Thin-Film Deposition and Characterization of a Sn-Deficient

Mar 13, 2016 - Synthesis of Lead-Free Perovskite Films by Combinatorial Evaporation: Fast Processes for Screening Different Precursor Combinations. Ma...
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Thin-Film Deposition and Characterization of a Sn-Deficient Perovskite Derivative Cs2SnI6 Bayrammurad Saparov,†,‡ Jon-Paul Sun,§ Weiwei Meng,∥ Zewen Xiao,∥ Hsin-Sheng Duan,† Oki Gunawan,⊥ Donghyeop Shin,† Ian G. Hill,*,§ Yanfa Yan,*,∥ and David B. Mitzi*,†,‡ †

Department of Mechanical Engineering and Materials Science and ‡Department of Chemistry, Duke University, Durham, North Carolina 27708, United States § Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada ∥ Department of Physics and Astronomy and Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, Ohio 43606, United States ⊥ IBM T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598, United States S Supporting Information *

ABSTRACT: In this work, we describe details of a two-step deposition approach that enables the preparation of continuous and well-structured thin films of Cs2SnI6, which is a one-half Sndeficient 0-D perovskite derivative (i.e., the compound can also be written as CsSn0.5I3, with a structure consisting of isolated SnI64− octahedra). The films were characterized using powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), UV−vis spectroscopy, photoluminescence (PL), photoelectron spectroscopy (UPS, IPES, XPS), and Hall effect measurements. UV−vis and PL measurements indicate that the obtained Cs2SnI6 film is a semiconductor with a band gap of 1.6 eV. This band gap was further confirmed by the UPS and IPES spectra, which were well reproduced by the calculated density of states with the HSE hybrid functional. The Cs2SnI6 films exhibited n-type conduction with a carrier density of 6(1) × 1016 cm−3 and mobility of 2.9(3) cm2/V·s. While the computationally derived band structure for Cs2SnI6 shows significant dispersion along several directions in the Brillouin zone near the band edges, the valence band is relatively flat along the Γ−X direction, indicative of a more limited hole minority carrier mobility compared to analogous values for the electrons. The ionization potential (IP) and electron affinity (EA) were determined to be 6.4 and 4.8 eV, respectively. The Cs2SnI6 films show some enhanced stability under ambient air, compared to methylammonium lead(II) iodide perovskite films stored under similar conditions; however, the films do decompose slowly, yielding a CsI impurity. These findings are discussed in the context of suitability of Cs2SnI6 for photovoltaic and related optoelectronic applications.

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INTRODUCTION There has been remarkable progress in the development of methylammonium lead iodide (CH3NH3PbI3) as a photovoltaic (PV) absorber material in recent years.1 Although devices incorporating CH3NH3PbI3 demonstrate high power conversion efficiencies (PCE) above 20% (certified),2 major challenges remain, including the presence of the toxic heavymetal lead (Pb), hysteresis, and instability under ambient air, irradiation, and heat exposure.3 One main focus of the efforts to address the stability issue has been deposition of protective layers.4−6 Altering the chemistry and crystallography of the absorber material provides an alternative solution to these problems.7−9 Perhaps the simplest chemical modification of CH3NH3PbI3 is the homovalent substitution of lead with another tetrel element (e.g., Sn and Ge). Such substitution addresses the issue of lead toxicity, and, in fact, working devices based on CH3NH3SnI3 have been fabricated;7 however, the lighter tetrel elements Sn and Ge normally prefer the © 2016 American Chemical Society

tetravalent state, and the stability of the divalent Sn- and Geanalogs is therefore a concern. An alternative modification pathway can be demonstrated by the example of (PEA)2(CH3NH3)2Pb3I10 (where PEA = phenylethylammonium), which is a 2-D layered derivative of the CH3NH3PbI3 perovskite. Due to its layered crystal structure, in which hydrophobic organic PEA+ cations occupy the interlayer space between the inorganic perovskite [Pb 3 I 10 ] 4− layers, (PEA)2(CH3NH3)2Pb3I10 demonstrates an enhanced stability in moist (52% relative humidity) air, an open-circuit voltage of 1.18 V, and a PCE of 4.73%.8 It should be noted, however, that the dimensional reduction influences the band structure of the lead halide perovskites−lower dimensional perovskites generally have larger band gaps, narrower bandwidths, and Received: January 30, 2016 Revised: March 10, 2016 Published: March 13, 2016 2315

DOI: 10.1021/acs.chemmater.6b00433 Chem. Mater. 2016, 28, 2315−2322

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Chemistry of Materials

for the valence band states are not strong in all directions within the Brillouin zone, leading to high hole effective masses. We discuss the potential of Cs2SnI6 as a solar cell absorber and electron/hole-transport material based on our combined experimental and theoretical results.

