One-Step Synthesis of SnI2&#183;(DMSO)xAdducts for High-Performance Tin Perovskite Solar Cells

Tin Perovskite Solar Cells

Article

One-Step Synthesis of SnI ·(DMSO) Adducts for High-Performance

Xianyuan Jiang, Hansheng Li, Qilin Zhou, Qi Wei, Mingyang Wei, Luozhen Jiang, Zhen Wang,

Zijian Peng, Fei Wang, Zihao Zang, Kaimin Xu, Yi Hou, Sam Teale, Wenjia Zhou, Rui Si, Xingyu Gao,

Edward H. Sargent, and Zhijun Ning*

Cite This: J. Am. Chem. Soc. 2021, 143, 10970 −10976

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sı Supporting Information

ABSTRACT: Contemporary thin-ﬁlm photovoltaic (PV) materi-

als contain elements that are scarce (CIGS) or regulated (CdTe

and lead-based perovskites), a fact that may limit the widespread

impact of these emerging PV technologies. Tin halide perovskites

utilize materials less stringently regulated than the lead (Pb)

employed in mainstream perovskite solar cells; however, even

today’s best tin-halide perovskite thin ﬁlms suﬀer from limited

carrier diﬀusion length and poor ﬁlm morphology. We devised a

synthetic route to enable in situ reaction between metallic Sn and

I₂in dimethyl sulfoxide (DMSO), a reaction that generates a

highly coordinated SnI ·(DMSO) adduct that is well-dispersed in the precursor solution. The adduct directs out-of-plane crystal

orientation and achieves a more homogeneous structure in polycrystalline perovskite thin ﬁlms. This approach improves the electron

diﬀusion length of tin-halide perovskite to 290 ± 20 nm compared to 210 ± 20 nm in reference ﬁlms. We fabricate tin-halide

perovskite solar cells with a power conversion eﬃciency of 14.6% as certiﬁed in an independent lab. This represents a ∼20% increase

compared to the previous best-performing certiﬁed tin-halide perovskite solar cells. The cells outperform prior earth-abundant and

heavy-metal-free inorganic-active-layer-based thin-ﬁlm solar cells such as those based on amorphous silicon, Cu ZnSn(S/Se) , and

Sb (S/Se) .

INTRODUCTION

Previous research on tin-halide perovskites has focused on

14,15

■

10−13

reducing Sn oxidation,

improving defect passivation,

16,17

Thin-ﬁlm solar cells have attracted research interest in view of

their high power conversion eﬃciency (PCE) and low

fabrication cost. The PCEs of several thin-ﬁlm photovoltaic

and engineering interfaces.

Template crystal growth using n-

propylammonium iodide additives recently led to a certiﬁed

PCE of 11.22% and excellent device stability. Regulating the

technologies, including CdTe, Cu(In,Ga)(S,Se) (CIGS), and

Goldschmidt tolerance factor using ethylamine enabled tin-

lead-halide perovskites, have advanced rapidly in recent years,

now approaching the performance of crystalline silicon (c-Si)

halide PSCs with a PCE of 13.24%. Phenylhydrazine cations,

−

combined with a halogen anion strategy (Cl and Br ), led to tin

solar cells. These photovoltaic technologies enable ﬂexible,

perovskites with a PCE of 13.4%. However, a lack of control

over crystallization has kept tin-perovskite ﬁlms from having the

desired combination of high crystallinity, an advantageous (out-

of-plane) crystal orientation, and a high degree of ﬁlm

manufacturable, and potentially lightweight solar modules.

The development of thin-ﬁlm solar cells is impeded by

scarcity (CIGS contains indium and gallium; CdTe relies on

tellurium), which has the potential to curtail growth to terawatt-

scale capacity, and by concerns over the use of regulated

substances, such as in the case of lead-halide perovskites. It is of

interest to develop thin-ﬁlm solar cells based on earth-abundant

21,22

continuity.

