Mobile Ions in Organohalide Perovskites: Interplay of Electronic

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Mobile Ions in Organohalide Perovskites: Interplay of Electronic Structure and Dynamics Edoardo Mosconi, and Filippo De Angelis ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00108 • Publication Date (Web): 23 May 2016 Downloaded from http://pubs.acs.org on May 24, 2016

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Mobile Ions in Organohalide Perovskites: Interplay of Electronic Structure and Dynamics Edoardo Mosconi,a,b* Filippo De Angelisa,b* a

Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di

Scienze e Tecnologie Molecolari (ISTM-CNR), Via Elce di Sotto 8, 06123, Perugia, Italy. b

CompuNet, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.

E-mail: [email protected], [email protected]

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ABSTRACT Perovskite photovoltaics have made giant leaps in efficiency in just a few years from their inception. The employed solution synthesis techniques lead to inherently “soft” structures, which are properly sampled by dynamical approaches. The presence of two types of mobile ions, i.e. the (organic) A-cations and ion/defects species in both the organic and inorganic lattices, give rise to a broad spectrum of dynamical features. A charge localization mechanism due to fluctuations of the A-cations is proposed to screen carrier recombination. Defect / ion migration probably underlies the slow materials response under light irradiation related also to solar cell hysteresis. We show how the dynamics of the organic cations and that related to ion/defect migration are essentially coupled, with the methylammonium cations providing a local screening mechanism which may further speed up the ionic migration. The use of less polar and less orientationally mobile A-cations may possibly slow down ion migration.

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Since the pioneering work by Kojima et al. and Park et al.,1,2 organohalide lead perovskites have revolutionized the field of emerging photovoltaic and optoelectronic technologies. Replacing the liquid electrolyte in such dye-sensitized solar cells formulations with a solid-state hole conductor represented a giant step forward, with solid state perovskite solar cells (PSCs) reaching efficiency values of ∼10%.3,4 Lee et al. demonstrated that substituting the TiO2 substrate with a mesoporous scaffold made of Al2O3, an insulating oxide, produced similar conversion efficiencies to TiO2-based devices, suggesting that the perovskite itself works as electron transporter.4 At the same time, Etgar et al. showed that the perovskite could also transport holes,5 which prompted the development of new PSC architectures. Over the last four years, PSCs have experienced an unprecedented development,6-8 with a certified 22.1% efficiency value. One of the main advantages of this class of materials is the possibility to resort to solution synthesis techniques, paving the way to the production of low cost devices. However, solution processed materials have an inherently “soft” structure. In organohalide perovskites such “softness” is characteristic to the inorganic lead-halide matrix but the presence of mobile (organic) cations, such as cesium, methylammonium (MA) or formamidinium (FA), provides additional fluxional degrees of freedom. The orientational dynamics of the MA cation in the cubo-octahedral cavity of methylammonium lead iodide (hereafter MAPbI3) has been recognized long time ago.9-10 Recent evidence pointed to a sub-picosecond dynamics related to MA librational modes11 and a 3-5 ps reorientional time.12-14 The presence of mobile organic cations has stimulated several hypotheses concerning the peculiar optoelectronic properties of perovskites, including a possible role of ferroelectric domains related to the MA dipolar cations in limiting electron-hole recombination,15 an exciton screening mechanism assisted by rotation of the MA cations16 and a possible role of MA re-orientation in contributing to the slow photoconductivity response of MAPbI3.17 Various computational modeling studies based on ab-initio or classical molecular dynamics simulations have been performed to catch the salient features of the organic cation dynamics and their relation to the perovskite electronic structure.13,

18-24

Despite the widely

varying computational level and simulation conditions, all these works found common features of MA or FA dynamics, with fast (sub-ps) librational modes and longer (3-7 ps) reorientational dynamics. It was furthermore noticed that in the tetragonal MAPbI3 phase at room temperature the MA cations are still partially constrained to specific orientations by hydrogen bonding to iodine atoms, inducing the typical tilting of the PbI6 octahedra.13 In the high temperature cubic