potentially higher effective masses of charge carriers along the interlayer direction.9 A third alternative has also been recently proposed, involving isoelectronic substitution of Sn2+ and Pb2+ with Sb 3+ and Bi 3+ . 9,10 Since these substitutions are heterovalent, the Sb- and Bi-based compositions involve either deficient perovskite structures, such as in the 1/3 Sb-deficient layered perovskite Cs3Sb2I9,9 or mixed-anion compounds, such as in the case of the hypothetical CH3NH3BiSeI2.11 In fact, the Sb- and Bi-based compositions are attracting increased attention as promising alternatives to CH3NH3PbI3.12−15 In this work, we highlight another possible strategy for addressing the Pb toxicity and instability issues of CH3NH3PbI3. The use of tetravalent Sn instead of divalent Pb could simultaneously address the Pb toxicity and also enhance the stability of the material, as Sn4+ should be more stable against oxidation compared to Sn 2+. Since the substitution of Pb2+ with Sn4+ is heterovalent, the composition needs to be modified accordingly to preserve the charge balance. An example of such substitution is Cs2SnI6, which is a 50% Sn deficient perovskite derivative featuring isolated [SnI6]2− octahedra (i.e., this can be considered a 0-D compound from a structural point of view). Interestingly, the actual tetravalent oxidation state of Sn in Cs2SnI6 has been questioned, as density functional theory (DFT) band structure calculations suggest the presence of 5s states well below the Fermi level and the Bader charge analysis also seems to support the divalent nature of Sn from the DFT standpoint.16 Notwithstanding this observation, the formal oxidation state concept still applies from charge balance and chemical trends viewpoints. In fact, according to the literature, Cs2SnI6 is a promising air- and moisture-stable hole transport material.17 The authors reported preparation of Cs2SnI6 in ambient air from solution, with a surprisingly dispersive valence band (VB) and conduction band (CB) and the band gap value of 1.3 eV.17 Computational work18 on this material indicates that iodine vacancies and interstitial tin are the dominant defects that give intrinsic n-type behavior in Cs2SnI6. The reported stability in ambient moist air and Pb-free composition, coupled with the surprisingly dispersive frontier states, suggests that Cs2SnI6 could be interesting for application as an absorber in solar cell devices. Here, we report a two-step deposition approach that enables the preparation of continuous thin films of Cs2SnI6. Our Cs2SnI6 films demonstrate enhanced air-stability compared to CH3NH3PbI3 films, corroborating the earlier experimental results; however, both thin film and especially bulk Cs2SnI6 powder samples are prone to decomposition in air, leaving behind CsI as an impurity. The films of the purely inorganic Cs2SnI6 exhibit a band gap of 1.6 eV based on UV−vis spectroscopy and photoluminescence, in contrast to the 1.3 eV gap previously reported for a bulk sample.17,19 We discuss our findings in conjunction with the DFT modeling and results from photoelectron spectroscopy measurements. According to ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) results, the Fermi level is located ∼1.5 eV above the valence band maximum (VBM) and within a few hundred meV of the conduction band minimum (CBM), further supporting the ∼1.6 eV band gap and suggesting deeply n-type behavior. The n-type behavior of our Cs2SnI6 films is also confirmed using Hall effect measurements, which give a majority carrier density of 6(1) × 1016 cm−3 and mobility of 2.9(3) cm2/V·s. More detailed analysis of the DFT results also show that the band dispersions

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EXPERIMENTAL AND COMPUTATIONAL SECTION

