Optical and Electrical Properties of Perovskite Variant (CH3NH3)2SnI6

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C: Energy Conversion and Storage; Energy and Charge Transport 3

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Optical and Electrical Properties of Perovskite Variant (CHNH)SnI Fuji Funabiki, Yoshitake Toda, and Hideo Hosono

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01820 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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The Journal of Physical Chemistry

Optical and Electrical Properties of Perovskite Variant (CH3NH3)2SnI6 Fuji Funabiki*†, Yoshitake Toda†, and Hideo Hosono†

†

Materials Research Center for Element Strategy, Tokyo Institute of Technology,

4259 Nagatsuta, Yokohama 226-8503, Japan

*Corresponding author E-mail address: [email protected] Phone number: +81-45-924-5128

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT In recent years lead halide perovskites have emerged as excellent photovoltaic materials for solar power generation. However, since they are toxic and chemically unstable in air, lead-free perovskites are also being investigated. In this study, the perovskite variant (CH3NH3)2SnI6 was studied. Polycrystalline films of (CH3NH3)2SnI6 were prepared using the thermal evaporation method. The films had a direct band gap of 1.81 eV with a strong absorption coefficient of ~7 × 104 cm−1. In addition, the films were n-type with a carrier concentration of ~2 × 1015 cm−3 and an electron mobility of ~3 cm2 V−1 s−1. Moreover, the conductivity was increased by a factor of four under simulated solar illumination (100 mW cm−2). These results indicate that (CH3NH3)2SnI6 is a lead-free optical semiconductor suitable for solar cell applications.

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INTRODUCTION Solar power generation is a clean energy way of producing electricity using sunlight. Several photovoltaic materials such as Si, GaAs, and CdTe are currently used in solar panels. In recent years, lead-based perovskites of the type APbX3 (where A = monovalent cations and X = halogens) have become of great interest to researchers for their use as novel photovoltaic materials. Current lead perovskite solar cells can achieve high power conversion efficiencies (PCEs) of up to 22.7%.1 However, they are toxic and chemically unstable in air. Therefore, lead-free perovskites are also being investigated.2,3 Tin-based perovskites of the type ASnI3 are relatively nontoxic materials.4–17 Current solar cells using CH3NH3SnI3 exhibit good PCEs of up to 8.12%.9,10,14,16,17 In addition, the perovskite variant Cs2SnI6 is an air-stable material.18–31 The reason is that Sn4+ is resistant to oxidation reaction compared with Sn2+. However, according to density functional theory (DFT) calculations, the oxidation state of Sn in Cs2SnI6 is nearly 2+.20,26 Thus, the chemical state of tin is controversial. The first tested solar cell using Cs2SnI6 showed a small PCE of 0.96% in air.29 Meanwhile, the perovskite variant (CH3NH3)2SnI6 was found to be an impurity in CH3NH3SnI3. Although this material is a novel perovskite

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variant, only a few researches have been conducted into its properties so far.31 To better understand the perovskite family, it is necessary to evaluate this material. In this study, a high quality thin film of (CH3NH3)2SnI6 was successfully prepared via thermal evaporation, and its optical and electrical properties were examined. We report here that (CH3NH3)2SnI6 acts as an optical semiconductor.

EXPERIMENTAL Materials and Methods. First, pure SnI4 was prepared by mixing SnI2 (99.9%, Kojundo, Japan) and iodine (99.99%, Kojundo, Japan) at a temperature of 100 °C. Next, SnI4 was manually dry mixed with CH3NH3I (>98.0%, Wako, Japan) in a molar ratio of 1:2 in a mortar at room temperature. The color of SnI4 immediately turned from orange to black, and finally pure (CH3NH3)2SnI6 powder was formed. Then, the powder was evaporated in a tungsten boat at 120 °C in a vacuum chamber at a pressure of 10−4 Torr to deposit the film on silica glass or silicon wafer substrates.

Structural Characterization. The surface of the film was observed using a field-emission scanning electron microscope (FESEM; JSM-7600F, JEOL,

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Japan). The X-ray diffraction patterns of the powder and the resulting films were recorded using an X-ray diffractometer with Cu Kα radiation (D8 Advance, Bruker, Germany). The crystal structure parameters were determined by a Rietveld refinement program (TOPAS, Bruker, Germany). The film thickness was measured by a stylus profilometer (D-120, KLA-Tencor, USA) and also by simulating the interference pattern observed in the optical transmission spectrum. The energy levels of the electrons in the films were measured using photoelectron spectroscopy in air (PESA; AC-2, Riken Keiki, Japan) and hard X-ray photoelectron spectroscopy (HAXPES) at the BL15XU beamline (hν = 6 keV) of SPring-8 (Kobe, Japan). Since HAXPES is bulk-sensitive, information on bulk was thus obtained.