Insights from Pb-based perovskites include avenues to control

crystal growth realized through a Lewis acid−base adduct

approach. PbI coordinates with solvent molecules, pseudo-

halogens, or organic molecules, to form complexes. These

materials such as Cu ZnSn(S/Se) (CZTS), Sb (S/Se) , and

lead-free halide perovskites.

Tin-halide perovskites have optoelectronic properties that

include high optical absorption and a suitable bandgap;

Received: March 21, 2021

Published: July 1, 2021

−8

however, there exists a performance gap between tin-halide

perovskite and lead-halide perovskite solar cells, and this has

been traced to a lower carrier diﬀusion length and poor ﬁlm

morphology in polycrystalline tin-halide perovskite ﬁlms.

2021 American Chemical Society

Journal of the American Chemical Society

J. Am. Chem. Soc. 2021, 143, 10970−10976

0970

Figure 1. TSS and OSS synthetic routes. (a) TSS and (b) OSS methods for SnI precursors and perovskite ﬁ lm preparation.

Article

Figure 2. Characterization of TSS and OSS SnI ﬁlms. SEM images of SnI ﬁlms formed using the TSS (a) and OSS (b); the scale bar is 10 μm. (c)

Photographs of SnI solutions (0.1 M) in toluene (TL) and chlorobenzene (CB). SnI made from OSS is well dispersed in solution, while SnI

precipitated in a TSS solution. (d) XRD patterns of SnI ﬁlms fabricated using TSS and OSS solutions (DMSO−DMF = 1:4, v/v), annealing at 80 °C

for 10 min; the inset shows the peaks assigned to the phase of SnI -3DMSO. (e) XAFS spectra (Sn K-edge) of OSS and TSS SnI solutions. (f) FT-IR

measurement of DMSO solutions with diﬀerent I concentrations.

enable more uniform nucleation, and they modulate crystal-

lization kinetics, ultimately improving the ﬁlm quality in the

perovskite active layer. Early works saw needle-shaped structures

in lead perovskite ﬁlms when using N,N-dimethylformamide

In tin-halide perovskites, even the adoption of this strategy, so

eﬀective in Pb-based perovskites, has nevertheless kept the PCE

21,27

below 14%.

We posited that the higher Lewis acidity of Sn

compared to that of Pb may prevent coordination among

Lewis base molecules and form monodispersed adducts in the

precursor. We reasoned that by characterizing the coordination

state of the Sn perovskite precursor, as well as the crystal growth

kinetics of tin perovskite ﬁlm, we could elucidate the mechanism

underpinning limited device performance of tin perovskite solar

cells and thus pursue remedies.

(

DMF) as a sole solvent, a ﬁnding assigned to the presence of

one-dimensional edge-sharing PbI crystals in solution not

suﬃciently separated by solvent molecules. The addition of

DMSO with higher coordination ability than DMF enabled

homogeneous crystallization, leading to a breakthrough in

perovskite eﬃciency and continued progress in perovskite

eﬃciency.

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Journal of the American Chemical Society

Article

We sought to study, and control, the chemistry of the tin

adducts, all with the goal of regulating crystal growth toward

more eﬃcient tin-halide PSCs. We explored a one-step synthesis

kept in the precursor, the OSS solution maintains chemical

stability in ambient air for half a year (Supplementary Figure

5 ).

(

OSS) of SnI -dimethyl sulfoxide (DMSO) colloidal complexes.

We performed XAFS measurements to analyze the coordina-

tion (CN) state of both TSS and OSS solutions in DMF−

DMSO mixed solvent (Figure 2e). The CN values of Sn−I in

both solutions is 1.9 for the OSS, lower than the 2.5 of the TSS,

while Sn −O is 2.4 for the OSS, higher than the 1.8 of the TSS

We reacted, in situ, tin metal and I -DMSO in solution, which

eliminates the unsolvated edge-sharing SnI nanocrystals in the

conventional solvation approach. This chemical approach

avoids SnI segregation and enables uniform perovskite ﬁlms

with increased out-of-plane crystal orientation. As a result, we

report an electron diﬀusion length in tin-halide perovskites that

now reaches 290 ± 20 nm compared to 210 ± 20 nm in control

ﬁlms. This enables a certiﬁed PCE of 14.6% in an independent

lab, showing a ∼20% increase compared to the previous best-

(Supplementary Table 1), which conﬁrms that SnI coordinates

with more DMSO molecules in the OSS solution. This is

consistent with the better solubility of the OSS precursor and the

absence of SnI segregation in the ﬁlm. It needs to be noted that

the CN value represents the coordination number, not the

stoichiometric ratio of ions in the solution considering the

existence of free ions. We propose the following reaction route

to SnI ·(DMSO) in OSS:

