Role of Organic Counterion in Lead- and Tin ... - ACS Publications

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Role of Organic Counterion in Lead- and Tin-Based Two-Dimensional Semiconducting Iodide Perovskites and Application in Planar Solar Cells Lingling Mao,† Hsinhan Tsai,§,∥ Wanyi Nie,§ Lin Ma,†,‡ Jino Im,⊥ Constantinos C. Stoumpos,† Christos D. Malliakas,† Feng Hao,†,‡ Michael R. Wasielewski,†,‡ Aditya D. Mohite,*,§ and Mercouri G. Kanatzidis*,†,‡ Chem. Mater. 2016.28:7781-7792. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/24/18. For personal use only.

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Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208, United States § Materials Physics and Application Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ∥ Materials Science and Nano Engineering, Rice University, Houston, Texas 77005, United States ⊥ Center for Molecular Modeling and Simulation, Korea Research Institute of Chemical Technology, Daejeon, 34114, Korea ‡

S Supporting Information *

ABSTRACT: Hybrid halide perovskites are emerging semiconducting materials, with a diverse set of remarkable optoelectronic properties. Besides the widely studied three-dimensional (3D) perovskites, two-dimensional (2D) perovskites show significant potential as photovoltaic (PV) active layers while exhibiting high moisture resistance. Here, we report two series of new 2D halide perovskite solid solutions: (HA)Pb1−xSnxI4 and (BZA)2Pb1−xSnxI4 (x = 1, 0.75, 0.5, 0.25, 0), where HA stands for the organic spacer histammonium and BZA stands for benzylammonium cations. These compounds are assembled by corner-sharing octahedral [MI6]4− units stabilizing single-layered, anionic, inorganic perovskite sheets with organic cations filled in between. The optical band gaps are heavily affected by the M−I−M perovksite angles with the band gap steadily decreasing when the angle approaches 180°, ranging from 2.18 eV for (BZA)2PbI4 to 2.05 eV for (HA)PbI4. We find an anomalous trend in electronic band gap in the mixed compositions (HA)Pb1−xSnxI4 and (BZA)2Pb1−xSnxI4. When Sn substitutes for Pb to form a solid solution, the band gap further decreases to 1.67 eV for (HA)SnI4. The minimum band gap is at x = 0.75 at 1.74 eV. For BZA, the irregular trend is more intense, as all the intermediate compounds (BZA)2Pb1−xSnxI4 (x = 0.75, 0.5, 0.25) have even slightly lower band gaps than (BZA)2SnI4 (1.89 eV). DFT calculations confirm the pure Pb and Sn compounds are direct band gap semiconductors. Relatively shorter photoluminescence (PL) lifetime in (BZA)2PbI4 than (HA)PbI4 is observed, suggesting faster recombination rates of the carriers. Solution deposited thin films of (HA)PbI4 and (BZA)2PbI4 show drastically different orientations with (HA)PbI4 displaying a perpendicular rather than parallel growth orientation with respect to the substrate, which is more favorable for PV devices. The higher potential in PV applications of the HA system is indicated by device performance, as the champion air stable planar device with the structure ITO/PEDOT:PSS/2D-perovskite/PCBM/Al of (HA)PbI4 achieves a preliminary power conversion efficiency (PCE) of 1.13%, featuring an open-circuit voltage (VOC) of 0.91 V.

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INTRODUCTION Hybrid organic−inorganic perovskites with three-dimensional structures, a class of semiconductors with versatile compositions and high tunability, have attracted strong attention because of their excellent performance as light absorber in solar cells. The power conversion efficiency (PCE) of perovskite solar cells has reached over 20% since 2009.1−8 The renaissance of these materials has motivated us to explore new territories of the chemical and physical properties.9,10 In the generic threedimensional (3D) perovskite formula ABX3, A can be CH3NH3+, HC(NH2)2+, or Cs+, B is usually Pb2+, Sn2+, Ge2+, etc., while X refers to Cl−, Br−, or I−. By changing the cationic © 2016 American Chemical Society

species, metal ion, or halides, the semiconducting properties of the perovskites can be tuned. The most well studied materials to date include CH 3 NH 3 PbI 3 , 11 CH 3 NH 3 PbBr 3 , 12,13 CH3NH3SnI3,14−16 CsSnI3,17 and HC(NH2)2PbI3.18−20 These compounds have suitable band gaps and favorable electrical and electronic properties for photovoltaic applications.3,7,21−23 Halide perovskites have been used as efficient emissive species in light-emitting diodes (LEDs)24,25 and lasing applications Received: July 26, 2016 Revised: October 1, 2016 Published: October 3, 2016 7781

DOI: 10.1021/acs.chemmater.6b03054 Chem. Mater. 2016, 28, 7781−7792

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Chemistry of Materials with low lasing thresholds.26,27 Another remarkable aspect of the 3D perovskites comes from Ge-based compounds which have polar crystal structures; CH3NH3GeI3, HC(NH2)2GeI3, and CsGeI3 exhibit strong second harmonic generation (SHG) response.28 Two-dimensional (2D) perovskites have layered crystal structure and offer greater synthetic versatility; they allow for more specialized device implementation because of tunability of the crystal structure. For example, functionalities of the 2D perovskites can be easily tuned by incorporating a wide array of organic cations into the 2D framework, in contrast to the 3D analogues, which have limited scope for structural engineering. There are three main types of 2D structures in general, the (100)-oriented, the (110)-oriented, and the (111)-oriented type, with the orientation labels referring to the cleavage plane of the ideal 3D cubic perovskite unit cell from where they derive.29 By far, the most common type is the (100)-oriented 2D perovskite, which has the formula of (RNH3)2(MA)n−1BnX3n+1,3031,32 and it is formed by stacking n perovskite layers along the (100) direction of the parent 3D structure, separated by the long spacer ammonium cations. In the less common (110)-oriented perovskite, the octahedra are connected in a zigzag manner forming stacks of the perovskite along the (110) of the parent 3D structure.33−35 The (111)oriented perovskite involves trivalent metal ion such as Sb3+ and Bi3+ and has a different general formula of A3M2X9.36−39 2D perovskites with thicker layers have shown great potential for solar cell applications because of their high stability and film quality. One of the most representative multilayered 2D materials, (C6H5(CH2)2NH3)2(CH3NH3)2Pb3I10, has shown high moisture endurance and a power conversion efficiency (PCE) of 4.73%.32 Recently, we utilized the (C4H9NH3)2(CH3NH3)n−1PbnI3n+140 series as light absorbers in mesoscopic solar cells, where the three-layered (n = 3) material stood out among them with a PCE of 4.02%41 Moving further in this system, using the more sophisticated deposition method of “hot-casting”, we were able to demonstrate an efficiency of 12.52% for the n = 4 member, showing high reproducibility and excellent operating stability.42 The single layered (n = 1) 2D perovskite compound, which is the only member of the series that does not contain the CH3NH3+ cation, has the highest band gap within the series. However, by substituting the terminal iodide anion with SCN−, a slightly lower band gap43−45 can be achieved in the single-layered (100)-oriented (CH3NH3)2Pb(SCN)2I2.43 2D halide perovskites can also form with transition metal ions such as Cu(II),46,47 Cr(II),48 Mn(II),49 and Fe(II)50,51 which exhibit interesting magnetic properties. Other applications of 2D perovskites lie in the area of field effect transistors (FETs)52,53 and light-emitting diodes (LEDs). Two-dimensional (2D) perovskites are much more versatile and functional as a class of materials than the 3D homologues because they offer greater tunability and property control. The layered structure and the extra organic cation populating the interlayer space offer greater synthetic versatility and allow for more device types and phenomena to be explored. In this work, we describe two series of new 2D compounds incorporating the histammonium dication (+2) and the benzylammonium monocation (+1) (Scheme 1), henceforth termed (HA) and (BZA), respectively. These are the first members of a homologous series of so-called Ruddelsden-Popper type structures of the general formula AxMI4 (x = 1 for HA or x = 2 for BZA, M = Pb, Sn) similar to the multilayered

