Article pubs.acs.org/cm Properties of CH3NH3PbX3 (X = I, Br, Cl) Powders as Precursors for Organic/Inorganic Solar Cells L. Dimesso,*,† M. Dimamay,† M. Hamburger,‡ and W. Jaegermann† † Technische Universitaet Darmstadt, Materials Science Department, Jovanka-Bontshits-Strasse 2, D-64287 Darmstadt, Germany Ruprecht-Karls-Universität Heidelberg, Institute of Organic Chemistry, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany ‡ S Supporting Information * ABSTRACT: CH3NH3PbX3 (X = Cl, Br, I) perovskites were prepared by a selforganization processes using different precursor solutions. The XRD analysis indicates the formation, at room temperature, of a tetragonal structure (space group I4/mcm) for X = I, of a cubic structure (space group Pm3̅m) for X = Br, and of centro-symmetric cubic structure (space group Pm3m) for X = Cl, respectively. The structural analysis revealed the formation of CH3NH3Cl as secondary phase in the Cl-containing system. The morphological investigation revealed the formation of rhombo-hexagonal dodecahedra crystallite for X = I, Br, whereas cube-like aggregates were observed for X = Cl. The thermogravimetric analysis performed in air did not reveal any loss until 250 °C for X = I and 300 °C for X = Br, respectively, whereas the differential thermal analysis (DTA) detected two endothermic thermal events (at 336 and 409 °C) for X = I and one only (379 °C) for X = Br, respectively. The infrared spectra (IR) of the powders conformed to the 3-fold symmetry of the methylammonium ion which rotates around the C−N axis. Optical absorption measurements indicated that the CH3NH3PbX3 systems behave as direct-gap semiconductors with energy band gaps of 1.53 eV for X = I, 2.20 eV for X = Br, and 3.00 eV for X = Cl, respectively, at room temperature. The direct-gap semiconductivity for X = I and X = Br was confirmed by the photoluminescence emission measurements, whereas the compound for X = Cl is inactive. I-containing powders were dissolved in an organic solvent (dimethyl-formamide, DMF). The dispersion (100−300 μL) was dropped on glassy substrates on which thick films were obtained by spin-coating and thermal treatment at 120 °C for ca. 5 min. The preparation of the layers was performed in air at room temperature. materials. By synthesizing IO-composites, the best of both worlds can potentially be obtained within a single material. Among these hybrids, self-organized low-dimensional (0D, 1D, and 2D) IO structures, derived from component 3D networks of R-MX3 (R-organic amine and MX-metal halide) type perovskite, have attracted much attention because of their unique crystal structures and the modified optical properties.1−9 When the R-site in the perovskite formula, R-MI3, is occupied by a monovalent cation, such as Rb, Cs, methylammonium (MA+ hereafter), or formamidinium, a 3D framework is obtained.10−14 Among them, MAPbX3 (X = Cl, Br, I) perovskites described by Weber14 are compounds unusual in several respects. Their color intensifies rapidly from the cream white chloride to the reddish-orange bromide and black iodide, pointing to a charge-transfer character of the Pb−X bonds and possibly to photoconduction. Despite the widening literature on the photovoltaic use of hybrid lead perovskites,8,9 many questions, concerning their peculiar structural, electronic chemistry, and material response to light-induced processes, remain to be addressed. Because perovskites are likely to play a 1. INTRODUCTION The growing demand for renewable energy sources has led to considerable development in many areas related to the research and manufacture of solar cells. In an effort to generate more cost-effective technology, the field of thin film solar photovoltaics (PV) presents a promising avenue toward highefficiency solar energy conversion. Hybrid inorganic−organic (IO) semiconductors are opening up a new insight into lowdimensional PV nanostructures. They deliver a unique replacement of their inorganic and organic counterparts in advanced device structures and provide significant opportunity as multifunctional materials for many electronic and optoelectronic applications. IO hybrids are thus a technologically important class of materials, offering the possibility of combining useful properties of both organic and inorganic components within a single molecular composite. Optical and electrical properties of organic materials, for example, can be tuned relatively easily by modifying their molecular structure. Their ease of processing, plasticity, and low price make organic materials attractive for a number of applications (e.g., fieldeffect transistors, light-emitting devices, in photovoltaics for organic solar cells). Lack of robustness, thermal stability, and low electrical conductivity, however, inhibit their use in many applications. These latter properties are offered by inorganic © 2014 American Chemical Society Received: September 3, 2014 Revised: November 10, 2014 Published: November 14, 2014 6762 dx.doi.org/10.1021/cm503240k | Chem. Mater. 