www.sciencemag.org/cgi/content/full/science.aav7911/DC1

Supplementary Materials for

Carrier lifetimes of >1 μs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells

Jinhui Tong, Zhaoning Song, Dong Hoe Kim, Xihan Chen, Cong Chen, Axel F. Palmstrom, Paul F. Ndione, Matthew O. Reese, Sean P. Dunfield,

Obadiah G. Reid, Jun Liu, Fei Zhang, Steven P. Harvey, Zhen Li, Steven T. Christensen, Glenn Teeter, Dewei Zhao, Mowafak M. Al-Jassim, Maikel F. A. M. van Hest, Matthew C. Beard, Sean E. Shaheen, Joseph J. Berry,* Yanfa Yan,* Kai Zhu*

*Corresponding author. Email: kai.zhu@nrel.gov (K.Z.); joe.berry@nrel.gov (J.J.B.); yanfa.yan@utoledo.edu (Y.Y.)

Published 18 April 2019 on Science First Release DOI: 10.1126/science.aav7911

This PDF file includes: Materials and Methods Supplementary Text Figs. S1 to S16 Tables S1 and S2 References

1

Materials and Methods

Low-band-gap perovskite film preparation

The (FASnI3)0.6(MAPbI3)0.4 precursor was prepared by mixing formamidinium iodide (FAI)

(Dyesol, 0.6 mmol), SnI2 (Alfa, 0.6 mmol), SnF2 (Sigma-Aldrich, 0.06 mmol), CH3NH3I (MAI)

(Dyesol, 0.4 mmol), PbI2 (Alfa, 0.4 mmol) in 800 µL dimethyl formamide (DMF) (anhydrous,

Sigma-Aldrich) and 200 µL dimethyl sulfoxide (DMSO) (anhydrous, Sigma-Aldrich). For

perovskites modified by guanidinium thiocyanate (GuaSCN), the GuaSCN powder (Sigma-

Aldrich) was directly added to the low-band-gap perovskite powders in different molar ratios with

respect to MAI. Different thicknesses of the perovskite films were realized by changing the

precursor concentration. For example, 1.6 M and 2.0 M precursors were used to prepared low-band-

gap perovskite films with ~580 nm and ~1 µm, respectively. For perovskite precursors with high

concentrations, thermal annealing at 65°C for about 30 min was applied to assist dissolving the

precursors. The perovskite precursors with different molar concentrations were spin-coated onto

the ITO/PEDOT:PSS substrate at 5,000 rpm for 30 s. During the spin-coating, 400 µL of toluene

was dropped onto the spinning substrates. The resulting perovskite films were then annealed at

100°C for 10 min to form (FASnI3)0.6(MAPbI3)0.4 or GuaSCN-modified (FASnI3)0.6(MAPbI3)0.4

perovskite thin films. For time-resolved photoluminescence (TRPL) measurement, the samples

were coated on glass substrates and covered by a thin layer (~30 nm) of poly(methyl methacrylate)

(PMMA).

Wide-band-gap perovskite film preparation

The Cs0.05FA0.8MA0.15PbI2.55Br0.45 precursor solution (1.5 M) was prepared by mixing CsI

(0.05 mmol), FAI (0.8 mmol), MABr (0.15 mmol), PbBr2 (0.15 mmol), PbI2 (0.85 mmol) in

anhydrous DMF: DMSO (533 uL: 133 uL) co-solvent. The perovskite precursor was spin-coated

onto the poly(triaryl amine) (PTAA)-coated ITO substrate at 5,000 rpm for 30 s. During the spin-

coating, diethyl ether (DEE) was dropped onto the spinning substrate. The resulting perovskite

films were then annealed at 100°C for 10 min to form Cs0.05FA0.8MA0.15PbI2.55Br0.45 perovskite

thin films.

Low-band-gap perovskite solar cell

2

The pre-patterned ITO glass (15 Ω sq-1) was sequentially cleaned using acetone and

isopropanol alcohol, followed by ultraviolet–ozone cleaningfor 15 min. PEDOT:PSS were spin-

coated onto the ITO substrate in air at 2,000 rpm for 30 s and then annealed at 150°C for 30 min.

The (FASnI3)0.6(MAPbI3)0.4 or GuaSCN-modified (FASnI3)0.6(MAPbI3)0.4 perovskite thin films

were prepared on the ITO/PEDOT:PSS using the low-band-gap perovskite film fabrication method

described above. After the deposition of the perovskite film, C60 (30 nm)/BCP (6 nm)/Ag (100 nm)

were sequentially deposited by thermal evaporation.

Wide-band-gap perovskite solar cell

The pre-patterned ITO glass (15 Ω sq-1) was sequentially cleaned using acetone and

isopropanol alcohol, followed by ultraviolet–ozone cleaningfor 15 min. The PTAA (Sigma-

Aldrich) was dissolved in toluene with a concentration of 2 mg mL−1 and spin-coated on the

ITO/glass substrates at 5,000 rpm for 30 s. The spun PTAA films were annealed at 100°C for 10

min. The Cs0.05FA0.8MA0.15PbI2.55Br0.45 perovskite films were then deposited on the ITO/PTAA

substrates using the wide-band-gap perovskite film fabrication method described above. For

opaque wide-band-gap perovskite solar cells, C60 (30 nm)/BCP (6 nm)/Ag (100 nm) were

sequentially deposited by thermal evaporation.

