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Article Cite This: ACS Omega 2017, 2, 7016-7021
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Solvent-Coordinated Tin Halide Complexes as Purified Precursors for Tin-Based Perovskites Masashi Ozaki,† Yukie Katsuki,† Jiewei Liu,† Taketo Handa,† Ryosuke Nishikubo,‡ Shinya Yakumaru,† Yoshifumi Hashikawa,† Yasujiro Murata,† Takashi Saito,† Yuichi Shimakawa,† Yoshihiko Kanemitsu,† Akinori Saeki,‡ and Atsushi Wakamiya*,† †
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 566-0871, Japan
‡
S Supporting Information *
ABSTRACT: A series of solvent-coordinated tin halide complexes were prepared as impurity-free precursors for tin halide perovskites, and their structures were determined by single-crystal X-ray diffraction analysis. Using these precursors, the tin halide perovskites, MASnI3 and FASnI3, were prepared, and their electronic structures and photophysical properties were examined under inert conditions by means of photoelectron yield spectroscopy as well as absorption and fluorescence spectroscopies. Their valence bands (MASnI3: −5.02 eV; FASnI3: −5.16 eV) are significantly higher than those of MAPbI3 or the typical hole-transporting materials 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′spirobifluorene and poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine). These results suggest that to develop the solar cells using these tin halide perovskites with efficient hole-collection properties, hole-transporting materials should be chosen that have the highest occupied molecular orbital levels higher than −5.0 eV.
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INTRODUCTION
perovskite solar cells, the preparation and use of highly purified tin halide precursor materials should be even more important. Upon complexation of SnX2 (X = F, Cl, Br, and I) with coordinating solvents such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), we developed a series of purified tin halide materials [SnX2(S)n] (S = DMF and DMSO) in this study. Using the thus-obtained highly purified precursors, such as [SnI 2 (dmf)], [SnI 2 (dmso)], and [SnI2(dmso)2], the Sn-based perovskites MASnI3 and FASnI3 were prepared, and their electronic and optical properties were examined under inert conditions by means of photoelectron yield spectroscopy (PYS) and photophysical measurements.
Perovskite solar cells have attracted much attention as the potential next generation of photovoltaics. Especially, solar cells that use lead halide perovskites as light-harvesting materials have been intensively studied.1−4 Even though their performances with respect to power-conversion efficiencies (PCEs) and durability1−4 have been improved substantially within only a few years, the inherent toxicity of lead compounds remains a concern and bottleneck for practical applications. As potential alternative light-harvesting materials, Sn-based perovskites have attracted attention, particularly owing to their lower toxicity.5−7 Although the highest reported PCEs of Sn-based perovskite solar cells are 5−8%,8−11 their performance in photovoltaic devices is still lower than that of Pb-based perovskite solar cells and moreover suffers from reproducibility issues. Different from Pb-based solar cells,12 the inferior performance of Sn-based perovskite solar cells should probably be attributed to the lower quality of the perovskite layer.13 The major difference between materials based on Pb and Sn is the stability of their divalent ions. In contrast to Pb2+, Sn2+ is easily oxidized to the more stable Sn4+. Indeed, Sn-based perovskite materials, such as MASnI3 (MA: methylammonium, CH3NH3+) and FASnI3 (FA: formamidinium, CH(NH2)2+), are sensitive to oxidation upon exposure to air. The resulting Sn4+ should subsequently affect the device performance by self-doping.14,15 For the fabrication of efficient Pb-based perovskite solar cells with high reproducibility, we have demonstrated that the purity of the starting materials, such as PbI2, is crucial.16 For Sn-based © 2017 American Chemical Society
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RESULTS AND DISCUSSION Initially, we analyzed a commercially available sample of SnI2 (99.9%, trace metals basis, purchased from Kojundo Chemical Laboratory Co., Ltd.) by 119Sn magic-angle spinning (MAS) NMR spectroscopy, thermogravimetric analysis (TGA), and Karl Fischer titration. Surprisingly, the purchased sample of SnI2 contained up to 10 wt % SnI4, together with ∼10 000 ppm water. In the 119Sn MAS NMR spectrum of the commercial sample, a signal at −1743 ppm, which arises from the presence of SnI4, was observed in addition to the signals at −389 and −527 ppm corresponding to SnI2 (Figure 1a).17 TGA measurements revealed a significant weight loss (10.1%) at Received: September 2, 2017 Accepted: October 5, 2017 Published: October 20, 2017 7016
DOI: 10.1021/acsomega.7b01292 ACS Omega 2017, 2, 7016−7021
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Figure 1. 119Sn MAS NMR spectra and thermogravimetric curves of (a, b) commercially available SnI2 (99.9%, trace metals basis) and (c, d) sublimated SnI2.
