Role of Methylammonium Orientation in Ion Diffusion and Current

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Letter

The Role of Methylammonium Orientation in Ion Diffusion and Current-Voltage Hysteresis in the CH3NH3PbI3 Perovskite Chuan-Jia Tong, Wei Geng, Limin Liu, and Oleg V. Prezhdo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00659 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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ACS Energy Letters

The Role of Methylammonium Orientation in Ion Diffusion and Current-Voltage Hysteresis in the CH3NH3PbI3 Perovskite Chuan-Jia Tong1,2, Wei Geng1, Oleg V. Prezhdo*2, Li-Min Liu*1 1

Beijing Computational Science Research Center, Beijing 100193, China

2

Department of Chemistry, University of Southern California, Los Angeles, California 900089,

United States

ABSTRACT: Hybrid organic-inorganic perovskites, and particularly CH3NH3PbI3 (MAPbI3), have emerged as a new generation of photovoltaic devices due to low cost and superior performance. The performance is strongly influenced by current-voltage hysteresis that arises due to ion migration, and the challenge remains how to suppress the ion migration and hysteresis. Our first principles calculations demonstrate that the energy barriers to diffusion of the I-, MA+ and Pb2+ ions are greatly affected by dipole moments of the MA species. The energy barriers of the most mobile I- ion range from 0.06 to 0.65 eV, depending on MA orientation. The positively charged MA+ and Pb2+ ions diffuse along the dipole direction, while the negatively charged I- ion strongly prefers to diffuse against the dipole direction. By influencing ion migration, the arrangement of MA molecules can effectively modulate the current-voltage hysteresis intensity. The current work contributes to the fundamental understanding of the microscopic mechanism of ion migration in MAPbI3, and suggests means to suppress the hysteresis effect and optimize perovskite performance. By demonstrating in detail how the arrangement of the organic molecules can efficiently influence ion migration, and hence, amplitude of the current-voltage hysteresis, our results suggest that the hysteresis effect can be suppressed and the long-term performance of perovskites can be improved, if the organic molecules are arranged and stabilized in an anti-ferroelectric order.

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Solar cell materials based on hybrid organic-inorganic perovskites (HOIPs) have been attracting great attention due to the rapid rise in their solar-to-electricity conversion efficiencies.1-7 Since the first reported perovskite solar cell with power conversion efficiency (PCE) of 3.81% by Kojima and coworkers in 2009,8 its PCE quickly surpassed 20% after only 5 years of photovoltaic research.9-14 The state-of-the-art performance of HOIPs can be mainly attributed to their remarkable properties, including the suitable band-gap that can result in high optical absorption in the visible region,15-21 low effective mass and small exciton binding energy22-23 which allow for efficient charge-carrier generation, collection and transport.24-29 At the same time, the advances in low-cost, high-throughput manufacturing process make it possible of large-scale fabrication of high-efficiency HOIPs solar cell devices.30-33 All these excellent properties make perovskite one of the best promising candidate for the next-generation solar cells. Although the new perovskite materials employed in the solar cells show an outstanding performance, there are still some conundrums that need to be clearly solved such as current-voltage hysteresis34-37 and stability upon exposure to moisture38-44. Ion migration in perovskites constitutes a vital issue, because it is considered to be the source of many unusual phenomena, and in particular the current-voltage hysteresis that affects the device performance45-51. For example, Tress et al. observed the strong hysteresis effect in perovskite solar cells by measuring current-voltage curves at different sweep voltage, and they concluded that the hysteresis effect is responsible for a compensating field induced by the ion migration.52 Besides, Eames et al. reported that the diffusion of intrinsic ionic defects has important implications for both hysteresis and long-term stability.48 Therefore, multiple experimental and theoretical studies have explored ion diffusion in perovskite methylammonium lead triiodide (MAPbI3, MA=CH3NH3).53-60 However, the exact origin of the hysteresis effect is still poorly understood. The reported energy barriers for iodine ion/vacancy migration vary from 0.08 to 0.58 eV in different publications.48, 54, 57 The MA+ molecule can have different orientations, which will result in a local built-in dipole within MAPbI3. As reported in the previous studies, the orientation of the organic molecules can greatly influence, stability, performance, and ferromagnetism.61-64 Mosconi et al. suggested a synergistic effect of ion migration and orientation of MA+ molecules, which can assist the