For semitransparent devices, 40 nm C60 was first thermally evaporated on the perovskite

film, followed by deposition of 6 nm SnOx and 2-nm of Zn-doped SnOx (ZTO) with atomic layer

deposition (ALD), and finally coated with 250 nm IZO by sputtering. ALD processes for SnOx and

ZTO were adapted for a Beneq TFS200 ALD system from (41). SnOx was grown from

tetrakis(dimethylamino)tin(IV) (TDMASn) and water at 85°C. The TDMASn bubbler was heated

to 55°C, water was unheated, and the chamber and process nitrogen flow rates were 250 and 300

sccm, respectively. TDMASn was introduced to the chamber through a charge-pulse-purge

procedure. In this process, the bubbler was charged with nitrogen for 0.35 s, pulsed for 1 s, and

pulsed for 0.2 s with nitrogen flow through the bubbler. The SnOx deposition cycle was as follows:

TDMASn charge-pulse-purge/purge (6 s)/H2O pulse (0.2 s)/purge (6 s) with an observed growth

rate of 1.4 Å/cycle. ZTO was grown from diethylzinc (DEZ), TDMASn, and water under the same

reactor conditions as SnOx. The DEZ bubbler was unheated. ZTO was deposited through a

supercycle method; a single supercycle consisted of three cycles of zinc oxide (ZnO) and three

cycles of SnOx. The ZnO deposition cycle was: DEZ pulse (0.2 s)/purge (6 s)/H2O pulse (0.2

3

s)/purge (6 s). This process resulted in a ZTO growth rate of 10 Å/supercycle. The SnOx/ZTO

bilayer reported in our semitransparent devices was grown with 50 cycles of SnOx and 2

supercycles of ZTO. Finally, the IZO film was RF (13.56 MHz) sputtered from a 2”×11” indium

zinc oxide target (indium:zinc=80:20) at low power (100 W). The film was deposited at 5 mTorr

in a mixture of argon and oxygen (0.15 at% O2 in Ar). Deposition of the film took 60 min and was

performed at room temperature.

Solar cell characterizations

Devices were tested using a Newport Oriel Sol3A solar simulator with a xenon lamp. The

intensity of the solar simulator was calibrated to 100 mW/cm2 AM 1.5G. The light current density–

voltage (J–V) characteristic were taken from both forward bias to reverse bias and reverse bias to

forward bias, with the step size of 10–30 mV and step delay of 10 ms, unless otherwise stated. The

device area was 0.1 cm2 and was masked with a metal aperture to define an active area of 0.059

cm2. The stable power output (SPO) of the devices was measured by monitoring the photocurrent

current density output with the biased voltage set near the maximum power point. External

quantum efficiency (EQE) measurements were taken using a Newport Oriel IQE200.

For 4-terminal tandem cell measurements, the following procedure is used. The J–V curves

of semitransparent top cells were measured under 100 mW/cm2 AM 1.5G solar irradiation, and

their stable power output was obtained by measuring the photocurrent density output with bias

voltage set near the maximum power point. EQE spectra were performed using the Newport Oriel

IQE200. The device area for both the top and bottom cells was 0.1 cm2 and was masked with a

metal aperture to define an active area of 0.059 cm2. For 4-terminal tandem performance

measurement, the J–V curves, SPO efficiency, and EQE spectrum of low-band-gap bottom cells

were taken by using a semitransparent wide-band-gap top cell as the optical filter.

X-ray diffraction (XRD) and electron microscope characterizations

The crystal structures of prepared perovskite films were characterized using an X-ray

diffractometer (D-Max 2200, Rigaku). 2D-XRD was measured using a D8-Discover (Bruker) with

general area detector diffraction system (GADDS) four-circle detector. The morphologies and

microstructures of the prepared perovskite films and the cross-sectional structures and thickness

of the solar cells were investigated using a field-emission scanning electron microscopy (FESEM,

4

Nova 630 NanoSEM, FEI). High-resolution transmission electron microscopy observations were

performed using a FEI Tecnai ST30 TEM machine at an acceleration voltage of 300 kV. The optical

absorption spectra of perovskite films were measured using an ultraviolet-visible (UV-vis)

spectrophotometer (Cary-6000i, Agilent).

X-ray photoemission spectroscopy (XPS) characterization

XPS measurements were performed on a Physical Electronics 5600 photoelectron

spectrometer, which has been discussed in detail previously (42). Briefly, radiation was produced

by a monochromatic 350-W Al Kα excitation centered at 1,486.7 eV. XPS core-level spectra were

collected using a step size of 0.1 eV and pass energy of 11.75 eV. The electron binding-energy

scale was calibrated using the Fermi edge of a gold substrate, cleaned with argon-ion bombardment.

Peak areas were fit using a Gaussian-Lorentzian peak-fitting algorithm with a Shirley background.