Scheme 1. Molecular Structures, Formula, and Notation of HA and BZA

compounds that we studied earlier.41 We selected these two systems in order to better elucidate the structure−property relationships and explore the role of the organic counterion in the physical properties of the materials. In particular, we chose to compare the effects of the charge difference between the two cations (1+ for BZA vs 2+ for HA) as well as the variation of the functional groups (NH3+ vs imidazolium) in an attempt to better understand how these affect their respective properties. Unlike most of the single-layered (n = 1) 2D perovskites previously reported, the inorganic frameworks in HA and BZA 2D perovskites are rather undistorted. As a result, they have slightly lower band gaps and thus better potential as light absorbers in photovoltaic devices. In addition, we find that both 2D perovskites can form continuous solid solutions by alloying of the metal site, exhibiting an anomalous bandgap evolution. The anomalous bandgap has been observed previously for the 3D perovskites (CH3NH3)PbxSn1−xI3, showing a narrower bandgap for the intermediate compositions with respect to the parent end-members.14,54−56 In the 2D systems described here, we also find an anomalous bandgap evolution in solid solutions of (HA)Pb1−xSnxI4 and (BZA)2Pb1−xSnxI4. The title compounds have strong photoluminescence at room temperature with relatively long lifetimes. According to this finding, we used our materials in planar solar cells, and our preliminary results show (HA)PbI4 to be the best of the series, reaching a power conversion efficiency of 1.13% with a high open circuit voltage of 0.91 V.

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EXPERIMENTAL SECTION

Synthesis. All chemicals were purchased from Sigma-Aldrich, and unless otherwise stated, they were used as received. Histamine is denoted as HA and benzylamine as BZA for convenience. (HA)PbI4. An amount of 0.223 g (1 mmol) of PbO powder was dissolved in 3.4 mL of 57% w/w aqueous hydriodic acid and 1.7 mL of 50% aqueous H3PO2 by heating under stirring for 20 min at 120 °C until the solution turned bright yellow. 0.184 g (1 mmol) of histamine dihydrochloride was dissolved in 1.7 mL of 57% w/w aqueous hydriodic acid under heating. By slowly layering the later solution on top of the PbI2 solution, dark orange platelike crystals precipitated during slow cooling. Yield: 0.530 g (64.0% based on total Pb content). (BZA)2PbI4. (BZA)2PbI4 was prepared in the same fashion except benzylamine hydrochloride (0.287 g, 2 mmol) to PbO (0.223 g, 1 mmol) ratio was doubled to 2:1. Orange platelike crystals precipitated during slow cooling. Yield: 0.490 g (52.6% based on total Pb content). (HA)SnI4. An amount of 0.226 g (1 mmol) of SnCl2·2H2O was dissolved in 3.4 mL of 57% w/w aqueous hydriodic acid and 1.7 mL of 50% aqueous H3PO2 by heating under stirring for 20 min at 120 °C until the solution turned light yellow. An amount of 0.184 g (1 mmol) of histamine dihydrochloride was dissolved in 1.7 mL of 57% w/w aqueous hydriodic acid under heating. By slowly layering the later solution on top of the SnI2 solution, black platelike crystals 7782


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Chemistry of Materials precipitated during slow cooling. At times, a second phase of (HA)SnI4 was observed during the precipitation. A pure black phase can be obtained by leaving the dark red phase in the mother solution for a few days. The crystal structure and other characteristics of the second phase are described in the Supporting Information. Yield: 0.485 g (65.6% based on total Sn content). (BZA)2SnI4. (BZA)2SnI4 was prepared in the same fashion except benzylamine hydrochloride (0.287 g, 2 mmol) to SnCl2·2H2O (0.226 g, 1 mmol) ratio was doubled to 2:1. Dark red platelike crystals precipitated during slow cooling. Yield: 0.523 g (62.1% based on total Sn content). (HA)Pb1−xSnxI4 (x = 0.75, 0.50, 0.25). (HA)Pb1−xSnxI4 (x = 0.75, 0.50, 0.25) (Figure 1) was prepared by dissolving a stoichiometric

a noncollinear optical parametric amplifier (Spirit-NOPA, SpectraPhysics). The NOPA delivered tunable, high repetition rate pulses with pulse widths as short as 20 fs at 800 nm, which were frequency doubled to 400 nm using the built-in second harmonic generation crystal in the Spirit-NOPA. Optical Absorption Spectroscopy. Optical diffuse reflectance measurements were performed using a Shimadzu UV-3600 UV−visNIR spectrometer operating in the 200−2000 nm region at room temperature. BaSO4 was used as the reference of 100% reflectance for all measurements. The reflectance versus wavelength data generated were used to estimate the band gap of the material by converting reflectance to absorption data according to the Kubelka−Munk equation:58 α/S = (1 − R)2(2R)−1, where R is the reflectance and α and S are the absorption and scattering coefficients, respectively. Resistivity Measurements. Four-probe resistivity measurements were performed on crystals of the compounds at room temperature. Electrical contacts were applied through 100 μm copper wires adhered to the crystal specimens through colloidal graphite paste. Measurements were made for arbitrary current directions in the ab-plane using standard point contact geometry. A homemade resistivity apparatus equipped with a Keithley 2182A nanovoltometer, Keithley 617 electrometer, Keithley 6220 Precision direct current (DC) source, and a vacuum chamber controlled by a K-20 MMR system was used. Data acquisition was controlled by custom written software. Electronic Structure Calculations. Relaxation of internal coordinates and calculation of electronic structures of (HA)PbI4, (HA)SnI4, (BZA)2PbI4, and (BZA)2SnI4 were carried out by using first-principles density functional theory (DFT) with projectoraugmented wave method59 implemented in the Vienna Ab initio Simulation Package.60,61 Energy cut off for plane-wave basis was set to 450 eV, and force criterion was set to 0.01 eV/Å. The exchangecorrelation functional was treated by the generalized gradient approximation (GGA) within Perdew−Burke−Ernzerhof formalism62 for structural relaxation and basic electronic structure calculation. Solution Preparation and Device Fabrication. Various amounts of (HA)PbI4 and (BZA)2PbI4 were dissolved in DMF to form solutions with concentrations ranging from 0.115 to 0.45 M. The (HA)PbI4 and (BZA)2PbI4 solutions were stirred overnight at 70 °C before spin-casting on the substrate. Patterned indium tin oxide (ITO) substrates were cleaned by using soapy water in an ultrasonication bath followed by washing with deionized water, acetone, and isopropyl alcohol sequentially for 10 min. After drying on a hot plate in air at 120 °C for 30 min, the substrate surface was cleaned with oxygen plasma for 3 min under rough vacuum. The poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) solution was spin-coated on top of indium tin oxide (ITO) substrate with a spin rate of 5000 rpm for 45 s to form a 50 nm thick hole-transporting layer (HTL). The PEDOT:PSS films were then dried in air on a 120 °C hot plate for 30 min. After drying, the substrates were transferred to an argon-filled glovebox for further use. The 2D perovskite layer of (HA)PbI4 or (BZA)2PbI4 were spin-casted on the ITO/PEDOT:PSS substrate with spin speed of 5000 rpm for 20 s; then, the film was transferred to a 110 °C hot plate for 10 min. The [6,6]-phenyl-C61butyric acid methyl ester (PCBM) solution (20 mg/mL in chlorobenzene) was spin-coated at room temperature on top of the perovskite thin film at 1000 rpm for 45 s to form a 20 nm thick electron-transporting layer (ETL). Finally, the whole device was transferred to an inbuilt thermal evaporation chamber and pumped down to 1 × 10−7 Torr for aluminum electrode deposition. The aluminum top electrode (100 nm) was deposited through a shadow mask that defined the device active area as 0.035 cm2 for the solar cells. Device Characterization. Power Conversion Efficiency Measurement. All solar cells were measured inside an Ar filled chamber that was pumped down to 1 × 10−5 Torr. The shadow mask confined the device area of around 0.035 cm2 for cathode deposition. The same mask is used during device measurement to avoid edge effects for small area solar cells. Current−voltage sweeps were done using a Keithley 2100 unit under simulated air mass 1.5 irradiation (100 mW cm−2) and using a xenon-lamp-based solar simulator (Oriel LCS-100). A NIST calibrated monocrystalline silicon solar cell (Newport 532,