Materials. The Cs2SnI6 films were fabricated through evaporation of CsI using a Radak source in an Angstrom Engineering evaporator, followed by annealing of the CsI films with 100−150 nm thicknesses in SnI4 vapor in a nitrogen-filled glovebox with controlled H2O and O2 levels. During CsI evaporation, the substrate was not heated, and the base pressure preceding evaporation was ∼5 × 10−7 Torr. The films were deposited on glass, Fluorine-doped Tin Oxide (FTO), and FTO/ compact-TiO2 substrates (see the Supporting Information). The fabrication conditions such as annealing time and temperature (Tann), cooling rate, and quenching temperature were carefully controlled in order to obtain single-phase Cs2SnI6, due to the narrow chemical potential window for pure Cs2SnI6.18 It was determined that annealing the CsI films for 20−40 min at 190 °C in a preheated SnI4 (excess) atmosphere for 10−30 min, followed by a gradual cooling to 140 °C over ∼10 min, and then quenching give the best results. At an annealing temperature of 220 °C and above, the presence of CsI and CsSnI3 impurities was noted, and above 250 °C, Cs2SnI6 did not form. Characterization Methods. X-ray diffraction measurements were carried out on a PANalytical Empyrean powder X-ray diffractometer under ambient conditions using CuKα radiation. CrystalMaker software (version 9.2.5) was used to create crystal structure images. For investigation of air-stability characteristics of Cs2SnI6, thin films were stored in a dark cabinet in ambient air, with a day-to-day relative humidity fluctuation ranging from 25 to 50%. Within the time frame of this study, the relative humidity was generally above 40%. High resolution images of the films were obtained on a FEI XL30 Scanning Electron Microscope (SEM). Optical absorption measurements were performed on a Shimadzu UV-3600 spectrophotometer. A QE-R Quantum Efficiency/Reflectivity measurement system (Enlitech) was used to perform diffuse reflectance measurements on bulk Cs2SnI6 samples. An Horiba Jobi-Yvon LabRAM ARAMIS system was used to carry out photoluminescence measurements, using an excitation wavelength of 633 nm. The van der Pauw and Hall effect measurements were done on a ∼5 × 5 mm film with thickness 350 nm. The Hall measurement was carried out using an IBM rotating parallel dipole line (PDL) Hall system that generates an oscillating magnetic field and performs lock-in detection of the corresponding Hall signal.20,21 This technique is necessary to extract the small Hall signal in the relatively low mobility films used in this study. Thermogravimetric analysis (TGA) of Cs2SnI6 was conducted using a TGA Q50 system from TA Instruments. For photoelectron spectroscopy (PES) experiments, 200 and 300 nm thick Cs2SnI6 films were deposited on glass/FTO substrates using the procedure described above. Care was taken to transfer the samples into the chamber without air exposure. PES measurements, including ultraviolet photoelectron spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and X-ray photoelectron spectroscopy (XPS), were conducted using as-loaded samples and after receiving different length sputtering treatments (i.e., Ar ion source with an extractor voltage of 3000 V and a beam current of 5 μA, rastered over a 10 mm × 10 mm area). Reproducibility of spectra between the two film thicknesses was confirmed to ensure that sample charging was not an issue. The analysis chamber was equipped with a hemispherical energy analyzer (Specs Phoibos 150) for UPS and XPS studies. The UPS measurements were carried out using a He I (hν = 21.22 eV) source. The XPS measurements were carried out using both Al Kα (1486.6 eV) and Mg Kα (1253.6 eV) sources. IPES measurements were performed in the isochromat mode using a homemade spectrometer located in the PES analysis chamber, with a resolution of approximately 0.6 eV as determined by the width of the Fermi edge of clean polycrystalline silver. The positions of the Fermi edge were used to align the UPS and IPES energy scales. 2316

DOI: 10.1021/acs.chemmater.6b00433 Chem. Mater. 2016, 28, 2315−2322

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Chemistry of Materials Computational Details. The DFT calculations were performed for Cs2SnI6 with the projector augmented wave (PAW) method22 and the screened hybrid Heyd-Scuseria-Ernzerhof (HSE)23,24 functionals, as implemented in the VASP code.25 The cutoff energy was set to 275.4 eV. The primitive cell and a Γ-centered 4 × 4 × 4 k-mesh were employed. It has been reported that 25% (the typical value) and 34% of the exact Hartree−Fock exchange yield band gaps of 0.93 eV, and 1.26 eV, respectively.16 Here, we further increased the amount of the exact Hartree−Fock exchange to 37% to reproduce the experimental band gap of 1.62 eV presented in this study. The calculated density of states was convolved with a Gaussian distribution to explain the origins of the peaks appearing in the UPS valence band and IPES conduction band spectra. The effective electron masses (me*/m0) were calculated with the equation mh*/m0 = ℏ2(d2E/dk2)−1/m0, where ℏ is the reduced Plank constant and d2E/dk2 is the curvature of the CBM band E(k), and the effective hole masses (mh*/m0) were calculated from the VBM band E(k). The fractions of holes that are in the heavy and light bands along the Γ−X direction were calculated using the phh p