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MAPbI3 phase the system is on average symmetric but with strong instantaneous deviations which define the time-average band-gap.24 Many recent observations suggested that the organic cations are not the sole mobile ions in organohalide perovskites. Various unusual findings have in particular been ascribed to ion/defect migration involving also the inorganic perovskite matrix. Among the effects possibly related to ion motion we mention: (i) the slow electrical material response under light irradiation;25-26 (ii) the anomalous hysteresis of the current–voltage (J-V) curves shown by various PSCs devices;17, 27-31 and (iii) a giant switchable photovoltaic effect.32-33 Based on initial observations,33-34 ion/defect migration has been intensively investigated in the last year. Ionic transport is well documented in perovskites oxides10,

35-37

and metal halides,38 whose ionic

conductivity proceeds by means of defect-mediated hopping.35 Ion vacancies and interstitials are thus central to this discussion, since ion diffusion is otherwise hardly realized in a perfect crystal. In this perspective we analyze the contribution of mobile ions to the electronic properties of organohalide perovskites, comparing the results of ab-initio simulations to available experimental data. We revise the assignment of mobile species and critically review experimental data. While the A-cation orientational dynamics and ion/defect migration have been separately investigated, here we further show that these two peculiar perovskite properties are interconnected, adding further complexity to our description of organohalide perovskites.

A-cation dynamics The orientation of the organic A-cations is probably the most complex problem in the structural characterization of organohalide perovskites. In the low temperature MAPbI3 orthorhombic phase, the position of the MA cations was solved, showing these are oriented in a head-to-tail fashion such to maximize the hydrogen bonding between the positively charged ammonium ends and the iodine atoms.39 In the high temperature cubic phase of MAPbX3 perovskites, instead, the MA cations are dynamically disordered, moving in an isotropic potential at a rate approaching that of the freely rotating MA cation.40 The room-temperature stable tetragonal phase of MAPbI3 is expected to be somehow in between the orthorhombic and cubic phases. The assignment of the space group symmetry of this system is quite debated. Poglitsch and Weber proposed the I4/mcm space group,41 while Stoumpos et al. proposed the noncentrosymmetric I4cm space group.42 The C3v symmetry of the MA cation, however, is likely incompatible with structures belonging to both space groups, unless cation disorder is accounted for. Recent reports suggest that the MAPbI3 tetragonal phase retains a partial cation disorder even at low temperature, i.e. just above the tetragonal to orthorhombic transition.43-44 Notably, the organic and inorganic perovskite 4 ACS Paragon Plus Environment

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components are strictly linked, so that a change in the orientation of the organic cations also implies a restructuring of the inorganic part.13 The implications of such dynamical disorder on the optoelectronic properties of organohalide perovskites can be investigated by probing the time evolution of the electronic structure as the nuclei fluctuate under the effect of thermal agitation, as probed by ab-initio molecular dynamics simulations. An illustrative example is offered by the comparison between β-MAPbI3 and α-FAPbI3 in their tetragonal and trigonal phases whose dynamics was probed at ∼330K, see a summary of the results in Figure 1.


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Figure 1. a) Surface plot of the local DOS of the valence and conduction states scanned along the c-axis for a selected snapshot of the MAPbI3 2x2x2 tetragonal supercell. The color bar on the right side corresponds to the DOS in units of number of states/eV. b) and c) Evolution of the qi(t) function (i.e. the difference between the number of states calculated at the CBM and VBM energy, see text for definition) for tetragonal 2x2x2 MAPbI3 (b) and trigonal 2x2x2 FAPbI3 (c) structures. The arrows in b) represent the starting orientation of the MA cations. Reproduced from Ref. 19 with permission from the Royal Society of Chemistry. 6 ACS Paragon Plus Environment