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migration of charged defects.65 In this work, the environment for the ions is carefully investigated to establish the relation between the hysteresis and orientation of the organic species. The diffusion behavior of each ion in different environments is systemically examined. First-principles calculations were carried out to systematically study diffusion of the I-/MA+/Pb2+ defects (Schottky vacancies) in MAPbI3, taking account of the MA+ orientation effect. Our results reveal that the local dipole induced by MA+ orientations affects both the vacancy site and the diffusion direction of ion migration. The positively charged ions prefer to diffuse along with the dipole direction, whereas the negatively charged ions prefer to diffuse against the dipole direction. The different arrangements of MA+ induce distinct compensating fields, which can affect the hysteresis intensity. The reported results provide the vital step on understanding the ion migration and hysteresis in HOIPs, which is essential to improve the performance of perovskite solar cells. As the tetragonal phase of MAPbI3 is stable at the room temperature, here we use a 2 × 2 × 2 supercell to explore the effect of the MA+ orientation on the ion diffusion. The cell contains 384 atoms. In order to learn the effect of MA+ orientation, many MAPbI3 configurations with different MA+ arrangements were examined at first. Finally, the two most energetically stable structures, labeled as structure A and B, were considered (see Fig. 1(a)-(b)). B is about 0.07 eV/cell more stable than A. It is well known that MA+ has a strong molecular dipole of 2.3D with its direction from N-terminal to C-terminal.66 Therefore, both structures A and B have a compensated dipole moment in the XY plane, while different distributions of the MA+ alignments exist along the Z direction. The structure B (ferroelectric) has a uniform dipole polarization with all dipole pointing up along the Z direction, as shown by colored arrows in Fig. 1(d); The structure A (anti-ferroelectric) has no total dipole moment, since the MA+ molecule orientations in neighbor layers are opposite by up and down arrangements along the Z direction. Thus, the corresponding dipole moments compensate as shown by colored arrows in Fig. 1(c). However, it should be noted that the dipole polarization still exists in local regions, and its influence cannot be ignored. In addition, it is known that thermal fluctuations can induce rotation of MA+ molecules in perovskite.67-68 In order to check how temperature affects the models used in this work, we carried out several first-principles molecule dynamics (FPMD) simulations for both structure A and B. Our results indicate that both structures can be stable at a finite temperature, as discussed in


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Supporting Information and Fig. S1.

Figure 1. Side view of structure models in this work, 2 × 2 × 2 tetragonal supercells of MAPbI3 named as: (a) structure A with orientations of MA+ molecules in adjacent layers being opposite in the vertical direction; (b) structure B with orientations of all MA+ molecules in the whole structure being the same in the vertical direction. The colored triangle represent dipole direction from bottom (N-terminal) to top (C-terminal) of the MA+ molecule. Schematic diagrams of the perovskite with only MA+ arrangements in (c) structure A and (d) structure B. The colored arrows stand for the built-in dipole direction induced by MA+ molecule, projected in the vertical direction. (Dark gray - lead; purple - iodine; brown - carbon; blue - nitrogen; white - hydrogen)

Based on the two supercells, three typical vacancies, namely, missing iodide ion, lead ion or MA+ ion, are considered as Schottky vacancy defects. Here, we also investigate specific vacancy sites related to the MA+ orientation, considering that the same atom may stay in the different environments, which could affect diffusion pathways. For example, iodine could have two distinct types of diffusion pathways. One is along the Z direction connected to the two lead atoms in the upper and lower layers, labeled as V (vertical) site. The other locates in the XY plane, referred as P (parallel) site in the following. Moreover, the inverse built-in MA+ molecular dipole in structure A


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makes its surrounding atoms stay in completely different environments, thus nearly all equivalent ions experience different environments. All these different representative atom/vacancy sites are symbolically marked in Fig. 2(a)-(b) and representative vacancy formation energies (Evf) are summarized in Table 1.