∼150 °C, which corresponds to the sublimation of SnI4, before another substantial weight loss (63.9%) was observed at ∼330 °C, which should be attributed to SnI2 (Figure 1b). On the basis of these results, we purified SnI2 (10.7 g) by sublimation under reduced pressure (100 Pa). After removal of SnI4 (2.1 g, 3.4 mmol) as an orange crystalline powder at 150 °C, further heating to 330 °C afforded SnI2 (7.1 g, 19 mmol) as a red crystalline powder. According to 119Sn MAS NMR (−585 and −605 ppm)18,19 and atmospheric pressure chemical ionization mass spectrometry (MS) measurements (Figures S2 and S23), the residual dark brown solid (0.85 g) should consist mostly of SnO2. Although SnI4 could be removed by sublimation (Figure 1c,d), the sublimed SnI2 powder afforded yellow solutions upon dissolution in DMF or DMSO that contained insoluble small brown particles of SnO2, which was confirmed by a mass spectroscopic analysis (Figure S3). This result indicated that one sublimation should not be sufficient to purify SnI2. As the higher homologue PbI2 forms complexes with DMF16 and DMSO,20 we tried to further purify SnI2 by recrystallization from these solvents. For that purpose, filtered solutions of SnI2 in DMF or DMSO were layered with toluene or dichloromethane (CH2Cl2). The slow diffusion of toluene into a DMF solution of SnI2 afforded colorless crystalline needles of [SnI2(dmf)], which was confirmed by single-crystal X-ray diffraction (XRD) analysis (Figure 2). In these crystals, one molecule of DMF coordinates to the tin center [Sn−O: 2.209(2) Å], which results in Sn−I bond lengths of 2.9744(3) and 3.0065(4) Å for Sn(1)−I(1) and Sn(1)−I(2), respectively. The packing structure of [SnI2(dmf)] is characterized by a linear alignment. During the recrystallizations, the choice of antisolvent (less soluble solvent) was found to determine the nature of the SnI2 complex. For example, diffusion of CH2Cl2 into a DMF solution of SnI2 afforded orange crystalline needles
Figure 2. Molecular structure of [SnI2(dmf)]: (a) Oak Ridge thermal ellipsoid plot (ORTEP) drawing with thermal ellipsoids at 50% probability; (b) perspective view along the a axis. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Sn−O 2.209(2); Sn(1)−I(1) 2.9744(3); Sn(1)−I(2) 3.0065(4); Sn(1)−Sn(1)#1 4.4957(3); Sn(1)#1−I(1) 3.3972(3); Sn(1)#1−I(2) 3.4354(3).
of [Sn3I6(dmf)2] and the 3:2 ratio between SnI2 and DMF was confirmed by a single-crystal X-ray diffraction analysis (Figure S5). Slow diffusion of CH2Cl2 into a DMSO solution of SnI2 furnished colorless needles of [SnI2(dmso)], which were structurally characterized by single-crystal X-ray diffraction analysis (Figure 3). In the crystal structure of [SnI2(dmso)], one molecule of DMSO coordinates to the tin center [Sn−O: 2.167(3) Å] and the packing structure is defined by a linear alignment. Interestingly, when toluene was used as the antisolvent in the recrystallization from DMSO, colorless crystals of [SnI2(dmso)2] were obtained as the only product, in which two molecules of DMSO coordinate to the tin center 7017
DOI: 10.1021/acsomega.7b01292 ACS Omega 2017, 2, 7016−7021
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were prepared by a one-step solution method, that is, a 1.5 M DMSO solution of [SnI2(dmso)2] and MAI or FAI in a 1:1 ratio was deposited on a quartz substrate by spin-coating. After thermal annealing at 100 °C, black films were obtained. The formation of MASnI3 and FASnI3 was confirmed by X-ray diffraction (XRD) analyses of the crystalline powders and films using synchrotron and Cu Kα radiation, respectively (Figure 4b,d). The XRD patterns of the crystalline powder samples,
Figure 3. Molecular structure of [SnI2(dmso)]: (a) ORTEP drawing with thermal ellipsoids at 50% probability; (b) perspective view along the a axis. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Sn−O 2.167(3); Sn(1)−I(1) 2.9910(5); Sn(1)−I(2) 3.0587(5); Sn(1)#1−I(1) 3.3119(5); Sn(1)#1−I(2) 3.3965(5); Sn(1)−Sn(1)#1 4.4931(8).
(Figure S6). Kanatzidis and co-workers have reported that [SnI2(dmso)3] can be obtained by treating a DMSO solution of MASnI3 with CH2Cl2, which is regarded as an intermediate in the one-step tin perovskite spin-coating process21 and could also be potentially used for the purification of tin-based perovskite materials for solar cells. The advantage of using the [SnI2(dmf)], [SnI2(dmso)], and [SnI2(dmso)2] complexes obtained in the present study as precursor materials is that they are prepared with relative ease from SnI2 solutions. Although these complexes are also air sensitive, they can be stored in vial containers under inert condition (O2, H2O < 10 ppm). Although both of SnI2 and SnI4 are red solids (Figure S1), these SnI2 complexes are colorless crystalline solids. SnI2 complexes immediately turn reddish brown upon exposure to air, suggesting that the purity of these complexes can be checked by the naked eye. In analogy to the SnI2 complexes, several solvent-coordinated tin complexes containing other halides, such as [SnBr2(dmf)], [SnBr2(dmso)2], [SnCl2(dmf)], and [Sn2F4(dmso)2]22 were obtained in a similar fashion. Their structures were unambiguously determined by single-crystal X-ray diffraction analyses (Figures S7−S10), and their purity was confirmed by elemental analysis and 119Sn MAS NMR spectroscopy (see SI). All of these tin halide complexes can be used as purified precursor materials for tin-based perovskites. In addition to their high purity, these tin halide complexes offer the advantage that their solutions can be prepared rapidly. For example, the preparation of a 1.5 M solution of sublimed SnI2 in DMF or DMSO requires more than 30 min stirring, whereas [SnI2(dmf)], [SnI2(dmso)], [SnI2(dmso)2], or other such complexes dissolve immediately (