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By exploring the electronic Density of States (DOS) fluctuations during the nuclear dynamics, our calculations highlight that valence band (VB) and conduction band (CB) edges are clearly localized in different supercell planes, see Figure 1a for a random snapshot extracted from the MAPbI3 time evolution. A similar localization was found by Ma and Wang for a selected disordered MAPbI3 structure.45 The nuclear dynamics induces such localization, with simulations showing that the band edges may localize in spatially separated regions with a very fast, sub-ps (ca. 0.1 ps), dynamics. This short time corresponds to frequencies of the order of few hundred cm-1, which are typical to the librational dynamics of the MA cations. Notice that these short-time local fluctuations are observed in the bulk, not at interfaces, and they do not correspond to persistent polarization as would be observed in a ferroelectric material. To quantify the electronic localization, we define the qi(t,E) function, that corresponds to the difference between the number of states at the CB minimum (CBM) energy, DOSi(t,E=CBM), and the number of states at the VB maximum (VBM) energy, DOSi(t,E=VBM), for the i-th layer at a given time, namely: qi(t) = DOSi(t,E=CBM) – DOSi(t,E=VBM) Positive/negative values correspond to an excess of conduction/valence states in a given layer. The evolution of the qi(t,E) function is reported in Figure 1b and c for MAPbI3 and FAPbI3, respectively, and it shows that a different spatial localization of the VB/CB edges easily occurs both for MAPbI3 and FAPbI3. These results highlight a very local effect of the motion of the organic cations on the electronic perovskite properties, resulting in the separation of the electronic states within a few pseudo-cubic units. In general, we observe a more effective localization for the conduction states than for valence states, i.e. the qi(t) function is in general positive. This translates into the fact that photogenerated electrons in MAPbI3 may be more sensitive than holes to such electronic structure fluctuations. FAPbI3 shows in general smaller oscillations than MAPbI3, excluding sporadic cases with a very strong localization of the valence states. This results point out on different localization mechanisms for the tetragonal and the quasi-cubic phases of lead iodide perovskites. The rationale behind this result could be the weakening of directional hydrogen bonds due to the different acidity of MA and FA (pKa ∼11 and 17, respectively) which, beyond the cation size, may also induce MAPbI3 to be tetragonal and FAPbI3 to be quasi-cubic.46 This analysis would also predict a different behavior for even more symmetric organic (e.g. guanidinium)47-48 or spherical inorganic cations.


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The reported electronic structure picture is consistent with an activation barrier to carrier recombination induced by the structural rearrangement needed to bring the recombining electron hole in the same space region. As a matter of fact, Savenije et al.,49 showed charge recombination in tetragonal MAPbI3 to be thermally activated, with a characteristic barrier of 0.075 eV. The measured barrier, determined under light irradiation, is consistent with the calculated ground state barrier to collective MA rearrangement (0.15 eV),13 especially considering the anticipated enhanced rotational freedom of the MA cations in the excited state.25 We believe this mechanism could also be the basis for the recently proposed hypothesis by Zhou and Podzorov whereby charge carriers in organohalide lead perovskites might be protected as large polarons.50 The localization which we observe in our ab-initio molecular dynamics simulations could indeed easily be assimilated to shallow trapping of the photogenerated electrons. Ion/defect migration. Migration of iodine or of iodine-related defects51-53 as well as of MA (or again of MA-related defects)54 has been experimentally demonstrated in organohalide perovskites. Activation energies ascribed to ion/defect migration have also been experimentally determined, ranging from ∼0.2 eV (proposed iodine migration),53 to ∼0.4 eV (proposed MA migration)54 to ∼0.6 eV (proposed iodine migration).55 Almora et al. further reported an activation energy for electrode polarization in MAPbI3 devices of 0.45 eV.56 The variability in the experimental data is likely a result of the different experimental techniques used to probe ion migration and possibly of the different sample preparation route and conditions. Along with dedicated experiments, first principles computational modeling has been employed to investigate ion/defect migration in MAPbI3/FAPbI3 and in MAPbBr3.55,

57-59

Defect migration was investigated considering iodine/bromine vacancies (VI/Br), iodine interstitials (Ii), MA/FA vacancies (VMA/FA) and lead vacancies (VPb). A summary of relevant calculated and experimental activation energies for MAPbI3 is reported in Table 1. Table 1. Experimental and calculated activation energies (Ea, eV) for migration of defects in MAPbI3. Values are rounded to the first decimal. Experimental values are assigned to the migration of defects proposed in the original papers.