Figure 1. Powder samples of (HA)PbxSn1−xI4 (x = 1, 0.25, 0.5, 0.75, 1) demonstrating a color trend resulting from the anomalous band gap evolution. Optical images of (HA)SnI4 and (HA)PbI4. amount of PbO and SnCl2·2H2O in 3.4 mL of 57% w/w aqueous hydriodic acid and 1.7 mL of 50% aqueous H3PO2 under heating and stirring for 20 min at 120 °C until all dissolved. An amount of 1 mmol of histamine dihydrochloride was dissolved in 1.7 mL of 57% w/w aqueous hydriodic acid under heating. Dark red plate-like crystals precipitated during slow cooling. Yields: 0.503, 0.482, and 0. 485 g (62.4%, 61.5%, and 63.7% based on total metal content). (BZA)2Pb1−xSnxI4 (x = 0.75, 0.50, 0.25). (BZA)2Pb1−xSnxI4 (x = 0.75, 0.50, 0.25) was prepared in the same way described above except for switching the cation from histamine dihydrochloride to benzylamine hydrochloride. Dark red plate-like crystals precipitated during slow cooling. The crystals were separated by filtration. Yields: 0.574, 0.555, and 0. 539 g (63.1%, 62.6%, and 62.3% based on total metal content). Powder and Single Crystal X-ray Diffraction. Powder XRD analysis was performed using a Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) operating at 40 kV/15 mA with a Kβ foil filter. Single crystals of appropriate size were selected for X-ray diffraction experiments. After screening a few diffracted frames to ensure crystal quality, full sphere data were collected using either a STOE IPDS 2 or IPDS 2T diffractometer with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å), operating at 50 kV and 40 mA under N2. Integration and numerical absorption corrections on the data were performed using the STOE X-AREA programs. Crystal structures were solved by direct methods and refined by full-matrix least-squares on F2 using the OLEX2 program package.57 All data were collected at room temperature under nitrogen flow. Scanning Electron Microscopy Coupled with Energy Dispersive X-ray (SEM/EDX) Spectroscopy. SEM images and EDS analyses were obtained using a Hitachi S3400N-II scanning electron microscope equipped with an Oxford Instruments INCAx-act SDD EDS detector at accelerating voltage of 15 kV. Time-Resolved Photoluminescence. Time resolved photoluminescence data were collected from single crystals at room temperature using a streak camera system (Hamamatsu C4334 Streakscope). The instrument response function (IRF) is about 30 ps. After deconvolution fitting, the temporal resolution is ∼10 ps. The 400 excitation pulses were generated by a high repetition rate ultrafast laser system. A one box ultrafast amplifier (Spirit, Spectra-Physics) produced a 1040 nm pulse (100 kHz, 300 fs), which was used to pump 7783


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Chemistry of Materials ISO1599) was used for light intensity calibration before each measurement. Photocurrent Measurement. The external quantum efficiency was measured with a NIST calibrated monochromator (QEX10, 22562, PV measurement INC.) in the AC mode. The light intensity was first calibrated with a NIST calibrated photodiode (91005) as a reference before each measurement. During the measurement, the monochromator was chopped at a frequency of 150 Hz, and the photocurrent was collected at short circuit condition through a lockin amplifier. Quantum efficiency was calculated by software integrating the measured photocurrent for the device and the standard reference cell.

(HA)PbI4 crystallizes in the monoclinic space group P21/n. It is a layered perovskite templated by the divalent organic cation histammonium, (C 5 N 3 H 11 ) 2+, which contains the dual functionality of a primary ammonium and an imidazolium group. The layers are defined by the connecting corner-sharing of PbI6 octahedra along the ac crystallographic plane. The (HA)SnI4 analogue has the exact same crystal structure as the Pb2+ analogue. For the sake of simplicity, only the latter will be described in detail below. HA is a divalent cation and acts as a “double linker” bringing the layers closer up together, as the interlayer terminal iodide−iodide distance is 4.37 Å for the Sn compound (4.39 Å for (HA)PbI4), which gives a certain 3D character to the structure. The crystal structure of (BZA)2PbI4 was first reported by Papavassiliou et al. in 1999.64 In agreement with the previous finding, (BZA)2PbI4 crystallizes in the orthorhombic space group Pbca. Unlike (HA)PbI4, it has a common (RNH3)2MX4 structure type corresponding to the distorted K2NiF4-type of inorganic lattice with two univalent (C6H5CH2NH3)+ organic cations intercalating between the inorganic layers. The organic cation bilayers push apart the inorganic layers resulting in a long interlayer iodide−iodide distance of 9.14 Å. The lower halides of this family of perovskites are also known, with the chloride and bromide analogues crystallizing in the polar space group Cmc21, displaying ferroelectric polarization.65,66 The local environment of the organic ammonium groups, as shown in Figure 3a,b, consists of four hydrogen bonds per ammonium unit, formed between the protonated amine and the four iodide ions of the perovskite. In general, the hydrogenbonding interactions in both series of compounds are weak,67 and instead, the electrostatic interactions between the charged organic groups and the inorganic layers seem to dominate. In (HA)SnI4, the hydrogen bonds between the N−H on the imidazolium group and the apical iodide are stronger than the other iodide atoms, which is reflected by the donor−acceptor distances (3.474 and 3.556 Å). For comparison, in (BZA)2SnI4, the donor−acceptor distances are 3.570 and 3.666 Å, indicating a weaker interaction. A pronounced difference in the hydrogen bonding between HA compounds and BZA compounds is that the two N−H bonds on the imidazolium group orient toward diagonal iodides, while for the BZA compounds the two N−H groups coordinate to two adjacent iodides. The differences in the hydrogen bonding mode also affect the stacking of the inorganic layers. The inorganic layers in the HA compounds adopt a nearly eclipsed stacking mode with respect to one another, while the layers in the BZA compounds adopt a staggered motif. Both BZA and HA contain aromatic rings in their organic skeleton, which are well-known for getting stabilized through π−π stacking interactions.68 However, because of the large spatial separation and the offset between adjacent cations, the π−π interaction does not occur in either the BZA or the HA structures. Only in the case of BZA, a weak intralayer stabilizing force is present, where a “point-to face” π−σ attraction occurs, with the distance between the C−H vector and the ring centroid being 3.93 Å (Figure 4d). In the case of HA, no intermolecular interactions were found because of longer distances (centroid to centroid 6.88 Å) between the cations as seen in Figure 4c. In light of the optical properties, however, which remain unchanged for BZA with respect to the alkylammonium templated structures, it appears that the stabilization energy offered by this interaction is negligible.

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RESULTS AND DISCUSSION Synthesis. Previously, we investigated Pb and Sn solid solutions of the 3D perovskite and found an anomalous dependence of the energy band gap which yields mixed Pb/Sn compositions with red-shifted bandgaps relative to those of the end Pb and Sn members.9,55 We investigated the mixed Pb/Sn compositions in the 2D systems because they exhibit greater versatility in structure and properties than the 3D perovskites. Powder-X-ray diffraction (PXRD) patterns of the solid solutions of both series show that as x increases in both (HA)Pb1−xSnxI4 and (BZA)2Pb1−xSnxI4 (x = 1, 0.75, 0.5, 0.25, 0) series, the unit cell contracts because of the difference in ionic radii of Sn2+ (0.94 Å) and Pb2+ (1.18 Å),63 indicating that Sn is incorporated into the perovskite network. This is clearly seen in the PXRD patterns of the solid solutions which show a shift of the Bragg reflections to higher angles (Figure S2) as the Sn fraction increases without producing any second phases. SEM images of samples from both series reveal well-formed plate-like crystals, consistent with their 2D nature (Figure S4). The ratio of Pb and Sn in the products was determined using microprobe EDS analysis (Figure S4). We note that the Sn content is slightly higher than the one targeted for synthesis. This is attributed to the inferior solubility of Sn compared to Pb in concentrated HI, a property that facilitates the precipitation of Sn-rich compositions. All compounds were handled in air and remained stable during the characterization. Crystal Structure Description. The crystal structures of (HA)PbI4, (HA)SnI4, (BZA)2PbI4, and (BZA)2SnI4 are shown in Figure 2a−d, and crystallographic data are listed in Table 1.