18,28

performing certiﬁed tin-halide PSCs.

The cells outperform

prior earth-abundant and heavy-metal-free inorganic-active-

layer-based thin-ﬁlm solar cells such as those based on

amorphous silicon, CZTS, and Sb (S/Se) . From X-ray

(

1) Since I is a molecular crystal, the intermolecular force

absorption ﬁne structure (XAFS) measurements we found

between iodine molecules is weak. The high-polarity

that the OSS method increases the coordination of SnI with

solvent DMSO attracts I through the van der Waals force,

solvent molecules. Using in situ grazing-incidence X-ray

diﬀraction (GIXRD) to track ﬁlm growth, we witness the

forming a weak bond between the oxygen and iodine. This

is evidenced in Fourier transform infrared (FT-IR)

spectra (Figure 2f), where the absorption peaks

corresponding with the S−O bond of the DMSO

molecule are red-shifted due to the decrease in bond

avoidance of SnI segregation in ﬁlm growth.

RESULTS AND DISCUSSION

■

One-Step Synthesis of SnI ·(DMSO) Colloidal Com-

force when I is added to the solution.

plexes. We ﬁrst prepared a SnI precursor solution by the

conventional method. In the known two-step synthesis (TSS)

(

2) Following the addition of Sn powder, metallic Sn reacts

method for preparing SnI ·(DMSO) complexes, Sn impur-

with I −DMSO to form coordination compounds. This

ities are removed, enabling a PCE of 12.4%. In the TSS

process is a spontaneous, exothermic reaction (ΔH =

−143.5 kJ/mol). A SnI ·(DMSO) adduct therefore

method (Supplementary Figure 1a), SnI is synthesized based

on the reaction between Sn and I in solution, and the product is

forms in situ.

29,30

puriﬁed using a vacuum sublimation process.

The SnI is

To clarify the inﬂuence of solvent, we prepared SnI solutions

then dissolved gradually in DMSO and DMF mixed solvent

using pure DMSO. XAFS measurements indicate the CN of Sn−

O is enhanced in comparison to DMF−DMSO mixed solvent

due to the better coordinating capability of DMSO. The CN of

Sn−O in OSS-DMSO (3.7) is higher than that of TSS-DMSO

(

DMSO−DMF = 1:4, v/v) to form a precursor (Figure 1a).

From this precursor we prepared SnI ﬁlms, ﬁnding needle-

shaped structures (SEM, Figure 2a). An energy-dispersive

spectral (EDS) line-scan measurement (Supplementary Figure

(

3.2) (Supplementary Table 1). The CN of Sn−I in OSS-

) indicated that these needles consist of phase-segregated SnI₂.

DMSO is reduced to 1.3, lower than that of edge-sharing SnI

Since SnI has a one-dimensional edge-sharing crystal structure

Supplementary Figure 1b), we posit that phase segregation may

clusters. This indicates the formation of a well-dispersed Sn

adduct in OSS solution. Using both the OSS and TSS perovskite

precursors, we cast ﬁlms on substrates without the use of

antisolvent and annealing. For ﬁlms that used the TSS method,

arise due to incompletely separated edge-sharing SnI clusters in

the precursor. A similar phenomenon is observed for the PbI

precursor when using DMF as a sole solvent.

We sought a new chemical approach to synthesize SnI₂

adducts, an approach we term the one-step synthesis. Our goal

XRD shows a diﬀraction peak from SnI (Supplementary Figure

6 ), while this is not seen for ﬁlms based on the OSS. This

indicates using pure DMSO solvent in TSS cannot preclude

clusters in solution.