Figure 2. Possible vacancies and pathways with different equivalent sites. (a) and (b) represent the vacancy sites in structures A and B, respectively. (c) Scheme of specific I- ion migration mechanism in MAPbI3 projected onto the XZ plane, represented by dashed frame in (a) and (b). The dashed arrows in (c) stand for different diffusion pathways, referred to as L1 to L6, respectively. The colored solid arrows represent the MA+ molecular dipole arrangements along the vertical direction in structures A and B.

As shown in Table 1, the vacancy formation energy exhibits a large fluctuation for each kind of ion. The MA+ exhibits the largest Evf, ranging from 2.52 to 2.70 eV. The iodine prefers to form a vacancy at the P site, and the Evf of the P site are about 0.5 eV for structure A and 0.1-0.2 eV for structure B smaller than that of the V site. Such results suggest that the iodine defect is more easily formed in the P site of structure A, whereas the formation energies of iodine vacancy do not fluctuate too much for the different structures, and are within 0.22 eV. The vacancy formation energies of Pb also vary little between different sites (V1 or V2), but they do change by about 0.6 eV between structures A and B. The results indicate clearly that the environment of the ions greatly affects the formation energy of the vacancy. The difference in the formation energies is much larger in structure A, which


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should be attributed to built-in polarization field induced in opposite directions by different orientations of MA+. Thus, the orientations of MA+ molecules greatly affect the formation of vacancies through the built-in molecular dipole. Especially, the formation of iodine vacancy strongly depends on the MA+ orientation in structure A, with a relatively small energy of ~ 1.5 eV at the P site.

Table 1. Calculated vacancy formation energy (Evf) of different atom/molecule (I-/Pb2+/MA+) vacancy in structures A and B in different sites as shown in Fig. 2(a)-(b). (Unit: eV) VI

VPb

VMA

P1

P2

V1

V2

V1

V2

V1

V2

Structure A

1.57

1.56

2.10

2.05

1.22

1.19

2.53

2.52

Structure B

1.77

1.78

1.89

1.98

1.83

1.84

2.69

2.70

As discussed above, the vacancy sites are affected strongly by orientation of MA+ molecules; therefore, the vacancy-mediated diffusion may also be influenced by MA+. In order to explore this possibility, a systematic study on the ion diffusions was carried out. In this part, we only examine the diffusion pathway from a metastable site to a relative stable site. As shown in Fig. 2(a)-(b), the following possible diffusion pathways are considered: (i) I- migration from the metastable site P1/P2 to the relative stable site V1, and I- migration from P1 to P4, occurring essentially in the XY plane. (ii) MA+ migration both along the vertical direction ( direction, from V1 to V2) and in the XY plane ( direction). It should be noted that migration of MA+ can involve both lateral movement and rotation around the center.69 Here, we do not consider explicitly the effect of MA+ rotation on MA+ diffusion, since the rotation barrier was reported to be very low (~ 0.1 eV)61. We do check the MA+ rotation barrier, which is about 0.02 eV for structure A and 0.03 eV for structure B. Moreover, the inverse migration (from V2 to V1) is also considered, since the charged MA+ would behave differently when they diffuse along or against the local polarization field, particularly in the vertical direction. (iii) Pb2+ migration only along the body diagonal ( direction), because iodine atoms block the Pb2+ diffusion pathways in both the XY plane ( direction) and along the vertical direction ( direction).


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Some typical energy barriers and diffusion pathways along with the corresponding atomic configurations for initial, transition and final states are shown in Fig. 3. It can be found that the Eb values are different even for the same vacancy of either I- or MA+, while the ion diffuses from a distinct site. For example, when I- diffuses along the dipole direction, the energy barrier is 0.33 eV in structure A and 0.21 eV in structure B (see Fig. 3(a)-(b)). The values become 0.06 eV in structure A and 0.12 eV in structure B, when I- diffuses against the dipole direction (see Fig. 3(a)-(c)). The same phenomena occur for the diffusion of MA+. The energy barriers of structure A are more sensitive to the MA+ orientation than those of structure B.