Spectra taken with the Al source are typically assigned an uncertainty of 0.05 eV. Compositional

analyses are typically assigned an uncertainty of 5%.

Photothermal deflection spectroscopy (PDS) characterization

PDS measurements were carried out using a house-built system, equipped with a 100-W

halogen lamp and a three-grating monochromator as excitation light source. The excitation light

was chopped at a frequency of about 7 Hz. The perovskite thin films were immersed in a liquid

media of C6F14 (99%, Sigma-Aldrich), which is a non-polar solvent and compatible with

perovskite materials. A He-Ne laser was aligned to glance through the surface of the sample. The

laser is deflected when the refractive index of the liquid media is changed by the heat generated

from light absorption in perovskite films. The deflection was detected using a silicon diode

position detector and a lock-in amplifier (Stanford SR810). The system response was calibrated

by measuring a carbon-black-coated glass with constant absorption across the wavelength range

of interest.

Transient reflectance (TR) spectroscopy characterization

The detailed description of the TR methods can be found in previously published literature

(37). Briefly, the TR measurements were performed by a pump–probe spectrometer. A Ti:sapphire

amplifier was used to generate 800-nm light at 1-KHz repetition rate. The fundamental pulse was

5

split into two parts. One part was sent to an optical parametric amplifier for the various pump

wavelength generation. The pump is chopped at a frequency of 500 Hz and attenuated by neutral-

density filter wheels. The other part of the fundamental pulse was focused into a sapphire crystal

to generate a near-infrared continuum (800–1,400 nm) that was used as the probe. The probe pulses

was delayed in time with respect to the pump pulses using a motorized translation stage mounted

with a retroreflecting mirror. The pump and probe were spatially overlapped on the surface of the

sample. The incident angle for the pump was around 0° and the probe was around 45°. The

reflected probe pulses were directed to the multichannel complementary metal–oxide–

semiconductor sensor. The size of the focused spot at the sample position for the probe and pump

beams was around 200 µm and 600 µm, respectively. The total pump-photon flux was determined

by measuring the pump power at the sample position. The input photon flux was obtained by

subtracting the reflected photon flux from the total photon flux. The average excitation density

was calculated as the ratio of input photon flux to the pump penetration depth. To prevent sample

degradation in air, the sample was placed in an air-free container with quartz optical window for

all the transient-reflection measurement.

Time-resolved photoluminescence characterization

Films were measured in air with a fiber-fed system previously described (43) with an

excitation wavelength of 905 nm in the low-injection regime, where the excitation intensity is less

than 5×1010 photons/cm2. The photoluminescence decay was measured with a thermoelectrically

cooled photomultiplier tube, with its output processed by a digital oscilloscope in binning mode.

Samples were measured both from the film side and through the glass. Data are representative of

multiple measurements on each sample and between samples.

Time-of-flight secondary-ion mass spectrometry

An ION-TOF TOF-SIMS V Time of Flight SIMS (TOF-SIMS) spectrometer was used for

depth profiling and chemical imaging of the perovskite using methods outlined in detail elsewhere

(34). Analysis was completed using a 3-lens 30-kV BiMn primary-ion gun. 1D profiles were

completed utilizing the Bi3+ primary-ion beam, (0.8-pA pulsed beam current), and a 50×50-µm

area was analyzed with a 128:128 primary beam raster. High resolution imaging was completed

with 100-nm lateral resolution using a Bi3++ primary-ion-beam cluster (100-ns pulse width, 0.1-

6

pA pulsed beam current); a 25×25-µm large area was imaged using a 512:512 primary-beam raster.

Over-sampling like this can improve the image quality when dealing with low signal and materials

with small grain size. Sputtering for surface cleaning and depth profiling was accomplished with

1-kV oxygen and cesium ion sputter beams (~5-nA sputter current for each source) with a raster

of 200×200 µm. After completion of the SIMS measurements, the depth of the SIMS sputter craters

was determined by optical interference light microscopy to convert the SIMS sputter time scale to

a sputter depth scale.

7

Supplementary Text

Determination of diffusion constant and surface recombination velocity.

Transient reflectance spectroscopy measures the surface carrier dynamics (36, 37). The

distribution of charge carriers with different pump-photon energy can be determined from

absorption spectra. According to the Beer-Lambert law, a pump with higher photon energy always

leads to a larger gradient of the initial photocarrier density—and thus, a faster surface decay

dynamic due to a larger diffusion rate. To quantitatively describe the carrier-density evolvement

under various optical excitation conditions, a one-dimensional diffusion equation is employed,

𝜕𝜕𝜕𝜕(𝑥𝑥,𝑡𝑡)𝜕𝜕𝑡𝑡

= 𝐷𝐷 𝜕𝜕2𝜕𝜕(𝑥𝑥,𝑡𝑡)𝜕𝜕𝑥𝑥2

− 𝜕𝜕(𝑥𝑥,𝑡𝑡)𝜏𝜏𝐵𝐵

, (S1)

where N(x, t) is the carrier density as a function of depth (x) and time (t), D is the ambipolar

diffusion coefficient, and 𝜏𝜏𝐵𝐵 is the bulk carrier lifetime. In comparison with the timescale for

carrier diffusion, the pulsed carrier generation is assumed to be instantaneous, and the initial

condition for Equation S1 is then given by

𝑁𝑁(𝑥𝑥, 0) = 𝑁𝑁0 ∙ exp(−𝛼𝛼𝑥𝑥) , (S2)

where N0 is the initial surface carrier density, such that N0 = (1 − R)F0, where F0 is the pump

fluence and R is the reflectance at the pump-photon energy. If the traces are normalized, then N0 is

equal to 1. The values for the absorption coefficient at different pump-photon energies for both

samples are known.

