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Photovoltaic Hybrid Perovskites under Pressure Marek Szafra#ski, and Andrzej Katrusiak J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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The Journal of Physical Chemistry Letters

Photovoltaic Hybrid Perovskites under Pressure Marek Szafrański*1 and Andrzej Katrusiak2

1

Faculty of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznań, Poland; 2

Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland

Corresponding author’s e-mail address: Marek Szafrański - [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT: High-pressure studies on methylammonium trihaloplumbates, of general formula [CH3NH3]+PbX3ˉ (abbreviated MAPbX3, where X = Cl, Br. I) and its analogues shed new light on the materials for harvesting solar energy and open new perspectives for the photovoltaic science and technology. However, there are considerable discrepancies between the reported structural, calorimetric and spectroscopic results, and even between the results obtained by the same technique, for example of X-ray diffraction. The origins of the discrepancies and possible pitfalls in the diffraction and spectroscopic studies on MAPbX3 crystals have been investigated, so these important data can be more reliably used for the theory, predictions and technological applications.

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In recent years organic-inorganic hybrid perovskites of the general formula ABX3 [where A = CH3NH3+ (MA) or NH2CH=NH2+ (FA), B = Pb2+, Sn2+, Ge2+ and X = Cl, Br, I] have been generally recognized as extremely efficient and the most promising light-harvesting materials for solar energy-conversion and other optoelectronic devices. Two of these compounds, methylammonium lead triiodide, MAPbI3, and formamidinium lead triiodide, FAPbI3, exhibit superior photovoltaic performance, owing to their wide absorption spectra and high absorption coefficients, as well as to the good carrier mobility and long diffusion length. Within seven years following first report on hybrid perovskite solar cells by Kojima et al. in 2009,1 the power-conversion efficiency raised almost sixfold, from 3.8% to over 22%.2 This unprecedented progress, along with the ease and low-cost of fabrication, strongly motivate further intense studies in this field. The energy gap of semiconducting absorber is one of the crucial parameters determining the width of solar spectrum absorption. According to the Shockley-Queisser predictions, the maximum power-conversion efficiency for a singlejunction solar cell is 33.7% at the optimized direct bandgap energy of 1.34 eV.3,4 This optimum value is significantly smaller than those of MAPbI35,6 and FAPbI3,6 both close to 1.5 eV. By different methods the electronic structure of materials can be tuned closer to the optimal bandgap. One of possible tools is a chemical substitution or doping. For example, the hybrid perovskite structure modified by substituting Pb with Sn indeed reduces the energy gap to 1.3 eV, but at the same time it undesirably shortens the carrier lifetime, lowering the efficiency of the solar cells.7,8 Moreover, the tin halides are considerably less stable in the air atmosphere than their lead-based analogues. Another chemical modification, of iodine atoms partly substituted with Cl or Br, increases the stability of the mixed systems and improve their charge-transport properties, but it increases the bandgap.9-11 The structures of hybrid perovskites are formed from corner-sharing octahedra and organic cations, enclosed in the ideally or approximately cubic (hexahedral) cages, with the BXB



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edges either straight or bent. High pressure can modify bond lengths and valence angles in hybrid perovskites without a chemical interference, finely tuning the electronic structure responsible for basic properties of photovoltaic materials, like the energy gap and carrier diffusion length. In response to pressure the inorganic framework is modified by the contraction of BX bonds or/and bending of BXB bridges coupled with the BX6 octahedra tilting. Generally, the shortening of bonds BX narrows the bandgap, whereas it is widened by the BXB angle bending.11,12 However, the resultant bandgap change depends on the contributions of these competitive structural pressure effects. The first high-pressure studies of methylammonium lead halides in the 1990s were focused on their pT phase diagrams. Precise boundaries between the phases were derived by the differential thermal analysis (DTA) measurements up to 0.2 GPa.13 These phase diagrams were subsequently extended to about 0.7 GPa by studying dielectric properties under pressure.14 The earliest structural high-pressure investigations were initiated for methylammonium and formamidinium tin(II) iodides,15 well before the discovery of photovoltaic properties of perovskite hybrids.1 The structures of tin(II) iodides are topologically identical with those of lead-based analogues. The following sequences of pressure-induced phase transitions were revealed by synchrotron X-ray powder-diffraction: Pm3m  Im3  Immm  amorphization

in

MASnI3

and

Pm3m  Im3  I4/mmm

 amorphization in FASnI3. The powder neutron-diffraction study on MAPbBr316 pioneered the structural studies on pressure effects in hybrid lead halides, albeit with no reference to the optical properties. A more comprehensive approach, combining the pressure-induced structural and optical changes, has been presented only in recent two years by several independent groups.12,17-26 Most of these studies were performed by powder-diffraction. Unfortunately, several of these reports were inconsistent in the symmetry assignments, structural models, and the stability regions of phases, all most relevant and essential for 4 ACS Paragon Plus Environment

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understanding, optimizing and predicting the photovoltaic properties of the MAPbX3 perovskites. These inconsistencies could be resolved by other methods, including careful single-crystal pressure experiments, deemed as more accurate than the powder ones. Few such experiments were reported recently. Two of them postulated new space-group symmetries of MAPbI3 crystal phases.20,24 Therefore a thorough analysis of these results and the understanding of their inconsistencies with other experiments is required. In organic-inorganic hybrid perovskites a variety of configurations, associated with different lattice types and space-group symmetries,27 are generated through the interplay of (i) positions of organic cations, (ii) their interactions with the inorganic framework and (iii) the tilts of BX6 octahedra. A variety of symmetries is realized in the MAPbX3 phases depending on the thermodynamic conditions of temperature and pressure. At ambient pressure, two solid-solid phase transitions reduce the symmetry of MAPbI3: its cubic phase I at 327 K transforms to tetragonal phase II, which at 162 K transforms to orthorhombic phase III.28 Selected structural information on the MAPbI3 phases is summarized in Table 1. However, alone for phase II, the one most important because it is stable at ambient conditions, several symmetries (of tetragonal space groups I4/mcm, I4/m, I4cm, and orthorhombic Fmmm) have been reported. Such conflicting information was reported for all phases of MAPbI3, as well as for some of other MAPbX3 perovskites. Below we have analyzed and discussed the literature data on the structure and symmetry of lead halide perovskites juxtaposed with our singlecrystal high-pressure studies. Single-Crystal Diffraction Experiments. An apparent advantage of single-crystals diffraction is the measurement of coordinates (in the reciprocal space) and intensity of individual reflections. Compared to the powder-diffraction methods, it eliminates the effects of



