lead iodide perovskite, Cs2PbI2(SCN)

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A thiocyanate containing two-dimensional cesiumlead iodide perovskite, CsPbI(SCN); Characterization, photovoltaic application, and degradation mechanism 2

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Youhei Numata, Yoshitaka Sanehira, Ryo Ishikawa, Hajime Shirai, and Tsutomu Miyasaka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15578 • Publication Date (Web): 14 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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ACS Applied Materials & Interfaces

A thiocyanate containing two-dimensional cesiumlead iodide perovskite, Cs2PbI2(SCN)2; Characterization, photovoltaic application, and degradation mechanism Youhei Numata,*,†,§

Yoshitaka Sanehira,† Ryo Ishikawa,‡ Hajime

Shirai,‡ and Tsutomu Miyasaka*,† † Department of Engineering, Toin University of Yokohama, 1614 Kurgane-cho, Aoba, Yokohama, Kanagawa 225-8503 Japan. ‡ Department of Functional Materials Science, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama, 338-8570 Japan. AUTHOR INFORMATION Corresponding Authors Y. Numata, [email protected] and T. Miyasaka, [email protected]. KEYWORDS perovskite solar cells, 2-dimensional material, thiocyanate, degradation mechanism, thermal stability

ACS Paragon Plus Environment

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Abstract

We

explored

thiocyanate

(SCN)-based

two-dimensional

(2-D)

organometal lead halide perovskite families toward photovoltaic applications. Using SCN axial ligand and various cation species, we examined AA'PbI2(SCN)2 type 2-D perovskite by replacing the cation species (AA') between methylammonium (MA), formamidinium (FA), and cesium. Among various cation compositions, only allinorganic cesium-based SCN perovskite, Cs2PbI2(SCN)2, film showed high thermal stability compared to known 2-D perovskites. The perovskite solar cell (PSC) using Cs2PbI2(SCN)2 absorber yielded approximately 2% conversion efficiency on the mesoscopic device. Relatively low efficiency is attributed, in addition to optical properties (a large band gap (2.05 eV) and exciton absorption), to the orientation of perovskite layer parallel to the layered structure, preventing carrier extraction from the light absorber perovskite. In the device stability, the Cs-based 2-D perovskite was stable against oxygen (oxidation), while it was found to be unstable against humidity. XRD and XPS measurements showed that, unlike long-alkylammonium-based 2-D perovskite families such as BA2PbI4 (BA = butylammonium), the Cs-based 2-D perovskite can undergo hydrolysis due to the hydrophilic Cs cations.

1. Introduction


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Organic-inorganic established

hybrid

itself

as

lead

high

trihalide

performance

perovskite

photovoltaic

has light

absorbers enabling solar power conversion efficiency beyond 23%.1 It permits wide-range compositional development of new perovskite materials by solution-based chemical synthesis. To date, perovskite solar cells (PSCs) based on APbX3-type threedimensional perovskite families have been leading the best conversion efficiency. MAPbI3 and FAPbI3 (MA = methylamonium and FA = formamidinium) have been employed as popular 3-D light absorbers in solar cells.2-6 In an attempt to improve conversion efficiencies

and

cell

durability,

mixed

cation/halide

type

perovskites have been energetically explored, which include MAxFA1-xPbI3,7,8 doped,15

(FAPbI3)0.85(MAPbBr3)0.15,9,10

and

Cs-doped,11-14

perovskites.16-18

K-doped

Rb-

Crystallinity,

optoelectronic properties, and stability of these perovskites have been subjects of study to control physical properties and photovoltaic tolerance

performance.

factor

of

the

However, cation

according

and

anion

to

Goldshcmidt

sizes,

possible

cation/halogen combination to form ABX3-type 3-D perovskite structure is strictly limited for lead and/or tin trihalidebased perovskites.19-21 To explore other new candidate materials as

light

absorbers,

double

perovskites

(A2M(I)M'(III)X6),22

Bi

halide,23-27 Ag-Bi halide,28-30 Sb halide,31-36 and low-dimensional lead-halide

perovskite

analogues37-39

have

been

examined

to

realize stable and high efficiency solar cells.