For polycrystalline samples, the film thickness is no longer larger than the optical

penetration depth plus carrier diffusion length. In this case, the boundary conditions are described

as

𝜕𝜕𝜕𝜕(𝑥𝑥,𝑡𝑡)𝜕𝜕𝑡𝑡

|𝑥𝑥=0 = 𝑆𝑆𝐹𝐹𝐷𝐷𝑁𝑁(0, 𝑡𝑡) (S3)

and

8

𝜕𝜕𝜕𝜕(𝑥𝑥,𝑡𝑡)𝜕𝜕𝑡𝑡

|𝑥𝑥=𝐿𝐿 = −𝑆𝑆𝐵𝐵𝐷𝐷𝑁𝑁(𝐿𝐿, 𝑡𝑡) , (S4)

where L is the thickness of the polycrystalline thin film (~1 µm), and SF and SB are the surface

recombination velocities (SRVs) for the front and back surfaces, respectively. In this case, the

diffusion equation cannot be solved analytically; therefore, we solve it numerically. The kinetic

traces in Figs. 3C, D are simultaneously modeled using the numerical solution, N(0, t) in a global

fitting routine. To reduce the number of fitting parameters, we make SF and D vary freely as global

variables, and SB is set as the same as SF during the fitting process. As indicated in Fig. 3, the bulk

carrier recombination in the polycrystalline film is negligible in a 2-ns delay window; thus, the

contribution of bulk recombination in Equation S1 is neglected. By modeling the TR dynamics

with Equations S1 to S4 for different pump-photon energy, the diffusion constant (D) and surface

recombination velocity (S) can be obtained for perovskite thin-film samples.

9

Supplementary Figures

Fig. S1. Cross-sectional SEM image of a low-band-gap perovskite solar cell. Each layer of the device stack is labeled.

10

Fig. S2. GuaSCN concentration dependence of device characteristics. (A) Short-circuit photocurrent density (Jsc), (B) open-circuit voltage (Voc), (C) fill factor (FF), and (D) power conversion efficiency (PCE) of low-band-gap perovskite solar cells prepared with varying concentration of GuaSCN in the range of 0% to 50%.

11

Fig. S3. Comparison of device characteristics based on low-band-gap perovskite thin films (~580 nm) prepared without and with different additives as indicted.

12

Fig. S4. GuaSCN concentration dependence of the morphology of low-band-gap perovskite thin films.

13

Fig. S5. TOF-SIMS 2D images of element distribution: Pb, I, Gua, and SCN. The scale bar is 2 μm. Note the Gua and SCN signals were collected in different measurements as a different SIMS polarity was required to image each species. It is evident that both Pb and I are distributed uniformly across perovskite grains at both concentrations. In contrast, Gua and SCN tend to segregate to GBs at a low GuaSCN concentration. This is consistent with the observation of the 2D structure at GBs. At a high GuaSCN concentration, there are clear and significant amounts of Gua and SCN found in the bulk of grains, in addition to segregation of Gua and SCN at GBs.

14

Fig. S6. Carbon 1s core-level X-ray photoemission spectroscopy spectra and associated fits for low-band-gap perovskite thin films using different GuaSCN concentrations. Guanidinium added to the stock solution in the form of guanidinium thiocyanate (GuaSCN) successfully incorporates itself into the perovskite films. This is evidenced by the growth of a carbon 1s peak at ~288.6 eV that is clearly distinct from the peaks attributable to formamidinium (FA) at ~287.7 eV, methylammonium (MA) at ~285.6 eV, and chemisorbed carbon (C) at ~284.5 eV. As the amount of GuaSCN in the stock solution is increased, so is the signal from the peak attributable to guanidinium, and thus its inclusion.

15

Fig. S7. Pb 4f and Sn 3d core-level XPS spectra and full width half maximum (FWHM) analysis. As the GuaSCN content is increased from 0% to the optimal concentration (7%), the FWHM of the Sn core level decreases. This signifies that Sn is, on average, in a more uniform/homogeneous chemical environment. Further increasing the amount of GuaSCN results in two distinct peaks in the Sn core level, indicating two unique chemical environments for Sn and more heterogeneous in local chemical structure. Note that these effects are not present for Pb core levels in these samples, suggesting the interactions of GuaSCN within the material are complex and differ across Sn and Pb.

