Article pubs.acs.org/IC
Determination of Thermal Expansion Coefficients and Locating the Temperature-Induced Phase Transition in Methylammonium Lead Perovskites Using X‑ray Diffraction T. Jesper Jacobsson,*,† L. Josef Schwan,‡ Mikael Ottosson,‡ Anders Hagfeldt,†,‡ and Tomas Edvinsson‡ Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland ‡ Ångström Laboratory, Department of Chemistry, Uppsala University, Box 538, 75121 Uppsala, Sweden †
S Supporting Information *
ABSTRACT: Lead halogen perovskites, and particularly methylammonium lead iodine, CH3NH3PbI3, have recently attracted considerable interest as alternative solar cell materials, and record solar cell efficiencies have now surpassed 20%. Concerns have, however, been raised about the thermal stability of methylammonium lead iodine, and a phase transformation from a tetragonal to a cubic phase has been reported at elevated temperature. Here, this phase transition has been investigated in detail using temperature-dependent X-ray diffraction measurements. The phase transformation is pinpointed to 54 °C, which is well within the normal operating range of a typical solar cell. The cell parameters were extracted as a function of the temperature, from which the thermal expansion coefficient was calculated. The latter was found to be rather high (αv = 1.57 × 10−4 K−1) for both the tetragonal and cubic phases. This is 6 times higher than the thermal expansion coefficient for soda lime glass and CIGS and 11 times larger than that of CdTe. This could potentially be of importance for the mechanical stability of perovskite solar cells in the temperature cycling experienced under normal day−night operation. The experimental knowledge of the thermal expansion coefficients and precise determination of the cell parameters can potentially also be valuable while conducting density functional theory simulations on these systems in order to deliver more accurate band structure calculations.
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INTRODUCTION
There are, however, some important issues still to be addressed before lead halogen perovskites can become a competitive commercial technology. One of the most important points is the stability and long-time performance under operating conditions. Lead iodine perovskite, CH3NH3PbI3, is soluble in water, and the cells are not stable in a humid environment.8,9 This is, however, a problem that most likely can be solved with proper encapsulation. Another stability concern arises from the behavior under thermal stress. At room temperature, CH3NH3PbI3 crystallizes in a tetragonal structure.10 This is expected for perovskites with ideality factors, t, under 0.9,11 which is the case for CH3NH3PbI3, where the ions lack the proper relative size for forming an ideal cubic perovskite structure at room temperature. It is rather common for perovskites to undergo a phase transformation from a tetragonal or an orthorhombic phase to a cubic phase at elevated temperatures.12 For CH3NH3PbI3, the tetragonal-tocubic phase transition is reported to occur around 55 °C.13,14 This is highly interesting from a stability perspective because a solar cell module under working conditions could reach 80 °C
Some of the most promising alternative solar cell materials investigated at the moment are the organic/inorganic lead halogen perovskites, where CH3NH3PbI3 could be considered as the model compound that so far attracts the most attention. The first papers on the subject were published in 2009 by Miyasaka et al.,1,2 where CH3NH3PbI3 was utilized as a light absorber in a quantum-dot-sensitized solar cell. These first solar cells were highly unstable, the efficiencies were low, and the attention from the community was moderate. A few years later, the interest exploded after the publication of a number of advances,3−7 including efficiencies around 10% and stability improvements by exchanging the liquid electrolyte in favor of a solid-state hole conductor. The number of publications has since increased in an exponential manner, and the certified record efficiency is, according to the National Renewable Energy Laboratory’s Research Cells Record Efficiencies table, now above 20%. The record efficiencies for lead halogen perovskite solar cells thus surpass what have been achieved for all other emerging solar cell technologies and are starting to catch up with commercial thin-film technologies like CdTe and CIGS. © XXXX American Chemical Society
Received: July 3, 2015
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DOI: 10.1021/acs.inorgchem.5b01481 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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THEORY Methylammonium lead iodine, CH3NH3PbI3, crystallizes in a perovskite structure. A prototypical 1−1−3 perovskite is a compound with the formula ABC3 with the same crystal structure as CaTiO3, which is illustrated in Figure 1. A and B
during sunny days. The desired lifetime for a commercial solar cell is a few decades, which means that the perovskite may be cycled over this phase transformation up to a few thousand times. How this phase transformation affects the material properties is so far largely unexplored. Among the areas that would be valuable to investigate are how the structure changes, how the optoelectronic properties change, how the phonons behave, and how cycling affects the mechanical integrity of the crystals. In this paper, we try to illuminate a few of these questions, focusing on the fundamental properties of methylammonium lead iodine perovskite in relation to the phase transformation between tetragonal to cubic symmetry. This was done by measuring X-ray diffraction (XRD) in a temperature interval between 30 and 100 °C and by a detailed analysis of the lattice shifts and thermal expansion coefficients. The paper starts with a Theory section outlining what can be expected and what is already known for the CH3NH3PbI3 structure. This is followed by experimental results and extraction of the lattice shifts and thermal expansion coefficients.
