 40-μm condenser aperture. Cross-section TEM
samples were prepared using FEI Helios FIB operating at 30 kV Ga focused ion beam,
and the sample surface is fine-cleaned using the low acceleration of 2 kV beam. It is
worth noting that reducing total milling time in each step as short as possible is
recommended to have a better TEM sample for atomic-scale STEM imaging. Pure 2D
5
PbI2 sample in Fig. S9 was investigated to compare with the 2D phase at the current
paper, revealing interlayer dopants.
Multislice STEM image simulation
Dr. Probe software is utilized to generate STEM simulation for HAADF & ABF using
multi-slice simulation (32). Pixel size is over-sampled to have 10 pm/pixel. Semi-
collection angles of HAADF and ABF were set to be 90-200 and 10-23 mrad,
respectively, and acceleration voltage and convergence semi-angle were 200 keV and 24
mrad, respectively. RABF was generated by inverting the contrast of ABF using
Photoshop. Supercells of 5~8 nm thickness for PbI2 and PEA2PbI4 were generated using
crystal info file (CIF), and the detail of each simulation is summarized in Fig. S10F. To
satisfy weak phase object approximation (WPOA), each slice thickness is set below 1~2
Å to have a periodic atomic configuration so that it has small atomic potential vertically
in each slice along the wave direction.
Fig. S1.
The device structure of wide-bandgap perovskite solar cells. (A) Cross-sectional SEM
image of a full device. (B) Device configuration.
6
Fig. S2
(A) Statistics of PV parameters with various concentrations of PEA(I0.25SCN0.75)
additives (x = 1, 1.5, 2, 3) in 3D + 2 mol% Pb(SCN)2 + x mol% PEA(I0.25SCN0.75). (B)
Statistics of PV parameters with various concentrations of Pb(SCN)2 additive (x = 1, 1.5,
2, 3) in 3D + x mol% Pb(SCN)2 + 2 mol% PEA(I0.25SCN0.75).
7
Fig. S3
Optical properties of perovskite films. (A) Absorbance measured by UV-visible
spectroscopy. (B) Tauc plots.
8
Fig. S4
EQE spectrum and hysteresis behavior of the champion wide-bandgap perovskite solar
cell. (A) EQE spectrum. (B) J-V curves with different scan directions.
9
Fig. S5
J-V curves of wide-bandgap perovskite solar cells during long-term light stability test.
10
Fig. S6
Comparison of perovskite films with no additive and with Pb(SCN)2 additive.
(A and B) Plan-view SEM images. (C) XRD. (D) Light J-V curves. (E and F) Mobility
product values acquired from TRMC measurements.
11
Fig. S7
Comparison of FWHM values of (110) XRD peaks from perovskite films formed with
different 2D additives.
12
Fig. S8
(A) The cross-sectional TEM image of perovskite film with PEA(I0.25SCN0.75) and (B)
Selected area diffraction patterns taken from the circled areas in (A). The table in (B)
shows the measured interplanar d-spacings of 3D halide perovskite and 2D phase
compared against PbI2 reference. Interplanar spacings along the out-of-plane and in-plane
direction of the 2D phase are larger by 1.7% and 1% compared to pure PbI2.
13
Fig. S9
(A) HAADF, ABF and RABF of pure PbI2 on [1-10] zone axis. (B and C) Comparison
between pure PbI2 (left) and our 2D phase (right) on [1-10] zone axis. Please note that a
plane consisting of Pb is sandwiched by planes with only Iodine in the pure PbI2 while
the Pb plane in our 2D phase is sandwiched by layers of mixed I and Br. Some contrast
between PbI2 layers (indicated by red arrows) is apparent while no such contrast is
observed in the pure PbI2, which indicates the presence of interlayer dopants in our 2D
phase. Candidates of the interlayer dopants are SCN and Cs.
14
Fig. S10
(A-D) Simulated supercell, HAADF, ABF, RAFB of PbI2 on Z=[1-10] and PEA2PbI4 on
Z=[110] and [310]. (E) Structural information of PbI2 and PEA2PbI4. (F) Simulation
conditions used for multi-slice image simulation of Dr. Probe software. Note that
interlayer spacings of PbI2 and PEA2PbI4 are different significantly. While the contrast of
organic molecules located at the interlayers of PEA2PbI4 is weak in HAADF, it is more
pronounced in the RABF due to the phase contrast of the periodic configuration of
organic molecules. This suggests a possibility that interlayer contrast observed in RABF
of our 2D phase (Fig. 2J) would be from Cs atoms or organic molecules such as SCN-.
15
Fig. S11
EDS mapping of perovskite film with PEA(I0.25SCN0.75) displaying elemental
distributions of Pb, I, Br, Cs, S, and N. Note that the 2D phase on the surface is Pb, I, Cs,
N-rich but Br-poor compared to the grain of 3D perovskite underneath. Due to spectral
overlap between S and Pb, the distribution of S is affected by Pb.
16
Fig. S12
TEM images showing locations of 2D phases. (A) Perovskite film with Pb(SCN)2 only:
2D layers are only observed on the surface of the perovskite film. (B) Perovskite film
with Pb(SCN)2 and PEA(I0.25SCN0.75): 2D layers are observed both on the surface and at
grain boundaries.
17
Fig. S13
Series of X-ray diffraction patterns from perovskite films with differing ratio (mol%) of
the 2D additive to 3D perovskite precursors. (A) Pb(SCN)2 + PEAI. (B) Pb(SCN)2 +
PEA(I0.25SCN0.75). (C) Pb(SCN)2 + PEASCN.
18
Fig. S14
XRD pattern (normalized by low-dimensional perovskite peak, (PEA)2An-1PbnI3n+1 (n = 2
or 3)) of perovskite films formed with different 2D additives: PEAI, PEA(I0.25SCN0.75),
and PEASCN. The molar concentration of the 2D additives compared to 3D perovskite
precursors is 40 mol%.
19
Fig. S15
The light J-V curve of Si bottom cell with light filtered by wide-bandgap perovskite top
cell.
20
Fig. S16
Certified photovoltaic performance of the perovskite/Si tandem solar cell.
21
Table S1.
Summary of photovoltaic parameters of perovskite/Si 2T tandem solar cells reported in
the literature.
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