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CHNHCaI Perovskite: Synthesis, Characterization and First-Principles Studies José Ignacio Uribe, Daniel Ramirez, Jorge Mario Osorio Guillén, Jaime Osorio, and Franklin Jaramillo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04207 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016
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CH3NH3CaI3 Perovskite: Synthesis, Characterization and FirstPrinciples Studies Author(s), and Corresponding Author(s)* José Ignacio Uribea, Daniel Ramirezb, Jorge Mario Osorioa, Jaime Osorioa and Franklin Jaramillob*
Affiliation: a Grupo de Estado Sólido –(GES), Instituto de Física, Universidad de Antioquia UdeA, Calle 70 No. 52-51, Medellín, Colombia b
Centro de Investigación, Innovación y Desarrollo de Materiales – CIDEMAT, Universidad de Antioquia UdeA, Calle 70 No. 52-21, Medellín, Colombia *E-mail:
[email protected] Tel: (+574) 2196680
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ABSTRACT In the last years organo-lead-halide perovskites have emerged as promising new materials for photovoltaics reaching high efficiencies. The excellent photo-electronic properties and easy solution processing makes the lead perovskite an ideal ligth harvesting material in a solar cell. In spite of these great advantages, there are concerns about the lead contain in the material because its well-known toxicity characteristics. Obtaining new metal halide perovskites without lead is still a challenge, but until now, only few experimental reports have been published, and some other theoretical calculations replacing lead by most of the possible candidates of the periodic table. In this paper we show for the first time the synthesis of the calcium hybrid perovskite CH3NH3CaI3 and CH3NH3CaI3-xClx with the complementary studies based on the first-principles band structure calculation. The crystallographic analysis shows a pseudo-cubic structure, and the optical measurements confirms that this type perovskite absorbs light in the UV region, which is in a good agreement with the calculation that showed a band gap larger than 3.5 eV.
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INTRODUCTION Organo-lead-halide perovskites have emerged as a promising material for photovoltaic solar cell applications
1–7
. In the last few years, the power conversion efficiency (PCE) of
perovskite solar cells (PSCs) has raised from 3.8 to 22.1% which is higher than that achieved by other emerging thin film solar cell, and even than the commercial ones like CdTe and CIGS
8–10
. Perovskites cover a large family of compounds with the general
formula ABX3. The most extensive studied hybrid perovskite material is CH3NH3PbI3 and its analogous mixed halide formulation CH3NH3PbI3-xClx. Initially, the perovskite term was used to refer to a calcium titanium oxide mineral with the chemical formula CaTiO3, but as many materials can adopt this structure, it is now used for any material with similar structure 11. The ideal crystalline structure is cubic with A and B being two different cations and X an anion as illustrated in Figure 1a. For the case of a hybrid perovskite the A component can be a monovalent organic cation. In spite of the potential of the lead halide perovskite as a cheap and efficient photovoltaic material, there are concerns respect to the use of lead, because it is a toxic heavy metal. Replacing lead by other metallic cations in the hybrid perovskite is highly justified and is probably, at long term, more sensible rather than just finding strategies to avoid leaking as encapsulating the devices. There are different points of view respect the possible leakage of the lead present in the perovskite devices on the environment. As pointed out by different authors, Pb2+ (obtained after degradation of the cell) should not be released to the environment such as river (water) and plants because a human blood Pb2+ level above
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10µg/dL (~0.5µM) is considered potentially hazard, especially to children 12, and therefore the US Environmental Protection Agency (EPA) suggests that level of lead in drinking water cannot be higher than 1.5µg/dL 13.
One of most common strategies to explore the possible candidates to replace the lead is to use the concept reported by V.M. Goldschmidt
14
in 1926 and adapted also by others 15,16
in which a semi-empirical factor t (Tolerance Factor) was introduced to evaluate ionic size mismatches which the perovskite-structure will tolerate until a different structure-type is formed (eq. 1): = ( + )/√2( + ) (1)
Where rA, rB, and rx are the ionic radius of the A, B, and X ions respectively. For values of τ in the range 0.9 – 1.0, mostly cubic perovskites can be obtained, whereas values of 0.80 – 0.89 predominantly lead to distorted perovskites, resulting in an orthorhombic, a rhombohedral, or a tetragonal structure. In order to see the influence of the possible ions that can lead to a perovskite structure similar to that of the methylammonium lead iodide, we evaluated the tolerance factor as function of different size of ions (Figure 1). Different from the study made by Kieslich et al.