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Table 1. Ambient-Pressure MAPbI3 Phases I, II, III (Temperature Limits in Brackets) and AmbientTemperature Phases IV, V, VI (Pressure Limits in Brackets), as well as their Space-Group Symmetry Information Phase

Space group

Z

References

I (>327K)

Pm3m

1

[28-32]

II (162-327K)

I4/mcm

4

[25,28,30-32]

III (2.5GPa)

Im3

8

[25]

VI (>2.5 GPa)

Amorphous

n/a

[25]

overlapped reflections, low ratio of observed reflections over the number of parameters as well as other disadvantages, like the preferred orientation of powder grains. On the other hand, there are 'pitfalls' specific to the single-diffraction technique, too, which are apparent from inconsistencies in structural results reported for MAPbI3. Twinning and other Pitfalls. A possible hindrances in the single-crystal X-ray diffraction analysis is the sample-crystal twinning. In the family of MAPbX3 halides a strong tendency to twinning applies to its most prominent and intensely studied representative, MAPbI3 in its phase II. This twinning of MAPbI3 results of the ferroelastic relation between phases I and II. The synthesis of this compound is usually performed around 370 K and then the solution is cooled to about 315 K.28 During the crystallization most of the crystals are nucleated in the cubic phase I of space group Pm3m. The temperature lowering below 327 K results in the symmetry reduction to tetragonal of space group I4/mcm, which implies the occurrence of three orientational states, as illustrated in Figure 1. Consequently, the ambient-pressure phase II can be present in three orientational states (triplet components labeled a, b and c), with their orientation described by the twin (in this case triplet) rules:

 1/ 2 1/ 2 1/ 2  1 / 2 1 / 2  1 / 2    1/ 2 1/ 2  1/ 2        Ťa→b= 1 / 2 1 / 2 1 / 2  ; Ťa→c=  1 / 2  1 / 2 1 / 2  ; Ťb→c=   1 / 2 1 / 2 1 / 2  (1)  1  1  1 1 0  1 0  1 0    


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illustrated in Figure 1.

Figure 1. Three orientational states (triplet components) of tetragonal MAPbI3 phase II, mimicking a cubic lattice F (its unit cell is indicated by thin black lines with open-circle nodes). The unit cells of tetragonal triplet components (lattices I) are marked in red, green and blue.

The coordinates of the nodes in the reciprocal space (Figure 2) transform according to the same matrices describing the twin laws (i.e. the transformation of the unit vectors in the direct space) combining the three possible twin components (indicated with subscripts a, b and c in Eq. 1). In particular, the reflections systematically absent in space group I4/mcm due to the glide plane c perpendicular to the [x] and [y] axes in one twin component will superimpose with reflections not extinct for other components. For example, the reflection 013a extinct due to the glide plane c perpendicular to axis [xa] (cxa) in the twin component (a) will (nearly exactly) superimpose with not-extinct reflections -12-1b and 211c from twin components b and c. Thus, the overlaid diffraction images of three (or even two) twin components of phase II (space group I4/mcm) may merge into the composed diffraction pattern where the extinctions of glide planes c are masked by other reflections of non-zero intensity.



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(a)

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(b)

Figure 2. The overlapped reciprocal lattices of three orientational states (or twin components ‒ cf. Figure 1) of tetragonal MAPbI3 phase II: (a) perspective view; and (b) the projection along directions [110]a (red), [110]b (green) and [001]c (blue). For clarity, the reciprocal spaces of the pseudomerohedral twins have been idealized, in this way that the reciprocal nodes of the twin components superimpose nearly precisely; their indices are given in the colors corresponding to the lattices. In drawing (a) the centrally located three superimposed nodes, marked with an asterisk, have indices 101a (red), 011b (green) and 101c (blue).

It can be shown in the same way, by transforming the reflections in the reciprocal lattices of twin components through matrices Ťa→b, Ťa→c and Ťb→c, that no reflections systematically extinct due to the I-lattice condition (h+k+l=2n+1) overlay with any of nonextinct reflections from other twin components. For example, reflection 111a of the twin component a lies between nodes in the reciprocal lattices of twin components b and c: at their points 1/2 3/2 0b and ½ ½ 2c, respectively. It can be further shown that the three overlaid lattices a, b and c mimic the Bravais lattice F with unit-cell vectors {aFbFcF}={cacbcc} (Figure 1). The unit cells of triplet components (a-c) transform into this averaged F lattice according to matrices:


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 0 0 1   Ťa→F=  1  1 0  ;  1 1 0  