Exp. VI+

0.2a

0.6b

Theor. ---

0.3d

0.1e

0.3f


0.6b

0.2f

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Ii-

---

---

---

---

0.1e

---

---

---

VMA-

---

---

0.4c

---

0.5e

0.6f

0.8b

0.6f

VPb2-

---

---

---

---

0.8e

0.9f

2.3b

1.4f

a

Ref. 53 ; b Ref. 55 ; c Ref. 54 ; d Ref. 58 ; e Ref. 57 ; f Ref. 59

Azpiroz et al. reported very low activation energies, ∼0.1 eV, for VI+ and Ii- migration, see Table 1.57 Other authors reported activation energies for VI+ migration ranging from ∼0.3,58,59 to ∼0.6 eV

55

but the defect migration order consistently showed VI+ to be the fastest migrating

ion vacancy (Ii- migration was only considered in Ref. 57). As one may notice, a similar spread of experimental and calculated values is found. As a matter of fact Aziproz et al.57 and Haruyama et al.59 assigned the slow response typical of e.g. solar cell hysteresis in MAPbI3 to VMA- migration, in agreement with Ref.

54

. Eames et al.55 and Meloni et al.,58 on the other hand, assigned the

measured activation energies to VI+ migration based on the similarity between calculated and experimental quantities. The relatively wide range of reported values and the different proposed migrating species highlights a lack of general consensus on the energy barriers ruling of ion/defect migration. One should also notice that the identification of the migrating species (e.g. iodine or MA) and the determination of the activation energies have been so far determined from independent experiments. In other words, current experiments have measured an activation energy (e.g. under the influence of an electromagnetic field) and determined a posteriori the variation in the chemical composition of the same film. Thus we cannot be sure that the measured activation energy and chemical compositional changes are strictly related: one could possibly measure an activation energy related to the slowest process taking place on a certain time scale amenable to the chosen experimental set-up, and then observe a global compositional change due to the overlapping migration of various ions/defects, including faster processes which could simply not be observed. Also, considering the electro-neutrality of the crystal one should envisage at least migration of VI+ and Ii- or VMA- at the opposite device poles, with the associated different characteristic times scales. Accumulation of defects close to the selective contacts could potentially induce structural rearrangement of the perovskite material, affecting its optoelectronic properties. Simulations at a high defect density (∼1020 cm-3) predict that a VI+ in an apical position should lead to a sizable contraction of the unit cell volume (∼11%) due to the halide vacancy.57 The system would however roughly preserve a tetragonal symmetry, with the c/b≈a


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ratio decreasing from 1.486 to 1.451 when passing from the pristine to the defective crystal structure.

Coupled ion migration and A-cation dynamics. Since previous computational determination of activation migration energies relied on a static picture with a selected orientation of the methylammonium cations,

55, 57-59

here we further explored the dynamics of VI+ migration in

MAPbI3 by performing ab initio molecular dynamics simulations of a tetragonal 2x2x2 supercell containing a single iodine vacancy, thus corresponding to a defect density of ∼1019 cm-3. The presence of orientationally mobile MA cations may reveal additional features of the defect migration pathways.

Figure 2. Simulated migration of VI+ dynamics at 450 K. The upper panels show the structural evolution characterizing the defect migration. In the lower panels we show the time evolution of a selected Pb-I distance (see upper panel for atomic labels) related to VI+ migration from site to site along with the average value of the φ angle characterizing the local orientation of the closest methylammonium cation. Our simulations, whose results are summarized in Figure 2, show that VI+ migrate at 450 K on the ~25 ps time scale amenable to our simulations, as illustrated by the sudden variation in the selected Pb-I distance in Figure 2, consistent with the low (~0.1 eV) activation barrier to 10 ACS Paragon Plus Environment