Figure 2. Crystal structures of (a) (HA)PbI4, (b) (HA)SnI4, (c) (BZA)2PbI4, and (d) (BZA)2SnI4, respectively. 7784


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Chemistry of Materials Table 1. Crystallographic Data for (HA)PbI4, (HA)SnI4, (BZA)2PbI4, and (BZA)2SnI4 at 293 Ka empirical formula (C5N3H11)PbI4 formula weight temperature (K) wavelength (Å) crystal system space group

unit cell dimensions

volume (Å3) Z density (calc) (g/cm3) absorption coefficient (mm−1) F(000) crystal size (mm3) θ range for data collection (deg) index ranges

reflections collected independent reflections completeness to θ = 26° (%) refinement method data/restraints/parameters goodness-of-fit final R indices [I > 2σ(I)] R indices [all data] largest diff. peak and hole (e·Å−3)

827.96

monoclinic P21/n a = 8.9159(7) Å α = 90° b = 20.0335(15) Å β = 91.875(6)° c = 8.9925(6) Å γ = 90° 1605.4(2) 4 3.426 18.175 1424 0.0197 × 0.0180 × 0.0017 2.033−29.209 −12 ≤ h ≤ 12, −27 ≤ k ≤ 27, −12 ≤ l ≤ 10 15 307 4323 [Rint = 0.1026] 99.7 4323/0/120 1.027 Robs = 0.0541, wRobs = 0.1410 Rall = 0.0639, wRall = 0.1501 2.079 and −2.606

(C5N3H11)SnI4 739.46

(C14N2H20)PbI4

931.11 293(2) 0.71073 monoclinic orthorhombic P21/n Pbca a = 8.7411(4) Å a = 9.1561(5) Å α = 90° α = 90° b = 20.0449(7) Å b = 8.6894(3) Å β = 91.571(3)° β = 90.000(4)° c = 8.9845(3) Å c = 28.7762(15) Å γ = 90° γ = 90° 1573.62(10) 2289.46(19) 4 4 3.121 2.701 9.444 12.760 1296 1648 0.0470 × 0.0275 × 0.0102 0.2106 × 0.0584 × 0.0188 2.543−34.986 2.637−29.192 −14 ≤ h ≤ 14, −12 ≤ h ≤ 12, −32 ≤ k ≤ 31, −11 ≤ k ≤ 11, −14 ≤ l ≤ 14 −39 ≤ l ≤ 39 24 323 20 074 6770 [Rint = 0.0468] 3083 [Rint = 0.0652] 98.4 100 full-matrix least-squares on F2 6770/0/123 3083/0/98 1.096 1.099 Robs = 0.0583, Robs = 0.0575, wRobs = 0.0916 wRobs = 0.1308 Rall = 0.1111, Rall = 0.0727, wRall = 0.1087 wRall = 0.1398 0.908 and −1.737 0.863 and −3.958

(C14N2H20)SnI4 842.61

orthorhombic Pbca a = 9.0944(4) Å α = 90° b = 8.6613(6) Å β = 90° c = 28.7640(13) Å γ = 90° 2265.7(2) 4 2.470 6.575 1520 0.2369 × 0.2166 × 0.1043 2.833−29.203 −12 ≤ h ≤ 12, −11 ≤ k ≤ 11, −39 ≤ l ≤ 39 20 087 3046 [Rint = 0.0711] 99.5 3046/0/98 1.089 Robs = 0.0354, wRobs = 0.0852 Rall = 0.0469, wRall = 0.0894 0.708 and −1.099

a R = Σ∥Fo| − |Fc∥/Σ|Fo|, wR = {Σ[w(|Fo|2 − |Fc|2)2]/Σ[w(|Fo|4)1/2 and w =1/[σ2(Fo2) + (0.0925P)2] (HAPbI4), w = 1/[σ2(Fo2) + (0.0300P)2 + 5.0120P] (HASnI4), w = 1/[σ2(Fo2) + (0.0850P) 2 + 0.5560P] (BZA2PbI4), w = 1/[σ2(Fo2) + (0.0420P) 2 + 1.8510P] (BZA2SnI4), where P = (Fo2 + 2Fc2)/3.

spectroscopy measurements (Figure 5a,b). The spectra show sharp absorption edges indicating direct bandgap semiconductors. All compounds display exciton peaks near the absorption edge. This may be due to quantum confinement effect of the carriers, previously shown in other 2D semiconductors such as the (C4H9NH3)2(CH3NH3)n−1PbnI3n+131 series and (C6H5(CH2)2NH3)2(CH3NH3)2Pb3I10.32 As a result, the band gaps here are slightly underestimated. Also the excitonic features are more prominent in the Pb compounds compared to Sn. Within the HA series, (HA)SnI4 has the lowest band gap of 1.67 eV while (HA)PbI4 has the highest (2.05 eV). When making the Pb/Sn solid solutions, the band gaps are largely reduced by adding more Sn to the (HA)PbI4 system but do not follow smooth Vegard’s law dependence. This anomaly is even more pronounced in the BZA compounds, as the intermediate compositions (BZA)2Pb1−xSnxI4 (x = 0.75, 0.5, 0.25) show an anomalous trend with a bandgap minimum at x = 0.75 of 1.82 eV, Figure 5c. All intermediates have narrower band gaps (1.82, 1.84, and 1.86 eV) than the pure Sn compound (BZA)2SnI4 (1.89 eV). In the 3D solid solution CH3NH3Sn1−xPbxI3, anomalous band gap is attributed to the antagonistic effect between spin−orbit coupling (SOC)

The M−I−M angle is another important structural feature of these compounds, since the band gaps of these semiconductors are strongly affected by these angles.69 From the top-down view, the inorganic layer of the HA compounds is much less distorted compared to the BZA one (Figure 4a,b). The Sn−I− Sn angles in (HA)SnI4 are 160.0°and 178.5°. Compared to (HA)PbBr4,70,71 the Pb−I−Pb angles in (HA)PbI4 are larger than the Pb−Br−Pb angles (156.4° and 178.2° compared to 153.7° and 174.8°, repsectively). The difference in distortion stems from the difference in the strength of the H-bonding. Because of its larger ionic radius, the negative charge on the iodide atom is more delocalized and thus weakens the relative strength of H-bonding, leaving the structure less disrupted. On the other hand, as seen in Figure 4b, the Sn−I−Sn angle of (BZA)2SnI4 is 160.6°, similar to (C4H9NH3)2SnI4 (159.6°),69 but slightly larger than the similar compound phenylethylamine (PEA)2SnI4 (156.5°),52,72 in which the organic spacer is one CH2 group longer than benzylamine, indicating that the steric hindrance of the aromatic rings is also a significant factor in the deformation of the inorganic lattice. Optical Properties. Band Gaps. The optical band gaps of (HA)Pb1−xSnxI4 and (BZA)2Pb1−xSnxI4 (x = 1, 0.75, 0.5, 0.25, 0) series were determined from diffuse reflectance UV−vis 7785


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Figure 3. Coordination environment of (a) (HA)SnI4 and (b) (BZA)2SnI4; the Pb analogues of HA2+ and BZA+ have the exact same environments. (c) Top-down view of (HA)SnI4, “eclipsed” conformation; (d) “staggered” conformation of (BZA)2SnI4.