  1 1 0   Ťb→F=  0 0 1  ;  1 1 0  

 1 1 0   Ťc→F=   1 1 0   0 0 1  

(2) Thus the 111a reflection of twin component a, systematically extinct due to the crystal spacegroup symmetry I4/mcm (condition h+k+l=2n), has in this averaged pseudo-F lattice indices 102F, and its absence can be confused with the pseudo-F lattice systematic-absence simultaneous conditions h+k=2n, h+l=2n' and k+l=2n". Likewise, the reflections allowed for the I lattices of the twin components will also satisfy the pseudo-F lattice conditions; for example, the indices of reflection 110a transform to 002F. Naturally, the best approach is to perform the diffraction experiment on a truly single crystal, which was done for MAPbI3.25 The use of intense synchrotron beams for diffraction studies also enhances two other potentially misleading effects: of multiple scattering and higher-harmonics (eg. λ/2, λ/3 etc.) diffraction.33 Both these effects can cause the overlapping of different reflections, those systematically extinct with not extinct ones. This can suggest that there are no systematicextinctions, and no associated translational symmetry in the crystal. This can be confusing for the space-group determination. Fortunately, the effects of multiple scattering can be easily checked for a chosen reflection by performing its ψ-scan, which moves all reciprocal nodes off Ewald's sphere, except this of the main reflection; the effect of higher-harmonics can be eliminated by choosing a short wavelength at synchrotrons, or low accelerating voltage of the X-ray tubes. Most importantly, there are means of checking few reflections defying an extinction rule present in the collected data, particularly when they contradict previous spacegroup determinations. It suffices to show that such exceptional reflections disappear (by changing the sample, ψ-rotations or changing the radiation characteristics) to validate the extinction rule. Generally, if there are few exceptions of a systematic extinction, they require a careful check, before the corresponding translational symmetry can be eliminated. 9 ACS Paragon Plus Environment


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There are also other pitfalls of single-crystal diffraction analysis, several of which can be exemplified by a recent reinvestigation24 of high-pressure MAPbI3 phase IV. According to this high-pressure single-crystal synchrotron X-ray diffraction study, phase IV at 293 K/0.4 GPa is of orthorhombic space group Imm2,24 different than the cubic space group Im3 (cf. Table 1).20,25 The origin of the orthorhombic space-group assigned to phase VI is the low completeness of collected reflections (348, i.e. only 7.3%) from a small portion of the reciprocal space, and in particular that only the reflections of one sign of Bragg index l were collected. The opposite l indices would be required for compensating the possible centering errors,35,36 usually the largest along the DAC axis. Axis [z] of the sample crystal was oriented approximately along the DAC axis and the accuracy of lattice parameter c, 12.25(5) Å,24 is far below the accepted standards. For the experiments using an area detector the number of reflections collected is important for precisely determining the lattice parameters, as distinguished from their accuracy.34 The ESD of unit-cell parameter c measured in this way is large, so although c differs from a and b, the level of significance of this difference is not impressive. For space groups Im3 and Imm2 the space-group assignment could be based on the reciprocal-space symmetry (Laue classes) and unit-cell dimensions. The unit-cell dimensions are derived of the orientation matrix (UB matrix in Busing-Levy's notation37), and this UB matrix is essential for the location of reflections and correct evaluation of their intensities. Hence, the intensities are less reliable, too. Their systematic errors certainly affected the refinement, and the increased residue factors R could be reduced by increasing the number of parameters, which could be done by reducing the symmetry of the model, from cubic Im3 to orthorhombic Immm, and then even doubled again for non-centrosymmetric space group Imm2. In such cases the rigorous R-factor test, for estimating the significance of the effects of the symmetry reduction,38 can be biased by systematic errors. Nonetheless, even for the low-symmetry model of space group Imm2 the R factors are high (no information was


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provided on the R factors for higher-symmetry models). Due to the discussed experimental deficiencies (without mentioning other relevant details, like the possible non-hydrostatic stress in the silicone oil) the contesting of previously determined cubic symmetry of MAPbI3 phase IV is weakly grounded. Generally, the increased number of refined parameters improves the residue factors (R), however such a symmetry lowering should be validated by the R-factor test.38 Often the omission of inversion centre, approximately doubling the number of parameters, such as between tetragonal space groups I4/mcm and I4cm ascribed toMAPbI3 phase II, is solely justified by the least-squares residue R-factors without any support of the crystal properties. Unfortunately, a considerable confusion is caused by the claims of ferroelectricity based on the observation of conductance hysteresis loop,39 which have nothing in common with ferroelectricity, as evidenced recently.40 The structure and symmetry of crystals are essential for understanding their properties and vice versa. Incorrect structural models disseminated to the related research have far-reaching consequences. For example, space group Fmmm assigned to phase II of MAPbI3 entails the disorder of iodine atoms and distorted structural parameters. The electronic structure calculations based on such a structural model yielded the pressure dependence of energy gap opposite to that measured experimentally.20 Structural and Optical Pressure-Induced Changes in MAPbI3. The sequence of high-pressure phases and their transformations in MAPbI3 have been shown in Figure 3a, according to recently performed single-crystal high-pressure experiments.25



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Figure 3. High-pressure phases of MAPbI3 (a), MAPbBr3 (b), and MAPbCl3 (c), at 293 K.


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Figure 4. Compressibility of PbX bonds (a) and the pressure dependence of PbXPb angles (b) in methylammonium lead halides MAPbX3. Where not indicated, the errors are smaller than the symbols.

The MAPbI3 crystal compression across all three phases I, IV and V involves a linear shortening of bonds PbI (Figure 4a) combined with the tilting of the PbI6 octahedra (Figure 4b). In the structure of tetragonal phase II, corresponding to Glazer’s symbol 27 a0a0c, the octahedra are rotated in antiphase around two PbI bonds aligned along the crystal [z] axis. Throughout phase II these two PbIPb bonds remain straight and the crystal compression along [z] totally depends on their shortening. The compression of bonds Pb–I in other directions is very similar to that along [z] and these bonds shortening correlates with the decreasing energy gap of MAPbI3 in phase II (Figure 5). This dependence is consistent with the calculated electronic structure of hybrid halides, where the top of the valence band is mainly composed of the X p and Pb 6s orbitals, while the conduction-band bottom is dominated by the X s and Pb 6p orbitals.11,22,41 Therefore, the optical transitions near the absorption edge strongly depend on the PbX distances and PbXPb angles, i.e., the 13 ACS Paragon Plus Environment


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structural parameters defining the extent of the overlapping of atomic orbitals. At 0.3 GPa, the transition to phase IV is induced by the onset of the PbI6 octahedra tilts, composed of three orthogonal rotations described by Glazer as as a+a+a+. At the transition point the PbIPb angle sharply bends from 180o to 161o (Figure 4b) and the energy gap suddenly increases. Then, in phase IV the energy gap slightly increases monotonically to 1.2 GPa and at the still higher pressure it slightly decreases up to the next transition at 2.5 GPa. This behavior arises from the competing effects of bending PbIPb and shortening PbI (cf. Figure 4). At the isostructural phase transition at 2.5 GPa to phase V, the PbIPb angles straighten by about 7o simultaneously with a stepwise shortening of PbI bond and the strongly increased atomic