2014, 26, 6762−6770 Chemistry of Materials Article major role for impacting device performances, as well as for further advancements on this emerging research front, the possibility to tune the perovskite features through composition control is very appealing. However, even if outstanding photovoltaic performances have been obtained using MAPbX3 with X = I, little was said about the properties with X = Br, and the role of the chloride remains poorly investigated. The main purpose of this work is the investigation of the structural, morphological, optical, and electronic properties of the MAPbX3 powders as a function of the halide anion substitution (X = I, Br, Cl), which can be used as precursors for dispersions to thin films devices. 3. RESULTS AND DISCUSSION The crystalline structure of the MAPBX3 systems was investigated by powder XRD analysis. The X-ray diffractogramms of the CH3NH3+PbX3− (X = I, Br, Cl) perovskites are shown in Figure 1a−c. For X = I (Figure 1a), the crystalline 2. EXPERIMENTAL SECTION Lead halide hybrid perovskites were prepared by self-organization processes using different precursor solutions that were prepared by commercially available starting materials. In particular, PbCl2 (lead chloride, Alfa Aesar), Pb(CH3COO)2·2H2O (lead acetate dehydrate, Alfa Aesar), HCl (37% wt) (chloridric acid, Carl Roth GmbH + Co. KG), HI (47% wt, iodic acid stabilized with 1.5 wt % hypophosphorous acid, Alfa Aesar), HBr (47−49% wt, bromic acid, Alfa Aesar), and CH3NH2 (40% wt aqueous solution, methylamine, Alfa Aesar) were used without further purification. Methylammoniumtrihalogeno-plumbates (II)CH3NH3+PbX3−, (X = CI, Br, I)were synthesized by a modified self-organization process previously reported.15 For X = I, Br, the materials were prepared in concentrated aqueous solution of the acid HX which contained Pb2+ ions [from lead(II) acetate] and a respective amount of CH3NH3+ (by adding a 40% solution of CH3NH2 in water). Beautiful large crystals of size up to 1−2 mm are grown by cooling an aqueous solution from about 95− 105 °C to room temperature. However, in the case of MA+PbI3‑ the temperature must not be lowered below 40−45 °C, because there the formation of colorless crystals of MA+PbI3‑·H2O begins. On the other hand, for X = Cl, the perovskite was prepared by dissolving 1 g of PbCl2 in a solution of 10 mL of HCl (37% wt) and 20 mL of distillate water. Microcrystalline powder is grown by cooling down the aqueous solution from about 125−130 °C to room temperature after drop by drop addition of 10 mL of CH3NH3+ as 40% solution of CH3NH2 in water. Color of the perovskite type phases: X = CI: colorless; X = Br: orange; X = I: black. The structural analysis of the samples was performed, after thoroughly grinding, by X-ray powder diffraction (XRD) using a D8 Bruker powder diffractometer (Cu Kα1 + Cu Kα2 radiation) with a theta/2 theta Bragg−Bentano configuration. The diffractometer is equipped with an Energy Dispersion Detector Si(Li) to minimize the fluorescence effects. A scanning electron microscope (SEM) Philips XL 30 FEG was used to investigate the morphology of the samples. The thermal behavior of the samples was determined by differential thermal analysis (DTA) and thermogravimetry (TG): measurements were carried out from 30 to 500 °C, with a heating rate of 10 K/min in flowing air using a METTLER STARe SW 10.00 thermoanalyzer. Optical diffuse-reflectance measurements were performed using a PerkinElmer UV/vis/NIR double-monochromator spectrometer operating from 200 to 2500 nm double-monochromator spectrometer operating from 200 to 2500 nm. BaSO4 was used as a nonabsorbing reflectance reference. The generated reflectance-versus-wavelength data were used to estimate the band gap of the material by converting reflectance to absorbance data according to the Kubelka−Munk equation: α/S = (1− R)2/2R, where R is the reflectance and α and S are the absorption and scattering coefficients, respectively.16,17 The infrared (IR) absorption was measured with a Varian FourierTransformed-Infrared (FT-IR) spectrometer model no. IR1001M010. The spectral slit width was 1 cm−1 and the investigated spectral range was between 6000 and 400 cm−1. The photoluminescence (PL) measurements were performed by a VARIAN Cary Ellipse fluorescence spectrophotometer equipped with a Xe-arc as emission source. The emission was measured in the 400−900 nm range. All measurements were performed at room temperature. Figure 1. XRD-patterns of the CH3NH3+PbX3‑ perovskites for (a) X = I, (b) X = Br, and (c) X = Cl, respectively. reflections indicate the formation of a tetragonal structure at room temperature (space group I4/m or I4/mcm), which is confirmed by the data in literature.15,18−20 Perovskites have been the subject of great interest for many years, due to the large number of compounds which crystallize in this structure, the novel properties of certain perovskites, and the large number of phase transitions found in these systems. Many studies have been published on the substances with formula AMX3 such as BATiO3, NaNbO3, CsPbCl3.21,22 There is another class of true perovskites in which A is a molecular cation. Various amines may be placed