Figure 5. Optical absorption of fresh crystalline samples (a) (HA)PbxSn1−xI4 (x = 1, 0.75, 0.5, 0.25, 0) and (b) (BZA)2PbxSn1−xI4 (x = 1, 0.75, 0.5, 0.25, 0) obtained from diffuse reflectance measurements converted using the Kubelka−Munk function (α/S = (1 − R)2/2R). (c) Increasing x = Pb content in Pb/Sn solid solutions vs band gaps display anomalous band gap behavior.

compounds have lower band gaps than the BZAs, largely due to the less distorted inorganic framework (larger M−I−M angle) as discussed above, which under generic considerations makes them more promising materials to be applied in photovoltaics. Time-Resolved Photoluminescence. In order to evaluate the semiconducting properties of the layered perovskites, we performed a time-resolved photoluminescence (PL) experiment on single crystals (Figure 6). All four compounds exhibit strong photoluminescence at room temperature. The emission energies obtained for (HA)PbI4, (HA)SnI4, (BZA)2PbI4, and (BZA)2SnI4 are 2.31, 1.92, 2.35, and 2.01 eV, respectively. Fitting the single-wavelength PL decays to an exponential decay model produced the PL lifetimes for the crystals.73 In Figure 5c,d, three decay time constants were required to fit the experimental spectra of the BZA compounds whereas, for HA compounds, two decay constants were needed for (HA)PbI4 and one decay constant for (HA)SnI4. The fastest decay process (τ1) corresponds to radiative recombination of excitons, while the slower decay processes (τ2 and τ3) should be due to trap states formation in the layered perovskites.74 More specifically, for (BZA)2PbI4, a triexponential decay has a 130 ps component (71%), a 0.71 ns component (21%), and a 3.1 ns component (8%). For (HA)PbI4, a biexponential decay with a 340 ps component (71%) and a 1.49 ns component (21%) produces the best fit. Since the nature of the trap states is somewhat obscure in 2D perovskites, we only compare the τ1

Figure 4. Comparison between (a) (HA)SnI4 and (b) (BZA)2SnI4 shows the inorganic sheets of (BZA)2SnI4 are more distorted resulting from different hydrogen-bonding between the organic spacers and the inorganic frameworks. Detailed Sn−I−Sn angles are also given here; as in HA, the angles are 160.0° and 178.5°. In BZA, the only angle is 160.6°. (c) Distanced spacial arrangement of HA cations. (d) “Point-to face” π−σ attraction between BZA cations. The distance between the carbon atom in the C−H vector and the ring centroid is 3.93 Å.

resulting from alloying heavier Pb atoms and lighter Sn atoms and structural distortion due to phase transition.56 An interesting finding in the CH3NH3Sn1−xPbxI3 series is that the Sn s-orbital contribution is highly dominant to the VBM up to x = 0.875. Thus, we expect a similar physical mechanism to be operating in the 2D solid solutions. In the (BZA)2Pb1−xSnxI4 series, we did not observe a room temperature phase transition of the solid solutions, which suggests the SOC is the main cause of the anomaly. The (HA)2Pb1−xSnxI4 series on the other hand displays an anomaly in the band gap evolution; however, in that case, the intermediate compositions lie in between the two endmembers, with all composition adopting a band gap of ∼1.8 eV lying very close to the Sn band edge. Overall, the HA 7786


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compositions.55 The resistivities of the all the intermediates compounds range from 1014 Ω·cm to 1011 Ω·cm and are closer to the pure Pb compound (∼1015 Ω·cm) rather than to the Sn one (∼106 Ω·cm). Note that the room temperature resistivity of (BZA)2SnI4 (∼106 Ω·cm) is comparable to the reported value of (C4H9NH3)2SnI4 (∼105 Ω·cm) at room temperature.30 The slightly higher resistivity value is attributed to the use of the H3PO2 reducing agent in the synthesis. This suppresses selfdoping from any Sn4+ species. In contrast, in the synthesis of (C4H9NH3)2SnI4, no reducing agent was used.74 Electronic Structure Calculations. In order to assess the electronic structure of the layered perovskites, we performed DFT calculations. All materials have direct band gaps in the range of 1.1−1.4 eV (Figures 7 and 8). Since the generalized

Figure 6. Time-resolved fluorescence spectra and fits for the decay time (τ) of (a) (HA)PbI4, (b) (HA)SnI4, (c) (BZA)2PbI4, and (d) (BZA)2SnI4. Figure 7. DFT calculations of electronic band structures and projected density of states (DOS) of (a) (HA)PbI4 and (b) (HA)SnI4. The Fermi level is set to 0 eV and indicated by the horizontal gray line.

values which correspond to the radiative recombination process. In this respect, (BZA)2PbI4 shows a much more efficient exciton recombination than (HA)PbI4 which in turn suggests that the probability of generating free-carriers is diminished. The faster PL decay lifetime of (BZA)2PbI4 compared to (HA)PbI4 suggests fast carrier recombination, an observation that is consistent with the photovoltaic performance of the (BZA)2PbI4 devices that will be discussed below. Electrical and Electronic Properties. Resistivity. Room temperature electrical resistivity was measured for single crystal samples of the solid solutions of both HA and BZA series. There is a clear tendency (Table 2) that, from Sn-rich to Pbrich, the resistivities of (HA)Pb1−xSnxI4 and (BZA)2Pb1−xSnxI4 (x = 1, 0.75, 0.5, 0.25, 0) increase from Sn-rich to Pb-rich

gradient approximation (GGA) functional is known to underestimate the band gap, the deviation from the experimental band gaps by 0.5−0.7 eV is acceptable. The DFT calculations are in agreement with the experimental band gap trend, where (HA)SnI4 has the smallest band gap and (BZA)2PbI4 has the largest. The calculations show these compounds to be direct gap semiconductors, which is consistent with the observed steepness of the absorption edges in the spectra. Emission wavelengths of the PL spectra follow the same trend as the band gaps; for instance, (HA)SnI4 emits at the lowest energy among all compounds. Projected density of states (PDOS) plots show that all materials have a

Table 2. Band Gap (Eg) and Room Temperature Resitivity for the Single Crystals of the Solid Solutions formula

Eg (eV)

(HA)PbI4 (HA)Pb0.75Sn0.25I4 (HA)Pb0.5Sn0.5I4 (HA)Pb0.25Sn0.75I4 (HA)SnI4

2.05 1.78 1.76 1.74 1.67

resistivity (Ω·cm) 2.5 × 8.0 × 1.3 × 1.6 × NA

1016 1014 1013 1014

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formula

Eg (eV)

(BZA)2PbI4 (BZA)2Pb0.75Sn0.25I4 (BZA)2Pb0.5Sn0.5I4 (BZA)2Pb0.25Sn0.75I4 (BZA)2SnI4

2.18 1.86 1.84 1.82 1.89

resistivity (Ω·cm) 8.3 1.1 4.1 3.6 4.5

× × × × ×

1015 1013 1012 1011 106


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Figure 9. Powder-XRD pattern of thin films of (a) (HA)PbI4 and (b) (BZA)2PbI4 under room temperature as-cast and postanneal conditions.

indicative of the orientation along the (010) plane. This vertical orientation is the correct one for efficient carrier transport across the thickness of the films. Such an orientation is reminiscent of that obtained for films of the 3-layered 2D perovskites reported previously, leading to a significant photovoltaic performance.32,41 This unique film orientation is reported here for the first time for a single-layered 2D perovskite. As mentioned above, this may be due to the close distance (4.39 Å) between the inorganic layers. Having established the formability of the thin films, we proceeded to assemble photovoltaic devices with a simple planar architecture (Figure 10d). The optimization of the

Figure 8. DFT calculations of electronic band structures and projected density of states (DOS) of (a) (BZA)2PbI4 and (b) (BZA)2SnI4. The Fermi level is set to 0 eV and indicated by the horizontal gray line.