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migration calculated on the corresponding optimized structures.57 We furthermore demonstrate the role of the methylammonium cations in assisting defect migration by partial screening of the positive charge bore by the defect by means of a local methylammonium reorientation. This is testified by the time coincidence of the VI+ migration with a change in the orientation of the closest MA cation, as indicated by the value of the angle formed by the C-N axis of this MA cation with the reference crystal ab plane. Notably, the MA cation closest to VI+ is initially oriented with the positive NH3 end pointing towards an iodine atom close to VI+, then it couples to the motion of the migrating iodine and eventually it undergoes a 180° rotation to point with its CH3 end towards the vacant iodine site. Notice that in Ref. 58 the authors did not observe any VI+ migration by similar ab initio molecular dynamics, probably because of the different simulation time and temperature. This coupled migration pathway, connecting the A-cation and X-anion dynamics, may provide even faster or more effective migration pathways to defect migration in MAPbI3. Summary and Future Outlook. Perovskite photovoltaics have made giant leaps in efficiency in just a few years from their inception. Competitive thin-film technologies have required decades to accomplish similar performance increases. One of the main advantages of organohalide perovskites is the possibility to resort to low-cost solution synthesis techniques, leading however to inherently “soft” structures. The perovskite “softness” is characteristic to the inorganic lead-halide matrix but the presence of orientationally mobile (organic) cations, provides additional fluxional degrees of freedom. In this perspective, we have revisited the interplay of electronic structure and dynamical properties of organohalide perovskites by abinitio molecular dynamics, performing a direct comparison with available experimental data. There are essentially two types of mobile ions in organohalide perovskites, i.e. the (organic) A-cations; and ionic species who become mobile by the presence of defects in both the organic and inorganic lattices. While the orientational dynamics of the organic cations have long been investigated, with recent reports providing additional insight into the typical time scale of their librational and vibrational motions, defect-assisted ion migration in organohalide perovskites is a recently discovered phenomenon. The implications of both type of dynamical features are broad, and probably underlie most of the peculiar perovskite properties. A charge localization mechanism due to fluctuations of the organic cations is proposed to contribute to reduced carrier recombination in both MAPbI3 and FAPbI3, in line with reports of thermally activated carriers recombination. This mechanism, essentially a cation-induced shifting


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of the valence and conduction band edges in different spatial regions, may also underlie a possible polaronic transport mechanism whereby electrons could be shallowly trapped by such collective orientational cation dynamics. Defect / ion migration is now established in MAPbI3, and it probably underlies the slow materials response under light irradiation related also to solar cell hysteresis. While a relatively large spread of calculated values for the activation energies have been reported, from ∼0.1 to ∼0.6 eV, a general consensus emerges that the X halide (i.e. I or Br) is the fastest migrating species. Notably, a similar spread of measured activation energies is found in the literature, thus a direct comparison between theory and experiment in not straightforward. Also, experiments performed so far do not simultaneously, i.e. in real time, measure the migration barrier and the compositional chemical variation in the perovskite, so chemical variations measured a posteriori may not be strictly related to the measured activated process due to the different time scales expected for migration of different ions. Finally, we have shown how the dynamics and the organic cations and that related to ion/defect migration are essentially coupled. A clear correction between the dynamics of an iodine vacancy migration in MAPbI3 and the reorientation of a closest MA cation was disclosed by ab initio molecular dynamics simulations. The MA cation is shown to assist defect migration by reorienting its charge distribution along with the ionic motion, providing a local charge screening mechanism which may further speed up the ionic migration. In this respect, the use of less polar, less acidic and less orientationally mobile A-cations (e.g. FA or Cs) may slow down ion migration in organohalide perovskites, possibly leading to more stable opotoelectronic devices.

Models and Details. Car-Parrinello molecular dynamics simulations of combined defect migration and A-cation dynamics have been carried out within the Quantum Espresso software package.60 The PBE61 exchange-correlation functional is used along with ultrasoft, scalar relativistic pseudopotentials for all atoms. Plane waves cutoffs of 25 and 200 Ry are adopted, for expansion of the wave function and density, respectively, sampling the first Brillouin zone at the Γ point only. Electron−ion interactions were described by ultrasoft pseudopotentials with electrons from O, N, and C 2s2p, H 1s, Pb 6s6p5d, and I 5s5p electrons explicitly included in the calculations. The cell parameters are fixed to the experimental values reported by Poglitsch and Weber.41 To speed up the dynamics we employed the same nuclear masses for all atoms and an


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integration time step of 10 a.u. The total simulation length was 25 ps, with ca. 3 ps of equilibration.