on the A-site, such as ammonium NH4+,23 methylammonium, CH3NH3+,14,15,24,25 and formamidinium NH2CH=NH2+.26,27 These are particularly common in compounds based around lead and tin halides. In these compounds, the perovskite aristotype is a cubic Pm3m framework structure of composition ABX3, where A (Wyckoff position 1a) is commonly a large cation coordinated to 12 X (3c) anions, B (1b) is a smaller metal bonded to six X anions, and BX6 octahedra are corner-connected to form a threedimensional framework. The hinged octahedra allow for wide adjustment of the B−X−B bond angle, and several sets of cooperative rotations, known as tilt transitions, promote symmetry reduction of the aristotype (ref 19 and references therein). Rapid reorientation motions of MA+ were detected by 6763 dx.doi.org/10.1021/cm503240k | Chem. Mater. 2014, 26, 6762−6770 Chemistry of Materials Article on a strictly local levelin other words, the nanostructure builds itself. At this point, one may argue that any chemical reaction driving atoms and molecules to assemble into larger structures, such as precipitation, could fall into the category of SA. However, there are at least three distinctive features that make SA a distinct concept. These are order, interactions, and building blocks. First, the self-assembled structure must have a higher order than the isolated components, be it a shape or a particular task that the self-assembled entity may perform. This is generally not true in chemical reactions, where an ordered state may proceed toward a disordered state depending on thermodynamic parameters. The second important aspect of SA is the key role of slack interactions (e.g., van der Waals, capillary, π−π, hydrogen bonds) with respect to more “traditional” covalent, ionic, or metallic bonds. Although typically less energetic, these weak interactions play an important role in materials synthesis. For instance, they determine the physical properties of liquids, the solubility of solids, and the organization of molecules in biological membranes. The third distinctive feature of SA is that the building blocks are not only atoms and molecules but also a wide range of nano- and mesoscopic structures, with different chemical compositions, shapes, and functionalities.35 Recent examples of novel building blocks include “polyhedra” and “patchy particles”.36 Important examples of SA in materials science include the formation of molecular crystals, colloids, lipid bilayers, phase-separated polymers, and self-assembled monolayers.37,38 The results of the morphological investigation on the MAPbX3 systems are shown in Figure 2a−c. The single crystals of MAPbI3 generally formed as dodecahedra, with some examples exhibiting faceting consistent with rhombo-hexagonal variable temperature NMR that revealed two phase transitions as the temperature decreases, as a result of progressive ordering of the MA+ ions.28,29 It is expected that these structural distortions strongly affect the relevant PV properties, but there is a need of further systematic investigation to clarify the involved structure−properties relationships. In the Br-containing plumbate, the crystalline reflections indicate the formation of a cubic structure at room temperature (space group Pm3̅m) as shown in Figure 1b. Our XRD data are supported by the investigations in literature.15,30−34 Poglitsch and Weber,15 in the case of MAPbBr3, showed there existed four phases which they labeled α−δ but which subsequent workers have generally referred to as I−IV. NMR, calorimetry, and infrared spectroscopic were performed by Knop et al.,24 who examined the states of disorder for the phases of MAPX3 as a function of temperature. The authors found that transition temperatures for protonated material are I−II 235.1(2)K and II−III 154.2(2)K and III−IV 148.35(5)K. In the case of MAPbBr3, the structure can be rationalized as being, to first order, a framework-driven tilt instability, in which deformation of octahedra are not required but are allowed. These secondary distortions of the octahedral in MAPbBr3 are in fact quite large.24 The XRD data of the Cl-containing system (shown in Figure 1c) revealed the presence of MAPbCl3 as major crystalline phase; however, crystalline reflections (indicated with * in Figure 1c) revealed the formation of CH3NH3Cl as secondary phase. The inorganic sublattice of MAPbCl3 is built up by a three-dimensional array of PbCl6 octahedra and the methylammonium (MA+) cations (with C3v molecular symmetry) are situated in the cavities between the octahedra; to satisfy the site symmetry, these cations have to execute complex rotational and orientational motions and possess almost spherical statistical symmetry. In this structure, the MA+ cations and the Pb atom (one of each per unit cell) occupy Oh sites, and the three chlorine atoms lie on D4h sites.33 Due to this unusual structure, there is also an alternating long and short Pb−Cl bond along a, due to an off-center displacement of Pb within the octahedron. This suggests that the most rigid unit is actually the methylammonium cation, rather than the PbCl6 