valence band maximum (VBM) dominated by metal s-orbitals and I p-orbitals. The conduction band minimum (CBM) is dominated by empty metal p-orbitals. Therefore, qualitatively, the electronic structure is similar to that of the 3D perovskites. The bands of the 2D perovskites along the in-plane directions show strong dispersion resulting in large band widths, particularly for the conduction bands. As seen in Figures 7 and 8, the PDOS plots show that the LUMOs of HA are lower in energy than the BZA, implying that the aromatic nature of the imidazolium rings may introduce some additional charge−transfer interactions between the organic cations and the inorganic layers. The lower band gaps of the HA compounds compared to the BZA ones can be attributed to the higher HOMO in HA compounds, which is caused by larger M−I−M angles that destabilize the lone pair in the metal. Thin Film Fabrication and Application in Solar Cells. The favorable optical and semiconducting properties of the 2D perovskites described above prompted us to test them as photovoltaic materials in planar solid-state solar cells. Before completing the devices, we evaluated the quality of the solution-processed thin films obtained by means of XRD in both (HA)PbI4 and (BZA)2PbI4. In both cases, we were able to obtain good quality films suitable for further characterization. To our surprise, as shown in Figure 9a,b, the PXRD patterns reveal that there is a big difference in how the crystal structure orients itself in the films of the two compounds. Films of (BZA)2PbI4 display the typical 2D perovskite film pattern (Figure 9b), with the perovskite layers aligning preferentially parallel to the substrate. As a result, only the basal (00l) reflections are observed in the diffraction pattern with the reflections of other (hkl) planes strongly suppressed.41,75,76 Remarkably, the PXRD patterns of the (HA)PbI4 films also show a strong preferred orientation, but in this case, the layers orient perpendicular to the substrate (Figure 9a). This is evident from the diffraction pattern which exhibits exclusively two peaks corresponding to the (101) and (202) reflections,

Figure 10. EQE spectra of (a) (HA)PbI4 and (b) (BZA)2PbI4 under as-cast and postanneal conditions. (c) Photovoltaic J−V characteristics of (HA)PbI4 using the postanneal method of various (0.115 to 0.45 M) concentrations. (d) Device architecture of the fabricated solar cells.

devices consisted of a 2-point approach with the first step associated with the optimization of the film thickness and the second with the optimization of the deposition temperature. For the first step, different concentrations of DMF precursor solutions were prepared, from 0.115 M up to 0.45 M, with the molarity refers to total Pb2+ concentration. For the second step, we compared two deposition techniques for both systems: the conventional postanneal method and the hot-casting method.77 Rather expectedly, the BZA films showed poor photovoltaic performance (Table S5) owing to the “wrong” orientation of the structure (parallel to the substrate) in the sense that the electrodes in a standard planar are placed perpendicular to the perovskite layers. This is an unfavorable configuration because 7788


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time, display varying band gap trends for different cation systems. For the more distorted BZA system, the intermediate solid solution compositions also have lower band gaps than the pure Sn compound. For the less distorted HA system, the band gaps of intermediate compositions still fall in between the pure Pb and Sn compounds. Taking advantage of the solution processability, we fabricated thin films of the title compounds. The orientation of the thin films is cation-dependent, as the (HA)PbI4-based films are the first reported single-layered 2D perovskites that have the unusual “perpendicular” orientation with respect to the substrate as opposed to (BZA)2PbI4, which adopts the regular parallel growth direction. This is crucial to device performance, as it was shown in the case of the multilayered 2D perovskite compounds, enabling highly efficient photovoltaic devices. This implies that the use of bifunctional dications in the 2D system will be beneficial to construct new 2D perovskite materials with high optoelectronic performance.

the highest carrier mobility is expected to occur along the inorganic layers, where the carriers propagate through chemical bonds (or organic molecules), whereas in the direction perpendicular to the layers the carriers propagate “throughspace” through a “hopping” mechanism, a significant barrier which significantly lowers the effective carrier mobility. On the other hand, the HA films, which have the “correct” layer orientation (perpendicular to the substrate), display significantly better device performance (Table 3). Table 3. Photovoltaic Performance of (HA)PbI4-Based Solar Cells Pb2+ (M)

Voc (V)

Jsc (mA/cm2)

FF (%)

0.115 0.17 0.225 0.375 0.45

0.65 0.34 0.91 0.79 0.64

0.83 1.94 2.65 1.78 1.10

36.76 34.24 46.7 50.05 39.25

PCE (%) 0.20 0.23 1.13 0.70 0.28

± ± ± ± ±

0.03 0.04 0.04 0.03 0.05

thickness (nm) 89.8 135.7 175.2 232.6 266.1

± ± ± ± ±

10 25 40 30 50

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ASSOCIATED CONTENT

S Supporting Information *

Further optimization of the HA films showed the postanneal method78 to be more suitable than the hot-casting method, judging on overall performance despite the stronger preferred orientation observed from the latter method. The reason for better device performance in (HA)PbI4 with the postanneal method is the premier thin film quality (less pinholes and good coverage). This behavior is unexpected, since the hot-casting process has been known to be more effective in the case of the 3D perovskites, by effectively increasing the grain size and reducing the photovoltaic hysteresis. The optimal devices were found to be those fabricated with the postanneal method for a 0.225 M concentration of (HA)PbI4. Although the Voc for the devices approaches 1 V (similar to the 3D perovskite), the highest device efficiency is limited to 1.13% (Figure 10c) owing to the small photocurrent. The low photocurrent is expected, in view of the wide band gap of the compound, which limits the absorption capacity. In addition, in n = 1 layered perovskite, the strong exciton species creates a barrier to the electron−hole pair separation and permits a lot of carrier recombination in the thin films. The external quantum efficiency (EQE) measurements indicates that the band-edge of both materials is around 600 nm (Figure 10a,b). Compared to the previously reported (BA)2PbI4-based devices, which made use of the mesoporous TiO2 substrate and had very low PCE (0.01%), our (HA)PbI4based devices show sufficiently long electron and hole diffusion length for the planar configuration to reach much higher efficiencies. (HA)PbI4 affords an EQE of up to 25%, which is by far superior to (BZA)2PbI4 (∼4%), Figure 10a,b. This difference highlights the importance of the layer orientation during film fabrication.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03054. Powder X-ray diffraction patterns, SEM and EDS results of (HA)Pb1−xSnxI4 and (BZA)2Pb1−xSnxI4 (x = 1, 0.75, 0.5, 0.25, 0), selected bond length tables of (HA)PbI4, (HA)SnI4, (BZA)2PbI4, and (BZA)2SnI4, thermogravimetric analysis (TGA) of (HA)PbI4 and (BZA)2PbI4, characterization of the second phase of (HA)SnI4, and additional device data (PDF) Crystallographic files (CIF)

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences, under Grant SC0012541 (synthesis and characterization of materials, M.G.K.). The photoexcitation time-resolved studies were supported by the Argonne-Northwestern Solar Energy Research (MRW, ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under award number DESC0001059 (M.R.W.). The solar cell work (A.D.M.) acknowledges the LDRD Program at Los Alamos National Laboratory (LANL). This work made use of the (EPIC, Keck-II, and/or SPID) facility(ies) of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; the State of Illinois, through the IIN. We thank Prof. Joseph Hupp and Prof. Omar Farha for use of the TGA instrument.

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CONCLUSIONS Differences in organic spacers significantly affect the overall properties of lead and tin 2D perovskite iodide materials. By modifying the functional groups of the organic cations, the geometric distortion levels of the inorganic [PbI4]2− and [SnI4]2− layers are strongly impacted, leading to modulations in the optical and electronic properties. When the imidazolium group in HA is in the structure, the charge is better delocalized in the aromatic ring and the distortion levels of the inorganic layers are strongly reduced, resulting in lower band gaps. Pb/Sn solid solutions of the 2D system, demonstrated here for the first

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REFERENCES

(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051.