Figure 5. Pressure dependence of the optical energy gap in MAPbI3.25 The inset shows the correlation between Eg and the PbI bond length in phase II.

displacement parameters indicating the progressive amorphization. The averaged structural changes would suggest a decrease in Eg, however the opposite effect occurs due to the amorphization process. The amorphization is induced by strongly pressure-enhanced interionic and NH+···I- interactions between the MA+ cations, trapped at random positions, 14 ACS Paragon Plus Environment

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and the inorganic framework.25 The atomic positions derived from the X-ray diffraction data correspond to the averaged crystal structure, where the local strong distortions of bonds and angles are reflected in considerably increased atomic displacement parameters. All these locally increased structural distortions in phase V result in a progressive loss of the crystal long-range order and in the increased value of Eg. Single-Crystal X-ray Diffraction High-Pressure Study on MAPbBr3. The reported highpressure structural studies on MAPbBr3, carried out mainly by powder-diffraction methods, are inconsistent. In the neutron experiment on deuterated sample, CD3ND3PbBr3, with the hydrostatic conditions ensured by perdeuterated isopropanol (hydrostatic to 4.2 GPa), a continuous phase transition Pm3mIm3 between two cubic phases (hereafter labeled I and V, respectively), with the unit-cell volume eight-fold increased in phase V, was observed at 0.9 GPa.16 Furthermore, an onset of amorphization was noted around 2.8 GPa. These neutron-diffraction results were generally confirmed recently by powder X-ray diffraction for MAPbBr3 compressed in helium as the hydrostatic medium20 (hydrostatic to 11.3 GPa). The only important discrepancy concerns the character of the transition at 0.9 GPa, appearing to be discontinuous according to the X-ray data. These studies strongly contrast with the results of the synchrotron X-ray diffraction experiments performed on the sample without a hydrostatic medium.17 The latter work reported the cubic-to-cubic Pm3mIm3 transition, too, but at about 0.25 GPa and of clearly first-order type, with the associated abrupt volume change of about 6%. Surprisingly, the ambient-pressure structure was refined with the unitcell parameter a' = 8.4416(5) Å,17 which cannot be reconciled with the well established literature value of a = 5.93 Å and space group Pm3m.31 It is puzzling, although of no significance whatsoever, that a  a' / 2 . Another pressure-induced phase transition between cubic Im3 and orthorhombic Pnma phases around 1.5 GPa, as well as the amorphization starting above 2 GPa were reported, too. Those PXRD data cannot be correlated with the 15 ACS Paragon Plus Environment


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optical results presented in the same paper: the pressure dependence of the energy gap of MAPbBr3 indicates that the phase transition occurs around 0.8 GPa, and that it is immediately followed by the amorphization. Our single-crystal measurements of the unit-cell volume of MAPbBr3 (Figure 6) indicate that the Pm3mIm3 transition occurs at 0.75 GPa and that most probably it is continuous, as no volume discontinuity has been detected. In this respect our and the previous neutron16 results are consistent, unlike the synchrotron PXRD17,20 However, the most unexpected was the observation of ‘dark’ regions in the single-crystal (shown in the inset in Figure 6), formed during a 24-hour X-ray diffraction data collection at 0.81 GPa. A closer

Figure 6. Unit-cell volume of MAPbBr3 as a function of pressure. The insets show the crystal in a diamond-anvil cell (DAC) at 0.27 and 0.81 GPa, each after 24 h of the pressure application. The dark regions visible at 0.81 GPa indicate an occurrence of additional phase VI coexisting in a wide pressure range with phase V.

optical examination showed that the dark appearance occurred due to the strong scattering of the transmitting light through highly defected crystal areas, indicative of an unknown transition to a new phase, associated with a large lattice strain. This new phase, labeled VI, 16 ACS Paragon Plus Environment

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developed and coexisted with the cubic phase V over a wide range of pressure (Figure 7) up to about 2.7 GPa, when the cubic-symmetry breaking was evidenced by split reflections. Most importantly, the nucleation and development of the ‘dark’ phase VI was both pressure and time-dependent, and most probably because of its very slow kinetics, this process was overlooked in previous high-pressure experiments.16,17,20 The literature data are inconclusive regarding the MAPbBr3 high-pressure phase VII. Neither the neutron16 nor recent PXRD studies20 reported this phase, whereas the information on the transition to the orthorhombic phase at 1.5 GPa is heavily biased by the nonhydrostatic conditions of the high-pressure experiment.17 To support our single-crystal X-ray

Figure 7. Transmitting-illumination photographs of a single crystal of MAPbBr3 in the DAC chamber, illustrating the time evolution of the ‘dark’ phase VI and its coexistence with phases V above 0.8 GPa and with phase VII (note its brighter shade) at 2.95 GPa .



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Figure 8. A single crystal of MAPbBr3 in phase I at 0.2 GPa (a); in phase V with small inclusions of phase VI at 1.45 GPa (b); in the range of coexisting phases V, VI and VII 24 h after increasing pressure to 2.14 GPa (c); and in phases VI and VII at 2.7 GPa (d); as well as several other crystals at 2.74 GPa viewed in the balanced transmitting and reflected illumination (e) and in transmitting polarized light (f). The arrows indicate the areas of the ‘dark’ phase VI.

diffraction experiments we performed a series of optical examinations, which confirmed the occurrence of the lower-symmetry phase VII (Figure 8). It turns out that valuable new information about phase transitions can be obtain by just looking at a single crystal. The onset of the transition to phase VII has been found in this manner around 2.1 GPa. As illustrated in


