Structural Stability, Vibrational Properties, and Photoluminescence in

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Structural Stability, Vibrational Properties, and Photoluminescence in CsSnI3 Perovskite upon the Addition of SnF2 Athanassios G. Kontos,*,† Andreas Kaltzoglou,† Eirini Siranidi,† Dimitrios Palles,‡ Giasemi K. Angeli,§ Michalis K. Arfanis,† Vassilis Psycharis,† Yannis S. Raptis,∥ Efstratios I. Kamitsos,‡ Pantelis N. Trikalitis,§ Constantinos C. Stoumpos,⊥ Mercouri G. Kanatzidis,⊥ and Polycarpos Falaras*,† †

Institute of Nanoscience and Nanotechnology, NCSR Demokritos, Athens 15310, Greece Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Athens 11635, Greece § Department of Chemistry, University of Crete, Heraklion 71003, Greece ∥ Faculty of Applied Sciences, National Technical University of Athens, Athens 15780, Greece ⊥ Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ‡

ABSTRACT: The CsSnI3 perovskite and the corresponding SnF2-containing material with nominal composition CsSnI2.95F0.05 were synthesized by solid-state reactions and structurally characterized by powder X-ray diffraction. Both materials undergo rapid phase transformation upon exposure to air from the black orthorhombic phase (B-γ-CsSnI3) to the yellow orthorhombic phase (YCsSnI3), followed by irreversible oxidation into Cs2SnI6 within several hours. The phase transition occurs at a significantly lower rate in the SnF2-containing material rather than in the pure perovskite. The high hole-carrier concentration of the materials prohibits the detection of Raman signals for B-γ-CsSnI3 and induces a very strong plasmonic reflectance in the far-IR. In contrast, far-IR phonon bands and a rich Raman spectrum are observed for the Y-CsSnI3 modification below 140 cm−1 with weak frequency shift gradients versus temperatures between −95 and +170 °C. Above 170 °C, the signal is lost due to B-α-CsSnI3 re-formation. The photoluminescence spectra exhibit residual blue shifts and broadening as a sign of structural transformation initiation.

1. INTRODUCTION Halide perovskites have a AMX3 composition with monovalent A and divalent M cations and a halide (X = I−, Cl−, Br−) anion.1,2 The cation A can be organic or inorganic (e.g., CH3NH3+, Cs+) and the M cation either Ge2+, Sn2+, or Pb2+. The perovskite polymorphs are controlled by the size of the A cation and the mismatch between the average A−X and M−X bond lengths and usually crystallize in the high-symmetry cubic phase at high temperatures.3−5 Upon decreasing temperature, tilting and distortions of the lattice transform the crystal into several lower-symmetry phases, such as tetragonal and orthorhombic. The perovskite phase formed at room temperature and the size of the cation affect the electronic properties of the material, with larger cations resulting in reduced band gaps. If the cations are too small or too large, however, the perovskite phase is destabilized and there is a profound change in the structure type.6 The so-called hybrid inorganic−organic halide lead or tin perovskites are formed when the A group is an organic ammonium cation.2 Such hybrid perovskites have recently shown tremendous application potential as thin-film lightemitting diodes (LEDs)7,8 and efficient solar cells (SCs),9−11 where a record power conversion efficiency of 21.1% has been reported within the last 3 years of extensive research by mixing © XXXX American Chemical Society

organic cations such as methylammonium (MA) and formamidinium (FA) with Cs cations.12 The beneficial properties of the halide perovskites are attributed to their tunable direct band gap in the visible-to-IR region controlled by the halide composition and to their effective ambipolar transport properties. On top, they are easily solution-processed, which minimizes the cost of device fabrication. Replacement of the commonly used toxic Pb by the environmentally friendly but less stable Sn in the structure is a frontier of perovskite SC research.13,14 CsSnI3 and its counterpart CsPbI3 are typical inorganic perovskite compounds presenting very high hole mobility and long carrier mean free paths, as well as optimum band gaps for SC3 and LED15 applications. CsPbI3 processed with a lowtemperature synthesis route has been used instead of the hybrid organic−inorganic perovskite analogues as a light absorber in perovskite SCs, showing that the presence of the organic cation is not vital.16 Nevertheless, CsPbI3 is unstable and transforms to the nonperovskite yellow phase within hours. Furthermore, Special Issue: Halide Perovskites Received: September 23, 2016