Optical and Electrical Measurements. The optical transmission spectrum of the film was measured using an ultraviolet–visible–near-infrared spectrometer (U4000, Hitachi, Japan). The refractive index of the film was measured by simulating the thin film interference pattern.32 The photoluminescence (PL) spectrum of the film was measured in the wavelength range from 200 to 900 nm using a fluorescence spectrometer (F4500, Hitachi, Japan). The electrical

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properties of the film were measured in the dark in helium at temperatures of 180 to 293 K at a magnetic field of 350 mT using the van der Pauw method (ResiTest 8300, Toyo Corporation, Japan). For the measurement, Au electrodes (2 × 2 mm2) were initially deposited on the corners of a silica glass substrate (10 × 10 × 1 mm3) using an electron beam evaporator (SVC-700LEB, Sanyu Electron, Japan). The photoelectrical conductivity of the film was measured in air at 293 K with a semiconductor parameter analyzer (Agilent41550, Agilent Technologies, USA) and a solar simulator (HAL320, Asahi Spectra, Japan).

RESULTS AND DISCUSSION A bluish–black film was formed on a substrate via the thermal evaporation of black powders. The resulting films were kept in a cool and dry environment because of their poor thermal stability and high water solubility. Figure 1a shows the FESEM image of the film's surface. The film was composed of distorted grains of about 100 nm in size. Since the evaporation temperature of this material is much lower than those of other tin-based perovskites (e.g. ~270 °C for Cs2SnI6),24,27 the grain size was much smaller than for other tin-based perovskites

(generally

between

100

nm

6


and

several

tens

of

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micrometers).8,17,18,24,27,29 Figure 1b and c shows the X-ray diffraction patterns of the prepared powder and the film, respectively. According to our Rietveld refinement (see Supporting Information), they were identified to be a cubic K2PtCl6-type phase with the Fm-3m space group and to have no impurity phases such as CH3NH3I, SnI2, SnI4, or CH3NH3SnI3. In addition, the film was well oriented in the (111) plane. It is considered that (CH3NH3)2SnI6 is thermally decomposed into gaseous components in the chamber, and the (111) plane is energetically the most stable surface. Figure 1d shows the crystal structure of (CH3NH3)2SnI6. Unlike for perovskites, the SnI6 units do not bond to each other but form a face centered cubic structure, while the CH3NH3 molecules occupy the interstitial sites coordinated by four SnI6 units. The lattice constant of (CH3NH3)2SnI6 was measured to be 12.016 Å, which was larger than that of Cs2SnI6 (11.644 Å) because the CH3NH3 molecule is larger than the Cs atom. The intra-octahedral tin–iodine distances in (CH3NH3)2SnI6 and Cs2SnI6 are 2.864 and 2.857 Å, respectively. The intra-octahedral iodine–iodine distances in (CH3NH3)2SnI6 and Cs2SnI6 are 4.051 and 4.041 Å, respectively. The inter-octahedral iodine–iodine distances in (CH3NH3)2SnI6 and Cs2SnI6 are 4.445 and 4.193 Å, respectively. Since the bond strength decreases as the bond

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length increases, (CH3NH3)2SnI6 is likely more unstable than Cs2SnI6.

(a)

Intensity (a.u.)

(b)

200nm 0

10

20

30

40

50

60

2θ (deg)

(d)

(c) After 7days in air

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(222) (111)

As-prepared

(002)

0

10

(004)

20

30

(044)

40

(444)

50

60

2θ (deg)

Figure 1. (a) Field-emission scanning electron microscopy (FESEM) image of a (CH3NH3)2SnI6 film. X-ray diffraction patterns of (b) powder and (c) film samples of (CH3NH3)2SnI6. (d) Crystal structure of (CH3NH3)2SnI6. In the structure, red octahedrons represents SnI6 units and the blue sphere represent CH3NH3 molecules.

