Article pubs.acs.org/JPCC
Minute-Scale Degradation and Shift of Valence-Band Maxima of (CH3NH3)SnI3 and HC(NH2)2SnI3 Perovskites upon Air Exposure Ryosuke Nishikubo,† Naoki Ishida,† Yukie Katsuki,‡ Atsushi Wakamiya,‡ and Akinori Saeki*,†,§ †
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡
S Supporting Information *
ABSTRACT: The development of lead-free, tin-based perovskite solar cells is becoming a pervasive topic; however, the inherent instabilities of such cells have prevented a boost of their power conversion efficiency and a deeper understanding of their fundamental properties. By using the photoelectron yield spectroscopy (PYS) and flash-photolysis time-resolved microwave conductivity (TRMC) techniques, we investigate the effects of air exposure on the valence-band maxima (VBMs) and photoconductivities of tin iodide perovskites (methylammonium cation, MASnI3; formamidinium cation, FASnI3). These perovskites exhibit a shift of the VBM (e.g., from −5.02 eV at 0 min to −5.17 eV at 18 min), deterioration of the PYS profiles, and progressive decrease of the TRMC transients on the minute scale of air exposure. The addition of SnF2 was found to suppress the initial defect-related density of the filled electronic states of MASnI3 and FASnI3, as revealed by PYS, and to partly mitigate the degradation of MASnI3, as revealed by TRMC. A low-dimensional perovskite (MA2SnI6) composed of the oxidized form of Sn(IV) was also evaluated to explain the anomalous TRMC behavior of the air-exposed MASnI3. Our results provide an important basis for correlation with the degradation and energetics of a device.
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addition34 is also noteworthy. However, the dark carrier density is still much higher than that of MAPbI3 even after SnF2 doping (∼1017 cm−3),20,21 and the mechanism remains poorly understood. In this article, we report the electrochemical and optoelectronic properties of MASnI3 and FASnI3 with/without SnF2 additive. These properties were systematically evaluated using the photoelectron yield spectroscopy (PYS)35 and laser flash-photolysis time-resolved microwave conductivity (TRMC)36,37 techniques, particularly regarding the effect of air exposure. PYS is a type of ultraviolet photoelectron spectroscopy (UPS) that records the photocurrent extracted under a relatively high extraction voltage (10−100 V) upon UV light irradiation (shorter than 310 nm). Consequently, it allows for the evaluation of a wide range of samples including thick films and powders in a vacuum and even in air. PYS in air is referred to as photoelectron spectroscopy in air (PESA). It detects electron-attached oxygen molecules using an opencounter analyzer (examples are the model AC-2 and AC-3 photoelectron spectrometers of RIKEN Keiki Corp.). In the present study, PYS of powder samples was performed in a vacuum to precisely evaluate the effect of air exposure on
INTRODUCTION Because of the remarkable progress in the power conversion efficiencies (PCEs) of organic lead halide perovskite solar cells,1−8 the development of a lead-free perovskite material toward the creation of third-generation solar cells is underway.9,10 In this regard, less toxic tin, a lighter element directly above lead in the periodic table, is one of the most plausible alternatives.11−17 Although methylammonium tin iodide (MASnI3, where MA represents the methylammonium cation, CH3NH3+) has been launched with relatively good PCEs of 5− 6%,12,13 its instability and high carrier (hole) density in darkness have inhibited an expected increase in device performance.18−22 The former is associated with a reaction of moisture and oxygen,23 which is much more significant than in the prototype MAPbI3.24−26 The dark carrier density of tin perovskite (1017−1019 cm−3) inherently doped by conversion from Sn2+ to Sn4+ is many orders of magnitude higher than that of MAPbI3 (109−1011 cm−3),20,21 which causes charge recombination, shorting of the diode circuit, and deterioration of the open-circuit voltage.13,16,18,19,21,27,28 Because the addition of SnF2 to CsSnI3 was found to mitigate the initial p-doping,29 a relatively large amount of SnF2 (10−20 mol %) is mixed with MASnI3 or formamidinium-cation- [HC(NH2)2+-, FA-) based FASnI3, exhibiting PCEs of 2−6%.16,20,30−33 The suppression of p-doping by hydrazine treatment30 and a low-temperature vapor-assisted solution process (LT-VASP) instead of SnF2 © XXXX American Chemical Society
Received: June 27, 2017 Revised: August 23, 2017
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DOI: 10.1021/acs.jpcc.7b06294 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
photons cm−2 pulse−1). The photoconductivity transient Δσ was converted into the product of the quantum yield (φ) and the sum of the charge-carrier mobilities, ∑μ = μ+ + μ−, by φ∑μ = Δσ(eI0Flight)−1, where e and FLight are the unit charge of a single electron and a correction (or filling) factor, respectively. After the first measurement (= 0 min), the cavity was opened to the air (∼25 °C and 60% humidity), and measurements were conducted (the air exposure time is the time after opening of the cavity). Photoemission spectroscopy was performed using a Jasco FP-8300 spectrometer. A powder sample on a quartz plate was prepared in the same way as for the TRMC evaluation. The time evolution of the emission (λex = 500 nm, λem = 900 nm) was recorded from 1 min after the removal of a sample from the glovebox. Powder X-ray diffraction (XRD) data were collected on a Rigaku MiniFlex-600 instrument using Cu Kα radiation (λ = 1.54187 Å) at room temperature in the air.
the valence-band maximum (VBM). The use of powder instead of a thin film permits the slowing of the degradation process, which ensures contact with a grounded electrode through conductive tape and reduces fluctuations in the quality of the tin perovskite, which is sensitive to its environment and processing. The observed shift in VBM and decrease in transient conductivity are important when designing the device structure of a Sn-based perovskite solar cell and in deepening the understanding of the degradation process.
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EXPERIMENTAL SECTION Sample Preparation. Methylammonium iodide (MAI) and formamidinium iodide (FAI) were purchased from Tokyo Chemical Industry (TCI) Co. Ltd. SnF2 was purchased from Sigma-Aldrich Inc. These materials were used as received. Dehydrated ethanol (super dehydrated) and N,N-dimethylformamide (DMF, super dehydrated) were purchased from Wako Pure Chemical Industries Ltd. and Kanto Chemical. Co. Inc., respectively, and were further dried over molecular sieves and degassed by Ar gas bubbling for 1 h before use. As a purified precursor for tin perovskite materials, [SnI2(dmf)] complex was prepared by recrystallization from a DMF solution of SnI2 (sublimed, TCI) in an Ar-filled glovebox (∼10 ppm O2). Details on the preparation and characterization of the [SnI2(dmf)] complex will be reported elsewhere.38 In an Arfilled glovebox, MAI (or FAI), [SnI2(dmf)], and SnF2 in DMF (1 M) were dissolved in ethanol at the stoichiometric ratio (1:1:0 or 1:1:0.2, respectively, in mole fraction). The reaction mixture was stirred at 80 °C for 30 min. The obtained pale green solution was cooled to afford a blackish-green crystalline powder. After filtration and washing with ethanol, a powder of MASnI3 or FASnI3 with/without SnF2 was obtained. MA2SnI6 was prepared in the same fashion using MAI and a SnI4 precursor (purchased from Aldrich) at a 2:1 stoichiometry (no addition of SnF2). Photoelectron Yield Spectroscopy. PYS experiments were carried out using a Bunko Keiki BIP-KV201 photoemission spectrometer (accuracy = ±0.02 eV, extraction voltage = 10 V) in a vacuum (