Figure 3. The energy profiles of ion diffusion and the corresponding atomic structures. Energy profile of I- diffusion in the vertical direction (a). Configurations at the initial state (IS), transition state (TS) and final state (FS) when I- diffuses along the dipole direction (b) and against the dipole direction (c). Energy profile of MA+ diffusion in the direction (d). The IS, TS and FS configurations for MA+ diffusion along the dipole direction (e) and against the dipole direction (f). All configurations represent atomic arrangements for diffusion in structure B. The configurations in structure A are almost the same as in structure B, except for the different MA+ molecular


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arrangements. VI means iodine vacancy, and VMA means MA+ vacancy.

In order to fully understand the fundamental mechanisms of the ion diffusions, the relation between Eb and the effective diffusion length (Le) was explored. Here, we define Le as following:

=

−1, 1,

= ∗

against the dipole direction along the dipole direction

where LZ is the diffusion length projected along the Z (vertical) direction, f is the effective factor which equals to -1 or 1 when the charged ions diffuse against or along the MA+ dipole direction, respectively. As discussed above, the main difference between structures A and B is the MA+ molecular dipole arrangement in the vertical direction. Thus, Le can describe the effect of the MA+ orientation on the ion diffusion. 16 possible ion diffusion pathways are considered in total, as discussed above, and the relation between the effective diffusion length (Le) and the energy barrier (Eb) of each diffusion is carefully examined. As shown in Fig. 4, the diffusion barriers of I- (0.06-0.39 eV, consistent with previous reports50,

70

) are relatively small, while the barriers of both MA+ (0.61-1.09 eV) and Pb2+

(0.85-1.05 eV) are much larger. Such results suggest that I- is the most mobile ion in the MAPbI3 perovskite. MA+ exhibits a moderate energy barrier of 0.61 eV for a certain diffusion pathway. The largest migration barrier of Pb2+ indicates that it is the least mobile ion in the MAPbI3 Perovskite. Recently, Yuan et al. carried out photothermal-induced resonance (PTIR) measurements, and presented a direct visualization of macroscopic migration of the I- and MA+ ions under an electric field.56 Our results clearly reveal the origin of I- and MA+ diffusion, as observed in the experiment. Importantly, the energy barriers of ion diffusion are greatly affected by Le, especially in structure A. I- prefers to diffuse against the dipole direction (Le 0). The main reason is that MA+ and Pb2+ are positively charged, while I- is negative. When it comes to structure B, the barrier difference becomes relatively small. The energy barriers of MA+ diffusion along the vertical direction are 1.04 eV and 1.09 eV for Le < 0 and Le > 0, respectively. The corresponding difference in the energy barriers for Pb2+ diffusion along the direction is


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about 0.01 eV between Le < 0 and Le > 0. As mentioned above, all MA+ and Pb2+ are equivalent in structure B, since they stay in the same environment with all MA+ molecules in the same arrangements. However, the iodine atoms in structure B are nonequivalent, because the vertical and parallel sites are essentially different. Therefore, the energy barriers of I- diffusion from the parallel site to the vertical site differ. They are equal to 0.12 eV and 0.21 eV for Le < 0 and Le > 0 in structure B, indicating that I- diffusion is still strongly affected by the surrounding dipole polarization field. Finally, as shown in Fig. 4, when the value of Le is nearly equal to zero, corresponding to diffusion along the XY plane in both structures A and B, the energy barriers for I- and MA+ diffusion are close in the two structures. Namely, the barriers in structure A are 0.38 eV for I- and 0.63 eV for MA+. The corresponding values for structure B, 0.39 eV for I- and 0.66 eV for MA+, are quite close. The reason is that the dipole moments compensate here. The dipole orientations in the XY plane are exactly same, and the ion migrations experience similar barriers in the XY plane.

Figure 4. Relation between effective length (Le) and ion migration barrier (Eb). The hollow or solid symbols stand for migrations in structure A or structure B. Circle, triangle and diamond represents I-, MA+ and Pb2+, respectively. Le < 0 or Le > 0 implies that the ion diffuses along the Z axis, either against or along the MA+ molecular dipole direction. Le ≈ 0 indicates that the diffusion occurs in the XY plane.