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Figure 8c, the crystal kept for 24h at 2.14 GPa underwent a partial transformation. This transition is associated with a piezochromic effect and therefore the coexisting phases are seen as a mosaic of darker (phase V) and lighter (phase VII) fragments. The transition causes also numerous cracks of the crystal, but most importantly, the ferroelastic domain structure emerging within the regions of phase VII (cf. Figure 8f) is characteristic for the transitions between crystallographic systems. Interestingly, the transition to phase VII was complete only at around 2.7 GPa, as shown in Figure 8d. In another experiment, when pressure was quickly increased in several steps (each followed by the pressure calibration and visual inspection of the sample), the transition was detected at ~2.7 GPa. On releasing pressure the domain structure disappeared and the sample color deepened, marking the restoration of phase V around 1.9 GPa. The sequence of high-pressure phases and the transformations in MAPbBr3 are summarized in Figure 3b. The transition at 0.75 GPa between the cubic phases doubles the lattice parameters from 5.8393(2) Å at 0.68 GPa to 11.6330(3) Å at 0.81 GPa. In the prototypical phase I, of space group Pm3m, the PbBr bonds are directed exactly along the crystal axes, and such an arrangement of the PbBr6 octahedra is described by Glazer’s symbol27 a0a0a0. The pressureinduced transition to the cubic phase V of space group Im3, involving the rotations of PbBr6 octahedra about three orthogonal axis, is described by symbol a+a+a+. Thus in phase V the PbBrPb angle significantly diverges from 180o, as illustrated in Figure 4b. By singlecrystal X-ray diffraction we have followed the pressure dependence of structural parameters. In phase I, the inorganic framework responds to the applied pressure by shrinking PbBr bonds, whereas the dominant effect in phase V is the PbBr6 octahedra tilting, especially in the 0.751.25 GPa range where the PbBr bond anomalously lengthens with pressure (cf. Figure 4). Such a lengthening was not observed neither for iodide nor chloride analogues. The structural changes in compressed phase V of MAPbBr3, of the simultaneous PbBrPb 19 ACS Paragon Plus Environment


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bending and PbBr elongation, should increase the energy gap, which has been confirmed by high-pressure photoluminescence measurements (cf. Figure 9).17 It is noteworthy that the Eg magnitude strongly decreases in phase I, where the crystal compression is totally 'absorbed' by the PbBr bonds. The inset in Figure 9 shows that the energy gap linearly correlates with the PbBr bond length.

Figure 9. Energy gap of MAPbBr3 derived from the photoluminescence spectra as a function of pressure (data taken from Ref. 17). The vertical dashed line marks the transition pressure at 0.75 GPa determined in our diffraction and optical experiments. The inset correlates the energy gap magnitudes with compressed PbBr distances in phase I.

Single-Crystal X-ray Diffraction High-Pressure Study of MAPbCl3. The only structural pressure study published recently on MAPbCl3 was performed by synchrotron X-ray powder diffraction with silicon oil as a pressure-transmitting medium.26 Two first-order phase transitions were reported in this compound, one at 0.8 and the other at 2.0 GPa. It was postulated that the transition at 0.8 GPa is isostructural, between two cubic phases of space group Pm3m, and that the second transition reduces the cubic symmetry to the orthorhombic space group Pnma. 20 ACS Paragon Plus Environment

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Our single-crystal X-ray diffraction and optical experiments on MAPbCl3 in a DAC confirmed that around 2 GPa the crystal undergoes a symmetry-lowering transition to phase V. The associated multiple twinning hampered single-crystal analysis and we have not reinvestigated the postulated orthorhombic space group and the structure of phase V. However, our lower pressure results considerably differ from the previous report.26 First of all, no isostructural phase transition around 0.8 GPa has been detected. As illustrated in Figure 10, the unit-cell volume decreases linearly with increasing pressure without any discontinuities, which could be ascribed to the isostructural, and thus necessarily first-order, phase transition. However, the most intriguing result in this study on MAPbCl3 was the occurrence of its new phase, labeled IV, coexisting with the cubic phase I up to the transition at 2 GPa, in certain respects similar to MAPbBr3 'dark' phase VI occurring within phase V (cf. Figures 6-8). The coexistence of MAPbCl3 phases I and IV is clearly seen in the crystal squeezed at the hydrostatic pressure of 0.9 GPa, as illustrated in the inset in Figure 10. A careful microscopic examinations allowed us to determine that the nucleation of this new

Figure 10. Pressure dependence of the unit-cell volume of MAPbCl3 in cubic phase I. The insets show the single-crystal sample compressed in the DAC as indicated by red arrows. The



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dashed red line marks the nucleation threshold of phase IV, detected by microscopic observations.

Figure 11. Photographs of an initially single crystal of MAPbCl3 (top-left) in the DAC chamber (400 μm across), illustrating the time progress of the pressure-dependent isothermal (296 K) transition to phase IV. A ruby chip for pressure calibration is placed left of the MAPbCl3 crystal, both submerged in isopropanol (hydrostatic to 4.2 GPa).


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phase starts around 0.75 GPa. The dark fragments of the crystal are not due to the piezochromic effect, but these are cracked and highly defected regions strongly scattering the transmitting light, which appears as its extinction. Most importantly, the first seeds of phase IV were observed only several hours after pressure was increased above 0.75 GPa. So, the transformation to phase IV is characterized by an extremely slow kinetics, both time- and pressure-dependent, as illustrated in Figure 11. The single-crystal state of cubic phase I slowly 'eroding' into the polycrystalline phase IV, allowed us to determine the structural model of MAPbCl3 phase I up to 1.82 GPa. In this phase the Pb‒Cl‒Pb angle remains straight (Figure 4b), whereas the Pb‒Cl bond decreases linearly with increasing pressure, as illustrated in Figure 4a. This Pb‒Cl compression well correlates with the linear decrease in Eg in the pressure range up to 0.75 GPa, shown in Figure 12a. The Eg(p) plot has been derived from the absorption edge measurements on the polycrystalline sample.26 We have compared those data with our single-crystal structure determinations and optical observations and marked the stability regions of phases I, IV and V in Figure 12a. The linear correlation between the Pb‒Cl bond length and Eg within the pressure range to 0.75 GPa can be extended to the whole pressure range of phase I, as shown in the inset of Figure 12a. However, the pressure dependence of Eg predicted in this way above 0.75 GPa significantly diverts from the experiment. This dramatic change in the Eg(p) trend was previously ascribed to the postulated isostructural Pm3mPm3m phase transition at 0.8 GPa,26 but the increase in energy gap cannot be reconciled with the shortening Pb‒Cl bonds, which is the only possible compression mode of this Pm3m-symmetric structure. Our single-crystal data do not indicate such a phase transition, but instead at 0.75 GPa we detected a nucleation onset of phase IV, which successively developed at still higher pressure. The transition to phase IV has a diffused character and is associated with a large lattice strain, indicating a lowering of the crystal symmetry. It is plausible that the occurrence of this phase is responsible for the