16

Fig. S8. Dark carrier density from microwave conductivity measurement. Microwave cavity resonances for a bare quartz substrate (black circles), quartz with ~600-nm layer of (FASnI3)0.6(MAPbI3)0.4 (blue circles), and (FASnI3)0.6(MAPbI3)0.4 with 7% GuaSCN added (red circles). The black solid lines are Lorentzian fits to the data. The change in Lorentzian fit parameters relative to the bare quartz substrate are compared to a lookup table of numerical simulations of the microwave cavity used in these experiments to extract the conductance of each perovskite film, which are shown in the inset. Photoinduced microwave conductivity was used to determine the product of charge-carrier yield and mobility (φΣµ) for the same perovskite films; the peak yield-mobility product averaged over several measurements at different excitation fluences (at 600 nm) is also shown in the inset. The dark carrier density (N) of each sample can be calculated from the sample thickness, dark conductivity, and product of charge-carrier yield and mobility, and the results are also shown in the inset. The details of microwave conductivity data analysis and the measurement technique is given elsewhere (44).

17

Fig. S9. Comparison of 2D XRD patterns of low-band-gap perovskite thin films prepared without and with 7% GuaSCN additive. There is not significant change in the preferred orientation for these samples. The FWHM at about 14° (28°) along the Chi angle are about the same: 10.2° (10.8°) and 10.3° (11.1°) for the samples prepared without and with 7% GuaSCN, respectively.

18

Fig. S10. GuaSCN dependence of optical absorption. (A) UV-vis absorption spectra of low-band-gap perovskite prepared with different amount of GuaSCN additive in the range of 0% to 50%. (B) Tauc plots of the absorption spectra to determine the impact of GuaSCN additive on the band gaps of low-band-gap perovskite thin films.

19

Fig. S11. GuaSCN concentration dependence of XRD patterns of low-band-gap perovskite thin films. Increasing GuaSCN to 20% and 50% leads to an obvious shift of the reflection peaks to lower angles corresponding to (110) and (220) lattice planes, which denotes the expansion of the unit-cell volume.

20

Fig. S12. SEM image comparison of film morphology of low-band-gap perovskite the films prepared (A) without and (B) with annealing at 100°C for 10 min. 50% GuaSCN additive was used. It shows that the as-prepared perovskite film without annealing displays a relatively small grain morphology. Pinholes between these apparent grains can be observed. When the film is annealed at 100°C for 10 min, the morphology changes and displays a significantly larger grain structure, accompanied by more and larger pinholes observed. Although we cannot rule out the loss of GuaSCN during annealing, the SEM comparison suggests that more pinholes are formed when small grains are merged/grown into large grain during annealing.

21

Fig. S13. Statistical distribution of PV parameters of PSCs based on 1-µm low-band-gap perovskite films prepared with 7% GuaSCN additive.

22

Fig. S14. J–V curves and PV parameters of a 0.12-cm2 PSC based on 1-µm low-band-gap perovskite film prepared with 7% GuaSCN additive.

23

Fig. S15. Schematic of the device stack of a semitransparent perovskite solar cell.

24

Fig. S16. Transmittance spectrum of the semitransparent 1.63-eV perovskite solar cell.

25

Supplementary Tables Table S1. PV parameters of low-band-gap perovskite solar cells prepared with and without 7% GuaSCN additive.

Device Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

SPO (%)

Control (0% GuaSCN)

Forward 27.8 0.76 74.0 15.6 15.3

Reverse 27.7 0.76 74.3 15.6

7% GuaSCN Forward 28.7 0.82 79.0 18.6 18.4

Reverse 28.5 0.82 80.0 18.7 Table S2. Fitting results of TRPL transients for samples shown in Figs. 3A,B.

Additives a1 (f1)*

τ1 (ns)

a2 (f2)

τ2 (ns)

τavg (ns)

Control (0% GuaSCN)

0.111 (0.001) 15 0.955

(0.999) 139 139

7% GuaSCN 0.093 (0.032) 391 0.872

(0.968) 1260 1232

* ai is the prefactor of exponential decay function in φΣμ(t) = Σi ai exp(−t/τi), and fi is the fractional contribution of each time constant (τi)

References

1. A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-

light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 131, 6050–6051 (2009).

doi:10.1021/ja809598r Medline

2. N.-G. Park, M. Grätzel, T. Miyasaka, K. Zhu, K. Emery, Towards stable and commercially

available perovskite solar cells. Nat. Energy 1, 16152 (2016).

doi:10.1038/nenergy.2016.152

3. W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K.

Kim, J. H. Noh, S. I. Seok, Iodide management in formamidinium-lead-halide-based

perovskite layers for efficient solar cells. Science 356, 1376–1379 (2017).

doi:10.1126/science.aan2301 Medline

4. National Renewable Energy Laboratory, Best Research-Cell Efficiencies (2019); www.nrel.gov/pv/assets/pdfs/pv-efficiency-chart.20190103.pdf.