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Page 4 of 39

Low-dimensional (zero, one, and two) perovskite families have been successfully applied to absorber layers to enhance conversion efficiencies and device durabilities.40-43 Using a larger or divalent ammonium cations, e.g., butylammonium (BA), phenethylammonium (PEA), and ethylenediammonium (EDA),44,45 alternative 2-D layer structure with cationic layer and [PbX4]m sheet can be prepared as shown in Figure 1. Such 2-D perovskite families, called as Ruddlesden-Popper phase (with monovalent cation: A+) or Dion-Jacobson phase (with divalent cation: A2+), exhibit high anti-moisture stability compared with APbX3-type 3-D perovskite families because the organic cation layers with hydrophobic alkylammoniums protect lead halide sheet from humidity. However, the organic cation layer behaves as insulating layer and prevents interlayer carrier diffusion, resulting in low conversion efficiencies of PSCs.40,46 Recently, Kanatzidis et al. reported that by mixing small and large cations (MA and BA) in appropriate ratio, 2-D sheet based on multi-layered lead halide, A2A'n-1PbnI3n+1 (A > A', n = 1, 2, 3 ...) was synthesized (Figure 1) and applied to PSCs.40-43 Such multi-layer based 2-D perovskite families showed red-shift in absorption depending on number of [PbnI3n+1]m monolayer stacked in each sheet layer, and carrier diffusion was improved. Such multilayered 2-D structure perovskite yields efficiency up to 14% with good device durability.47 Furthermore, by replacing surface and grain boundary of 3-D perovskite with 2-D perovskite, stability of the 3D perovskite


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can be significantly improved. PSC-based on 2D-3D heterostructured perovskite, BA0.05(FA0.83Cs0.17)0.91Pb(I0.8Br0.2)3, exhibits improved stability and high conversion efficiency over 20%.43 These backgrounds of 2-D perovskite-based solar cells promoted exploration of new low-dimensional perovskites as a promising light absorber for PSCs. Additionally, the 2-D perovskite family is also applied to fabrication of (organic) light emitting diodes48,49 and photodetectors.50,51 2-D perovskite can be formed by two different ways; one is to use bulky or divalent cations in perovskite, and other is to cap surfaces of lead-halide sheet by non-bridging ligand such as SCN. Compared to A2PbX4-type perovskites, A2PbI2(SCN)2-type perovskites have been rarely studied as light absorber in solar cells despite wider spectral sensitivity (narrower band gap). Xu et al. firstly reported a quasi-halide thiocyanateincorporating perovskite as a light absorber of a PSC.52 They reported that the dark brown perovskite is a 3-D MAPb(SCN)2I, showing moderate conversion efficiency and excellent stability against humidity. This report was later corrected for the structure as a two-dimensional MA2PbI2(SCN)2 as confirmed by single crystal X-ray diffraction measurement.53 However, the XRD pattern and optoelectronic property of MA2PbI2(SCN)2 was completely different from the dark brown compound mentioned above.52 Currently, it is expected that the stable dark brown perovskite film is a 3-D MAPbI3; stable and high quality


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perovskite film can be obtained by adding a little amount of SCN salts as additives.54-56 First fully identified MA2PbI2(SCN)2 perovskite-based photovoltaic device was reported by Wang et al.57 In this report, we prepared the MA2PbI2(SCN)2 at room temperature process as a bright red film. Interestingly, the SCN-incorporating 2-D perovskite presents significant redshift of absorption onset up to 60 nm compared to the A2PbI4 type 2-D perovskite (A = BA and PEA), meaning that absorption property of the 2-D (multi-) layered perovskite can be improved by combination with quasi-halide such as SCN. In relation of our study, some characteristics of the thiocyanate-based perovskite were studied for solar cell application;58-60 however, to the best of our knowledge, development of new thiocyanate-based perovskite and its solar cell application have been never reported. Similar to the 3-D perovskite, we expected that 2-D perovskite could be stabilized by optimizing cation and/or halogen ions. Based on the expectation, we examined influence of various cation compositions (MA, FA, Cs, and their mixture) on formation of 2-D perovskite, its stability, and photovoltaic performance. In this report, we present preparation, development of PSC, and degradation mechanism of a Cs-based 2-D perovskite, Cs2PbI2(SCN)2. 2. Experimental Section 2.1 General