=

phh phh + plh

=

* )3/2 (mhh * )3/2 + (mlh*)3/2 (mhh

the latter, due to the charge balance requirements, the formal Bcation oxidation state is different in each member, assuming monovalent A and A′ cations and X = halide anions. Thus, the formal oxidation state of Sn is divalent in CsSnI3 (q = ∞) and tetravalent in Cs2SnI6 (q = 1), and in a q = 2 member such as the 2-D layered Cs3Sb2I9, Sb is trivalent.9 This is important, since while Sn2+ and Sb3+ are isoelectronic, Sn4+ is not, and the 5s lone pair electrons, at least formally, should move up from the valence band to the conduction band. Based on these simple ideas alone, Cs2SnI6 may feature less dispersive valence bands compared to that of Cs3Sb2I9, for example, though in real systems, decoupling the impacts of changes in electronic configurations, and changes in crystal structure and dimensionality, is often difficult. In the literature, Cs2SnI6 has been prepared from solution by reacting an aqueous acidic solution of CsI with a solution of SnI4 in warm ethanol.17 In another method, CsI and SnI2 were reacted in a hot 120 °C aqueous solution of HI and H3PO2, and the resultant precipitate was dissolved in acetone, from which crystals of Cs2SnI6 were obtained.17 A third method also exists in which Cs2SnI6 is gradually obtained by leaving CsSnI3 in a drying oven in air.19 Note here that this method, i.e., the preparation of Cs2SnI6 through oxidation of CsSnI3, supports the formal oxidation state of +4 in the former and +2 in the latter. In our attempts to fabricate high-quality Cs2SnI6 films, we have tried the literature preparation methods. The gradual oxidation of CsSnI3 proved difficult to control, and the films prepared using this method were visibly patchy. Depending on the scanned region, the presence of CsI and CsSnI3 impurities along with Cs2SnI6 was detected from powder X-ray data (PXRD) (see Figure S1). It is important to avoid the presence of the CsSnI3 impurity, in particular, in order to accurately determine the fundamental properties of Cs2SnI6 (e.g., the band gap, Eg) since both CsSnI3 and Cs2SnI6 have been reported to have Eg ≈ 1.3 eV in the literature.19 Therefore, it was decided that CsSnI3 or SnI2, which in combination with CsI gives CsSnI3, should be avoided as starting materials in the current study. We first prepared a bulk powder sample of Cs2SnI6 using the method described by Lee et al.17 All peaks in the PXRD pattern of the Cs2SnI6 powder prepared this way can be successfully assigned to the cubic Cs2SnI6 structure (Figure S2). Noticeable changes in the PXRD of the Cs2SnI6 powder were observed for samples that were left in moist air. The Cs2SnI6 peaks decrease in intensity, and, conversely, a strong CsI impurity peak emerges after only several days in air, in sharp contrast with the previously reported air-stable nature of the material.17 Consequently, in order to avoid the decomposition of the bulk powder sample in air, Cs2SnI6 samples, except when carrying out stability tests, were stored under inert atmosphere. A low concentration solution of the obtained Cs2SnI6 powder (50 mg Cs2SnI6/1 mL N,N-dimethylformamide (DMF)) was then drop-cast and gently heated at 110 °C following the reported recipe.17 Based on our results, this method also gives films with poor coverage and CsI impurities. Modifying the conditions of thin-film deposition from solution, such as the concentration and temperature, did not afford good quality films. Interestingly, despite its 0-D crystal structure featuring isolated SnI62− units and highly ionic bonding expected for halide perovskites, solubility of Cs2SnI6 in many polar solvents (e.g., water, ethanol, DMF) is limited, and, in fact, Cs2SnI6 forms a precipitate even in an acidic HI(aq) solution. Such a

relation, where hh and lh refer to

heavy and light hole bands.

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RESULTS AND DISCUSSION Crystal Structure, Synthesis, and Thin-Film Preparation. Perovskites are well-known for the tunability of their structural dimensionality, chemical content, and crystal/ electronic structures.26,27 Lower dimensional perovskites are grouped into families based on the fragmentation of the parent three-dimensional (3-D) perovskite structure along certain crystallographic directions, including the ⟨100⟩-oriented, ⟨110⟩oriented, and ⟨111⟩-oriented families (more exotic cuts are also possible). Cs2SnI6 is a q = 1 member of the ⟨111⟩-oriented perovskite family, which has a general formula of A′2Aq‑1BqX3q+3.26,27 The crystal structure of Cs2SnI6 is derived from the 3-D perovskite CsSnI3 by removing every second Sn layer along ⟨111⟩ (Figure 1). Note that such removal leaves

Figure 1. A schematic depiction of the relationship between the crystal structures of (a) the 3-D parent perovskite Cs2Sn2I6 (i.e., CsSnI3) and (b) the 0-D Cs2SnI6. Removal of every second Sn layer along the ⟨111⟩ direction of (a) CsSnI3 results in (b) Cs2SnI6. (c) Perspective view of the crystal structure of Cs2SnI6 emphasizing its cubic perovskite-derived crystal structure. Orange and red spheres represent Cs and I atoms, respectively; Sn-centered octahedra are shown in green and blue.

behind isolated SnI6 octahedra, resulting in a 0-D structure (in terms of connectivity of the tin iodide framework), and necessitates oxidation of the remaining Sn from a divalent to a tetravalent state in order to maintain the formal charge balance in (Cs+)2(Sn4+)(I−)6. A significant difference between the ⟨100⟩-, ⟨110⟩-, and ⟨111⟩-oriented families is the fact that, in 2317