A

DOI: 10.1021/acs.inorgchem.6b02318 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry it is found that, by alloying CsPbI3 with FAPbI317 and recently with FAPbI3 and MAPbI3,12 the stability of the photoactive perovskite phase is improved. As for the active layer for LED devices, CsPbX3 nanocrystals have recently demonstrated very narrow (12−42 nm), broadly tunable (410−700 nm), and very bright photoluminescence (PL) emission with quantum yields of up to 90%.15 CsSnI3 has led the perovskites race in photovoltaics since the pioneering work of Northwestern University in 2012,13 which described its use as an efficient hole-transporting material in solid-state dye-sensitized SCs. In the same work, doping of CsSnI3 with SnF2 was carried out as an efficient way to increase the efficiencies of the corresponding cells. Since then, several theoretical18−20 and experimental works21−23 have appeared dealing with the thermodynamic, structural, vibrational, and electronic properties of these materials, as well as their use in SCs. Recently, low-temperature-processed CsSnI3 has been used as a light absorber to develop Pb-free perovskite SCs.24 However, CsSnI3 alone is not effective because it presents metallic conductivity and is prone to form intrinsic defects, mainly Sn vacancies and Sn4+ centers.3,24 Thus, the addition of SnF2 is necessary to reduce the free carrier density presumably by filling the Sn vacancies with the extra Sn2+ ions and suppressing the formation of Sn4+ centers. The use of mixedhalide CsSnI3−xBrx perovskites25 is another method to lower the charge-carrier densities and improve cell photovoltages. In all of the above work, it is pointed out that understanding fundamental issues like dissolution, recrystallization, phase modification, and fast degradation of B-γ-CsSnI3 is indispensable for the efficient application of the material in SCs. The CsSnI3 perovskite exists in two orthorhombic polymorphs at room temperature:4 one has a 1D double-chain structure and is yellow (Y), and the other has a 3D perovskite structure and is black (B-γ). The B-γ phase is a highly conducting p-type direct semiconductor with a band gap of 1.3 eV, which is desirable for SCs, in contrast to the Y phase, which is an indirect semiconductor with a 2.55 eV band gap. When the Y phase is heated above 150 °C in an inert atmosphere, it transforms to the black cubic perovskite phase (B-α), which, upon subsequent cooling, converts to the black tetragonal phase (B-β) below 147 °C and the black orthorhombic phase (B-γ) at about 80 °C. The yellow phase is the thermodynamically stable one at room temperature. The orthorhombic black phase is a kinetically stabilized metastable phase that transitions to the yellow phase when perturbation occurs (oxygen, moisture, solvent, change of pressure when opening the silica tube, etc.). Prolonged exposure to air causes oxidation into the defect perovskite Cs2SnI6 (Figure 1). In this work, we present a thorough structural and vibrational investigation upon phase transformations of B-γ-CsSnI3 in YCsSnI3 and Cs2SnI6, and we study the role of SnF2 addition in improving the stability of the system. We also report on the irreversible broadening and shift of the PL emission peak upon slow degradation of B-γ-CsSnI3, which is of high significance for SCs.