Up to this point, it has been proven that the local dipole polarization field induced by MA+


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molecules plays an important role in ion migrations, and that I- is the most mobile ion in MAPbI3. In the previous section, we only focus on diffusion of I- from the metastable site (P) site to the stable site (V). However, I- ions may migrate throughout the whole MAPbI3 structure, considering the relatively small diffusion barrier. It is crucial to understand the general mechanism of Imigration. For this purpose, six possible I- diffusion pathways (see Fig. 2(c)) are considered for both structures A and B, and the corresponding calculated barriers are shown in Table 2. It can be concluded that MA+ orientations strongly affect the ion migration barriers, depending on both diffusion site and direction. First, as for the diffusion pathway along L1 and L2 in structure A, the barrier difference mainly stems from different dipole directions rather than the specific diffusion sites, both diffusions happening from a metastable (parallel) site to an energetically more favorable (vertical) site. As for L3, although its energy barrier (0.32 eV) is nearly the same as that of L2 (0.33 eV), the mechanism is totally different. In L3, the diffusion occurs from a stable (vertical) site to a less favorable (parallel) site. Thus, the effect of diffusion site here is negative, while the effect of the diffusion direction is positive, since I- diffuses against the dipole direction. Consequently, the overall effect of these two factors on the energy barrier of Idiffusion is the same as that in L2.

Table 2. Energy barriers (Eb) of I- along different pathways as shown in Fig. 2(c). (Unit: eV) Structure A

Structure B

L1

0.06

0.21

L2

0.33

0.12

L3

0.32

0.52

L4

0.65

0.40

L5

0.38

0.39

L6

0.38

0.39

Furthermore, both site and dipole take a negative effect in L4, which results in the highest energy barrier (0.65 eV) in structure A. Because migrations along pathways L5 and L6 occur between two equivalent sites in the XY plane without being affected by the dipole field in the


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vertical direction, the energy barriers are quite close in structures A (0.38 eV) and B (0.39 eV). The same phenomena occur in structure B. The difference is that the effects of site and dipole are less serious than those in structure A, because of the identical environment induced by the same arrangements of MA+ molecules in structure B. This explains well that the lowest diffusion barrier in B (0.12 eV) is still higher than that in A (0.06 eV), while the highest barrier in B (0.52 eV) is smaller than that in A (0.65 eV). It should be noted that in the model of Fig. 2(c), the effect of dipole direction on each diffusion pathway in structure B is opposite to that in structure A, which leads to the reverse relation among the energy barriers for the equivalent diffusion pathways in the vertical direction: L1L4 in structure B. Further, we observed diffusion of the iodine defect using a long FPMD trajectory 300 K. The simulation indicates that the iodine prefers to diffuse against the dipole direction, which is consistent with our conclusion obtained with the static model. The details and results of this FPMD simulation are presented in Supporting Information and Fig. S2. As established above, I- is the most mobile ion in the MAPbI3 perovskite, and its diffusion pathway is greatly related with the arrangements of polar MA+ molecules. Ion migration plays a key role in the current-voltage hysteresis in perovskite solar cells. Accordingly, it is important to explore the relation between the MA+ orientation and hysteresis effect. As shown in Fig. 5(a)-(b), by imposing the forward and reverse electric field (0.05 MV/cm) along the Z direction, all MA+ molecules adjust their orientations to redistribute along the external field direction, and MA+ molecules in structure A and B responds differently. The orientation changes of MA+ molecules (∆θ) along the Z-axis are examined under the forward and reverse direction. For structure A, the MA+ orientation changes slightly with ∆θ of about 1° (see Fig. 5(a)). For structure B, the MA+ orientation varies more significantly with ∆θ of about 3°, and the MA+ molecules deviate more from the Z-axis under the reverse electron field (see Fig. 5(b)). Moreover, as for structure B, the configuration under the forward electric field is energetically more stable (∆E = -0.04 eV) than the one under the reverse electric field. Interestingly, there is no energy difference (∆E = 0) in structure A when the field applied in the forward and reverse directions. Such results indicate that structure B is much more sensitive to the external electric field compared with structure A, because of the different MA+ arrangements. Under working conditions (see Fig. 5(d)), the electrons can transfer to the electron transport