octahedra, in agreement with existing spectroscopic data, and consequently, the size of the distortion of the PbCl6 octahedra is substantial for the physical properties of the material. To obtain pure materials with exact stoichiometries and lowest number of defects, the solution method is very suitable. Our synthetic procedure is a modification of a reported method15 and gives no observable impurities while it allows for easier manipulation of the materials. Self-organization is a process where some form of global order or coordination arises out of the local interactions between the components of an initially disordered system. This process is spontaneous: it is not directed or controlled by any agent or subsystem inside or outside of the system; however, the laws followed by the process and its initial conditions may have been chosen or caused by an agent. Self-organization is also relevant in chemistry, where it has often been taken as being synonymous with “self-assembly” (SA). Self-assembly in the classic sense can be defined as the spontaneous and reversible organization of molecular units into ordered structures by noncovalent interactions. The main property of a self-assembled system that this definition suggests is the spontaneity of the self-assembly process: the interactions responsible for the formation of the self-assembled system act Figure 2. Typical SEM images of MAPbX3 systems, showing the crystal habits of the respective compounds, obtained from the selforganizing solution method for (a) X = I, (b) X = Br, and (c) X = Cl, respectively (more details in Figure S1). 6764 dx.doi.org/10.1021/cm503240k | Chem. Mater. 2014, 26, 6762−6770 Chemistry of Materials Article dodecahedra,19 which is a typical crystal habit of a body centered tetragonal lattice, in agreement with the reported structure at room temperature (space group I4/mcm) (Figure 2a). A very similar behavior has been observed in MAPbBr3 as shown in Figure 2b. The crystals of the obtained materials have a well-defined habit which can even be modified depending on the temperature profile of the precipitation. Namely, the products vary from discrete polyhedral crystals when crystallization occurs by cooling slowly to room temperature and without stirring, to polycrystalline aggregates when precipitation starts at elevated temperatures (ca. 130 °C) under vigorous stirring, as clearly shown in Figure 2c for the MAPbCl3 system. A magnification of Figure 2c (shown in Figure S1 of Supporting Information) confirmed the formation of aggregated microcrystalline structures. To investigate the thermal stability of the MAPBX3 systems, DTA-TG measurements have been carried out in air from 30 to 500 °C. The DTA profiles and TG-curves are shown in Figure 3A for X = I and Figure 3B for X = Br, respectively; the values °C and T = 409 °C, respectively, have been detected. The first event (Tp1) can be related to the melting process of the material, which takes place together with its decomposition. Indeed, according to the thermal data by Mitzi (ref 39 and references therein), the decomposition of A2MX4 (A = organic amine, M = divalent metal, X = halogen) compounds often occurs through the simultaneous loss of A and HX from the compound below the melting temperature. In our case, however, the thermal events in air, at Tp1 in Figure 3A, correspond to the following reactions Step 1: CH3NH3PbI3 (s) → Liq + PbI 2 (s) Step 2: PbI 2 (s) → PbI 2 (liq) where “Liq” indicates the formation of a liquid phase as confirmed by the absence of a “plateau” in the TG-curve after the thermal event at Tp1, whereas the formation of solid PbI2 is supported by (a) optical observations (the material becomes yellow) and (b) by the thermal event at Tp2 which corresponds to the melting point of solid PbI2.40 A different thermal behavior has been observed in the Brcontaining system (shown in Figure 3B). In this case, DTA profile and TG-curve show only one thermal event occurring at Tp1 = 379 °C (as in Table 1), which also corresponds to the melting point of PbBr2 solid phase.40 This indicates that the decomposition and melting processes occur in one step only with the formation of a solid phase, which is stable until 500 °C as confirmed by the plateau in the TG curve shown in Figure 3B. The results obtained from our thermal investigations can open new perspectives concerning the formation and growth of MAPBX3 polycrystalline systems from (partially) melting conditions. During our investigation, we also observed stability of prepared powders under humidity (2−3 weeks, 60% relative humidity at room temperature). Accurate investigations on the effects of the humidity on MAPBX3 systems have not been performed yet; however, in any case, we speculate that the washing in diethyl ether after the separation from the mother liquor played an important role to reduce the degradation effects of the humidity on the samples. Infrared spectra of the MAPbX3 systems are shown in Figure 4a for X = I, Figure 4b for X = Br and Figure 4c for X = Cl, respectively. A summary of the vibrational group frequencies is shown in Table 