Figure 1. Crystal structures of B-γ-CsSnI3, Y-CsSnI3 (1 × 2 × 1 unit cell), and Cs2SnI6 and their phase transformation routes. Unit cell edges are drawn with thin black lines. then transferred to a vacuum line and evacuated to ca. 10−3 mbar, flame-sealed, and heated to 400 °C for 5 h, followed by quenching in cold water. The air-sensitive polycrystalline products were thoroughly ground in an agate mortar and stored in a glovebox. Samples were placed on an open glass holder and then loaded on a Rigaku SmartLab diffractometer, which operates with Cu radiation and a Ni filter in Bragg−Brentano geometry. Successive scans were performed as a function of the exposure time to ambient air. Data were collected over the angular range of 10° ≤ 2θ ≤ 100° in a 2θ scan rate of 4°/min for all patterns except for the “1240 min” and “1540 min” patterns for the pure perovskite sample and the “2820 min” pattern for the SnF2containing sample, which were recorded in 1°/min in order to obtain a better signal-to-background ratio. Inevitably, each sample was exposed to air for 10 min prior to the first scan on the diffractometer. The Rietveld method was used for both crystal structure refinement and quantitative analysis of the samples with the GSAS package.26 In order to make a qualitative estimation of the perovskites’ electrical conductivity, 50 mg of the SnF2-containing sample was dissolved in 1 mL of N,N-dimethylformamide in the glovebox. The clear yellow solution was spread dropwise on a microscope slide, the solvent was left to evaporate at 60 °C on a hot plate, and then selfadhesive Cu contacts were firmly attached on the sides so that the electrical resistance could be recorded. Far-IR reflectance (FIR) spectra were acquired with a quasi-normal configuration (11° angle of incidence), using a Hg lamp as a light source, a FIR beamsplitter and detector, and a gold mirror as the reference. Each spectrum was averaged over 2400 scans with a 4 cm−1 resolution. Measurements were carried out on powder samples pressed in the form of 10-mm-diameter and ca. 1-mm-thick disks inside the glovebox using a homemade stainless steel hand press. Micro-Raman spectra were measured in a backscattering configuration on a Renishaw inVia Reflex microscope using diode lasers emitting at λ = 514.4 and 785 nm, respectively, as excitation sources. PL experiments were performed on the same instrument. Excitation was done at 785 nm and power density was typically in the order of 1 μW/μm2. PL signal was recorded by averaging over relatively large areas of ≈50 μm2. Samples were examined in three conditions of different reactivity either chosen intentionally or directed by the experimental limitations of the specific characterization technique: (a) under the strictly inert conditions of vacuum in sealed silica tubes; (b) under a slightly reactive environment, inside a sealed THMS600PS Linkam temperature cell (heating−freezing stage) with optical windows that permit the acquisition of variable-temperature data; (c) in highly reactive open air. The laser beam was focused onto the samples by means of 5× long-distance microscopic objectives, while analysis of the scattered beam was performed on a 250-mm-focallength spectrometer along with a 1800 lines/mm (1200 lines/mm for

2. EXPERIMENTAL SECTION Reagents were handled in an Ar-filled glovebox with oxygen and humidity levels of 99.5%), SnI2 (Alfa-Aesar, 99.999% metal basis), and SnF2 (Aldrich, 99%) were mixed in molar ratios of 1:1:0 and 1:0.975:0.025 for the synthesis of pure perovskite and the SnF2-containing samples, respectively. The reaction mixtures were loaded into fused-silica tubes. The tubes were B

DOI: 10.1021/acs.inorgchem.6b02318 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Normalized PXRD patterns at room temperature for CsSnI3 (a) and SnF2-containing CsSnI3 (b) as a function of the exposure time to air. The histograms below refer to the theoretical patterns of the corresponding perovskites. the 785 nm line) diffraction grating and a high-sensitivity CCD detector. A near-excitation (NeXT) Rayleigh rejection filter, which permits the acquisition of Raman spectra at the very low frequency range (above 30 cm−1), was used when necessary. Spectra were acquired from different spots for each studied sample, while the frequency shifts were calibrated by an internal Si reference.