changes in the absorption spectrum of MAPbCl3. This conclusion is supported by the blue shift of the absorption edge observed as a function of time (Figure 12b) for the sample at 1.35 GPa. To get a better insight into the character and growing process of MAPbCl 3 phase IV we performed systematic powder X-ray diffraction (PXRD) experiments according to two

(b) p = 1.35 GPa

Absorbance (a. u.)

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2.80

0.5 h 62 h 111 h 280 h

2.85

2.90

2.95 Energy (eV)

3.00

3.05

3.10

Figure 12. (a) Pressure dependence of the energy gap in MAPbCl3 derived from the absorption edge measurements in Ref. 26 (black solid circles). The vertical dashed line at 0.75 GPa marks the threshold pressure, where the nucleation of a new phase IV was detected in our optical and X-ray diffraction experiments. The dashed line at ~2 GPa denotes the transition between the phases I and V. The inset shows the correlation established for the 24 ACS Paragon Plus Environment

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PbCl bond length and Eg measured in phase I up to 0.75 GPa, and extended to the whole pressure range of phase I. In the main plot this correlation has been used for simulating Eg values (red diamonds) expected in pristine phase I at the absence of phase IV(see the text). (b) Time evolution of the MAPbCl3 absorption edge at 1.35 GPa.

Intensity (a. u.)

(a)

phase I 0.24 GPa/0.5 h

phase V 2.79 GPa/24 h

4

8

12

16

20

24

28

2(deg)

(b)

1.41 GPa/1 h

1.41 GPa/18 h

Intensity (a.u.)

1.41 GPa/11 days

110

2.58 GPa/0.5 h

8

12

16

2(deg)

20

222 230 231

111

4

122

released 0.14 GPa/0.5 h 130 131

022

112

200

012

100

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24

28



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Figure 13. Powder X-ray diffraction patterns (λMoKα = 0.71073 Å) illustrating different transition paths of MAPbCl3: (a) the direct transition from phase I to phase V induced by pressure change from 0.24 to 2.79 GPa, and (b) time-dependent growth of phase IV at 1.41 GPa, and the diffraction patterns after pressure raised to 2.58 GPa (pressure range of phase V) and then released to 0.14 GPa (phase I).

scenarios. In the first one, after collecting the diffraction pattern of phase I at 0.24 GPa the pressure was increased in one step to 2.79 GPa. The diffraction patterns in Figure 13a show that phase I has directly transformed into phase V, and it remained in this form for days at least. In the second scenario, the MAPbCl3 sample was kept at 1.41 GPa and its diffraction patterns were monitored in order to observe the progress of phase IV growing in time. This series of PXRD patterns show that the reflections of phase IV appear after few hours and then slowly increase their intensity over days; however even after weeks the reflections of phase I were clearly present in the PXRD patterns and in the single-crystal diffraction images, albeit of much lower intensity. The preliminary indexing analysis suggests that the unit-cell parameters of phase IV doubled and its symmetry is orthorhombic or lower. After 11 days the pressure of this sample was increased to 2.58 GPa, i.e. into the pressure range of phase V, however no signs of phase IV transforming into phase V were detected. Moreover, our measurements and observations suggest that the free Gibbs energy of phase IV is lower than that of phase V, which would indicate that phase V is metastable. This conclusion is further supported by the observation that the compression of the MAPbCl3 sample from phase I to phase V above 2.5 GPa considerably slows, if not halts completely, the transformation to phase IV, which resembles the process of super-cooling or over-pressurizing phases. The sequence of high-pressure/ambient-temperature phases in MAPbCl3 has been shown is shown in Figure 3c. When the pressure of the MAPbCl3 sample, with most of its volume converted into phase IV, was released to 0.14 GPa, a full restoration of phase I took place, according to the 26 ACS Paragon Plus Environment

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subsequent PXRD measurement (Figure 13b). It shows that the reverse transformation from phase IV to phase I proceeds much quicker than in the other direction. Undoubtedly, the photovoltaic hybrid perovskites MAPbX3 reveal under pressure their general structure-property relations, allowing a rational search for new chemical compositions of materials of improved performance. Apart from the Eg-structure relationships and compression characteristics, also other important properties of the crystals have been discovered. In particular, the kinetic effects, extending into months, of transformations into new phases, have been found in the crystals of MAPbBr3 and MAPbCl3. Slow-kinetics effects can be easily overlooked in optical-spectroscopy and synchrotron X-ray diffraction experiments, when even microseconds are sufficient for acquiring full data sets. One can be tempted to take full advantage of such possibilities, however the time scale of investigated phenomena can fall outside.… the experiment. Also, the pressure on efficient research, often equated with high publishing frequencies, can be incommensurate with the slow kinetics of some phase transitions, like those in MAPbBr3 and MAPbCl3. It can be concluded that high-pressure, the most efficient method of modifying the crystal structure, can be used for fine tuning the whole range of strain variations, as well as for triggering drastic structural changes induced by phase transitions, all correlated with properties relevant to the photovoltaic activity. Together with the parameter of temperature, applied quite routinely, the independent thermodynamic parameter of pressure extends the structural variations into the two-dimensional space. So identified structural features associated with the photovoltaic characteristics can be then considered for engineering new materials of improved properties. However, high pressure by its nature, enhances the interactions between molecules and ions, and in this way the dynamics and some transformations in the structure are slowed down. This effect of pressure increases the significance of kinetic processes, often neglectable at ambient pressure. The slow