For the OSS method, low-polarity solvents such as toluene

and chlorobenzene with low coordination capability enable a

was to avoid the formation of SnI clusters in the solution

(

Figure 1b). In the OSS, I grains are dissolved in a DMSO−

DMF mixed solvent, and Sn powder is then added into the I -

DMSO−DMF solution to form the precursor solution at room

temperature. The formation of a SnI complex is seen in the

absorption spectra of the ﬁ ltered SnI precursor formed using

the OSS (Supplementary Figure 3 ). This method synthesizes

the SnI ·(DMSO) adduct directly, avoiding the edge-sharing

SnI clusters seen in the TSS. We used the OSS precursor to

prepare SnI ﬁlms, and needle-shaped structures were not

observed in SEM (Figure 2b). XRD of a SnI ﬁlm made using

well-dispersed SnI solution (Figure 2c). In contrast, SnI does

not dissolve in toluene nor in chlorobenzene using the TSS

method. This conﬁrms that the synthesis of a tin adduct via OSS

is crucial to form a well-dispersed SnI

edge-sharing SnI clusters in solution.

Tin-Halide Perovskite Film Formation. We investigated

precursor that eliminates

24,33

how the OSS and TSS inﬂuence crystal growth

by tracking

OSS precursors exhibited stronger peaks at 9.0° and 10.8°

perovskite growth kinetics using in situ GIXRD. We prepared

perovskite precursor solutions by mixing OSS vs TSS DMF−

DMSO solutions with formamidinium iodide (FAI) and

phenylethylamonium bromide (PEABr). We used the anti-

solvent dripping method. The photoluminescence (PL) spectra

of ﬁlms show peaks around 870 nm, consistent with the

characteristic value of tin perovskite (Supplementary Figure 7).

associated with SnI −3DMSO (Figure 2d). This suggests

better coordination of Sn with solvent molecules in the OSS

precursor. We carried out the reaction in a glovebox; this

approach does not require transfer, so avoids oxidation during

the transport of SnI required in the conventional method

(Supplementary Figure 4 ). When the unreacted Sn powder is

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Figure 3. Characterization of perovskite crystallization kinetics. (a) In situ GIXRD of a TSS ﬁlm (b) and OSS ﬁlm during ﬁlm growth. (c) Intensity

−

curves integrated azimuthally about the ring at q = 1.0 Å after ﬁlm annealing. The integrated areas of 0−20° and 80−100° represent in-plane and out-

of-plane orientated structures, respectively. (d) Absorption spectra of the OSS vs TSS perovskite ﬁlms before and after annealing. (e) XRD patterns of

perovskite ﬁlms made using the OSS and TSS. The abbreviations 1L and 2L indicate single- vs two-layer structures. (f) SEM images of perovskite ﬁlms

made using the OSS and TSS; the scale bar is 1 μm.

Regarding the TSS ﬁlms, we found that during the spin-

of pin holes in the ﬁlm. In sum, the OSS suppresses SnI₂

segregation through better coordination of tin adducts.

coating process, before dripping the antisolvent, a distinct peak

−

corresponding to the SnI phase (q = 0.9 Å ) appeared,

We therefore proceeded to characterize the structure of TSS-

and OSS-derived perovskite ﬁlms. From the GIXRD patterns

(Supplementary Figure 8) we can see that the (100)-diﬀraction

peak of TSS perovskite ﬁlms exhibited similar peak intensity at

azimuth angles near 0° and 90° (Figure 3c), indicating the

coexistence of in-plane (28%) and out-of-plane (72%) oriented

indicating phase segregation (Figures 1a, 3a). Prior to dripping

the antisolvent, there is no signature of perovskite crystallization.

Perovskite nucleation happened immediately after the deposi-

tion of the antisolvent, indicated by the emergence of perovskite

diﬀraction peaks (Figure 3a). However, the nucleation process

33,35

was limited by the phase segregation of SnI , evidenced by the

perovskite domains, respectively.