8.635(5) Å, b = 8.706(5) Å, and c = 12.384(6) Å for B-γCsSnI3, a = 10.372(5) Å, b = 4.7620(17) Å, and c = 17.718(7) Å for Y-CsSnI3, and a = b = c = 11.645(5) Å for Cs2SnI6. For the SnF2-containing sample after 190 min of exposure to air, the corresponding values are a = 8.637(4) Å, b = 8.687(4) Å, and c = 12.378(4) Å for B-γ-CsSnI3, a = 10.335(12) Å, b = 4.750(4) Å, and c = 17.697(16) Å for Y-CsSnI3, and a = b = c = 11.650(6) Å for Cs2SnI6. This is in agreement with the literature data because the only structurally characterized ternary Cs−Sn−F compound is Cs2SnF6 with a trigonal structure.27 In contrast to the well-studied solid solutions for CsSnCl3−xBrx and CsSnBr3−xIx,28 the formation of a mixedhalide CsSnI3−xFx is not expected because of the large radius difference of the two halide anions, namely, 1.33 Å for F− and 2.2 Å for I−. Nevertheless, no unreacted CsI, SnF2, or any decomposition products (such as SnO2) could be detected by powder X-ray diffraction (PXRD) analysis probably because of their very low concentration. Electrical Resistance Measurements. The electrical properties of the SnF2-containing sample were studied on films deposited on glass substrates. Despite the fact that this method is not quantitative, because various parameters such as the perovskite film thickness and density as well as the firmness of Cu contacts are not accurately controlled, a qualitative and reproducible view of the change in the electrical resistance upon phase transformations of the perovskites could be obtained. First, the perovskite samples that are stored in the glovebox remain in the B-γ phase, and their resistance is stable. Exposure of the B-γ perovskite to air then causes a gradual phase transformation with visible alteration of its color to yellow as well as a large increase of its electrical resistance (Figure 4), characteristic of its transformation to the Y phase.29 Upon prolonged exposure to ambient conditions, the film turns black again because of the formation of air-stable Cs2SnI6 and its resistance drops to values comparable with those initially obtained for the B-γ phase. Far-IR Spectroscopy. Far-IR measurements obtained directly after exposure of CsSnI3 to air showed a broad reflectance background signal at low frequency, which implies domination of the reflectance by the free carrier contribution (plasma mode),30−32 which is so strong that it masks the clear attendance of phonon modes. A few minutes later and for the next few hours, two additional strong far-IR phonon bands appear in the spectra, which peak at 62 and 119 cm−1 (marked with vertical dashed lines in Figure 5), that are attributed to the Y-CsSnI3 phase. Finally, after prolonged exposure of the pellets

3. RESULTS AND DISCUSSION Crystallographic Analysis. Both SnF2-free and SnF2containing CsSnI3 samples were structurally analyzed on the powder diffractometer at various exposure times to ambient air, as shown in Figure 2a,b. The results confirm quantitatively the previously reported trend4 of the gradual transformation from B-γ-CsSnI3 to Y-CsSnI3 and then irreversible oxidation into Cs2SnI6 (Figure 3). The SnF2-containing sample undergoes a

Figure 3. Phase distribution based on the PXRD analysis of the SnF2free and SnF2-containing samples as a function of the exposure time to air (in a logarithmic scale).

slower transformation by approximately a factor of 2 from black to yellow CsSnI3. On the other hand, the oxidation kinetics from Y-CsSnI3 to Cs2SnI6 (also black) is much less affected by the presence of SnF2. Both samples show the coexistence of all three phases (B-γ-CsSnI3, Y-CsSnI3, and Cs2SnI6) at the intermediate periods of 3−4 h after exposure to air. The lattice parameters of the perovskites in both SnF2-free and SnF2-containing samples are essentially the same within the standard 3σ deviation. In particular, the values obtained for the SnF2-free sample after 210 min of exposure to air are a = C

DOI: 10.1021/acs.inorgchem.6b02318 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Electrical resistance (in logarithmic scale) of a SnF2containing perovskite film versus the duration of exposure to air. The resistance is estimated in a relative scale to the mean value attained from the B-γ phase of the sample before taking it out of the glovebox. Inset: Photograph of the perovskite film with Cu contacts inside the glovebox.