17
in which lead iodides and manganese formiates
were evaluated in terms of the tolerance factor, in this study we combined 11 organic cations with valence of 1+, 20 metallic cations with valence of 2+ and iodine as halide anion with valence of 1-. Smaller halides like F, Cl and Br were also evaluated (Figure S1), but as the iodide perovskite is the most studied material due to its optimal properties as semiconductor and perfect size match, we only focused on this halide. Cl and Br have been
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also used experimentally to obtain methylammonium perovskites, but those perovskites do not have as good properties as the ones obtained from I. The information of the ionic radii of the organic cations used to evaluate the tolerance factor was obtained from reference 17, while the data for the halide ions and the metallic cations in octahedral coordination was taken from reference 18. As shown in Figure 1 the most promising elements to replace lead are Ba, Sr and Ca, and another transitions metals like Sn. Until now, Sn has been used instead of Pb with some successful devices reaching efficiencies up to 6% but with high instability because this metal prefer to be in the 4+ oxidation state rather than in the 2+, a state needed to configure a more stable 3D perovskite structure. There are also some exploratory papers with elements like Sr but they are only theoretical and the calculations have not been experimentally corroborated yet non-toxicity, low cost
22
19–21
. Because of the high abundance of calcium in nature,
and adequate size capable to form a perovskite structure, we
decided to evaluate its possible semiconductor properties in a perovskite-like structure CH3NH3CaI3 and CH3NH3CaI3-xClx, therefore we performed first-principles studies on this structure and also obtained this material experimentally using CaI2 and CaCl2 as precursors. The structural and electronic properties of the material, both simulated and experimentally (proven for first time), show that obtaining a Pb-free perovskite with ideal semiconducting properties still remains a challenge.
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Figure 1. Tolerance factor as function of organic and metallic cations with iodine as halide anion. The numbers in the plot indicate the organic cations. 1=[NH4]+, 2=[H3NOH]+, 3=[(CH3)NH3]+, 4=[H3N-NH2]+, 5=[(CH2)3NH2]+; 6=[NH2(CH)NH2]+; 7=[C3N2H5]+; 8=[(CH3)2NH2]+; 9=[(C2H5)NH3]+; 10=[C(NH2)3]+; and 11=[(CH3)4N]+.
EXPERIMENTAL AND THEORETICAL METHODS First-principles band structure calculation. All total energies, ionic forces, and stress tensor components were calculated via first-principles Density Functional Theory (DFT) using the projector augmented wave (PAW) method 24,25
23
as implemented in the VASP code
. We have used the Generalized Gradient Approximation (GGA), specifically PBEsol,
for the exchange-correlation energy functional
26
. The PAW atomic reference
configurations are: 1s1for H, 2s2 2p2 for C, 2s2 2p3 for N, 3p64s2 for Ca, 5s2 5p5 for I and 6s2 6p2 for Pb, where only electrons treated as valence electrons are explicitly enumerated. The energy cut-off in the plane waves expansion is 520 eV. All structural parameters have been optimized by simultaneously minimizing all atomic forces and stress tensor components via
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the conjugate gradient algorithm. Brillouin-zone integration has been performed on a Monkhorst–Pack 4x4x4-k-mesh with a Gaussian broadening of 0.01 eV for relaxation (ionic forces are converged to 1 meV/Å) and a -centered 6x6x6-k-mesh using the tetrahedron method including Blöchl corrections for the calculation of the total energy (this is converged to 1 meV/formula unit) and the density of states. Synthesis of CH3NH3ICaI3 and CH3NH3CaI3-xClx. An equimolar precursor solution of CH3NH3I and CaI2 was used to obtain the CH3NH3CaI3 material, while a molar ratio of 1:3 (CaCl2:CH3NH3I) was used for the case of CH3NH3CaI3-xClx. Due to the difference in solubility between the calcium salts and methylammonium iodide, the choice of the solvent is very crucial. Attempts were performed using N-N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) or a ratio of 0.7/0.3 (DMF/DMSO) in volume, under magnetic stirring at 60°C. The concentration of the solution was 20wt%, in all three cases a transparent solution was achieved after 3h indicating that probably no reaction was taking place because all of the salts are white powders in solid state. The solutions precipitated when the stirring and heating stopped. Nevertheless, they were left under stirring at 60°C for 5 days. At this time the solution containing the mixture of solvents became slightly yellow (similar to the precursor solution of the lead perovskite) indicating that a reaction was taking place, while the other two solutions in pure DMF and DMSO were still transparent. The need of prolonged stirring is an indicative that these solvents are not the right choices. This may be one of the reasons why Jaccobson et al 21 were not able to obtain CH3NH3SrI3 from CH3NH3I and SrI2 in DMF. We performed one more attempt using gamma butyrolactone (GBL) and toluene in a ratio of 0.9/0.1, respectively. In this case the