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Article

Chemistry of Materials (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643−647. (3) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (4) Snaith, H. J. Perovskites: The Emergence of a New Era for LowCost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623− 3630. (5) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319. (6) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H.-j.; Sarkar, A.; Nazeeruddin, M. K.; Gratzel, M.; Seok, S. I. Efficient Inorganic−Organic Hybrid Heterojunction Solar Cells Containing Perovskite Compound and Polymeric Hole Conductors. Nat. Photonics 2013, 7, 486−491. (7) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. (8) Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y.; Grätzel, M.; Han, H. A Hole-Conductor−Free, Fully Printable Mesoscopic Perovskite Solar Cell with High Stability. Science 2014, 345, 295−298. (9) Stoumpos, C. C.; Kanatzidis, M. G. The Renaissance of Halide Perovskites and Their Evolution as Emerging Semiconductors. Acc. Chem. Res. 2015, 48, 2791−2802. (10) Stoumpos, C. C.; Kanatzidis, M. G. Halide Perovskites: Poor Man’s High-Performance Semiconductors. Adv. Mater. 2016, 28, 5778−5793. (11) Jung, H. S.; Park, N. G. Perovskite Solar Cells: From Materials to Devices. Small 2015, 11, 10−25. (12) Cai, B.; Xing, Y.; Yang, Z.; Zhang, W.-H.; Qiu, J. High Performance Hybrid Solar Cells Sensitized by Organolead Halide Perovskites. Energy Environ. Sci. 2013, 6, 1480−1485. (13) Ryu, S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Yang, W. S.; Seo, J.; Seok, S. I. Voltage Output of Efficient Perovskite Solar Cells with High Open-Circuit Voltage and Fill Factor. Energy Environ. Sci. 2014, 7, 2614−2618. (14) Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P.; Kanatzidis, M. G. Lead-Free Solid-State Organic−Inorganic Halide Perovskite Solar Cells. Nat. Photonics 2014, 8, 489−494. (15) Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A.-A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B.; et al. Lead-Free Organic−Inorganic Tin Halide Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 3061−3068. (16) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019−9038. (17) Chung, I.; Lee, B.; He, J.; Chang, R. P.; Kanatzidis, M. G. AllSolid-State Dye-Sensitized Solar Cells with High Efficiency. Nature 2012, 485, 486−489. (18) Koh, T. M.; Fu, K.; Fang, Y.; Chen, S.; Sum, T.; Mathews, N.; Mhaisalkar, S. G.; Boix, P. P.; Baikie, T. Formamidinium-Containing Metal-Halide: An Alternative Material for near-IR Absorption Perovskite Solar Cells. J. Phys. Chem. C 2014, 118, 16458−16462. (19) Pang, S.; Hu, H.; Zhang, J.; Lv, S.; Yu, Y.; Wei, F.; Qin, T.; Xu, H.; Liu, Z.; Cui, G. NH2CHNH2PbI3: An Alternative Organolead Iodide Perovskite Sensitizer for Mesoscopic Solar Cells. Chem. Mater. 2014, 26, 1485−1491. (20) McMeekin, D. P.; Sadoughi, G.; Rehman, W.; Eperon, G. E.; Saliba, M.; Hörantner, M. T.; Haghighirad, A.; Sakai, N.; Korte, L.; Rech, B.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. A Mixed-Cation Lead Mixed-Halide Perovskite Absorber for Tandem Solar Cells. Science 2016, 351, 151−155.

(21) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (22) Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y.; et al. A Hole-Conductor−Free, Fully Printable Mesoscopic Perovskite Solar Cell with High Stability. Science 2014, 345, 295−298. (23) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506−514. (24) Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright LightEmitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (25) Kim, Y. H.; Cho, H.; Heo, J. H.; Kim, T. S.; Myoung, N.; Lee, C. L.; Im, S. H.; Lee, T. W. Multicolored Organic/Inorganic Hybrid Perovskite Light-Emitting Diodes. Adv. Mater. 2015, 27, 1248−1254. (26) Stranks, S. D.; Snaith, H. J. Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices. Nat. Nanotechnol. 2015, 10, 391−402. (27) Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D.D.; Higler, R.; Hüttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; et al. High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. J. Phys. Chem. Lett. 2014, 5, 1421−1426. (28) Stoumpos, C. C.; Fraser, L.; Clark, D. J.; Kim, Y. S.; Rhim, S. H.; Freeman, A. J.; Ketterson, J. B.; Jang, J. I.; Kanatzidis, M. G. Hybrid Germanium Iodide Perovskite Semiconductors: Active Lone Pairs, Structural Distortions, Direct and Indirect Energy Gaps, and Strong Nonlinear Optical Properties. J. Am. Chem. Soc. 2015, 137, 6804− 6819. (29) Saparov, B.; Mitzi, D. B. Organic−Inorganic Perovskites: Structural Versatility for Functional Materials Design. Chem. Rev. 2016, 116, 4558−4596. (30) Mitzi, D. B.; Feild, C. A.; Harrison, W. T. A.; Guloy, A. M. Conducting Tin Halides with a Layered Organic-Based Perovskite Structure. Nature 1994, 369, 467−469. (31) Stoumpos, C. C.; Cao, D. H.; Clark, D. J.; Young, J.; Rondinelli, J. M.; Jang, J. I.; Hupp, J. T.; Kanatzidis, M. G. Ruddlesden−Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors. Chem. Mater. 2016, 28, 2852−2867. (32) Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I. A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability. Angew. Chem. 2014, 126, 11414− 11417. (33) Cheng, Z.; Lin, J. Layered Organic−Inorganic Hybrid Perovskites: Structure, Optical Properties, Film Preparation, Patterning and Templating Engineering. CrystEngComm 2010, 12, 2646− 2662. (34) Li, Y.; Lin, C.; Zheng, G.; Cheng, Z.; You, H.; Wang, W.; Lin, J. Novel < 110>-Oriented Organic−Inorganic Perovskite Compound Stabilized by N-(3-Aminopropyl)Imidazole with Improved Optical Properties. Chem. Mater. 2006, 18, 3463−3469. (35) Li, Y.; Zheng, G.; Lin, J. Synthesis, Structure, and Optical Properties of a Contorted < 110>-Oriented Layered Hybrid Perovskite: C3H11SN3PbBr4. Eur. J. Inorg. Chem. 2008, 2008, 1689− 1692. (36) Saparov, B.; Hong, F.; Sun, J.-P.; Duan, H.-S.; Meng, W.; Cameron, S.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Thin-Film Preparation and Characterization of Cs3Sb2I9: A Lead-Free Layered Perovskite Semiconductor. Chem. Mater. 2015, 27, 5622−5632. (37) Zaleski, J.; Pietraszko, A. Crystal Structure and Phase Transitions of [C(NH2)3]3Sb2Cl9•0.9H2O. Acta Crystallogr., Sect. B: Struct. Sci. 1996, 52, 287−295. (38) Ishihara, H.; Watanabe, K.; Iwata, A.; Yamada, K.; Kinoshita, Y.; Okuda, T.; Krishnan, V.; Dou, S.-Q.; Weiss, A. NQR and X-Ray 7790