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transformations in MAPbCl3 and MAPbBr3 occur at relatively low pressure and can be induced even by the strained interfaces or by bending flexible devices. This can lead to the undesirable modifications of optoelectronic characteristics in the time scale of weeks or months after the device fabrication. Of course it is essential that all, equally ambient and high-pressure, experimentally determined structural and symmetry information is correct, so it will not mislead the theoretical interpretation, derived structure-property relations, and the envisaged directions of chemical synthesis research. Unfortunately, there is still a considerable noise of conflicting structural information for the MAPbX3 materials. Consequently, when taking into account the inconsistent results and the slow-kinetic processes, there is still a wide perspective for the careful research on the MAPbX3 photovoltaics, for improving its performance and for finding other even more efficient and environment-friendly analogues.

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. (2) See https://www.nrel.gov/pv/assets/images/efficiency-chart.png, at National Renewable Laboratory, Best Research-Cell Efficiencies. Accessed January 17, 2016. (3) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p‐n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510. (4) Sha, W. E. I.; Ren, X.; Chen, L.; Choy, W. C. H. The Efficiency Limit of CH3NH3PbI3 Perovskite Solar Cells. Appl. Phys. Lett. 2015, 106, 221104. (5) Baikie, T.; Barrow, N. S.; Fang, Y.; Keenan, P. J.; Slater, P.R.; Piltz, R. O.; Gutmann, M.; Mhaisalkar, S. G.; White, T. J. A Combined Single Crystal Neutron/X-Ray Diffraction and


Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Solid-State Nuclear Magnetic Resonance Study of the Hybrid Perovskites CH3NH3PbX3 (X=I, Br and Cl). J. Mater. Chem. A 2015, 3, 9298–9307. (6) Eperon, G. E.; Stranks, S. D.; Menelaou, C.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. Formamidium Lead Trihalide: a Broadly Tunable Perovskite for Efficient Planar Heterojunction Solar Cells. Energy Environ. Sci. 2014, 7, 982–988. (7) Noel, N. K.; Stranks, S. D.; Abate, A.; Wehrenfennig, C.; Guarnera, S.; Haghighirad, A.; Sadhanala, A.; Eperon, G. E.; Pathak, S. K.; Johnston, M. B.; et al. Lead-Free OrganicInorganic Tin Halide Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 3061−3068. (8) Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P. H.; Kanatzidis, M. G. Lead-Free Solid-State Organic−inorganic Halide Perovskite Solar Cells. Nat. Photonics 2014, 8, 489−494. (9) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colour, Efficient, and Stable Inorganic-Organic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764−1769. (10) Colella, S.; Mosconi, E.; Fedeli, P.; Listorti, A.; Gazza, F.; Orlandi, F.; Ferro, P.; Besagni, T.; Rizzo, A.; Calestani, G.; et al. MAPbI3-xClx Mixed Halide Perovskite for Hybrid Solar Cells: The Role of Chloride as Dopant on the Transport and Structural Properties. Chem. Mater. 2013, 25, 4613−4618. (11) Liu, X.; Zhao, W. Cui, H.; Xie, Y.; Wang, Y.; Xu, T.; Huang, F. Organic-Inorganic Halide Perovskite Based Solar Cells – Revolutionary Progress in Photovoltaics. Inorg. Chem. Front. 2015, 2, 315–335. (12) Liu, G.; Kong, L.; Gong, J.; Yang, W.; Mao, H.-k.; Hu, Q.; Liu, Z.; Schaller, R. D.; Zhang, D.; Xu, T. Pressure-Induced Bandgap Optimization in Lead-Based Perovskites with



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Prolonged Carrier Lifetime and Ambient Retainability. Adv. Funct. Mater. 2017, 27, 1604208. (13) Onoda-Yamamuro, N.; Yamamuro, O.; Matsuo, T.; Suga, H. p-T Phase Relations of CH3NH3PbX3 (X = Cl, Br, I) Crystals. J. Phys. Chem. Solids 1992, 53, 277–281. (14) Gesi, K. Effect of Hydrostatic Pressure on the Structural Phase Transitions in CH3NH3PbX3 (X = Cl, Br, I). Ferroelectrics 1997, 203, 249–268. (15) Lee, Y.; Mitzi, D. B.; Barnes, P. W.; Vogt, T. Pressure-Induced Phase Transitions and Templating Effect in Three-Dimensional Organic-Inorganic Hybrid Perovskites. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 020103(R). (16) Swainson, I. P.; Tucker, M. G.; Wilson, D. J.; Winkler, B.; Milman, V. Pressure Response of an Organic-Inorganic Perovskite: Methylammonium Lead Bromide. Chem. Mater. 2007, 19, 2401–2405. (17) Wang, Y.; Lü, X.; Yang, W.; Wen, T.; Yang, L.; Ren, X.; Wang, L.; Lin, Z.; Zhao, Y. Pressure-Induced Phase Transformation, Reversible Amorphization, and Anomalous Visible Light Response in Organolead Bromide Perovskite. J. Am. Chem. Soc. 2015, 137, 11144– 11149. (18) Wang, K.; Liu, R.; Qiao, Y.; Cui, J.; Song, B.; Liu, B.; Zou, B. Pressure-Induced Reversible Phase Transition and Amorphization of CH3NH3PbI3. arXiv:1509.03717, 2015. (19) Ou, T.; Yan, J.; Xiao, C.; Shen, W.; Liu, C.; Liu, X.; Han, Y.; Ma, Y.; Gao, C. Visible Light Response, Electrical Transport, and Amorphization in Compressed Organolead Iodine Perovskites. Nanoscale 2016, 8, 11426–11431. (20) Jaffe, A.; Beavers, C. M.; Voss, J.; Mao, W. L.; Karunadasa, H. I. High-Pressure SingleCrystal Structures of 3D Lead-Halide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties. ACS Cent. Sci. 2016, 2, 201–209.