DOI: 10.1021/acs.chemmater.6b00433 Chem. Mater. 2016, 28, 2315−2322

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perovskite structures, respectively, one might expect Cs2SnI6 to be the more readily infiltrated and impacted by polar solvents such as water, compared to the higher-dimensional systems. In the literature, Cs2SnI6 has been reported to be an air-stable material;17,30 however, as discussed above, PXRD patterns within the current study suggest that a powder sample prepared using the solution approach shows strong signs of decomposition in days (Figure S2). Given the relative importance of a material’s air stability for PV applications, the current Cs2SnI6 films were monitored using periodic X-ray diffraction measurement (Figure 4). Two sets of films, deposited on glass and FTO/c-TiO2 substrates, were left in moist air (with relative humidity of 25−50%) in the dark for several months. A gradual evolution of a CsI impurity peak was noticed for both sets of films during this time period (Figure 4); however, the decomposition of Cs2SnI6 films occurs at a slower rate compared to the bulk powder samples obtained from solution (Figure S2), perhaps due to the more compact nature of the grains in the films. The observed behavior and decomposition time scale of the films are similar to that of Cs3Sb2I9.9 Both Cs3Sb2I9 and Cs2SnI6 exhibit enhanced stability in moist air compared to CH 3 NH 3 PbI3 films stored under similar conditions.9 Notwithstanding the fact that a cross-comparison of stabilities of different materials is difficult (i.e., given possible different film grain structures and morphologies), most CH3NH3PbI3 films demonstrate visible signs of decomposition within several weeks.8,9 It is also noteworthy that the Cs2SnI6 peaks of the air-exposed films become sharper, similar to the reported observations for CH3NH3PbI3.9,31 This is especially noticeable for the films deposited on FTO/c-TiO2 substrates, for which the full width at half-maximum (fwhm) of the (040) peak, for example, goes from 0.279° to 0.128°. In comparison, for Cs2SnI6 films on glass, the fwhm value decreases from 0.116° to 0.070° for the (222) peak. Halide perovskites are known for low formation and decomposition energies.6,32 Since thermal stability is also an important parameter for prospective applications, we carried out thermogravimetric analysis (TGA) of a Cs2SnI6 powder sample, prepared using the solution approach (Figure 5).17 The thermal decomposition of Cs2SnI6 starts at ∼250 °C (i.e., extrapolated onset temperature is found to be To = 284 °C), which is consistent with our observations during the thin-film deposition procedure  i.e., annealing CsI in SnI4 vapor at 250 °C and above does not yield Cs 2 SnI 6 . Finally, after decomposition of Cs2SnI6, the leftover powder corresponds to ∼46% of the initial sample weight (Figure 5), indirectly suggesting that the white powder is CsI, which constitutes 45.34% of the total weight in Cs2SnI6. PXRD measurement on this white powder confirms the conjecture that Cs2SnI6 decomposes into CsI and volatile SnI4. Optical Properties. Optical absorption measurements using UV−vis spectroscopy on our darkly colored Cs2SnI6 films give a characteristic two hump feature30 (Figure 6) due to the specifics of the band structure.16,17 The calculated band gap from the Tauc fit of the absorbance data for a direct band gap semiconductor was found to be 1.62 eV. In comparison, the Tauc plot assuming an indirect gap gives a value of 1.39 eV. In the literature, there are conflicting reports regarding the nature of the band gap in Cs2SnI6 − one study claims that this material is a direct band gap semiconductor with ∼1.3 eV band gap17 and another suggests an indirect band gap of ∼1.3 eV,19 both obtained from diffuse reflectance data. The DFT results indicate that Cs2SnI6 possesses a direct band gap at the Γ

limited solubility of Cs2SnI6 is a major challenge for using solution deposition methods to fabricate thin films. To deposit high-quality Cs2SnI6 films, then, we decided to carry out the deposition of a CsI film on glass and FTO substrates followed by annealing in preheated SnI4 vapor, in analogy to the recently described preparation of the related Cs3Sb2I9 films.9 The reaction of the CsI film with SnI4 vapor at 190 °C affords black films that are visibly better than those obtained from our attempts using the solution-based processing. The higher quality is further established from PXRD data, which indicate the successful preparation of single phase Cs2SnI6 (Figure 2). The unit cell parameter was refined

Figure 2. Powder X-ray diffraction pattern for a Cs2SnI6 film (black) on a glass substrate, which was prepared by annealing a CsI film in SnI4 vapor at 190 °C. Pawley fit (red) to the cubic K2PtCl6-type (Fm3m) structure28 gives a = 11.6425(2) Å; the difference plot is shown in blue.