2. The FT-IR absorption spectra, recorded in the wavenumber range between ν/6000 cm−1 and ν/400 cm−1, give information on the vibrational behavior of the MA group in the systems only. Information on the lattice dynamical properties of the Pb-X groups in the octahedral coordination can be obtained by measuring at higher frequencies (lower than ν/400 cm−1). These measurements have been reported by Preda34 for Pb−I and by Carabatos-Nedelec41 for Pb−Br and Pb−Cl groups, respectively. Internal vibrations of an MA ion are classified into A1, A2, and E modes according to the C3v symmetry32 of the molecular ion. Fundamental vibrations were assigned by comparison with well documented spectra of the MA ion in other compounds (ref 42 and references therein). By observing the FT-IR spectra of the MAPbX3 compounds, some points are important to note. First, the spectra for X = I (Figure 3A) and X = Br (Figure 3B) are very similar to the exception of the widths of the group bands in the Br-containing system. Moreover, fundamental vibrations above ν/4000 cm−1 have not been observed in FT-IR spectra; this can be explained by the lower electronegativity of Br and I anions, by the lower distortion and consequently by the lower polarization along the Figure 3. DTA heating scans and TG curves (10 K/min) of the MAPbX3 systems for (A) X = I, and (B) X = Br, respectively. Temperatures of the thermal events and weight losses are reported in Table 1 Table 1. Summary of the Values of the Thermal Events for the MAPbX3 Systemsa X ther. event no. Ton‑set (°C) Tp (°C) endo/exo Δw/w (%) I 1 2 1 331 406 363 336 409 379 endo endo endo 18.83 2.85 22.20 Br a Data are obtained from the data in Figure 3A (X = I) and in Figure 3B (X = Br), respectively, after thermal analysis performed in air from 30°C to 500°C. of the peak temperatures (Tp) are summarized in Table 1. From the thermal profiles in Figure 3 and the data in Table 1, some important points are to note. First, a high thermal stability of the materials in air can be observed. Indeed, no thermal events were detected up to 260 °C for X = I (blue arrow in Figure 3A) and up to 285 °C for X = Br, respectively (blue arrow in Figure 3B). Second, the thermal behavior of the MAPbX3 depends upon the nature of the anion in the system. In the I-containing system, two endothermic events at T = 336 6765 dx.doi.org/10.1021/cm503240k | Chem. Mater. 2014, 26, 6762−6770 Chemistry of Materials Article Table 2. FT-IR Group Frequencies/Assignments for the MAPbX3 Systems Obtained from the Spectra Shown in Figure 4a−c X no. I 1 group frequency (cm‑1) 3955−3766 3761−3468 2 3 4 Br Cl Figure 4. FT-IR spectra of the MAPbX3 systems (measured at room temperature) for (a) X = I, (b) X = Br, and (c) X = Cl, respectively. C−N···X (X = Br, I) axis. Second, the spectrum for X = Cl shows wider and more intense vibrational bands as well as a vibration mode in the NIR (near-infrared) range. A very detailed discussion and assignment of the frequency bands of Cl-containing compound FT-IR spectra has been reported by Waldron.43 The authors found out a fundamental vibration at ν/5470 cm−1 (in our work ν/5411 cm−1, as shown in Figure 4C and in Table 2), which was assigned to the degenerate E overtone absorption bands which are polarized perpendicular to the c crystallization axis (which corresponds to the C−N···Cl axis). However, the fundamental vibrations of MAPbCl3 are overlapped by the vibrational modes of methylammonium iodide as secondary phase, and this makes a correct interpretation of the spectrum more of a challenge. The differences in the structural and morphological features have a dramatic influence on the optical properties of the prepared materials. In Figure 5A, the optical properties of the MAPbX3 systems are shown. The Cl- and Br-containing systems display a strong, abrupt absorption in the visible spectral region, whereas the I-containing system displays an absorption in the near-infrared attributed to the energy gap between the conduction and the valence bandl; the energy values of the band-gaps as a function of the anions (and their ionic radii44) have been summarized in Table 3. Assuming a bulk-like behavior of the prepared samples, the values of the band energy gap, in electronvolt (eV) have been calculated according to the equation: Eg = 1239.84/λon − set a 2916 2849 2361−2342 1592−1301 5 6 1 905 719; 677; 668; 649; 617 3958−3773 3764−3228 2 3228−2590 3 4 5 6 1 2 2362; 2343; 2334 1722−1287 904 720; 677; 669; 648; 618 5411 3921−3560 3 2947; 2773 4 5 6 7 8 2477; 2362; 2342 1660−1350 1258; 1185 1000; 954 707−634 functional group/assignment primary ammines; N−H asymmetric stretching primary ammines; N−H symmetric stretching methyl C−H asymmetric stretching methyl C−H symmetric stretching H−C−N stretching C−N asym. bending + N−H asym. bending C−N rocking N−H rocking + C−H rocking N−H asymmetric stretching primary ammines; N−H asymmetric stretching methyl C−H asym. stret. + Methyl C−H symm. stret. H−C−N stretching C−N stretching + C−H bending C−N rocking N−H rocking + C−H rocking polarized torsion modea primary ammines; N−H symmetric