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Figure 2a shows the PESA spectrum of the (CH3NH3)2SnI6 film. The valence band maximum (VBM) was found at 5.47 eV below the vacuum level. Figure 2b– d shows the HAXPES spectra of the (CH3NH3)2SnI6 film. The Fermi level was found at 1.69 eV above the VBM. The core levels of Sn3d5/2 and Sn3d3/2 were found at 487.6 and 496.03 eV below the Fermi level, respectively. Further, the core levels of I3d5/2 and I3d3/2 were found at 620.11 and 631.60 eV below the Fermi level, respectively. Each peak could be fitted well with a single Gaussian, which confirms that the Sn atoms have only one chemical state, as is the case with iodine atoms. This result agrees with the fact that there is no impurity phase in the film.

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(b)

Yield1/3 (a.u.)

Intensity (a.u.)

(a)

1.69 eV

5.47 eV 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

0 1 2 3 4 5 6 7 8 9 10

Photon energy (eV)

Binding energy (eV)

(d)

Gaussian fitting Sn 3d5/2 487.60 eV Sn 3d3/2 496.03 eV

Intensity (a.u.)

(c) Intensity (a.u.)

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Gaussian fitting I 3d5/2 620.11 eV

I 3d3/2 631.60 eV

475 480 485 490 495 500 505

610 615 620 625 630 635 640

Binding energy (eV)

Binding energy (eV)

Figure 2. (a) Photoelectron spectrum in air (PESA) and (b–d) hard X-ray photoelectron (HAXPES) spectra of a (CH3NH3)2SnI6 film.

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Figure 3a shows the optical transmission spectra of the (CH3NH3)2SnI6 films with different thicknesses. Broad absorption bands appear in the visible region, and thin film interference patterns can be clearly observed in the near-infrared region. Figure 3b shows the absorption spectrum obtained by subtraction of the thin film interference pattern. There are two absorption peaks at 2.0 and 3.0 eV. This spectral shape is similar to that of a Cs2SnI6 film (see Supporting information). The absorption coefficient was found to be ~7 × 104 cm−1 at the peak. The 620-nm-thick film could absorb 64% of the sunlight's energy. Figure 3c shows the band gaps of the (CH3NH3)2SnI6 films. The band gap was found to be 1.81 eV, which is larger than that of Cs2SnI6 (1.63 eV). This result is consistent with DFT calculations that show that the band gap increases when the size of the monovalent cation (A) in A2SnI6 compounds is increased.25 On the other hand, this value is considerably larger than the value reported using diffuse reflectance spectroscopy (1.35 eV).31 This result may be related to the fact, for Cs2SnI6, that the band gap measured by transmission spectroscopy (1.48 to 1.62 eV)24,28,29 is larger than that measured by diffuse reflectance spectroscopy (1.23 to 1.27 eV).18,19,23,31 Figure 3d shows the refractive indexes of the (CH3NH3)2SnI6 films. The refractive index was found to be 1.92

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irrespective of the film thickness. Meanwhile, PL was not observed from our films, although

PL

has

been

observed

from

most

tin-based

perovskites.4,9,11,13,14,17,24,28,29 It has been indicated that PL activity changes with the annealing,4,8 the sample quality,8 and the sample form.23 Therefore, we expected the (CH3NH3)2SnI6 powder and the annealed films to show PL, but could not observe any PL from them. This question was investigated in the photoconductivity measurement.

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80

(b)

10 8

60 40 nm 100 nm 140 nm 310 nm 620 nm

40 20

6

(αhν)2

100

α (104 cm-1)

(a)

4 2

0

1.81 eV

0 0

500 1000 1500 2000 2500

1

2

3

4

5

Photon energy (eV)

1.85 1.80 1.75

(d)

2.4

Refractive index

Wavelength (mm)

(c) 1.90 Band gap (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Transmission (%)

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2.2

1.70

2.0 1.8 1.6 1.4

0 100 200 300 400 500 600 700

0 100 200 300 400 500 600 700

Film thickness (nm)

Film thickness (nm)

Figure 3. (a) Optical transmission spectra of the (CH3NH3)2SnI6 films with different thicknesses. (b) Absorption spectrum obtained by subtracting the thin film interference pattern of the 310-nm-thick film. (c) Band gaps and (d) refractive indexes of the films.