to a = 11.6425(2) Å in the space group Fm-3m, in good agreement with the published structural data,28,29 through a Pawley fit with a reliability factor of Rp = 9.96% (weighted Rfactor wRp = 12.32%) and goodness-of-fit = 1.62. The films display preferred orientation, with enhanced {111} peak intensities. Interestingly, films deposited on FTO/c-TiO2 substrates show a different type of preferred orientation with increased {020} peak intensities (Figure S3). Scanning Electron Microscopy (SEM) investigations suggest that our two-step deposition approach gives continuous smooth films, with grain sizes in the 100−900 nm range (Figure 3). Stability. Considering the formal tetravalent oxidation state of Sn in Cs2SnI6, this material is expected to be stable against oxidation in air. On the other hand, given the fact that CH3NH3PbI3, Cs3Sb2I9, and Cs2SnI6 have 3-D, 2-D, and 0-D

Figure 3. Scanning Electron Microscopy (SEM) images of a Cs2SnI6 film deposited on a FTO/c-TiO2 substrate at (a) lower and (b) higher magnification. Cs2SnI6 films deposited on glass substrates yield similar SEM images. 2318

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Figure 4. In ambient air, a slow evolution of the CsI impurity peaks is observed for Cs2SnI6 films deposited on (a) glass and (b) FTO/c-TiO2 substrates.

that typically occurs for halide perovskites including CH3NH3PbI3 and Cs3Sb2I9.9 Photoelectron Spectroscopy (XPS, UPS, IPES). Photoelectron spectroscopy measurements were conducted in order to further investigate the band gap and electronic structure of the Cs2SnI6 films prepared in the current study using the SnI4 annealing process. The as-loaded samples showed small amounts of carbon and oxygen, which could be removed by a short 5 or 10 s sputtering treatment (Figures S4−S5). The impact of the 5−10 s sputtering on the UPS spectra involves the shift of the onset to lower binding energy by ∼0.3 eV, similar to the ∼0.2 eV shift observed for the 30 s sputtered Cs3Sb2I9 films (Figure S6).9 Note that He I emission onsets for our films (Figure S6a) are clear and sharp, in contrast to multiple onsets, which are indicative of sample heterogeneity, as observed for solution-processed Cs2SnI6.17 Usually, the presence of multiple emission onsets makes the differentiation of VBMs of minority and majority components difficult. Based on the UPS spectra, the ionization energy of the as-loaded samples corresponds to 6.1 eV, while the sputtered samples are closer to 6.4 eV (Figure 7), with the valence band maximum ∼1.5 eV below the Fermi level (Figure S6b). The fact that the

Figure 5. Thermogravimetric Analysis (TGA) data for a powder sample of Cs2SnI6, obtained from solution.17

Figure 6. (a) Tauc fit of the absorbance data assuming a direct band gap for Cs2SnI6, yielding a 1.62 eV gap (inset shows the essentially black and uniform top surface of a corresponding thin film). (b) The photoluminescence spectrum (633 nm excitation wavelength) for Cs2SnI6 features a peak at 1.56 eV.

point.16,17 Assuming a direct band gap for Cs2SnI6, the 1.6 eV value from absorbance data on our films is significantly larger than the reported 1.26−1.3 eV band gap values in the literature. Photoluminescence (PL) measurements (Figure 6) on the same films yield a relatively broad peak with fwhm of 0.27 eV and a maximum at 1.56 eV, consistent with the band gap obtained from the absorbance data. Notably, the PL peak intensity for Cs2SnI6 is considerably higher than that of Cs3Sb2I9,9 which features a slightly indirect band gap and deep defects, and, conversely, it is lower than that of a CH3NH3PbI3 film measured under similar conditions. Such a high PL peak intensity also seems to support the suggested direct nature of the band gap for Cs2SnI6 from DFT results. Note that, during PL measurements, care must be taken to avoid the laser damage

Figure 7. Combined UPS (He I)/IPES spectra (top, black and green curves) showing the onset of photoemission, in the valence and conduction band regions of the 10 s-sputtered 200 nm Cs2SnI6 film on glass/FTO substrates. The Fermi level (EF), shown as a dotted red line, is ∼1.5 eV above the valence band maximum based on the UPS results. Below, Gaussian broadened DFT-calculated density-of-states (DOS) plot (blue curve), using the HSE06 hybrid functional with 37% of exchange, is provided for comparison. The green curve connecting the IPES data (black dots) is shown as a guide to the eye. 2319

DOI: 10.1021/acs.chemmater.6b00433 Chem. Mater. 2016, 28, 2315−2322

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Table 1. Calculated Band Gap and Effective Masses of Electrons and Holes (me*/m0 and mh*/m0) from Band Structures Calculated by HSE06 Hybrid Functional with 34% and 37% of Exchange, Respectivelya me*/m0 34% 37%

mh*/m0

band gap (eV)