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Article
Figure 1. (a) Unit cell of an idealized cubic methylammonium lead iodide perovskite. (b) Same structure as that in part a but illustrating the 6-fold coordination of Pb2+ and the corner-sharing of the PbI6 octahedrals. (c) Same structure as that in part b but seen along the c axis. The structures are drawn using the Vesta software.20
are two different cations, and C is an anion binding the cations together. There is an entire class of compounds related to the CH3NH3PbI3 perovskite, where A is CH3NH3+, B is Pb2+, and C is I−. The iodine has been replaced with other halogens like bromine15 and chlorine,16 methylammonium by other small ions like formamidinium17 and ethylammonium,2 and lead by tin.18,19 Here we choose to focus entirely on CH3NH3PbI3, which could be considered as the model compound most research up until now has focused upon and for which most of the best solar cell results have been reported. The basic structural motifs of this perovskite are cornersharing lead−iodine octahedrals, where the methylammonium ions have a 12-fold coordination symmetry and are located in the cuboctahedral voids between the PbI6 octahedrals, as illustrated in Figure 1. Central for the structure of perovskites is the tolerance factor, t, which was introduced by Goldschmidt in the 1920s.11 The tolerance factor relates the structure to the ionic radius according to eq 1, where rA, rB, and rC are the ionic radii of the A, B, and C ions, respectively. rA + rC t= 2 (rB + rC) (1)
EXPERIMENTAL METHODS
Methylammonium lead iodine perovskite, CH3NH3PbI3, was synthesized according to an established protocol3 based on spin coating of an equimolar solution of PbI2 and CH3NH3I in dimethylformamide (DMF). In a typical synthesis, 1 g of CH3NH3I (6.29 mmol) and 2.89 g of PbI2 (6.29 mmol) were dissolved in 6.2 mL DMF at 80 °C under magnetic stirring for 3 h. This forms an orange-yellow solution containing 40 wt % salts. Methylammonium was bought from Dyesol, and the other chemicals were bought from Sigma-Aldrich. The chemicals were used as received. The solution preparation and all syntheses were performed under ambient atmosphere. The lead salts are poisonous, and care should be taken during processing and while taking care of contaminated waste. Thin films of the perovskite were deposited by both spin and drop coating from the DMF precursor solution. Spin-coated films were deposited on soda lime glass at 2000 rpm under 60 s and subsequently annealed at 150 °C in an oven for 10 min. The drop-coated films, which are thicker and optically nontransparent, were deposited on soda lime glass by dropping the precursor solution on the substrate, followed by annealing at 150 °C in an oven for a few hours. UV−vis absorption measurements were performed on an Ocean Optics HR-2000+ spectrophotometer using deuterium and halogen lamps. In all measurements, a full spectrum from 190 to 1100 nm with 2048 evenly distributed points was collected. A total of 100 consecutive spectra were averaged in order to obtain good statistics. At room temperature, XRD measurements were measured using a Bruker D8 Advanced Twin diffractometer. The Bragg−Brentano geometry was used with a Lynxeye XE position-sensitive detector. Cu Kα with a wavelength of 1.54 Å was used as the X-ray source. 2θ scans between 10° and 90° were collected, using a step size of 0.0015°. Temperature-dependent XRD was measured using a Bruker D8 Advanced Bragg−Brentano diffractometer with a Johansson germanium primary monochromator yielding pure Cu Kα1 radiation and a Lynxeye position-sensitive detector. An Anton Paar temperature chamber was used to control the temperature. 2θ scans between 12° and 60° were collected, using a step size of 0.0092°. The temperature program was initiated at room temperature, and diffractograms were collected at the following 17 temperatures: 35, 40, 43, 46, 49, 51, 53, 55, 57, 60, 63, 66, 69, 72, 77, 87, and 97 °C. Each measurement lasted around 30 min, and time was given for thermal equilibration before each measurement. After the temperature program, the sample was naturally cooled to room temperature and a new scan was collected to ensure the reversibility of the observations and that the heat treatment had not degraded the sample.