Figure 6. Evolution of Raman spectra for the SnF2-containing sample after exposure to ambient conditions.

hours, but the spectrum is considerably affected after prolonged exposure of the sample to ambient conditions for about 1 day. After such long time, the sample is transformed to the Cs2SnI6 phase. This change is verified by recording Raman spectra under NIR (785 nm) excitation. Under these conditions, Cs2SnI6 is excited under resonance, while Y-CsSnI3 is offresonance. Thus, the Raman scattering efficiency of Cs2SnI6 is strongly enhanced, and the spectrum gained (see Figure 6, top) perfectly matches that obtained in the literature. Considering the X-ray diffraction (XRD) and electrical conductivity analysis, we interpret both Raman and far-IR spectra in a consistent way. Under ambient conditions, the black orthorhombic phase remains pure only for a short time. For pure CsSnI3, the Y phase appears just after a few minutes, while for the SnF2-containing sample, the B-γ phase is dominant for almost half an hour, before the Y phase is observed. For the B-γ phase, there are no detectable phonon modes because the large number of free carriers screen the radiation from inserting in the material bulk and induce strong plasmon reflectance in the far-IR. After exposure of the sample to ambient air for a few minutes up to several hours, the lowsymmetry Y phase is formed and exhibits rich far-IR and Raman spectra below 140 cm−1. According to group theory34 and theoretical calculations,18 a total number of 22 IR-active and 30 Raman-active modes are expected for this material, all of them at frequencies below 140 cm−1. Table 1 summarizes the outcome of the structural and vibrational characterization of the B-γ phase and of its structural transformation upon exposure to air. In turn, after transformation of the SnF2-containing sample to the yellow phase, its Raman signal was studied from −95 to +170 °C (Figure 7a). Up to 155 °C, the distinct Raman signal of the yellow phase was observed. At temperatures of up to 110 °C, the samples are not particularly sensitive and a relatively high laser power density (0.5 mW/μm2) was used, whereas above 110 °C and up to 155 °C, a very low laser power density (0.05 mW/μm2) was used to avoid photoinduced effects on the Raman spectra. No signals could be detected at 170 °C, characteristic of the complete phase transformation of the

Figure 5. Evolution of FIR spectra for SnF2-free and SnF2-containing CsSnI3 samples after exposure to ambient conditions.

to the ambient conditions for 19−22 h, the bands of Cs2SnI633 appear superimposed to those of Y-CsSnI3. Raman Spectroscopy. Micro-Raman experiments were carried out to characterize the black orthorhombic (B-γ) phase of the SnF2-free and SnF2-containing CsSnI3 materials. The samples were initially sealed in silica tubes. Attempts were made to observe Raman signals from these samples by performing a series of experiments at various irradiation levels but resulted only in a broad background increasing toward low frequencies. Then, the samples were transferred from the silica tubes into the sealed temperature cell (slightly reactive environment). In this way, safe loading of the material under an Ar atmosphere was achieved; however, very strict inert conditions of Raman experimentation are not secured because of a gradual mild air contamination of the cell’s environment. However, a Raman signal from the perovskite was first observed after the sample was stored in the temperature cell for some days. The above observation led us to probe by Raman spectroscopy the rapid degradation of B-γ-CsSnI3 with its direct exposure to ambient conditions. A trend similar to that observed in the far-IR spectra is also presented for evolution of the Raman spectra versus the duration of SnF2-containing sample exposure to air, as shown in Figure 6. In the spectra recorded immediately after air exposure of the perovskite, there is no Raman signal apart from a background scattering continuum at low frequencies. After approximately 30 min, a rich Raman peak is observed below 140 cm −1 . The spectroscopic characteristics remain well observed for some D

DOI: 10.1021/acs.inorgchem.6b02318 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Structural and Vibrational Data for the CsSnI3 and Cs2SnI6 Phases

a

phase

space group

B-γ-CsSnI3 Y-CsSnI3

Pnma Pnma

Cs2SnI6

Fm3̅m

Raman peaks (cm−1) not observable 36, 51, 68, 91, 116, 127, 138 78, 92, 126

far-IR peaks (cm−1)