Γ−X

Γ−K

Γ−L

Γ−Xheavy

Γ−Xlight

Γ−K

Γ−L

1.27 1.62

0.56 0.63

0.56 0.63

0.56 0.63

32.90 (0.993) 10.09 (0.957)

1.18 (0.007) 1.27 (0.043)

3.34 2.77

2.58 2.31

The fractions of holes that are in the heavy and light bands along the Γ−X direction are given in the brackets (see Experimental and Computational Section for details). a

the deeply n-type nature of our Cs2SnI6 films and modest electrical transport characteristics for the majority carriers. The carrier density of 6(1) × 1016 cm−3 for Cs2SnI6 is significantly higher and, conversely, the mobility of 2.9(3) cm2/ V·s is significantly lower than that obtained for the related 3-D perovskite semiconductors. For example, the well-studied PV perovskite CH3NH3PbI3 is reported to be slightly p-doped with a carrier concentration of ∼109 cm−3 and mobility of 66−140 cm2/V·s (although a wide range of values can be found in the literature).28,33 Further comparison can also be made with chalcogenide PV absorber films such as CdTe, which can be prepared as n-type or p-type depending on the deposition conditions34,35 and, irrespective of this fact, demonstrate higher mobilities in the 15−40 cm2/V·s range.34 All of these comparisons clearly indicate that the mobility value of 2.9(3) cm2/V·s is lower compared to that of other high-performance thin-film PV materials. Although this could present a challenge for the application of Cs2SnI6 as a PV absorber, we note that Cu2ZnSn(S,Se)4 (CZTSSe)-based devices with power conversion efficiencies >10% have been fabricated, with the majority (hole) carrier mobility in the range of 0.5−1.3 cm2/ V·s.36 Finally, it is important to note that the calculated relatively high hole (minority carrier) effective masses in Cs2SnI6 (Table 1) are in contrast with the previously reported dispersive VB.17 The HSE-calculated band structure with 37% of the exact Hartree−Fock exchange is similar to that calculated with 34% of the exact Hartree−Fock exchange reported in the literature,18 except for an enlarged band gap (Table 1, Figure S7a). The VBM is particularly flat along the Γ−X direction (see the Brillouin zone shown in Figure S7b), which leads to the large hole effective mass and, consequently, low mobility along this direction. The large minority carrier effective mass makes it more difficult to extract photogenerated carriers from a prospective absorber layer made of this material. The flat band along the Γ−X direction is explained by the isolated nature of the [SnI6]2− octahedra caused by the removal of half of the Sn atoms in the Cs2SnI6 structure.18 Interestingly, the CBM along the Γ−X direction remains dispersive, and the calculated electron effective masses (Table 1) are low. As shown previously,16 the bottom of the conduction band is derived from the unoccupied I p states, whereas the top of the valence band is derived from the occupied I p states. The occupied I p states do not overlap at sites where SnI62− octahedra do not share corners due to their localized wave functions. However, the unoccupied I p states can still have some overlap due to their more delocalized wave functions. Therefore, the hole mass is much heavier than the electron mass along the Γ−X direction, i.e., the direction with the largest separation between two adjacent SnI62− octahedra. Note that the band structure along the Γ−X direction was not provided in the earlier work by Lee et al.,17 leading to the description of the VB as being generally dispersive. In fact, for

Fermi level is 1.5 eV above the valence band maximum (VBM) supports the measured optical band gap of 1.6 eV for the Cs2SnI6 films. This also suggests that our Cs2SnI6 films exhibit n-type behavior, in agreement with the defect properties calculated using DFT methods, according to which the dominant VI and Sni defects are responsible for the intrinsic n-type conduction.18 These findings are further supported by the IPES results, which suggest that the Fermi level is within a couple of hundred meV of the CBM. The combination of our UPS and IPES results, therefore, places the band gap value just above ∼1.6 eV (Figure 7). The combined UPS/IPES spectra for our Cs2SnI6 films are also in a good agreement with the various calculated DOS plots.17,18 Note that, although the UPS spectra show some changes with sputtering, the IPES features and onsets do not change after sputtering. The ionization energy value of 6.4 eV reported here is significantly higher than the values reported in the literature. In two separate studies, which use the different solution-based approaches discussed above, the ionization energy values of 5.49 eV17 and 5.94 eV19 were reported. We attribute this variation to the different film deposition methods used in these studies and to the Cs2SnI6 film quality. Additionally, surface preparation and residual contamination (e.g., from solvents, prolonged air exposure) tend to decrease the ionization energy of the surface. To probe whether bulk material changes are occurring during our PES (UPS, IPES, and XPS) measurements or associated sample handling, we have also performed PXRD and UV−vis spectroscopy measurements after PES analysis. Based on PXRD and absorbance data, no noticeable changes were observed for the Cs2SnI6 films used for the PES measurements. Electronic Transport Properties. According to the literature,17 a polycrystalline pellet of Cs2SnI6 annealed at 200 °C yielded an n-type semiconductor with a high electron mobility of 310 cm2/V·s and a carrier concentration of ∼1 × 1014 cm−3.17 Room temperature Hall effect measurements on our Cs2SnI6 films suggest that, in the current case, the material is an n-type semiconductor with a carrier density of 6(1) × 1016 cm−3 and mobility of 2.9(3) cm2/V·s. In comparison, carrier densities calculated using the UPS/IPES data (i.e., considering the Fermi level being 0.12 eV below the CB edge) and effective masses of electrons from DFT band structure calculations (Table 1) yield a consistent value to that determined from Hall data, i.e. in the range of 1 × 1017 cm−3. Only slight shifting of the Fermi level to 0.13−0.14 eV below the CB edge leads to a calculated carrier concentration of 5.6−6.9 × 1016 cm−3. Interestingly, according to the DFT results, a carrier density of 6(1) × 1016 cm−3 corresponds to a sample prepared under Ipoor synthetic conditions.18 Further findings from the Hall effect measurements include the sheet resistance and the average Hall coefficient, which are 9.7(3) × 105 Ω/sq and −98(16) cm3/C, respectively. The obtained Hall data confirm 2320