stretching C−H symm. stret.; C−H asym. stret. H−C−N stretching C−N stretching + C−H bending H−C−N bending C−N stretching; C−N rocking N−H rocking + C−H rocking ref 43. Figure 5. (A) Normalized absorption spectra of MAPbX3 systems measured at room temperature, (B) plots of (αhν)2 for direct transitions in MAPbX3 systems (in inset X = Cl), where α is absorption coefficient and hν is photon energy. Band gaps Eg are obtained by extrapolation to α = 0. The data are reported in Table 3. (1) where Eg is measured in eV, “1239.84 (eV·nm)” is a constant, λon‑set (nm) is the frequency value at which the photons absorption begins, obtained by fitting the linear part of the absorbance spectrum.45 The materials display optical band gaps 6766 dx.doi.org/10.1021/cm503240k | Chem. Mater. 2014, 26, 6762−6770 Chemistry of Materials Article Semiconductors semiconductors with low defect concentration have been prepared. For X = I, the band gap has a value of 1.50 eV which is very consistent with the data reported in the literature (e.g., ref 20 and references therein). The authors found also out that the Pb-based phases have band gaps which are independent of the preparation method, observed a narrowing of the band gap upon symmetry lowering as well as a sharp absorption edge suggesting a direct band gap. Overall, the iodide perovskite materials absorb in the spectral region between 1.2 and 1.7 eV. Semiconductors with direct energy gaps between 1.0 and 1.5 eV are highly sought after, because they are optimal for solar cell applications yielding the maximum theoretical conversion efficiency. Second, for X = Br, MAPbBr3 showed a lower band gap (2.17 eV) than that reported by Kitazawa48 (2.35 eV) by reparing films through spin-coating from a 0.5%wt N,N-dimethyl-formamide solution and by Edri30 (∼3.5 eV until ∼2.4 eV) who prepared films through a solution process in organic solvent. The authors attributed the high values to the nonuniformity of the films. Third, unlike MAPbI3 and MAPbBr3 systems, MAPbCl3 system showed a sharp absorption edge in the visible region of the spectrum (∼400−700 nm) with the band gap energy of 2.94 eV, which is close to 3.11 eV reported in ref 48. This can depend upon the peculiar crystalline structure of the material. Indeed, in the case of the methylammonium lead halides, there are both molecular cations and anions. As the anions are totally corner-bonded, clearly the anions possess strong direct interactions between one another. However, cation−cation interactions will be mainly indirect, as they are isolated from one another in the perovskite cages. Chi et al. (ref 33 and references therein) showed that orientational ordering of the methylammonium units and the hydrogen bonding between methylammonium and chloride ions significantly distorts the network of octahedra. Finally, by comparing to the values obtained from Figure 4B, the extrapolation gave Eg = 1.53 eV for MAPbI3, Eg = 2.20 eV for MAPbBr3, and Eg = 3.00 eV for MAPbCl3 respectively. The measurements clearly indicate (a) MAPbX3 polycrystalline materials with grain size in μm range behave like bulk-materials, (b) for X = Cl the presence of optical inactive secondary phases does not influence the properties of optical active materials. The PL properties of specimens of the MAPbX3 systems were measured at room temperature using an emission wavelength of 380 nm. PL emissions were observed for Iand Br-containing compounds (shown in Figure 6a,b) whereas the Cl-containing system was PL inactive. The emission wavelength of MAPbI3 (peak maximum at ∼754 nm as in Figure 6a) is consistent with our optical absorption data and supported by Stoumpos49, providing further evidence for the direct nature of the band gap. On the other hand, the emission wavelength of the MAPbBr3 (peak maximum at ∼568 nm, as in Figure 6b) is consistent with our optical absorption data but it is slightly higher than the values (∼550 nm) reported by Edri,30 Kitazawa,48 and Schmidt.50 In the case of comparative studies between continuous thin films and nanopowdered materials, the sizes of primary particles can play a prominent role in the final properties. The fact that the PL spectra exhibit a definitive red shift of the peak maxima from 550 (in films48) to 568 nm (in powders) is consistent with band-edge emission and reminiscent of the size-dependent PL emission observed from Ge-nanocrystals (ref 51 and references therein). The absence of PL emission from well-formed solutiongrown MAPbCl3 nanocrystalline system at room temperature, Table 3. Band Gap Energy Values for the MAPbX3 Systems X ionic radius (Ǻ )c λon−set (nm) Eg (eV)a Eg (eV)b I Br Cl 2.20 1.96 1.81 824 571 422 1.50 2.17 2.94 1.53 2.20 3.00 a Results are obtained from the data in Figure 5A, and the energy values have been calculated by using λon−set obtained by fitting the linear part of the absorbance spectra; bIn Figure 5B, the energy values have been calculated according to the model proposed in refs 46,47. c ref 44; coordination number: 6; state: high spin. in good agreement with the different colors of the solids. The data at wavelengths