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change in color was observed only within one day of stirring. In order to precipitate the material, the solution was dropped into a beaker filled with toluene. It is important to mention here that the material did not precipitate as a powder, but in a gel like form. In order to evaporate the remaining solvent, the material was left inside a globe box in a dry and non-reactive nitrogen atmosphere for one week at room temperature. Characterization. Both CH3NH3CaI3 and CH3NH3CaI3-xClx resulted to be very unstable materials and degraded rapidly in the presence of water. Therefore in order to perform the characterization, encapsulation was required. X-ray diffractogram was collected from the obtained powders in a PANalytical diffractometer. The samples were scanned from 2θ=10° to 60° in a Bragg-Brentano geometry, using Cu Kα (1.5408 Å) radiation with a step size of 0.04° and a speed of 5° per minute. To prevent degradation, the samples were prepared inside the globe box. The powders were put into the holder and covered with an x-ray transparent Mylar film. Raman measurements were performed to the precursor salts and calcium perovskite materials. Raman spectra were recorded using a Horiba Yvonjobin dispersive micro-Raman spectrophotometer using a 632.8nm laser (He–Ne laser) for the excitation radiation with a total acquisition time of 50s for the range of 3500–50cm-1. As a micro-Raman equipment was used, the focusing of the beam directly to the sample is allowed and therefore the glass does not interfere in the spectrum. The UV-vis Reflectance spectrum was taken at room temperature on a UV-vis spectrophotometer Cary 100 from Varian Inc between 300 and 800 nm. Films were tried to obtain by spin coating onto glass substrates, but even encapsulated, the simple degraded fast.
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RESULTS AND DISCUSSION CH3NH3CaI3 structure To describe what happens when the Pb cation is replaced by Ca, the CH3NH3CaI3 and CH3NH3PbI3 crystal structures were relaxed using as the starting crystal structure the pseudo cubic model
27–30
. In order to relax efficiently the crystal structure, we used a
selective dynamics procedure, where in a first step I and Ca (Pb) ionic positions were relaxed and CH3NH3 was frozen, in the second step was allowed CH3NH3 to relax and I and Ca (Pb) were frozen, and in the third step all the crystal structure was fully relaxed.
The optimized geometry of the CH3NH3CaI3 is shown in Figure 2, the CH3NH3CaI3 presented a pseudo orthorhombic structure (a distorted orthorhombic structure) with the Ca atoms located at the center of the unit cell and the I atoms are placed at the center of the faces. The calculated lattice parameters of CH3NH3CaI3 and CH3NH3PbI3 are presented in Table 1. The changes of the b and c lattice constants respect to CH3NH3PbI3 are 1.5% and 1.9%, which are fairly large, but the increment in a is small (0.7%), the resulting unit cell volume increment is 4.2%. All the angles are close to 90o, showing a pseudo orthorhombic structure. In general, there is not a huge change in the crystallographic properties of the structure when we replace Pb by Ca as is expected because the ratio between the ionic radius of calcium and lead is 1.2. This indicates that the synthesis of the material is possible, and the diffraction pattern should be similar to the lead perovskite. Using the software Vesta
31
we observed that the CH3NH3 molecule is oriented almost perpendicular
to the c axis, with a dihedral angle of 4o with respect the I-I bond for CH3NH3CaI3, whereas for CH3NH3CaI3PbI3 we obtained 14o for the same dihedral angle. In Figure 2b it can be
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observed that the position of the Ca atoms is almost in the center of the distorted octahedron build by the I atoms, which shows clearly the perovskite-type structure.
Figure 2. Fully relaxed crystal structure for the CH3NH3CaI3, the yellow lines shows the unit cell used in the calculation, blue represent Ca atoms, purple I atoms, brown C atoms, gray N atoms and pink H atoms. On the left, the figure shows a polyhedral model of the crystal structure; it is clear how the CH3NH3 molecule is in the center of the tetrahedral space among the network of share-corner octahedrons.
