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: jykim.mse@snu.ac.kr (J.Y.K.); kai.zhu@nrel.gov (K.Z.);

donghoe.k@sejong.ac.kr (D.H.K.); byungha@kaist.ac.kr (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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