Acknowledgment: The research leading to these results has received funding from the European Union Seventh Framework Programme [FP7/ 2007%2013] under Grant Agreement No. 604032 of the MESO project. REFERENCES

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Reorientation in Methylammonium Lead Iodide Perovskites. J. Phys. Chem. Lett. 2015, 6, 36633669. (15) Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells. Nano Lett. 2014, 14, 2584-2590. (16) Even, J.; Pedesseau, L.; Katan, C. Analysis of Multivalley and Multibandgap Absorption and Enhancement of Free Carriers Related to Exciton Screening in Hybrid Perovskites. J. Phys. Chem. C 2014, 118, 11566-11572. (17) Chen, H.-W.; Sakai, N.; Ikegami, M.; Miyasaka, T. Emergence of Hysteresis and Transient Ferroelectric Response in Organo-Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 164-169. (18) Mosconi, E.; Quarti, C.; Ivanovska, T.; Ruani, G.; De Angelis, F. Structural and electronic properties of organo-halide lead perovskites: a combined IR-spectroscopy and ab initio molecular dynamics investigation. Phys. Chem. Chem. Phys. 2014, 16, 16137-16144. (19) Quarti, C.; Mosconi, E.; De Angelis, F. Structural and electronic properties of organohalide hybrid perovskites from ab initio molecular dynamics. Phys. Chem. Chem. Phys. 2015, 17, 9394-9409. (20) Weller, M. T.; Weber, O. J.; Frost, J. M.; Walsh, A. Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α-[HC(NH2)2]PbI3, at 298 K. J. Phys. Chem. Lett. 2015, 6, 32093212. (21) Frost, J. M.; Butler, K. T.; Walsh, A. Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells. APL Mater. 2014, 2, 081506. (22) Mattoni, A.; Filippetti, A.; Saba, M. I.; Delugas, P. Methylammonium Rotational Dynamics in Lead Halide Perovskite by Classical Molecular Dynamics: The Role of Temperature. J. Phys. Chem. C 2015, 119, 17421-17428. (23) Carignano, M. A.; Kachmar, A.; Hutter, J. Thermal Effects on CH3NH3PbI3 Perovskite from Ab Initio Molecular Dynamics Simulations. J. Phys. Chem. C 2015, 119, 8991-8997. (24) Quarti, C.; Mosconi, E.; Ball, J. M.; D'Innocenzo, V.; Tao, C.; Pathak, S.; Snaith, H. J.; Petrozza, A.; De Angelis, F. Structural and optical properties of methylammonium lead iodide across the tetragonal to cubic phase transition: implications for perovskite solar cells. Energy Environ. Sci. 2016, 9, 155-163. (25) Gottesman, R.; Haltzi, E.; Gouda, L.; Tirosh, S.; Bouhadana, Y.; Zaban, A.; Mosconi, E.; De Angelis, F. Extremely Slow Photoconductivity Response of CH3NH3PbI3 Perovskites Suggesting Structural Changes under Working Conditions. J. Phys. Chem. Lett. 2014, 5, 26622669. (26) Juarez-Perez, E. J.; Sanchez, R. S.; Badia, L.; Garcia-Belmonte, G.; Kang, Y. S.; MoraSero, I.; Bisquert, J. Photoinduced Giant Dielectric Constant in Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 2390-2394. (27) Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T.-W.; Wojciechowski, K.; Zhang, W. Anomalous Hysteresis in Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 1511-1515. (28) Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Mohammad K., N.; Grätzel, M. Understanding the Rate-Dependent J-V Hysteresis, Slow Time Component, and Aging in CH3NH3PbI3 Perovskite Solar Cells: The Role of a Compensated Electric Field. Energy Environ. Sci. 2015, 8, 995-1004. (29) Kim, H.-S.; Park, N.-G. Parameters Affecting I−V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer. J. Phys. Chem. Lett. 2014, 5, 2927-2934. (30) Unger, E. L.; Hoke, E. T.; Bailie, C. D.; Nguyen, W. H.; Bowring, A. R.; Heumuller, T.; Christoforo, M. G.; McGehee, M. D. Hysteresis and Transient Behavior in Current-Voltage 14 ACS Paragon Plus Environment