Table 1 Calculated lattice parameters of CH3NH3CaI3 and CH3NH3PbI3. Material
Volume (Å3)
a ( Å)
b ( Å)
c (Å)
α
Β
γ
CH3NH3CaI3
250.44
6.2632
6.2780
6.3692
90.004
89.4788
90.0017
CH3NH3PbI3
240.37
6.2192
6.1840
6.2508
90.0012
89.0446
90.0021
Figure 3a and 3b shows the XRD of the two perovskites synthetized from CaI2 (CH3NH3CaI3) and CaCl2, (CH3NH3CaI3-xClx) respectively. The pure iodine perovskite showed a pronounced peak at 2θ = 14.13° corresponding to the (100) diffraction plane of
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the cubic structure, similar to the mayor peak of a CH3NH3PbI3, while the perovskite obtained from the chlorine salt not only showed this peak, but also showed other two at 2θ = 17.00° and 26.02°. The 17.00° peak can be attributed to a CH3NH3CaCl3 perovskite because is very close to the peak already reported for the CH3NH3PbCl3
32
. The match of
the 14.13° experimental peak indicates that the CH3NH3CaI3 perovskite was obtained and therefore validate the theoretical calculations. The Raman spectra of the two materials are shown in Figure 3c and Figure 3d. While the Raman spectra of the precursor salts are included in the Figure S2. It can be observed that when using CaI2 three bands are found at 79, 114 and 146 cm-1, while for the mixed perovskite there are the same bands at 81, 115 and 146 cm-1. The shape of these spectra is very similar to the one reported for the hybrid lead perovskite manly corresponding to the vibration of the inorganic part of the structure 33,34, but the position is not the same because compared to lead, calcium is a lighter element and therefore the molecular vibrations should be achieved at larger shifts. Figure 3e shows the images of the CH3NH3PbI3 and CH3NH3CaI3 perovskites. The lead perovskite is dark brown while the calcium perovskite is white, which suggest a high band gap for the lead-free perovskite.
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Figure 3. Diffraction pattern of (a) CH3NH3CaI3 and (b) CH3NH3CaI3-xClx; Raman spectra for The CH3NH3CaI3 Synthetized from (c) CaCl3 and (d) CaI2, and (e) images comparing the color of the CH3NH3PbI3 and CH3NH3CaI3 pervoskites. Electronic structure. The calculated dispersion relations (E(k)) and the total and the site projected density of states (DOS) for CH3NH3ICaI3 and CH3NH3PbI3 are shown in Figures 4a, 4b and Figures S3a, S3b, respectively. We can observe from Figure 4a that CH3NH3ICaI3 is an indirect gap insulator with the valence band maximum (VBM) and the conduction band minimum (CBM) located at the R and Г points, respectively; the calculated band gap is 3.4 eV. Due to the fact that semi-local functionals such as PBEsol always underestimate the calculated gap and the appearance of a light cation such as Ca, which does not introduce a large correction of the calculated gap due to the spin-orbit
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interaction (SOI); we expect a band gap larger than the obtained one at this level of the computational framework, contrary to the results previously observed for Pb perovskites35. On the other hand, for the reference system CH3NH3PbI3, Figure S3a shows that the band gap is direct and it is located at the R point in agreement with a previous work by Brivio et al.
36
; in the case our calculated gap is 1.56 eV. This value is similar to the experimental
one obtained in the literature
37
because a fortuitous cancelation of errors, where the SOI
negative correction and the positive quasi-particle correction (GW) cancel each other out 38. We can also notice that CH3NH3CaI3 calculated bands have less dispersion than the CH3NH3PbI3 ones, especially around the CBM and VBM. Further, the lowest conduction band is almost dispersionless in the Г-X direction indicating a large electronic effective mass. In general, the topology of the dispersion relation for CH3NH3CaI3 point out to larger electronic and hole effective masses than the CH3NH3PbI3 compound. As the mobility is indirectly proportional to the effective mass, the larger effective masses of CH3NH3CaI3 are an indication of a possible poor conductivity, especially for electrons, in this system. The topology difference of the dispersion relation between these two compounds is due to the orbital character of the nominal Ca2+ and Pb2+ ions building the upper valence and lowest conduction bands. For CH3NH3CaI3 the CBM is build up solely by Ca 3dyz orbitals and in other points of the reciprocal space, e.g. R, the lowest conduction band is mainly made up by Ca 3d orbitals. The presence of Ca 4s orbitals occur at higher conduction bands. This dominance of the Ca 3d orbitals of the low conduction bands results in this flat conduction band topology. On the contrary, for CH3NH3PbI3 the CBM is built up by the hybridization of Pb pz orbitals (~85%) and I s and p orbitals (~15%), this orbital character of the CBM and in general the low conduction bands results in the good dispersion of those
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bands. On the other hand, the orbital character of the VBM of CH3NH3CaI3 is made up only by I p orbitals as it is almost the case for the remaining reciprocal points of the upper valence band. In contrast, the VBM of CH3NH3PbI3 is the result of the hybridization of I p orbitals (~62%) and Pb s orbitals (~38%), given again a good dispersion of the valence band around R. The calculated projected DOS confirms the above discussion of the orbital character of the dispersion relation of these compounds (Figures 4b and S3b). Further, we can observe that the deeper valence band at 4.7 eV (CH3NH3CaI3) and 5.2 (CH3NH3PbI3) are made up by the s and p states which contribute the CH3NH3 molecule. These results show how the halogen atom defines mainly the VBM and the metallic cation define the CBM, whereas the organic molecule has not contribution to the states close the Fermi level. The role of the organic molecule have been studied in some works 39 where the influence of the molecular orientation in the electronic properties has been extensively studied, but the experimental control of the molecular orientation during the synthesis is almost impossible, so the main role of the CH3NH3 in this type of perovskite is the stability of the structure and to facilitate the solution processing used for film formation.