Eg (eV)a

not observable 62, 119

1.30 2.55

46, 83, 166

1.48

Energy-gap values are taken from the literature.4,35

material to the black phase. The transformation was evident by the black color of the powder after opening the cell. We should emphasize here that, below 170 °C, no phase transformation is observed, apart from that the sample becomes very photosensitive above 110 °C. This further verifies that we actually record the spectrum of the Y phase and not the B-γ phase, which is expected to transform from orthorhombic to tetragonal above 99 °C. The resemblance of the Raman spectra characteristics from −95 to +170 °C shows that the Y phase is thermodynamically stable over this temperature range. The major peaks present minor red temperature shifts below 0.013 cm−1, as shown in Figure 7b. Shifts of the low-frequency lattice modes are relatively stronger. Similar frequency shifts were observed in the orthorhombic CsSnCl3,36 which also undergoes no phase transition between −188 and +21 °C. It is also worth noting that CsSnI3 allows the recording of strong Raman scattering signals. This shows that they are not particularly sensitive to light-induced degradation, in contrast to the hybrid organic−inorganic MAPbI3 perovskite, which is very photosensitive and can only be examined under off-resonance excitation in Raman.37 PL Spectroscopy. PL data were recorded from B-γ CsSnI3 in the evacuated silica tubes and the in the temperature cell to monitor the initial degradation steps of the material. Characteristic PL spectra are shown in Figure 8 with black solid lines. Emission is observed at about 955 nm for SnF2-containing sample. For the SnF2-free sample inside the temperature cell a

Figure 8. Normalized PL spectra of SnF2-free and SnF2-containing CsSnI3 before (solid black lines) and after (dashed red lines) thermal treatment at 210 °C. Irreversible PL effects inside the sealed temperature cell and reversible PL effects for the samples enclosed in silica tubes are observed.

blue shift to 935 nm as well as broadening of PL are observed. The spectral lines are relatively broad and for the SnF2containing sample their shape is asymmetric. This behavior is in good corroboration with previous reports of asymmetric PL spectral line38,39 and indicates inhomogeneous PL emission stemming from the nanocrystalline character of the material. In fact, perovskite nanocrystals with quantum sizes are expected to present electron confinement which results in band gap widening and consequent blue shifting of the PL emission.15 A thermal treatment of the perovskite was performed by heating the material at 210 °C for 20 min with raising and lowering temperature ramps of 2 °C/min. Recorded PL spectra at room temperature after the thermal cycle are shown in Figure 8 with red dashed lines. A complex hysteresis effect is seen for both SnF2-free and SnF2-containing materials in the sealed temperature cell, where ideal inert conditions are not secured. However, this permitted us to follow the initial stage of the materials phase transformation. On the other hand, PL emission is highly reversible for the sample enclosed in the

Figure 7. (a) Micro-Raman spectra for polycrystalline SnF2/CsSnI3 as a function of the temperature. (b) Frequency shifts versus temperature for some characteristic Raman peaks. Temperature slopes are obtained with the least squares method. E

DOI: 10.1021/acs.inorgchem.6b02318 Inorg. Chem. XXXX, XXX, XXX−XXX

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4. CONCLUSIONS SnF2-free and SnF2-containing samples of the CsSnI3 perovskites have been synthesized by a solid-state reaction and thoroughly studied for their structural and PL properties. According to the PXRD analysis, the perovskite structure is formed in the black orthorhombic B-γ phase and contains no fluorine as an iodine substitute. The high density of free carriers prohibits detection of vibrational modes in this phase and induces very strong plasmonic reflectance. Raman scattering and far-IR spectroscopic data verify the transformation from the black phase to the yellow phase in accordance with the XRD analysis and conductivity measurements, which perceive large changes in the carrier concentration. Moreover, all experimental data confirm that the SnF2-containing material presents significantly higher chemical stability than pure CsSnI3. The Raman spectrum of the Y phase shows many lowfrequency phonon modes (