less than 730 nm for X = I, 510 nm for X = Br and 295 nm for X = Cl have been omitted due to dispersion and diffusion effects. For semiconductors, the absorption of photons with energy similar to that of the band gap, Eg ≅ hν, leads to an optical transition producing an electron in the conduction band and a hole in the valence band (exciton). Such an electronic transition is subject to the selection rule such that the wave vector (k) must be conserved (i.e., Δk = 0).46,47 Semiconducting materials, in which the wave vector is conserved for optical transitions, are known as direct band gap semiconductors. Indeed, strong size confinement effects can be observed in nanocrystalline systems when their crystal size becomes smaller than the exciton Bohr diameter. In this case, the electron−hole carriers (excitons) are spatially confined by the dimension of the particle, allowing quantization of the electronic states of conduction and valence bands. This nanoscale reduction does not allow for broad electronic bands to develop and this inability to develop broad bulk-like electronic bands is the reason for the well-known blue shifts observed in conventional nanocrystals. As nanoparticles in powders tend to agglomerate and consequently to develop bulk-like bands, the relationship between the adsorption coefficient (α) near the absorption edge and the optical band gap (Eg) for direct interband transitions obeys the following relationship46 (αhν)2 = A(hν − Eg ) (2) where A is the parameter that relates to the effective-masses associated with the valence and conduction bands and hν is the photo energy. Hence, the optical band gap for the absorption can be obtained by extrapolating the linear portion of (2) (αhν)2 − hν to α = 0, as shown in Figure 5B for X = I and X = Br and in Figure 5B inset for X= Cl due to a different scale of the ordinates. The values of the energy gaps obtained from eq 2 by extrapolating to α = 0 are reported in Table 3. By observing Table 3, some important points are apparent. First, according to the optical absorption measurements shown in Figure 4A, an increase of the band gap energy by decreasing the ionic radius of the anion in the MAPbX3 systems can be observed. In addition, the pregap region does not show strong absorption tails, indicating that high quality semiconductors with low defect concentration have been prepared. For X = I, the band gap has a value of 1.50 eV, which is very consistent with the data reported in the literature (e.g., ref 20 and references therein). The authors found also out that the Pbbased phases have band gaps, which are independent of the preparation method, observed a narrowing of the band gap upon symmetry lowering as well as a sharp absorption edge suggesting a direct band gap. Overall, the iodide perovskite materials absorb in the spectral region between 1.2 and 1.7 eV. 6767 dx.doi.org/10.1021/cm503240k | Chem. Mater. 2014, 26, 6762−6770 Chemistry of Materials Article Figure 6. Photoluminescence spectra of MAPbX3 systems, measured at room temperature and emission at 380 nm, for (a) X = I and (b) X = Br, respectively. Figure 7. (A) Top view and (B) tilted view SEM images of a MAPbI3 layer deposited on FTO glass substrate by spin coating, then by thermal annealing at 110 °C for 5 min from 10 wt % dispersion in DMF. The thickness of the film is around 30 μm. The red arrows indicate the FTO glass substrate. observed and reported by Kitazawa48 during the investigation of CH3NH3PbBr3−xClx films (x = 3) as well, is hard to explain. A rough attempt to account for this is to suppose that the material is relatively PL-inactive in their higher symmetry phase, where the distortions are small. If PL originates from defects, it should increase as the symmetry is lowered via twinning (as by lowering the temperature). Other possible explanations may be related to the particular chemical structure of the Cl-containing system compared to the others, higher polarization along the C−N···Cl axis as well as to the method of preparation which allowed a fast growth of polycrystalline materials with very small grain size. The small grain sizes (and grain size distribution) give origin to vacancies, defects, selfinterstitial(s), vacancy aggregates which can affect the PL behavior of the material without to affect different decay mechanisms. The next step of our investigation has been the preparation of layers using the prepared powders as precursors. As example, 1.23 g of CH3NH3PbI3, dissolved in different amounts of DMF, resulted in very homogeneous and stable dispersions with concentrations of 30 wt %, 10, 5, and 1 wt % of MAPbI3 respectively. Typically, 100−300 μL of the dispersion has been dropped on fluorine-doped tin oxide coated (FTO-coated) glass substrates, then deposited by spin-coating, and finally thermally treated at 100−110 °C for 5−10 min in order to evaporate the solvent and consequently to favor the recrystallization of the MAPbI3 phase on the substrates. The results of the morphological investigation on a typical MAPbI3 layer are shown in Figure 7a,b. The tilted SEM image (Figure 2B) confirms the formation of a layer on the FTO glass substrate (indicated with red arrows), whereas the top view SEM image (Figure 7A) confirms the homogeneity of the layer, however, with the presence of small hollows possibly due to the fast evaporation of the solvent during the thermal treatment. The crystallization of the MAPbI3 phase in the layer has been confirmed by PL measurements (not reported here). The PLprofile showed a broad peak in the range 750−800 nm, as supported by the literature.52 These results encouraged us to continue our investigation in order to optimize the preparation of the layers and to build up solar cells devices with enhanced properties. 