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Reagents and chemicals were purchased from Japanese chemical companies (Tokyo Chemical Industry Co., Ltd (TCI), Wako Pure Chemical Industries, Ltd, and Sigma-Aldrich Japan). CsI, MAI, FAI, were purchased from TCI. Pb(SCN)2 was bought from Mitsuwa Chemicals Co., Ltd, Japan. The reagents and solvents were used without any purification. FTO glass substrate and an aqueous suspension

of

brookite

TiO2

nano-particles

(pecc-01)

were

provided from Peccel Technologies. 2.2 Materials preparation For a precursor solution of 2-D perovskite, AI (A: corresponding cation) and Pb(SCN)2 (2:1 in molar ratio) were dissolved in a DMF and DMSO (4:1 v/v) mixture (1 M) and stirred at 80 °C for 1h, (for a CsI, need to stir for overnight because of low solubility). For a hole transport layer (HTL), a solution of 2,2',7,7'tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD) (36 mg) in chlorobenzene (300 L) was stirred at 70 °C for 1 h. The solution was cooled to r.t., and added with 4-tert-butylpyridine (3.6 L) and a 0.6 M acetonitrile solution of lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) (15.2 L) as additives. 2.3 Device Fabrication On a cleaned FTO substrate, a 0.15 M i-propanol solution of [Ti(acac)2(iPrO)2]

(acac

=

acetylacetonato

and

iPrO

=

i-

propoxido) was spin-coated (3000 rpm / 30 sec) and dried at 100


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Page 8 of 39

°C for 10 min, followed by spin coating twice with 0.3 M [Ti(acac)2(iPrO)2] solution. The substrate was sintered at 450 °C for 15 min to prepare compact TiO2 layer (CL).

The CL-coated

substrate was treated by UV-O3 treatment, and a suspension of TiO2 particles in EtOH (1:4 in wt) was spin-coated (3000 rpm / 30 sec) and dried at 150 °C for 60 min. The TiO2 mesoporous films were cleaned by UV-O3 treatment before cell fabrications. The perovskite precursor solutions and TiO2 substrates were heated at 70 °C before spin coat. On to the TiO2 substrate, the precursor solution was spin-coated (wait for 30 sec → 3000 rpm / 20 sec), and as-prepared film was annealed at 100 °C for 40 sec.

On to the pervskite film, HTM was spin-coated by drop a

HTM solution on spinning substrate (500 rpm / 5 sec → 4000 rpm / 30 sec)). The cell was kept under dark for overnight to promote oxidation of the HTL. Finally, gold was vacuum deposited as a counter electrode. 2.4 Characterization SEM measurements were performed by a SU8000 (Hitachi HighTechnologies Co.) XRD patterns were measured by a D8 DISCOVER (Bruker-AXS

K.

K.)

with

Cu

K

radiation

under

operation

condition of 40 kV, 40 mA. The perovskite film was protected by polymethylmethacrylate (PMMA) film. Photovoltaic measurements. Cell active area (3 × 3 mm2) was defined by a black metal mask. Photocurrent density-voltage (JV) curves were recorded using with a PEC-L01 solar simulator