5. M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Efficient hybrid solar

cells based on meso-superstructured organometal halide perovskites. Science 338, 643–

647 (2012). doi:10.1126/science.1228604 Medline

6. M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu, S. M. Zakeeruddin, J.-P.

Correa-Baena, W. R. Tress, A. Abate, A. Hagfeldt, M. Grätzel, Incorporation of rubidium

cations into perovskite solar cells improves photovoltaic performance. Science 354, 206–

209 (2016). doi:10.1126/science.aah5557 Medline

7. X. Zheng, B. Chen, J. Dai, Y. Fang, Y. Bai, Y. Lin, H. Wei, X. C. Zeng, J. Huang, Defect

passivation in hybrid perovskite solar cells using quaternary ammonium halide anions

and cations. Nat. Energy 2, 17102 (2017). doi:10.1038/nenergy.2017.102

8. H. Tan, A. Jain, O. Voznyy, X. Lan, F. P. García de Arquer, J. Z. Fan, R. Quintero-Bermudez,

M. Yuan, B. Zhang, Y. Zhao, F. Fan, P. Li, L. N. Quan, Y. Zhao, Z.-H. Lu, Z. Yang, S.

Hoogland, E. H. Sargent, Efficient and stable solution-processed planar perovskite solar

cells via contact passivation. Science 355, 722–726 (2017). doi:10.1126/science.aai9081

Medline

9. D. P. McMeekin, G. Sadoughi, W. Rehman, G. E. Eperon, M. Saliba, M. T. Hörantner, A.

Haghighirad, N. Sakai, L. Korte, B. Rech, M. B. Johnston, L. M. Herz, H. J. Snaith, A

mixed-cation lead mixed-halide perovskite absorber for tandem solar cells. Science 351,

151–155 (2016). doi:10.1126/science.aad5845 Medline

10. D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A.

Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H.

Sargent, O. M. Bakr, Low trap-state density and long carrier diffusion in organolead

trihalide perovskite single crystals. Science 347, 519–522 (2015).

doi:10.1126/science.aaa2725 Medline

11. Y. Lin, B. Chen, F. Zhao, X. Zheng, Y. Deng, Y. Shao, Y. Fang, Y. Bai, C. Wang, J. Huang,

Matching Charge Extraction Contact for Wide-Bandgap Perovskite Solar Cells. Adv.

Mater. 29, 1700607 (2017). doi:10.1002/adma.201700607 Medline

12. J. Ávila, C. Momblona, P. P. Boix, M. Sessolo, H. J. Bolink, Vapor-Deposited Perovskites:

The Route to High-Performance Solar Cell Production? Joule 1, 431–442 (2017).

doi:10.1016/j.joule.2017.07.014

13. G. E. Eperon, T. Leijtens, K. A. Bush, R. Prasanna, T. Green, J. T.-W. Wang, D. P.

McMeekin, G. Volonakis, R. L. Milot, R. May, A. Palmstrom, D. J. Slotcavage, R. A.

Belisle, J. B. Patel, E. S. Parrott, R. J. Sutton, W. Ma, F. Moghadam, B. Conings, A.

Babayigit, H.-G. Boyen, S. Bent, F. Giustino, L. M. Herz, M. B. Johnston, M. D.

McGehee, H. J. Snaith, Perovskite-perovskite tandem photovoltaics with optimized band

gaps. Science 354, 861–865 (2016). doi:10.1126/science.aaf9717 Medline

14. Z. Yang, A. Rajagopal, A. K.-Y. Jen, Ideal Bandgap Organic-Inorganic Hybrid Perovskite

Solar Cells. Adv. Mater. 29, 1704418 (2017). doi:10.1002/adma.201704418 Medline

15. D. Zhao, Y. Yu, C. Wang, W. Liao, N. Shrestha, C. R. Grice, A. J. Cimaroli, L. Guan, R. J.

Ellingson, K. Zhu, X. Zhao, R.-G. Xiong, Y. Yan, Low-bandgap mixed tin–lead iodide

perovskite absorbers with long carrier lifetimes for all-perovskite tandem solar cells. Nat.

Energy 2, 17018 (2017). doi:10.1038/nenergy.2017.18

16. G. Kapil, T. S. Ripolles, K. Hamada, Y. Ogomi, T. Bessho, T. Kinoshita, J. Chantana, K.

Yoshino, Q. Shen, T. Toyoda, T. Minemoto, T. N. Murakami, H. Segawa, S. Hayase,

Highly Efficient 17.6% Tin-Lead Mixed Perovskite Solar Cells Realized through Spike

Structure. Nano Lett. 18, 3600–3607 (2018). doi:10.1021/acs.nanolett.8b00701 Medline

17. F. Hao, C. C. Stoumpos, R. P. H. Chang, M. G. Kanatzidis, Anomalous band gap behavior in

mixed Sn and Pb perovskites enables broadening of absorption spectrum in solar cells. J.