For OSS-derived perov-

weak intensity of the perovskite diﬀraction peaks and reduced

ﬁlm absorbance (Figure 3d). A subsequent thermal annealing

skite ﬁlms, the ratio of in-plane oriented structure is three times

reduced (9%), suggesting that out-of-plane crystal orientation

process then converts the remaining SnI and halide salts to tin-

dominates (Figure 1b). Avoiding the segregation of SnI also led

halide perovskite. This is consistent with a strong absorbance

enhancement (Figure 3d) following ﬁlm annealing.

to weaker X-ray diﬀraction (XRD) peaks from two-layer (2L,

PEA FASn I ) and one-layer (1L, PEA SnI ) perovskites,

2 7

For the OSS ﬁlms, no SnI precrystallization was observed

whereas TSS ﬁlms show a stronger peak from 1L perovskites

(Figure 3e). The OSS ﬁlms were more compact (Figure 3f),

while TSS showed square grains and pinholes.

prior to antisolvent dripping, consistent with increased DMSO

coordination. Following antisolvent dripping, the intensities of

perovskite-ascribed diﬀraction peaks are stronger than in TSS-

derived ﬁlms (Figure 3b), consistent with better nucleation of

the perovskite structure. The absorbance of the perovskite ﬁlms

increased only slightly after thermal annealing, in agreement

with the full conversion of the intermediate complexes to

We then investigated carrier diﬀusion using the quenching

,36,37

method.

From the change in photoluminescence lifetime

with the addition of the quenching layer (Supplementary Figure

9 ), we estimate 290 ± 20 nm for the electron diﬀusion length

from the OSS and 210 ± 20 nm for the TSS.

perovskite during antisolvent dripping, while thermal

annealing removes solvent molecules and enhances perovskite

crystallinity. The absorbance of the OSS ﬁlm is slightly higher

than that of the TSS ﬁlm, which can be ascribed to a more

complete growth in perovskite structure and a decreased density

Device Performance of Tin-Halide Perovskite Solar

Cells. We used the device architecture: indium-doped tin oxide

(ITO)−glass/PEDOT/tin-halide perovskite/indene-C bisad-

duct (ICBA)/bathocuproine (BCP)/Ag (Figure 4a). The OSS

and TSS ﬁlms have similar thicknesses (∼200 nm), while the

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https://pubs.acs.org/doi/10.1021/jacs.1c03032.

Article

Figure 4. Performance of perovskite devices. (a) Cross-sectional SEM images of OSS and TSS devices; the scale bar is 200 nm. (b) Current density−

voltage (J−V) under simulated AM1.5 G illumination. (c) EQE spectra and integrated J . (d) Statistical PCE and J from 40 OSS and 40 TSS

devices.

OSS has a more homogeneous morphology. We sent an

encapsulated OSS device for certiﬁcation at the Shanghai

Institute of Microsystem and Information Technology (Figure

We encapsulated devices and found that OSS devices retained

96% of the initial performance following a 100-day storage under

an N atmosphere (Supplementary Figure 14). For steady-state

PCE measurements, we saw less than 5% relative decay in PCE

following 1 h of continuous operation (Supplementary Figure

b, Supplementary Figure 10). Device performance was

−

measured under 100 mW cm AM1.5 G illumination. From

reverse current density−voltage (J−V) curves, the best-

performing OSS tin-halide PSCs had a PCE of 14.63%, with a

15 ).

CONCLUSION

short-circuit current (J ), open-circuit voltage (V_O_C), and ﬁll

■

−

factor (FF) of 20.6 mA·cm , 0.91 V, and 77.1%, respectively.