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(Peccell Technologies) under AM1.5G condition (100 mW cm–2). Measurement condition; Step voltage: 0.01 V, search delay: 0.05 sec, and hold time: 0.05 sec. External quantum efficiency (EQE) spectra

were

observed

by

a

PEC-S20

spectrometer

(Peccell

Technologies). Photoluminescence (PL) spectrum was measured using a C11367 Quanturas-Tau

compact

fluorescence

lifetime

spectrometer

(Hamamatsu Photonics). The perovskite film was covered with PMMA film to protect from humidity during measurements. XPS spectra were measured using an AXIS Nova (Kratos Analytical Ltd.) equipped with a monochromatic Al K X-ray source (150 W), and the pass-energy of the spectrometer was set to 160 or 40 eV for the survey or core spectrum measurement, respectively. The perovskite film was covered with PMMA. IR

spectra

were

observed

using

an

IRPrestige-21

Fourier

transform infrared spectrophotometer (SHIMADZU). 3. Results and discussion 3.1 Preparation of A2PbI2(SCN)2 films A2PbI2(SCN)2 films were prepared by using MA, FA, Cs, and their mixture as a cation A. 1.0 M precursor solution was prepared by stoichiometrically mixing AI, A'I, and Pb(SCN)2 in DMF and DMSO (4:1) mixture, to form (A1-xA'x)2PbI2(SCN)2 (A, A' = FA and MA; 2:0, 1:1. A, A' = Cs and FA or MA; 2:0, 1:1, 1.6:0.4) 2-D perovskite. The mixtures were stirred at 70 °C for 1 h. For the Cs-containing precursors, they needed to be stirred for few hours due to the low solubility of Cs salt.


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Page 10 of 39

All the precursor solutions were casted on TiO2 mesoporous substrates on spin-coating and annealed at 100 °C for 2 min under

dry

condition.

As

shown

in

Figure

S1,

only

Cs-rich

compositions (Cs2, Cs1.6FA0.4, and Cs1.6MA0.4) gave red films, which is expected as 2-D perovskite. Pure FA-based film, FA2PbI2(SCN)2 was colorless even after annealing at 100 °C for 30 min. Other recipes; MA, FA, and their mixtures produced hazy and pale color films. Among the all the combinations, the Cs2PbI2(SCN)2 showed significantly higher thermal stability compared to other SCN incorporating films and A2PbI4 type 2-D perovskite families. Therefore, Cs2PbI2(SCN)2

we

accurately and

observed

MA2PbI2(SCN)2

as

thermal a

stability

comparison.

A

of red

MA2PbI2(SCN)2 film (prepared from DMF solution and dried at r.t.) became black above 60 °C, and over 100 °C turned into hazy yellow (Figure S2 and S3). After cool down to room temperature, the black film turned into hazy and pale red. The hazy yellow film was not changed after cool down to room temperature. In contrast, Cs2PbI2(SCN)2 film retained vivid red color up to 120 °C, and some transparent pinholes appeared over 130 °C. 3.2 Characterization of Cs2PbI2(SCN)2 film Obtained A2PbI2(SCN)2 films were characterized by XRD and UV spectroscopy measurements. Figure 2a shows XRD chart of the Cs2PbI2(SCN)2 film on a TiO2 mesoporous substrate as an example, because of the best film quality among all obtained films. Unfortunately, we could not obtain a good single crystal of Cs2PbI2(SCN)2 applicable to XRD measurement for crystal structure


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determination; therefore, we compared the obtained data with those of MA2PbI2(SCN)2; orthorhombic, a-axis is perpendicular and b-c

plane

is

parallel

to

the

2-D

lead-halide

sheet,

respectively.53 Diffraction peaks appeared at 2 = 9.83, 19.73, 29.78, and 40.08 °. Compared with the reported crystal structure of MA2PbI2(SCN)2, it is expected that this red film possesses 2-D structure similarly to MA2PbI2(SCN)2, and the diffraction peaks correspond to (200), (400), (600), and higher order indexes; (h00) of the Cs2PbI2(SCN)2. Its intersheet distance is 9.00 Å calculated

from

Bragg's

comparable

to

the

difference

of

MA+

law,

2dsin

MA2PbI2(SCN)2,

9.31

pm)20,61

and

(217

=

n. Å.