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(21) Jiang, S.; Fang, Y.; Li, R.; Xiao, H.; Crowley, J.; Wang, C.; White, T. J.; Goddard III, W. A.; Wang, Z.; Baikie, T.; et al. Pressure-Dependent Polymorphism and Band-Gap Tuning of Methylammonium Lead Iodide Perovskite. Angew. Chem. Int. Ed. 2016, 55, 6540–6544. (22) Capitani, F.; Marini, C.; Caramazza, S.; Postorino, P.; Garbarino, G.; Hanfland, M.; Pisanu, A.; Quadrelli, P.; Malavasi, L. High-Pressure Behavior of Methylammonium Lead Iodide (MAPbI3) Hybrid Perovskite. J. Appl. Phys. 2016, 119, 185901. (23) Wang, L.; Wang, K.; Zou, B. Pressure-Induced Structural and Optical Properties of Organometal Halide Perovskite-Based Formamidium Lead Bromide. J. Phys. Chem. Lett. 2016, 7, 2556-2562. (24) Kong, L.; Liu, G.; Gong, J.; Hu, Q.; Schaller, R. D.; Dera, P.; Zhang, D.; Liu, Z.; Yang, W.; Zhu, K.; et al. Simultaneous Band-Gap Narrowing and Carrier-Lifetime Prolongation of Organic-Inorganic Trihalide Perovskites. Proc. Natl Acad. Sci. USA 2016, 113, 8910–8915. (25). Szafrański, M.; Katrusiak, A. Mechanism of Pressure-Induced Phase Transitions, Amorphization, and Absorption-Edge Shift in Photovoltaic Methylammonium Lead Iodide. J. Phys. Chem. Lett. 2016, 7, 3458–3466. (26) Wang, L.; Wang, K.; Xiao, G.; Zeng, Q.; Zou, B. Pressure-Induced Structural Evolution and Band Gap Shifts of Organometal Halide Perovskite-Based Methylammonium Lead Chloride. J. Phys. Chem. Lett. 2016, 7, 5273–5279. (27) Glazer, A. M. The Classification of Tilted Octahedra in Perovskites. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1972, 28, 3384–3392. (28) Poglitsch, A.; Weber, D. Dynamic Disorder in Methyammoniumtrihalogenoplumbates (II) Observed by Millimetre-Wave Spectroscopy. J. Chem. Phys. 1987, 87, 6373–6378. (29) Mashiyama, H.; Kurihara, Y.; Azetsu, T. Disordered Cubic Perovskite Structure of CH3NH3PbX3(X=Cl, Br, I). J. Korean Phys. Soc. 1998, 32, 156–158.



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(30) Kawamura, Y.; Mashiyama, H.; Hasebe, K. Structural Study on Cubic-Tetragonal Transition in CH3NH3PbI3. J. Phys. Soc. Jpn. 2002, 71, 1694–1697. (31) Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Graetzel, M.; White, T. J. Synthesis and Crystal Chemistry of the Hybrid Perovskite (CH3NH3)PbI3 for Solid-State Sensitized Solar Cell Applications. J. Mater. Chem. A 2013, 1, 5628–5641. (32) Weller, M. T.; Weber, O. J.; Henry, P. F.; Di Pumpo, A. M.; Hansen, T. C. Complete Structure and Cation Orientation in the Perovskite Photovoltaic Methylammonium Lead Iodide between 100 and 352 K. Chem. Commun. 2015, 51, 4180–4183. (33) Kirschbaum, K.; Martin, A.; Pinkerton, A. λ/2 Contamination in Charge-Coupled-Device Area-Detector Data. J. Appl. Crystallogr. 1997, 30, 514–516. (34) Herbstein, F. H. How Precise Are Measurements of Unit-Cell Dimensions from Single Crystals? Acta Crystallogr., Sect. B: Struct. Sci. Cryst. Eng. Mat. 2000, 56, 547–557. (35) Dera, P.; Katrusiak, A. Diffractometric Crystal Centering, Appl. Crystallogr. 1999, 32, (36) Budzianowski A.; Katrusiak, A. In High-Pressure Crystallography; Katrusiak, A.; McMillan, P. F., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2004; pp 101–112. (37) Busing,W. R.; Levy, H. A. Angle Calculations for 3- and 4-circle X-ray and Neutron Diffractometers, Acta Crystallogr. 1967, 22, 457464. (38) Hamilton, W. C. Significance Tests on the Crystallographic R Factor. Acta Crystallogr. 1965, 18, 502–510. (39) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and NearInfrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019–9038.


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(40) Belisten-Edmands, J.; Eperon, G. E.; Johnson, R. D.; Snaith, H. J.; Radaelli, P. G. NonFerroelectric Nature of the Conductance Hysteresis in CH3NH3PbI3 Perovskite-Based Photovoltaic Devices. Appl. Phys. Lett. 2015, 106, 173502. (41) Umebayashi, T.; Asai, K.; Kondo, T.; Nakao, A. Electronic Structure of Lead Iodide Based Low-Dimensional Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 155405.

Quotes: 1. High pressure can modify bond lengths and valence angles in hybrid perovskites without a chemical interference, finely tuning the electronic structure responsible for basic properties of photovoltaic materials, like the energy gap and carrier diffusion length.

2. Generally, if there are few exceptions of a systematic extinction, they require a careful check, before the corresponding translational symmetry can be eliminated.

3. The structure and symmetry of crystals are essential for understanding their properties, and vice versa.

4. It turns out that valuable new information about phase transitions can be obtain by just looking at a single crystal.

5. Slow-kinetics effects can be easily overlooked in optical-spectroscopy and synchrotron Xray diffraction experiments, when even microseconds are sufficient for acquiring full data sets.


Photovoltaic Hybrid Perovskites under Pressure - The Journal of

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