Am. Chem. Soc. 136, 8094–8099 (2014). doi:10.1021/ja5033259 Medline

18. R. Prasanna, A. Gold-Parker, T. Leijtens, B. Conings, A. Babayigit, H.-G. Boyen, M. F.

Toney, M. D. McGehee, Band Gap Tuning via Lattice Contraction and Octahedral Tilting

in Perovskite Materials for Photovoltaics. J. Am. Chem. Soc. 139, 11117–11124 (2017).

doi:10.1021/jacs.7b04981 Medline

19. S. Shao, Y. Cui, H. Duim, X. Qiu, J. Dong, G. H. Ten Brink, G. Portale, R. C. Chiechi, S.

Zhang, J. Hou, M. A. Loi, Enhancing the Performance of the Half Tin and Half Lead

Perovskite Solar Cells by Suppression of the Bulk and Interfacial Charge Recombination.

Adv. Mater. 30, e1803703 (2018). doi:10.1002/adma.201803703 Medline

20. G. E. Eperon, M. T. Hörantner, H. J. Snaith, Metal halide perovskite tandem and multiple-

junction photovoltaics. Nat. Rev. Chem. 1, 0095 (2017). doi:10.1038/s41570-017-0095

21. T. Leijtens, K. A. Bush, R. Prasanna, M. D. McGehee, Opportunities and challenges for

tandem solar cells using metal halide perovskite semiconductors. Nat. Energy 3, 828–838

(2018). doi:10.1038/s41560-018-0190-4

22. J. J. Berry, J. van de Lagemaat, M. M. Al-Jassim, S. Kurtz, Y. Yan, K. Zhu, Perovskite

Photovoltaics: The Path to a Printable Terawatt-Scale Technology. ACS Energy Lett. 2,

2540–2544 (2017). doi:10.1021/acsenergylett.7b00964

23. D. Zhao, C. Chen, C. Wang, M. M. Junda, Z. Song, C. R. Grice, Y. Yu, C. Li, B. Subedi, N.

J. Podraza, X. Zhao, G. Fang, R.-G. Xiong, K. Zhu, Y. Yan, Efficient two-terminal all-

perovskite tandem solar cells enabled by high-quality low-bandgap absorber layers. Nat.

Energy 3, 1093–1100 (2018). doi:10.1038/s41560-018-0278-x

24. M. T. Hörantner, T. Leijtens, M. E. Ziffer, G. E. Eperon, M. G. Christoforo, M. D. McGehee,

H. J. Snaith, The Potential of Multijunction Perovskite Solar Cells. ACS Energy Lett. 2,

2506–2513 (2017). doi:10.1021/acsenergylett.7b00647

25. A. Rajagopal, Z. Yang, S. B. Jo, I. L. Braly, P.-W. Liang, H. W. Hillhouse, A. K.-Y. Jen,

Highly Efficient Perovskite-Perovskite Tandem Solar Cells Reaching 80% of the

Theoretical Limit in Photovoltage. Adv. Mater. 29, 1702140 (2017).

doi:10.1002/adma.201702140 Medline

26. D. Forgács, L. Gil-Escrig, D. Pérez-Del-Rey, C. Momblona, J. Werner, B. Niesen, C. Ballif,

M. Sessolo, H. J. Bolink, Efficient Monolithic Perovskite/Perovskite Tandem Solar Cells.

Adv. Energy Mater. 7, 1602121 (2017). doi:10.1002/aenm.201602121

27. D. Zhao, C. Wang, Z. Song, Y. Yu, C. Chen, X. Zhao, K. Zhu, Y. Yan, Four-Terminal All-

Perovskite Tandem Solar Cells Achieving Power Conversion Efficiencies Exceeding

23%. ACS Energy Lett. 3, 305–306 (2018). doi:10.1021/acsenergylett.7b01287

28. L. Ma, F. Hao, C. C. Stoumpos, B. T. Phelan, M. R. Wasielewski, M. G. Kanatzidis, Carrier

Diffusion Lengths of over 500 nm in Lead-Free Perovskite CH3NH3SnI3 Films. J. Am.

Chem. Soc. 138, 14750–14755 (2016). doi:10.1021/jacs.6b09257 Medline

29. W. Liao, D. Zhao, Y. Yu, C. R. Grice, C. Wang, A. J. Cimaroli, P. Schulz, W. Meng, K. Zhu,

R.-G. Xiong, Y. Yan, Lead-Free Inverted Planar Formamidinium Tin Triiodide

Perovskite Solar Cells Achieving Power Conversion Efficiencies up to 6.22. Adv. Mater.

28, 9333–9340 (2016). doi:10.1002/adma.201602992 Medline

30. S. J. Lee, S. S. Shin, Y. C. Kim, D. Kim, T. K. Ahn, J. H. Noh, J. Seo, S. I. Seok, Fabrication

of Efficient Formamidinium Tin Iodide Perovskite Solar Cells through SnF2-Pyrazine

Complex. J. Am. Chem. Soc. 138, 3974–3977 (2016). doi:10.1021/jacs.6b00142 Medline

31. A. Rajagopal, P.-W. Liang, C.-C. Chueh, Z. Yang, A. K. Y. Jen, Defect Passivation via a

Graded Fullerene Heterojunction in Low-Bandgap Pb–Sn Binary Perovskite

Photovoltaics. ACS Energy Lett. 2, 2531–2539 (2017).

doi:10.1021/acsenergylett.7b00847

32. T. Yokoyama, D. H. Cao, C. C. Stoumpos, T.-B. Song, Y. Sato, S. Aramaki, M. G.

Kanatzidis, Overcoming Short-Circuit in Lead-Free CH3NH3SnI3 Perovskite Solar Cells

via Kinetically Controlled Gas-Solid Reaction Film Fabrication Process. J. Phys. Chem.