This

value

is

Taking

ion

size

(188

pm)

into

Cs+

consideration, the shorter intersheet distance is reflecting a smaller Cs ion size which locates at intersheet spaces. Figure 2b shows absorption and photoluminescent (PL) spectra of the Cs2PbI2(SCN)2 film prepared on a glass substrate (UV spectra of the all prepared cation composition films are shown in Figure S4). For the absorption spectrum, a sharp peak was observed at 590 nm (2.10 eV), which was assigned to the exciton absorption due to the quantum well in the 2-D lead halide sheet structure,57 and absorption onset elongated to approximately 740 nm (1.68 eV). The PL spectrum showed a sharp peak at 694 nm. The exciton absorption

onset

is

slightly

red-shifted

compared

with

MA2PbI2(SCN)2, 580 nm. Based on an atmospheric photo-electron measurement, an ionization potential of Cs2PbI2(SCN)2 film was


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Page 12 of 39

estimated as 5.84 eV. This energy level of Cs2PbI2(SCN)2 is appropriate to combine with TiO2 as an electron transport layer (ETL) and spiro-OMeTAD as a HTL in perovskite solar cell as shown in Figure 2c. For the MA-based perovskite, the obtained film annealed at 100 °C was grayish red similar to the reported compound,52 but the absorption spectra and XRD chart of the film was different from fully characterized bright red MA2PbI2(SCN)2 by single crystal X-ray crystallographic analysis (Figure S3, S4, and S5). 3.3

Characterization

and

photovoltaic

properties

of

Cs2PbI2(SCN)2-based PSC We examined various combinations of A-site cation composition in film preparation. However, as described above, only Cs-rich recipes (Cs2, Cs1.6FA0.4, and Cs1.6MA0.4) produced pinhole-free and uniform red perovskite films (Figure S1). Therefore, we applied the Cs-rich perovskites to fabrication of perovskite solar cells by using brookite TiO2 mesoporous layer as ETL. Unfortunately, the mixed-cation films were immediately decomposed by spincoating a HTM solution (LiTFSI and tBP) on the perovskite films. Thus, we were only able to prepare pure Cs-based perovskite device. Figure 2d and S6 show cross-sectional SEM image of the mesoscopic PSC device and surface SEM image of the Cs2PbI2(SCN)2 film on the mesoporous substrate. The perovskite film presents flat and pinhole-free film surface. For the cross-sectional SEM image, thicknesses of each layer are 270 nm (HTL), 550 nm


12

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(perovskite), and 220 nm (TiO2 mesoporous layer), and 100 nm (TiO2 CL), respectively. Figure 3 shows photocurrent density–voltage (J–V) curves and external quantum efficiency (EQE) spectrum of the cells. The PSCs showed conversion efficiency of 1.69±0.28 % with shortcircuit current (JSC) of 4.29±0.77 mA cm–2, open-circuit voltage (VOC) of 0.90±0.04 V, and fill factor (FF) of 0.44±0.03 in reverse scans (1.34±0.30 % with JSC of 4.78±0.80 mA cm–2, VOC of 0.87±0.05 V, and FF of 0.32±0.02 in forward scans). A champion efficiency was 2.04 % with JSC 4.55 mA cm–2, VOC 0.94 V, and FF 0.48 in reverse scan. The low efficiency is controlled by low JSC value and low FF. The onset of EQE spectrum reached 735 nm and a peak assigned to exciton of two-dimensional perovskite material at 590 nm was observed reflecting the optical absorbance spectrum. The EQE maximum is approximately 40%. Integrated photocurrent value calculated from the EQE data was 5.23 mA cm–2, which is well matched with 5.16 mA cm–2 in a forward J-V curve. On the other hand, the high series resistance (RS), over 800 , is apparently responsible to the low JSC and FF values. It is expected that these values are affected by orientation of the 2-D lead halide sheet same as the case of the A2PbI4 type perovskite families.40,46 For

2-D

perovskite-based

PSCs,

EQE

response

and

resulting

photocurrent value were generally very low compared to 3-D perovskite-based PSCs because of influence of low interlayer carrier transport properties.46 The interlayer carrier transport