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Measurements of Hybrid-Perovskite Absorber Solar Cells. Energy Environ. Sci. 2014, 7, 36903698. (31) Chen, B.; Zheng, X.; Yang, M.; Zhou, Y.; Kundu, S.; Shi, J.; Zhu, K.; Priya, S. Interface Band Structure Engineering by Ferroelectric Polarization in Perovskite Solar Cells. Nano Energy 2015, 13, 582-591. (32) Park, N.-G. Perovskite solar cells: Switchable photovoltaics. Nat. Mater. 2015, 14, 140– 141. (33) Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices. Nat. Mater. 2014, 14, 193-198. (34) Dualeh, A.; Moehl, T.; Tétreault, N.; Teuscher, J.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Impedance Spectroscopic Analysis of Lead Iodide Perovskite-Sensitized Solid-State Solar Cells. ACS Nano 2014, 8, 362-373. (35) Mun, A. B.; Ritzmann, A. M.; Pavone, M.; Keith, J. A.; Carter, E. A. Oxygen Transport in Perovskite-Type Solid Oxide Fuel Cell Materials : Insights from Quantum Mechanics. Acc. Chem. Res. 2014, 47, 3340-3348. (36) Ritzmann, A. M.; Mun, A. B.; Pavone, M.; Keith, J. A.; Carter, E. A. Ab Initio DFT+U Analysis of Oxygen Vacancy Formation and Migration in La1-xSrxFeO3-d (x = 0, 0.25, 0.50). Chem. Mater. 2013, 25, 3011-3019. (37) Ritzmann, A. M.; Pavone, M.; Muñoz-García, A. B.; Keith, J. a.; Carter, E. a. Ab Initio DFT+U Analysis of Oxygen Transport in LaCoO3: The Effect of Co3+ Magnetic States. J. Mater. Chem. A 2014, 2, 8060-8074. (38) Misuzaki, J.; Arai, K.; Fueki, K. Ionic Conduction of the Perovskite-Type Halides. Sol. St. Ionics 1983, 11, 203-211. (39) Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Grätzel, M.; White, T. J. Synthesis and crystal chemistry of the hybrid perovskite (CH3NH3)PbI3 for solidstate sensitised solar cell applications. J. Mater. Chem. A 2013, 1, 5628-5641. (40) Wasylishen, R. E.; Knop, O.; Macdonald, J. B. Cation rotation in methylammonium lead halides. Solid State Commun. 1985, 56, 581-582. (41) Poglitsch, A.; Weber, D. Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter-wave spectroscopy. J. Chem. Phys. 1987, 87, 6373-6378. (42) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019-9038. (43) 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. (44) Ren, Y.; Oswald, I. W. H.; Wang, X.; McCandless, G. T.; Chan, J. Y. Orientation of Organic Cations in Hybrid Inorganic–Organic Perovskite CH3NH3PbI3 from Subatomic Resolution Single Crystal Neutron Diffraction Structural Studies. Crystal Growth & Design 2016, 16, 2945-2951. (45) Ma, J.; Wang, L.-W. Nanoscale Charge Localization Induced by Random Orientations of Organic Molecules in Hybrid Perovskite CH3NH3PbI3. Nano Lett. 2014, 15, 248-253. (46) Amat, A.; Mosconi, E.; Ronca, E.; Quarti, C.; Umari, P.; Nazeeruddin, M. K.; Grätzel, M.; De Angelis, F. Cation-Induced Band-Gap Tuning in Organohalide Perovskites: Interplay of Spin–Orbit Coupling and Octahedra Tilting. Nano Lett. 2014, 14, 3608-3616. (47) Giorgi, G.; Fujisawa, J.-I.; Segawa, H.; Yamashita, K. Organic–Inorganic Hybrid Lead Iodide Perovskite Featuring Zero Dipole Moment Guanidinium Cations: A Theoretical Analysis. J. Phys. Chem. C 2015, 119, 4694-4701.