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The electrical data of the (CH3NH3)2SnI6 films with different thicknesses measured at room temperature are summarized in Table 1. All films were n-type semiconducting. As the film thickness was increased, the carrier concentration decreased and the mobility increased. The 600-nm-thick film had a resistivity of ~1 × 103 Ω cm, a carrier concentration of ~2 × 1015 cm−3, and a mobility of ~3 cm2 V−1 s−1. According to the literature, Cs2SnI6 can be either n-type (with a carrier concentration of ~1 × 1014 to ~6 × 1016 cm−3 and a mobility of 2.78 to 310 cm2 V−1 s−1)18,23,24,30 or p-type (with a carrier concentration of ~1 × 1014 to 3.65 × 1019 cm−3 and a mobility of 42 to 382 cm2 V−1 s−1).18,28 Our previous DFT calculations on Cs2SnI6 predicted that the intrinsic defects, VI and Sni, could be electron donors with a small formation enthalpy of 0.28 and 0.93 eV, respectively, and a shallow donor level at 0.52 and 0.11 eV below the conduction band minimum (CBM), respectively.21 They likely formed in (CH3NH3)2SnI6 because (CH3NH3)2SnI6 structurally resembles Cs2SnI6. Figure 4a shows the temperature dependence of the carrier concentration, in which the concentration decreases rapidly with increasing temperature. In semiconductor physics, carrier concentration varies according to the Arrhenius equation because of defect activation at low temperatures and band gap excitation at high temperatures. In

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this study, the activation energy for the carrier concentration was observed to be 0.21 eV, which is smaller than the band gap of 1.81 eV, but larger than the suitable donor level of less than 0.1 eV. This result roughly agrees with the fact that the energy difference between the CBM and the Fermi level is 0.12 eV. However, Figure 4b shows the temperature dependence of the mobility, in which the mobility increases slowly with increasing temperature. In semiconductor physics, mobility changes proportionally to T1.5 at low temperatures because of ionized impurity scattering and proportionally to T−1.5 at high temperatures because of lattice scattering. In this study, the mobility was observed to be proportional to T1.42, probably because of ionized impurity scattering. For the same reason, increasing carrier concentration decreases mobility, as shown in Table 1.

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Table 1. Electrical Data of the (CH3NH3)2SnI6 Films Measured at Room Temperature. resistivity

carrier concentration

mobility

thickness

type 15

-3

2

-1

-1

(Ω cm)

(10

cm )

(cm V s )

100 nm

1090 ± 363

5.25 ± 2.28

1.29 ± 0.25

n

300 nm

976 ± 210

2.93 ± 1.06

2.43 ± 0.43

n

600 nm

1114 ± 232

1.95 ± 0.55

3.05 ± 0.56

n

1017 10

16

10

15

(b)

5 µ0Ta of a=1.42

n0exp(-E/kT) of E=0.21eV µ (cm2 V-1 s-1)

(a)

n (cm-3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1014 1013

4 3 2 1

1012 1011

0 160

200

240

280

320

160

200

240

280

320

T (K)

T (K)

Figure 4. Temperature dependences of (a) carrier concentration and (b) mobility of 600-nm-thick (CH3NH3)2SnI6 film.

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Figure 5 shows the photoconductivity (σpc) of the (CH3NH3)2SnI6 films under simulated sunlight. The photoconductivity increases as the irradiance increases. The incident photon flux is ~5.4 × 1017 cm−2 s-1 at an intensity of 100 mW cm−2; the photo-excitation density (G) is ~1.2 × 1022 cm−3 s−1, which is calculated by multiplying the incident photon flux by the absorption coefficient of the film. According to the literature,33 the σpc and the G are in a power-law relation, σpc∝ Ga, with the exponent a=1 or 0.5, and the photocarrier density (n) obeys the following rate equation. ௗ௡ ௗ௧

௡

= ߢ‫ ܩ‬− ఛ − ߛ݊ଶ = 0.

(1)

೟ೝ

Here, κ is the photocarrier generation efficiency (≤100%), τtr is the trap-limited carrier lifetime, and γ is the coefficient of electron-hole recombination; τr≡(γn)-1 is the electron-hole recombination lifetime. In this study, the σpc is observed to be proportional to G0.9, probably because the carrier lifetime was dominated by the defect’s trapping prior to the electron-hole recombination (i.e. τtr

Optical and Electrical Properties of Perovskite Variant (CH3NH3)2SnI6

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