13


in

the

2-D

layered

perovskite

is

Page 14 of 39

prevented

by

insulating

interlayers of organic cations (and/or metal ions) as shown in Figure 1. To investigate [PbI2(SCN)2]2+n sheet orientation in our PSC device, we carried out in-plane and grazing-incident (GI) out-of-plane XRD measurements. In-plane and out-of-plane GI-XRD measurements can observe crystal configuration perpendicular and parallel to a substrate, respectively. In the GI out-of-plane XRD chart, other than (200) diffraction peak at 2 = 9.83 ° disappeared or significantly weakened, and some

new

peaks

appeared

(Figure

S7).

In

the

Cs2PbI2(SCN)2

perovskite film, it is expected that 2-D sheets are directed parallel to (h00) plane. In contrast, in the in-plane XRD chart the (200) peak almost disappeared, and new diffraction peaks appeared at 2 = 20.2 and 28.4 °. These results indicate that 2-D

[PbI2(SCN)2]2+n

substrate,

in

sheet

which

arrangement

configuration,

lied

parallel

photo-generated

to

the

carrier

cannot transport across the insulating Cs+ layer. Interestingly, SCN-containing 2-D perovskite solar cells showed better

photovoltaic

properties,

in

particular

higher

photocurrent values,40,46,57 compared to performance of PSCs using A2PbI4-type

single-layered

2-D

perovskites,

except

for

some

special cases such as using nano-wire array.46,62 It is expected that interlayer distances between [PbI2(SCN)2]n sheets in the SCN-based perovskite is significantly shorter than [PbI4]n cases because of small cation sizes that form thinner insulating


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layers (Figure 1). This is assumed to enable better interlayer carrier hopping is expected. 3.4 Degradation of Cs2PbI2(SCN)2 film We tried to evaluate degradation mechanism of the Cs2PbI2(SCN)2 to

improve

stability

of

this

material.

Shelf

life

and

degradation of Cs2PbI2(SCN)2 films were investigated by storing films for long time under different conditions. Figure 4 shows photographs of Cs2PbI2(SCN)2 films kept under inert, dry, and humid atmospheres. The perovskite films kept in a glove box (GB); dew point (DP) < –100 °C, N2, remain bright red color after 2 years passed (Figure 4a). Therefore, under inert atmosphere, the perovskite is highly stable. Contrastively, a MA2PbI2(SCN)2 film was decomposed within few days although it was stored in the GB (Figure S8). A perovskite film was also kept under dry condition (DP < –33 °C); here, oxygen exists but humidity is almost zero. In such dry booth, the perovskite film remained bright red color over two days (Figure 4b). Contrastively, when a perovskite film was left under ambient humid condition, the film was quickly degraded as shown in Figure 4c. Just after the perovskite film was taken out from GB, yellow spots randomly appeared on the film. Then, number of the yellow spot increased and the spot sizes were expanded from the center of the yellow spots. Finally, the film completely became colorless after 1 h., Additionally, when the perovskite film was encapsulated by PMMA polymer to measure XRD and PL spectra, the it turned to be stable and retained the red color for few days under the ambient


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Page 16 of 39

condition same as in Figure 4c. Therefore, we concluded that this

color

deterioration

is

caused

by

water

in

the

air.

Interestingly, the red and degraded perovskite films were easily dissolved into water. Figure 5a shows IR spectra of the fresh and the decomposed perovskite films (red and colorless films shown in Figure 4c). For the fresh film, a sharp singlet peak assigned to the stretching mode of N=C=S– was observed at 2085 cm–1. After decomposition,

intensity

of

the

N=C=S–

stretching

peak

significantly decreased and two peaks appeared at 2070 and 2050 cm–1. Compared with a reported of MA2PbI2(SCN)2 structure,53,57 it is expected that the Cs-based perovskite is alternative layered structure comprising 2-D -I-Pb-I- sheet with axial two SCN ligands and Cs+ layers.