DOI: 10.1021/acs.chemmater.6b00433 Chem. Mater. 2016, 28, 2315−2322

Chemistry of Materials the p-type Cs2SnI6, a hole mobility of 42 cm2/V·s, which is comparable to that of CH3NH3PbI3, has been reported;17 however, considering the contrasting results presented here and the propensity for secondary phase formation within this system, such a high hole mobility value should be further validated. Notwithstanding this important difference in how the data are presented and analyzed, in other respects, the results of the present band structure calculations are largely consistent with the previously reported band structures.17,18

CONCLUSIONS In summary, we report a vacuum-based deposition approach for preparing high quality thin films of Cs2SnI6, which yield a direct band gap of 1.6 eV for the semiconductor (a value that is larger than the 1.3 eV value previously reported in the literature for solution-processed samples). The already more oxidized nature of Sn in Cs2SnI6 makes this material more resistant to further oxidation compared to compounds based on divalent Sne.g., the black B-γ perovskite polymorph of CsSnI3 degrades in 1 h when exposed to air.19,37 The described two-step Cs2SnI6 thinfilm preparation procedure in this work yields an n-type semiconductor with a carrier density of 6(1) × 1016 cm−3, presumably due to the presence of dominant VI and Sni defects.18 Besides defects, another potential challenge for prospective PV application of Cs2SnI6 arises from the relatively low measured electron (majority carrier) mobility of 2.9(3) cm2/V·s and, given the high hole effective masses from DFT results, expected even lower hole (minority carrier) mobility. Finally, based on our combined UPS − IPES spectra, the ionization potential (IP) and electron affinity (EA) of the material are 6.4 and 4.8 eV, respectively. In comparison, EA values for CH3NH3PbI3 and CH3NH3PbBr3 on TiO2 are 3.7 and 3.6 eV, respectively.38 Such large IP and EA values for Cs2SnI6 explain the n-type behavior and the relative difficulty of p-type doping in Cs2SnI6. In addition, due to the high IP, using Cs2SnI6 as a high-performance hole transport material for a CH3NH3PbI3-based solar cell device, for example, would likely prove challenging. In principle, Cs2SnI6 may be considered as an electron transfer material considering the dispersive CB and lower electron effective mass; however, given the very high EA, band alignment engineering would be necessary. The detailed characterization results presented here for single-phase Cs2SnI6 films may further help to elucidate potential suitability of this semiconductor for other applications beyond PV.

ACKNOWLEDGMENTS

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b00433. Details of solution-based thin-film deposition, compact TiO2 layer deposition, HSE-calculated band structure, further details of photoelectron spectroscopy measurements and results, Figures S1−S7, and Table S1 (PDF)

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The information, data, or work presented herein was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0006712. One of the authors (B.S.) acknowledges support from a Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE contract number DE-AC05-06OR23100. I.G.H. acknowledges funding from NSERC of Canada under the Discovery Awards program (RGPIN 04809) and the Canada Foundation for Innovation. J.P.S. acknowledges funding from the NSERC Postgraduate Scholarships program, NSERC CREATE DREAMS, and the Killam Trusts. All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of DOE, ORAU, or ORISE. See the Supporting Information for more information.

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AUTHOR INFORMATION

Corresponding Authors

*For I.G.H: E-mail, [email protected]. *For Y.Y.: E-mail, [email protected]. *For D.B.M: E-mail, [email protected]. Notes

The authors declare no competing financial interest. 2321

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