Article


Enables Broadening of Absorption Spectrum in Solar Cells. J. Am. Chem. Soc. 2014, 136, 8094−8099. (56) Im, J.; Stoumpos, C. C.; Jin, H.; Freeman, A. J.; Kanatzidis, M. G. Antagonism between Spin−Orbit Coupling and Steric Effects Causes Anomalous Band Gap Evolution in the Perovskite Photovoltaic Materials CH3NH3Sn1−XPbxI3. J. Phys. Chem. Lett. 2015, 6, 3503− 3509. (57) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A.; Puschmann, H. Olex2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339−341. (58) Kortüm, G.; Braun, W.; Herzog, G. Principles and Techniques of Diffuse-Reflectance Spectroscopy. Angew. Chem., Int. Ed. Engl. 1963, 2, 333−341. (59) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (60) Kresse, G.; Hafner, J. Norm-Conserving and Ultrasoft Pseudopotentials for First-Row and Transition Elements. J. Phys.: Condens. Matter 1994, 6, 8245−8257. (61) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (62) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (63) Shannon, R. t. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751−767. (64) Papavassiliou, G. C.; Mousdis, G. A.; Raptopoulou, C.; Terzis, A. Preparation and Characterization of [H3N(CH2)6NH3]PbI4 and Similar Compounds with a Layered Perovskite Structure. Z. Naturforsch., B: Chem. Sci. 1999, 54, 1405−1409. (65) Braun, M.; Frey, W. Crystal Structure of Bis(Benzylammonium) Lead Tetrachloride, (C7H7NH3)2PbCl4. Z. Kristallogr. - New Cryst. Struct. 1999, 214, 331−332. (66) Liao, W.-Q.; Zhang, Y.; Hu, C.-L.; Mao, J.-G.; Ye, H.-Y.; Li, P.F.; Huang, S. D.; Xiong, R.-G. A Lead-Halide Perovskite Molecular Ferroelectric Semiconductor. Nat. Commun. 2015, 6, 7338. (67) Steiner, T. The Hydrogen Bond in the Solid State. Angew. Chem., Int. Ed. 2002, 41, 48−76. (68) Janiak, C. A Critical Account on π−π Stacking in Metal Complexes with Aromatic Nitrogen-Containing Ligands. Dalton Trans. 2000, 3885−3896. (69) Knutson, J. L.; Martin, J. D.; Mitzi, D. B. Tuning the Band Gap in Hybrid Tin Iodide Perovskite Semiconductors Using Structural Templating. Inorg. Chem. 2005, 44, 4699−4705. (70) Li, Y.; Lin, C.; Zheng, G.; Lin, J. Synthesis, Structure and Optical Properties of New Organic-Inorganic Haloplumbates Complexes (C5H10N3)PbX4 (X = Br, Cl), (C2H2N4)PbBr3. J. Solid State Chem. 2007, 180, 173−179. (71) Mitzi, D. B. Synthesis, Crystal Structure, and Optical and Thermal Properties of (C4H9NH3)2MI4(M = Ge, Sn, Pb). Chem. Mater. 1996, 8, 791−800. (72) Papavassiliou, G. C.; Koutselas, I.; Terzis, A.; Whangbo, M.-H. Structural and Electronic-Properties of the Natural Quantum-Well System (C6H5CH2CH2NH3)2SnI4. Solid State Commun. 1994, 91, 695−698. (73) Trinh, M. T.; Wu, X.; Niesner, D.; Zhu, X.-Y. Many-Body Interactions in Photo-Excited Lead Iodide Perovskite. J. Mater. Chem. A 2015, 3, 9285−9290. (74) Wu, X.; Trinh, M. T.; Niesner, D.; Zhu, H.; Norman, Z.; Owen, J. S.; Yaffe, O.; Kudisch, B. J.; Zhu, X.-Y. Trap States in Lead Iodide Perovskites. J. Am. Chem. Soc. 2015, 137, 2089−2096. (75) Takahashi, Y.; Obara, R.; Nakagawa, K.; Nakano, M.; Tokita, J.y.; Inabe, T. Tunable Charge Transport in Soluble Organic−Inorganic Hybrid Semiconductors. Chem. Mater. 2007, 19, 6312−6316. (76) Liang, K.; Mitzi, D. B.; Prikas, M. T. Synthesis and Characterization of Organic−Inorganic Perovskite Thin Films Prepared Using a Versatile Two-Step Dipping Technique. Chem. Mater. 1998, 10, 403−411.

Studies of [N(CH3)4]3M2X9 and (CH3NH3)3M2X9 (M = Sb, Bi; X = Cl, Br). Z. Naturforsch., A: Phys. Sci. 1992, 47, 65−74. (39) Lehner, A. J.; Fabini, D. H.; Evans, H. A.; Hébert, C.-A.; Smock, S. R.; Hu, J.; Wang, H.; Zwanziger, J. W.; Chabinyc, M. L.; Seshadri, R. Crystal and Electronic Structures of Complex Bismuth Iodides A3Bi2I9 (a = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics. Chem. Mater. 2015, 27, 7137−7148. (40) Stoumpos, C. C.; Cao, D. H.; Clark, D. J.; Young, J.; Rondinelli, J. M.; Jang, J. I.; Hupp, J. T.; Kanatzidis, M. G. Ruddlesden−Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors. Chem. Mater. 2016, 28, 2852−2867. (41) Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G. 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications. J. Am. Chem. Soc. 2015, 137, 7843−7850. (42) Tsai, H.; Nie, W.; Blancon, J.-C.; Stoumpos, C. C.; Asadpour, R.; Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak, S.; Pedesseau, L.; Even, J.; Alam, M. A.; Gupta, G.; Lou, J.; Ajayan, P. M.; Bedzyk, M. J.; Kanatzidis, M. G.; Mohite, A. D. HighEfficiency Two-Dimensional Ruddlesden−Popper Perovskite Solar Cells. Nature 2016, 536, 312−316. (43) Daub, M.; Hillebrecht, H. Synthesis, Single-Crystal Structure and Characterization of (CH3NH3)2Pb(SCN)2I2. Angew. Chem., Int. Ed. 2015, 54, 11016−11017. (44) Jiang, Q.; Rebollar, D.; Gong, J.; Piacentino, E. L.; Zheng, C.; Xu, T. Pseudohalide-Induced Moisture Tolerance in Perovskite CH3NH3Pb(SCN)2I Thin Films. Angew. Chem., Int. Ed. 2015, 54, 7617−7620. (45) Xiao, Z.; Meng, W.; Saparov, B.; Duan, H.-S.; Wang, C.; Feng, C.; Liao, W.; Ke, W.; Zhao, D.; Wang, J.; Mitzi, D. B.; Yan, Y. Photovoltaic Properties of Two-Dimensional (CH3NH3)2Pb(SCN)2I2 Perovskite: A Combined Experimental and Density Functional Theory Study. J. Phys. Chem. Lett. 2016, 7, 1213−1218. (46) Willett, R.; Place, H.; Middleton, M. Crystal Structures of Three New Copper(II) Halide Layered Perovskites: Structural, Crystallographic, and Magnetic Correlations. J. Am. Chem. Soc. 1988, 110, 8639−8650. (47) Polyakov, A. O.; Arkenbout, A. H.; Baas, J.; Blake, G. R.; Meetsma, A.; Caretta, A.; van Loosdrecht, P. H.; Palstra, T. T. Coexisting Ferromagnetic and Ferroelectric Order in a CuCl4-Based Organic-Inorganic Hybrid. Chem. Mater. 2012, 24, 133−139. (48) Staulo, G.; Bellitto, C. (C6H5CH2NH3)2CrBr3.3I0.7 - a New Insulating Ferromagnet with a Curie-Temperature of 51K. J. Mater. Chem. 1991, 1, 915−918. (49) Van Amstel, W.; De Jongh, L. Magnetic Measurements on (CH3NH3)2MnCl4, a Quasi Two-Dimensional Heisenberg Antiferromagnet. Solid State Commun. 1972, 11, 1423−1429. (50) Han, J.; Nishihara, S.; Inoue, K.; Kurmoo, M. High Magnetic Hardness for the Canted Antiferromagnetic, Ferroelectric, and Ferroelastic Layered Perovskite-Like (C2H5NH3)2[FeIICl4]. Inorg. Chem. 2015, 54, 2866−2874. (51) Nakajima, T.; Yamauchi, H.; Goto, T.; Yoshizawa, M.; Suzuki, T.; Fujimura, T. Magnetic and Elastic Properties of (CH3NH3)2FeCl4 and (C2H5NH3)2FeCl4. J. Magn. Magn. Mater. 1983, 31-34, 1189− 1190. (52) Mitzi, D. B.; Dimitrakopoulos, C. D.; Kosbar, L. L. Structure and Optical Properties of Several Organic-Inorganic Hybrids Containing Corner-Sharing Chains of Bismuth Iodide Octahedra. Chem. Mater. 2001, 13, 3728−3740. (53) Kagan, C.; Mitzi, D.; Dimitrakopoulos, C. Organic-Inorganic Hybrid Materials as Semiconducting Channels in Thin-Film FieldEffect Transistors. Science 1999, 286, 945−947. (54) Ogomi, Y.; Morita, A.; Tsukamoto, S.; Saitho, T.; Fujikawa, N.; Shen, Q.; Toyoda, T.; Yoshino, K.; Pandey, S. S.; Ma, T.; et al. CH3NH3SnxPb(1−X)I3 Perovskite Solar Cells Covering up to 1060 Nm. J. Phys. Chem. Lett. 2014, 5, 1004−1011. (55) Hao, F.; Stoumpos, C. C.; Chang, R. P.; Kanatzidis, M. G. Anomalous Band Gap Behavior in Mixed Sn and Pb Perovskites 7791


Article

Chemistry of Materials (77) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H.-L.; Mohite, A. D. High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains. Science 2015, 347, 522−525. (78) Xie, Y.; Zhou, W.; Yin, J.; Hu, X.; Zhang, L.; Meng, X.; Ai, Q.; Chen, Y. Post-Annealing to Recover the Reduced Open-Circuit Voltage Caused by Solvent Annealing in Organic Solar Cells. J. Mater. Chem. A 2016, 4, 6158−6166.

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