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(48) Marco, N. D.; Zhou, H.; Chen, Q.; Sun, P.; Liu, Z.; Meng, L.; Yao, E.-P.; Liu, Y.; Schiffer, A.; Yang, Y. Guanidinium: A Route to Enhanced Carrier Lifetime and Open-Circuit Voltage in Hybrid Perovskite Solar Cells. Nano Lett. 2016, 16, 1009-1016. (49) Savenije, T. J.; Ponseca, C. S.; Kunneman, L.; Abdellah, M.; Zheng, K.; Tian, Y.; Zhu, Q.; Canton, S. E.; Scheblykin, I. G.; Pullerits, T., et al. Thermally Activated Exciton Dissociation and Recombination Control the Carrier Dynamics in Organometal Halide Perovskite. J. Phys. Chem. Lett. 2014, 5, 2189-2194. (50) Zhu, X. Y.; Podzorov, V. Charge Carriers in Hybrid Organic–Inorganic Lead Halide Perovskites Might Be Protected as Large Polarons. J. Phys. Chem. Lett. 2015, 6, 4758-4761. (51) De Bastiani, M.; Dell'Erba, G.; Gandini, M.; D'Innocenzo, V.; Neutzner, S.; Kandada, A. R. S.; Grancini, G.; Binda, M.; Prato, M.; Ball, J. M., et al. Ion Migration and the Role of Preconditioning Cycles in the Stabilization of the J-V Characteristics of Inverted Hybrid Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, 1501453. (52) Yang, T.-Y.; Gregori, G.; Pellet, N.; Grätzel, M.; Maier, J. The Significance of Ion Conduction in a Hybrid Organic–Inorganic Lead-Iodide-Based Perovskite Photosensitizer. Angew. Chemie Int. Ed. 2015, 54, 7905-7910. (53) Li, C.; Tscheuschner, S.; Paulus, F.; Hopkinson, P. E.; Kießling, J.; Köhler, A.; Vaynzof, Y.; Huettner, S. Iodine Migration and its Effect on Hysteresis in Perovskite Solar Cells. Adv. Mater. 2016, 28, 2446-2454. (54) Yuan, Y.; Chae, J.; Shao, Y.; Wang, Q.; Xiao, Z.; Centrone, A.; Huang, J. Photovoltaic Switching Mechanism in Lateral Structure Hybrid Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1500615. (55) Eames, C.; Frost, J. M.; Barnes, P. R. F.; O'Regan, B. C.; Walsh, A.; Islam, M. S. Ionic transport in hybrid lead iodide perovskite solar cells. Nat Commun 2015, 6, 7497. (56) Almora, O.; Zarazua, I.; Mas-Marza, E.; Mora-Sero, I.; Bisquert, J.; Garcia-Belmonte, G. Capacitive Dark Currents, Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 1645-1652. (57) Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F. Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation. Energy Environ. Sci. 2015, 8, 2118-2127. (58) Meloni, S.; Moehl, T.; Tress, W.; Franckevicius, M.; Saliba, M.; Lee, Y. H.; Gao, P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Rothlisberger, U., et al. Ionic polarization-induced current-voltage hysteresis in CH3NH3PbX3 perovskite solar cells. Nat Commun 2016, 7, 10334. (59) Haruyama, J.; Sodeyama, K.; Han, L.; Tateyama, Y. First-Principles Study of Ion Diffusion in Perovskite Solar Cell Sensitizers. J. Am. Chem. Soc. 2015, 137, 10048-10051. (60) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I., et al. QUANTUM ESPRESSO: A Modular and OpenSource Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. (61) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868.

Quotes to be highlighted in the paper: …a cation-induced shifting of the valence and conduction band edges in different spatial regions may also underlie a possible polaronic transport mechanism…


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ACS Energy Letters

We show how the dynamics and the organic cations and that related to ion/defect migration are essentially coupled. The use of less polar and less orientationally mobile A-cations may slow down ion migration, possibly leading to more stable optoelectronic devices.


ACS Energy Letters

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Authors biographies

Edoardo Mosconi obtained his PhD in theoretical chemistry in 2011 from University of Perugia. From January 2012 he wss a Post-doctoral Researcher at CNR-ISTM in Perugia, Italy. His main research interests are in the field of new generation photovoltaics, employing DFT and ab initio Car-Parrinello molecular dynamics and GW methods. Filippo De Angelis is senior research scientist and deputy director at CNR-ISTM in Perugia, Italy. He is the founder and leader of the Computational Laboratory for Hybrid/Organic Photovoltaics. He earned a PhD in Theoretical Inorganic Chemistry in 1999 from the University of Perugia. His main contributions are in the modeling of materials and processes in hybrid/organic photovoltaics. He is the 2007 recipient of the Nasini Gold Medal of the Italian Chemical Society.


Mobile Ions in Organohalide Perovskites: Interplay of Electronic

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