Therefore, it is expected that NCS

moiety was decomposed or molecular symmetry was changed. As shown in Figure 5b, after exposure to humid air, almost all diffraction peaks disappeared, meaning that none of crystalline phase of perovskite, PbI2 and other degraded products are not remained unlike the case of 3-D perovskite that presents -phase or PbX2 structures.63,64 Figure 6 and S9 show XPS spectra of the Cs2PbI2(SCN)2 perovskite films before and after decomposition for Pb, Cs, S, N, and I elements. For Pb atom, characteristic 4f (7/2 at 143.3 eV and 5/2 at 138.4 eV) were observed. Fresh sample, only these two strong peaks were observed. In contrast, the decomposed sample showed new small shoulder peaks at slightly lower energy region


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at 136.81 and 141.81 eV. We considered that such low energy peaks can be assigned to formation of lead hydroxide and/or aqua complex or metal Pb by reduction of Pb(II) with I– ions under ambient and light irradiation condition.65,66 Therefore, based on IR, XRD and XPS results, it is expected that infinite 2-D -IPb-I- sheet structure is shredded into oligomeric fragments by insertion

or

reaction

with

water

molecules.

For

S

atom,

intensities of S 2p peaks at 163 and 164 eV decreased compared with fresh sample. For N 1s, similar with S atom, the peak intensity

was

decreased

after

decomposition.

Oppositely,

intensity of Cs peak at 159.01 eV was slightly increased after decomposition.

In

contrast,

the

intensity

of

I

peaks

was

significantly increased different from other elements after decomposition, indicating that iodine concentration increased at the film surface. Therefore, a possible degradation mechanism is that I– was removed from Pb atom and the free I– ions and/or oxidized I2 molecules are exhausted to the surface of the film during decomposition. The glass substrate, on which red perovskite film was almost washed off, seems transparent and no perovskite remains (Figure S10), meaning the Cs2PbI2(SCN)2 is highly soluble in water. Therefore, it is likely that the lead atoms were partly converted to lead aqua complexes such as Pb(SCN)2(H2O)x and Csx[Pb(I)x(SCN)2(H2O)y]n. Replacement and cleavage of the ligand, I– and SCN–,

coordinating

to

central

lead

atom

are

competitive


17


Page 18 of 39

reactions. Because of strong -back donation from Pb(II) to SCN–, I– is preferentially cleaved and observed by XPS measurement (Figure S11). Based on the above IR, XRD, and XPS results, we proposed the degradation mechanism of Cs2PbI2(SCN)2 as summarized in Figure 7. Water molecules approached perovskite surface from the air. Then, the water molecules were intercalated into the 2-D layer and replaced with the bridging I– ions. At last, the 2-D sheet structures are shredded, and the Pb(SCN)2 moieties were partly cut out from the 2-D sheets as a Pb(SCN)2(H2O)n. At same time, free CsI (or Cs+ and I– ions) exhausted to the film surface as detected by surface XPS measurement. 4. Conclusions We attempted to prepare various ammonium and/or cesium-based lead-iodide-thiocyanate perovskite families (A2PbI2(SCN)2), and successfully obtained Cs2PbI2(SCN)2 perovskite with high thermal stability compared with other 2-D perovskites. Except for pure Cs material, all mixed and FA-based families did not form stable 2-D

perovskite

thermally

stable

structure. over

100

The °C,

Cs2PbI2(SCN)2 and

perovskite

possesses

is

appropriate

electronic structure to combine with TiO2 and spiro-OMeTAD for solar cells application. The PSC based on Cs2PbI2(SCN)2 was achieved 2% conversion efficiency, which is considerably high efficiency among the PSCs based on A2PbI4-type 2-D perovskite families. This result may be attributed to shorter intersheet distances between lead-halide sheets.