4. CONCLUSIONS In this work, perovskitic CH3NH3PbX3 (X = Cl, Br, I) systems (as powders) have been investigated. The XRD analysis indicates the formation, at room temperature, of a tetragonal structure (space group I4/mcm) for X = I, of a cubic structure (space group Pm3̅m) for X = Br and of centrosymmetric cubic structure (space group Pm3m) for X = Cl, respectively. Furthermore, crystalline reflections due to CH3NH3Cl phase as secondary phase have been observed in the Cl-containing system. The morphological investigation revealed the formation of rhombo-hexagonal dodecahedra crystallite for X = I, Br, whereas polycrystalline aggregates have been observed for X = Cl. The analysis confirmed the crystallization of a secondary phase in the Cl-containing compound. The thermal stability of the MAPBX3 systems has been investigated by the DTA-TG analyses, which have been carried out in air from 30 to 500 °C. From the thermal profiles, the following important points were observed: (1) no thermal events were detected up to 260 °C for X = I and up to 285 °C for X = Br respectively; (2) a dependence of the thermal behavior from the nature of the anion. In the I-containing system, two endothermic events at T = 336 °C and T = 409 °C, respectively, have been detected. The first event (Tp1) has been related to melting and decomposition processes which bring to the formation of liquid and lead iodide as solid phase, whereas the second event corresponds to melting process of the lead iodide. On the other hand, in the Br-containing system only one thermal event was observed at Tp1 = 379 °C, which also corresponds to the melting point of PbBr2. 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By decreasing the ionic radius of the halogen, a spectral splitting is observed which may be partially explained by C−N···X (X = I, Br, Cl) interactions in the unit cell. However, the presence of the secondary did not allow a clear assignment of the vibrational modes of the spectrum. Optical absorption measurements indicate that, assuming a bulk-like nature, the CH3NH3PbX3 systems behave as directgap semiconductors with energy band gaps of 1.50 eV for X = I, 2.17 eV for X = Br, and 2.94 eV for X = Cl, respectively, at room temperature. On the other hand, the energy band gaps obtained from the extrapolation plots (αhν)2 for direct transitions in MAPbX3 systems were 1.53, 2.20, and 3.00 eV for X = I, X = Br, and X = Cl, respectively. The results obtained indicate a small effect of size confinement effects in polycrystalline systems and/or strong agglomerated nanocrystalline compounds. The direct-gap semiconducting behavior of MAPbI3 and MAPbBr3 has been confirmed by the photoluminescence emission measurements whereas MAPbCl3 is inactive, possibly related to the higher polarization along the C−N···Cl axis. The prepared powders were used as precursors to prepare layers deposited on FTO-coated glass substrates by spin coating and thermal treatment. The morphological investigation revealed the formation of a layer of MAPbI3 with the presence of small hollows possibly due to the fast evaporation of the solvent during the thermal treatment. ■ ASSOCIATED CONTENT S Supporting Information * In Figure S1, a magnification of Figure 2C is shown. The arrows indicate crystallites of CH3NH3PbCl3. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: ldimesso@surface.tu-darmstadt.de. Tel.: +49 6151 1669667. Fax: +49 6151 166308. Author Contributions All authors have given approval to the final version of the manuscript. Funding This work has been supported by the Federal Ministry of Research and Development of Germany. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Many thanks are owed to Mr. J.-C. Jaud for the technical assistance during the XRD analysis. The authors thank the Federal Ministry of Research and Development (BMBF) for the financial support during this work. ■ ■ REFERENCES (1) Kagan, C. R.; Mitzi, D. B.; Dimitrakopoulos, C. Science 1999, 286, 945. 6769 dx.doi.org/10.1021/cm503240k | Chem. Mater. 2014, 26, 6762−6770 Chemistry of Materials Article (42) Oxton, I. A.; Knop, O.; Duncan, J. L. J. Mol. Struct. 1977, 38, 25. (43) Waldron, R. D. J. Chem. Phys. 1953, 21, 734. (44) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (45) Ghobadi, N. International Nano Letters 2013, 3, 2. (46) Tsunekawa, S.; Fukuda, T.; Kasuya, A. J. Appl. Phys. 2000, 87, 1318. 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