The eﬃciency from the forward scan is 14.5%, indicating

inconsequential hysteresis. A PCE of 14.3% is achieved for a

This study demonstrates increased eﬃciency in tin-halide

perovskite solar cells that rely on a tin adduct. The one-step

synthesis of the SnI precursor facilitates the formation of SnI ·

large-area device (0.09 cm ) (Supplementary Figure 11 ). The

(DMSO)_xcomplexes with enhanced coordination. This

approach regulates tin-halide perovskite growth, avoiding SnI₂

phase segregation. Perovskite ﬁlms produced from this method

contain fewer pinholes, are more uniformly orientated, and

exhibit a diﬀusion length of 290 ± 20 nm compared to 210 ± 20

nm for controls. We report a certiﬁed eﬃciency of 14.6%, higher

than all other material-abundant Pb/Cd-free thin-ﬁlm solar cells.

This work examines the inﬂuence of the coordination state of

precursors on Sn perovskite growth and explored this via in situ

measurements of crystal growth dynamics. This study includes

salient insights for the future development of Sn perovskite

optoelectronic devices.

best-performing TSS tin-halide PSCs showed a Jsc of 16.7 mA

−

cm , V of 0.96 V, and FF of 76.4%, and thus a PCE of 12.2%

(

Figure 4b). The integrated current densities from external

−

quantum eﬃciency (EQE) curves are 20.1 and 16.3 mA cm for

OSS and TSS devices, respectively, well matched with the Jsc

calculated from J−V curves (Figure 4c). The high J_S_Cof OSS

solar cells is due to the enhanced diﬀusion length owing to the

better orientation of the perovskite ﬁlm. For comparison,

control devices based on the TSS method using commercial SnI₂

obtained an eﬃciency of 11.9% (Supplementary Figure 12a).

We further studied device reproducibility and stability in tin-

halide PSCs. We fabricated 40 devices using each method

ASSOCIATED CONTENT

(Figure 4d, Supplementary Figure 13). For OSS devices, 80% of

■

cells exhibited PCEs exceeding 13.1%, with an average eﬃciency

of 13.5%. TSS devices showed eﬃciencies were distributed

between 9.5% and 12.2%.

sı Supporting Information

The Supporting Information is available free of charge at

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J. Am. Chem. Soc. 2021, 143, 10970−10976

Journal of the American Chemical Society

orcid.org/0000-0001-5966-0175

Article

Author Contributions

X.J. and H.L. contributed equally.

Experimental details and additional supplementary ﬁgures

(PDF )

Notes

AUTHOR INFORMATION

The authors declare no competing ﬁnancial interest.

■

Corresponding Author

ACKNOWLEDGMENTS

Zhijun Ning − School of Physical Science and Technology,

ShanghaiTech University, Shanghai 201210, China;

orcid.org/0000-0002-9130-3490; Email: ningzhj@

shanghaitech.edu.cn

■

The authors thank Dr. Peihong Cheng, Na Yu, and Rong Gao for

their kind assistance. The authors gratefully acknowledge

ﬁnancial support from the National Key Research and

Development Program of China (under Grant No.

Authors

016YFA0204000), ShanghaiTech start-up funding, 1000

Xianyuan Jiang − School of Physical Science and Technology,

ShanghaiTech University, Shanghai 201210, China;

Hansheng Li − School of Physical Science and Technology,

ShanghaiTech University, Shanghai 201210, China;

orcid.org/0000-0001-5065-1838

Qilin Zhou − School of Physical Science and Technology,

ShanghaiTech University, Shanghai 201210, China;

orcid.org/0000-0003-3359-5963

Qi Wei − School of Physical Science and Technology,

ShanghaiTech University, Shanghai 201210, China;

orcid.org/0000-0002-5506-0495

Young Talent Program, the National Natural Science

Foundation of China (61935016, 92056119), and Shanghai

Key Research Program (16JC1402100). This work is partially

supported by the BL14B1 and BL14W1 beamlines of Shanghai

Synchrotron Radiation Facility (SSRF), Shanghai, China; the

Centre for High-Resolution Electron Microscopy (CℏEM),

SPST, ShanghaiTech University under Contract No.

EM02161943; and the Analytical Instrumentation Center,

SPST, ShanghaiTech University under Contract No. SPST-

AIC10112914.

Mingyang Wei − Department of Electrical and Computer

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