The ideal perovskite structure is cubic, as illustrated in Figure 1. For the structure to conform to cubic at room temperature, the tolerance factor must be in the range 0.9−1. For smaller tolerance factors, between 0.7 and 0.9, the B ions are too large or the A ions too small, favoring an orthorhombic, a rhombohedral, or a tetragonal structure over the ideal cubic structure. The ionic radius is 0.132 nm for Pb2+, 0.206 nm for I−, and 0.18 nm for CH3NH3+.21 According to eq 1, this gives a tolerance factor of 0.81, which is consistent with a noncubic structure. It is also found experimentally that CH3NH3PbI3 crystallizes in a tetragonal structure with space group I4cm at room temperature.10 The tetragonal room temperature structure is illustrated in Figure 2. The structure and atomic coordinates are based on single-crystal data from Dang et al.10 The cell parameters are found to be a = b = 8.893 Å, c = 12.637 Å, and α = β = γ = 90°, with four formula units in the chosen unit cell. The temperature is an important parameter for the phase stability, and a phase B
DOI: 10.1021/acs.inorgchem.5b01481 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
comes from Constantinos et al.14 Indexation of the major peaks is given in the figure, and full indexation of all peaks is given in the Supporting Information. At a first glance, it is apparent that several peaks are located at almost the same place, even though the indexation differs. There are, however, a few peaks that disappear when the tetragonal phase transforms into the cubic structure, as well as some double peaks that merge into single peaks. In this study, XRD has been measured as a function of the temperature, and a detailed analysis of the phase transformation, peak positions, and indexation is given in the Results section. In addition to the changes in the XRD signal related to the phase transformation, there is a peak shift with the temperature resulting from thermal expansion of the lattice. This peak shift can be used to extract the thermal expansion coefficients, which are valuable parameters upon evaluation of the thermal stress that the material will undergo upon temperature cycling.
Figure 2. (a) Unit cell of the tetragonal phase. (b) Tetragonal phase seen along the c axis. (c) Tetragonal phase seen along the a axis, which is equivalent with the b axis. (d) Tilted view of the tetragonal phase. (e) Unit cell of the cubic phase. (f) Cubic phase seen along the c axis. Compared with the tetragonal phase in part b, the rotation of the unit cell in the ab plane is evident. (g) Cubic phase seen along one of the primary cell axes. Compared with the tetragonal phase in part c, distortion in the tetragonal structure is seen. (h) Tilted view of the cubic phase, which compared to part d illustrates the rotation of the unit cell associated with the phase transition.
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RESULTS Room temperature XRD was measured on both the drop- and spin-coated films. In both cases, the experimental data match the simulated diffractogram for the tetragonal phase, as illustrated in Figure 4 for the drop-coated film. There is one peak at 13° that cannot be attributed to the perovskite but instead indicates the remaining unreacted PbI2 in the film. Besides the PbI2 peak, no crystalline phases except the perovskite were observed. There is a clear difference between the XRD spectra for the drop- and spin-coated samples. A few strong peaks dominate the XRD spectra for the spin-coated sample, indicating that spin coating introduces a texture to the film, where the tetragonal crystals will have an increased tendency to be oriented with the c axis perpendicular to the substrate. A comparison of the XRD data for the spin- and drop-coated films is given in the Supporting Information. Spin coating gives higher-quality films from the perspective of device production and would be the method of choice when pursuing high-efficiency solar cell devices. Upon examination of the crystallographic phase transformation, this is, however, not a concern. Therefore, in order to decrease problems related to texture and to obtain reliable data for as many reflections as possible, drop-coated samples were used for the temperaturedependent XRD measurements. XRD was measured as a function of the temperature between 35 and 95 °C. A contour plot over all measurements is given in Figure 5b, and the diffractogram for the lowest, first measured, temperature is given in Figure 5a. In the measured range, there are 15 peaks, or groups of peaks, that are intense enough to be analyzed in detail. These 15 groups are indicated in Figure 5a and in Table 1, where the Miller indices, simulated peak
transformation from a noncubic to a cubic perovskite structure is often seen at elevated temperatures. For CH3NH3PbI3, such a transformation has been reported around 55 °C.13,14 In Figure 2, the structures of the cubic and tetragonal phases are compared. The cubic structure is illustrated as an idealized cubic cell, with one formula unit per unit cell and with the cell parameters a = b = c = 6.3115 Å, based on data collected at 130 °C by Constantinos et al.14 In the tetragonal structure, the c axis is prolonged as a consequence of both the