18

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The Cs2PbI2(SCN)2 perovskite was quickly decomposed under ambient condition with high humidity. By comparison of the experiments under

inert

and

dry

conditions,

it

was

revealed

that

the

decomposition process was significantly accelerated by water molecules. It is expected that reactivity of the lead increased by replacing axial iodide with thiocyanate, which possesses strong -accepter nature, and makes lead atoms unstable to the hydration and consequent hydrolysis reactions. Oppositely, our result implies a possibility of stabilizing the 2-D perovskite by means of quasi-halides and other as axial ligands; and combination

with

high

electron-withdrawing

Br–

and

Cl–

as

bridging ligands. In addition, band gap of the 2-D perovskite can be controlled depending on the -accepter properties and electron affinity of axial quasi-halides and/or bridging halides same with 3-D perovskites. Further examinations to obtain stable and highly efficient 2-D perovskite families as a light absorber in PSCs are now undergoing.


19


Page 20 of 39

Figure 1. Crystal structures of 2-D lead-halide perovskite families with different thickness.53,40


20

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Figure 2. (a) XRD chart of Cs2PbI2(SCN)2 film on a glass substrate. (b) UV (red) and PL (blue) spectra of Cs2PbI2(SCN)2 film on the glass substrate. (c) Energy diagram of a mesoscopic Cs2PbI2(SCN)2 PSC. (d) Cross-sectional SEM images of the Cs2PbI2(SCN)2 based PSC.


21


Page 22 of 39

Figure 3. (a) J–V curve and (b) EQE spectrum of Cs2PbI2(SCN)2 based PSC.


22

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Figure 4. Photographs of the perovskite films kept under different conditions; (a) in a glove box, (b) in a dry booth, and (c) ambient condition, respectively.

Figure 5. (a) IR spectra and (b) XRD chart of fresh and decomposed Cs2PbI2(SCN)2 films on glass and mesoporous TiO2 substrates, respectively.


23


Page 24 of 39

Figure 6. XPS spectra of the perovskite before (red) and after (blue) humidity irradiation for (a) Pb 4f, (b) Cs 4p, S 2p, (c) N 1s, and (d) I 3d. Allows indicate newly appeared peaks for Pb.


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Figure 7. Schematic illustrations of expected mechanism for degradation of Cs2PbI2(SCN)2 2-D perovskite by humidity. Red (Pb), purple (I), yellow (S), black (C), blue (N), green (Cs), gray square ([PbI4S2] octahedron), and water molecules.

ASSOCIATED CONTENT Supporting Information. Photographs and UV spectrum of AA’PbI2(SCN)2 films (A = MA, FA, and Cs); Photographs and UV spectrum of thermal treated A2PbI2(SCN)2 films (A = MA and Cs); (GI-) XRD chart of A2PbI2(SCN)2 films (A = MA, FA and Cs); Surface SEM image and XPS chart of Cs2PbI2(SCN)2 films before and after decomposition (PDF) AUTHOR INFORMATION Corresponding Author


25


Page 26 of 39

*(Youhei Numata) [email protected]. *(Tsutomu Miyasaka) [email protected]. ORCID Youhei Numata: 0000-0001-8350-7226 Yoshitaka Sanehira: 0000-0003-2030-2690 Ryo Ishikawa: 0000-0002-6924-2580 Hajime Shirai: 0000-0002-1416-9742 Tsutomu Miyasaka: 0000-0001-8535-7911 Present Addresses § Research Center of Advanced Science and Technology (RCAST), The University of Tokyo, 4-6-1 Komaba, Meguro, Tokyo 153-8904 Japan. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Advanced Low Carbon Technology Research and Development Program (ALCA) by Japan Science and Technology Agent (JST) Grant-in-Aid for Scientific Research C (17K05968) of Japanese Society for Promotion of Science (JSPS) Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by Advanced Low Carbon Technology Research and Development Program (ALCA) by Japan Science and Technology Agent (JST) and Grant-in-Aid for Scientific Research C

(17K05968)

of

Japanese

Society

for

Promotion

of

Science

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Properties A

of

Combined

Two-Dimensional Experimental

and


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lead iodide perovskite, Cs2PbI2(SCN)

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