Lett. 7, 776–782 (2016). doi:10.1021/acs.jpclett.6b00118 Medline

33. W. Ke, C. Xiao, C. Wang, B. Saparov, H.-S. Duan, D. Zhao, Z. Xiao, P. Schulz, S. P.

Harvey, W. Liao, W. Meng, Y. Yu, A. J. Cimaroli, C.-S. Jiang, K. Zhu, M. Al-Jassim, G.

Fang, D. B. Mitzi, Y. Yan, Employing Lead Thiocyanate Additive to Reduce the

Hysteresis and Boost the Fill Factor of Planar Perovskite Solar Cells. Adv. Mater. 28,

5214–5221 (2016). doi:10.1002/adma.201600594 Medline

34. S. P. Harvey, Z. Li, J. A. Christians, K. Zhu, J. M. Luther, J. J. Berry, Probing Perovskite

Inhomogeneity beyond the Surface: TOF-SIMS Analysis of Halide Perovskite

Photovoltaic Devices. ACS Appl. Mater. Interfaces 10, 28541–28552 (2018).

doi:10.1021/acsami.8b07937 Medline

35. M. H. Kumar, S. Dharani, W. L. Leong, P. P. Boix, R. R. Prabhakar, T. Baikie, C. Shi, H.

Ding, R. Ramesh, M. Asta, M. Graetzel, S. G. Mhaisalkar, N. Mathews, Lead-free halide

perovskite solar cells with high photocurrents realized through vacancy modulation. Adv.

Mater. 26, 7122–7127 (2014). doi:10.1002/adma.201401991 Medline

36. Y. Yang, Y. Yan, M. Yang, S. Choi, K. Zhu, J. M. Luther, M. C. Beard, Low surface

recombination velocity in solution-grown CH3NH3PbBr3 perovskite single crystal. Nat.

Commun. 6, 7961 (2015). doi:10.1038/ncomms8961 Medline

37. Y. Yang, M. Yang, D. T. Moore, Y. Yan, E. M. Miller, K. Zhu, M. C. Beard, Top and bottom

surfaces limit carrier lifetime in lead iodide perovskite films. Nat. Energy 2, 16207

(2017). doi:10.1038/nenergy.2016.207

38. W. J. Royea, A. Juang, N. S. Lewis, Preparation of air-stable, low recombination velocity

Si(111) surfaces through alkyl termination. Appl. Phys. Lett. 77, 1988–1990 (2000).

doi:10.1063/1.1312203

39. R. Cohen, V. Lyahovitskaya, E. Poles, A. Liu, Y. Rosenwaks, Unusually low surface

recombination and long bulk lifetime in n-CdTe single crystals. Appl. Phys. Lett. 73,

1400–1402 (1998). doi:10.1063/1.122169

40. X.-H. Zhao, M. J. DiNezza, S. Liu, C. M. Campbell, Y. Zhao, Y.-H. Zhang, Determination of

CdTe bulk carrier lifetime and interface recombination velocity of CdTe/MgCdTe double

heterostructures grown by molecular beam epitaxy. Appl. Phys. Lett. 105, 252101 (2014).

doi:10.1063/1.4904993

41. K. A. Bush, A. F. Palmstrom, Z. J. Yu, M. Boccard, R. Cheacharoen, J. P. Mailoa, D. P.

McMeekin, R. L. Z. Hoye, C. D. Bailie, T. Leijtens, I. M. Peters, M. C. Minichetti, N.

Rolston, R. Prasanna, S. Sofia, D. Harwood, W. Ma, F. Moghadam, H. J. Snaith, T.

Buonassisi, Z. C. Holman, S. F. Bent, M. D. McGehee, 23.6%-efficient monolithic

perovskite/silicon tandem solar cells with improved stability. Nat. Energy 2, 17009

(2017). doi:10.1038/nenergy.2017.9

42. C. L. Perkins, F. S. Hasoon, Surfactant-assisted growth of CdS thin films for photovoltaic

applications. J. Vac. Sci. Technol. A 24, 497–504 (2006). doi:10.1116/1.2194929

43. I. L. Repins, B. Egaas, L. M. Mansfield, M. A. Contreras, C. P. Muzzillo, C. Beall, S. Glynn,

J. Carapella, D. Kuciauskas, Fiber-fed time-resolved photoluminescence for reduced

process feedback time on thin-film photovoltaics. Rev. Sci. Instrum. 86, 013907 (2015).

doi:10.1063/1.4905535 Medline

44. O. G. Reid, D. T. Moore, Z. Li, D. Zhao, Y. Yan, K. Zhu, G. Rumbles, Quantitative analysis

of time-resolved microwave conductivity data. J. Phys. D 50, 493002 (2017).

doi:10.1088/1361-6463/aa9559a