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Figure 4. (a) Calculated dispersion relation and (b) total and site projected DOS for CH3NH3CaI3. The energy is shifted to the VBM.
The optical band gap of the CH3NH3CaI3 was calculated from the kubelka-munk 40 graphic for the diffuse reflectance (Figure S4). The intercept of the lineal region gives the gap of the material, for this case the obtained result was 3.78 eV, which is in good agreement with the DFT results. Another lineal region was also observed in the plot and can be attributed to some minor crystalline structure that could be achieved during the synthesis of the material because of the complexity of the reagents solubility. CONCLUSION The CH3NH3CaI3 material was experimentally studied for the first time as a candidate for lead free perovskite solar cells. Due to the theoretical and experimental results indicating high band gap and low mobility, we can conclude that this calcium based perovskite is not a good candidate as light harvesting material in solar cells. Despite of the results, and considering the obtained band gap and the nature of other Ca perovskites reported in
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literature, other electronic properties such as dielectric and ferroelectricity could be further explored for this type of material.
ACKNOWLEDGMENTS We would like to thank to “Departamento Nacional de Planeación” from Colombia, SGR collaborative project 2013000100184 between Empresas Públicas de Medellín, Andercol S.A., Sumicol S.A. and Universidad de Antioquia for supporting this work.
SUPPORTING INFORMATION Additional tolerance factor curves, Raman spectroscopy of the precursor materials, electronic structure of CH3NH3PbI3, and optical spectroscopy of the CH3NH3CaI3 is included in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.
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TOC: Raman spectrum, geometry and density of states of CH3NH3CaI3
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TOC 203x161mm (150 x 150 DPI)
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Figure 1. Tolerance factor as function of organic and metallic cations with iodine as halide anion. The numbers in the plot indicate the organic cations. 1=[NH4]+, 2=[H3NOH]+, 3=[(CH3)NH3]+, 4=[H3NNH2]+, 5=[(CH2)3NH2]+; 6=[NH2(CH)NH2]+; 7=[C3N2H5]+; 8=[(CH3)2NH2]+; 9=[(C2H5)NH3]+; 10=[C(NH2)3]+; and 11=[(CH3)4N]+. 129x92mm (220 x 220 DPI)
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Figure 2. Fully relaxed crystal structure for the CH3NH3CaI3, the yellow lines shows the unit cell used in the calculation, blue represent Ca atoms, purple I atoms, brown C atoms, gray N atoms and pink H atoms. On the left, the figure shows a polyhedral model of the crystal structure; it is clear how the CH3NH3 molecule is in the center of the tetrahedral space among the network of share-corner octahedrons. 279x105mm (150 x 150 DPI)
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Figure 3. Diffraction pattern of (a) CH3NH3CaI3 and (b) CH3NH3CaI3-xClx; Raman spectra for The CH3NH3CaI3 Synthetized from (c) CaCl3 and (d) CaI2, and (e) images comparing the color of the CH3NH3PbI3 and CH3NH3CaI3 pervoskites. 173x111mm (150 x 150 DPI)
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Figure 4. (a) Calculated dispersion relation and (b) total and site projected DOS for CH3NH3CaI3. The energy is shifted to the VBM. 349x150mm (150 x 150 DPI)
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