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medium (ETM), while the holes move to the hole transport medium (HTM). At the same time, the positive iodine vacancy (VI+) migrates to the HTM, and the negative iodide ion (I-) migrates to the ETM. In addition, this migration process needs a relatively low energy barrier, considering that Imigrates against the dipole direction. Finally, the positive/negative charges accumulate at the HTM/ETM, which induces an inverse built-in field, Ein. This new built-in field could hinder the injection of holes/electrons to the HTM/ETM. Consequently, the J-V hysteresis phenomenon appears. The charges accumulated at the HTM/ETM behave differently in structures A and B, considering that MA+ molecules have distinct responses to the external field. Since the MA+ molecules in structure A are not uniformly aligned in one direction, and are coupled in an anti-ferroelectric manner in the Z direction, the MA+ orientation in structure A is not sensitive to the external field compared with the that in structure B (see Fig. 5(a)). As a result, MA+ molecules respond only slightly to the photo-generated field E0, and remain primarily in their original arrangements. As shown in Fig. 5(c)-(d), the dipoles are directed in the opposite ways in structure A, and in the same way in structure B. Considering that the positive/negative ions prefer to diffuse along/against the dipole direction of MA+, they will diffuse in the up or down direction. However, the ions get trapped in the energetically favorite sites in structure A (see Fig. 5(c)), while they can continue to diffuse in one direction in structure B. Therefore, the ions in structure A can only diffuse over a short range, while the ions in structure B can continue to migrate until reaching the HTM/ETM. Thus, structure B has the potential to accumulate more charges in the ETM than structure A. Accordingly, the built-in field Ein in structure A will be weaker than the one in structure B, and structure A can effectively suppress the hysteresis effect. This important result suggests that perovskites materials capable of anti-ferroelectric arrangements of dipolar organic cations should exhibit decreased hysteresis and better long-term photovoltaic performance.


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Figure 5. Shift (∆θ) in the angle between the MA+ molecules and the Z axis due to application of forward and reverse electric fields in (a) structure A and (b) structure B. (The solid colored triangles illustrate MA+ arrangements under the forward field, while the dashed triangles represent MA+ arrangements under the reverse field. ∆E stands for energy difference under the forward and reverse fields.) Microscopic mechanism of hysteresis induced by iodine ion/vacancy migration in (c) structure A and (d) structure B. (E0 is photo-generated external field. Ein is built-in field induced by charges accumulated at electrodes. HTM is hole transport medium. ETM is electron transport medium. e- is electron. h+ is hole. VI+ is iodine vacancy.)

Next, let us discuss how the scanning direction and rate affect the hysteresis of the J-V curves. Consider structure B as an example. Once the external scanning voltage reverses, the MA+ molecules will respond and change their orientations to redistribute along the external field. As discussed above, the MA+ molecules in structure B prefer to rotate to the opposite direction under the reverse electron field, gaining about 0.04 eV of energy. As the MA+ molecules rotate to the


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reverse direction, I- tries to migrate along the MA+ molecular dipole direction, which needs a much larger energy barrier compared the one in the forward field. If the scanning rate is fast, the cations/anions do not have enough time to reach the HTM/ETM to compensate the external field. As a result, the hysteresis effect becomes weaker under the fast scanning rate. Recently, Azpiroz et al.54 used DFT calculations to examine the origin of the relatively small hysteresis of the J-V curves at a fast scanning rate. They suggested that the decreased hysteresis at the fast scanning rate is related to rate-determining defects, which do not have time to migrate towards the electrodes under the fast voltage sweep. Our results both confirm their suggestion, and provide a deeper understanding of the phenomenon by establishing that the hysteresis intensity is strongly influenced by the MA+ orientation. The presented analysis cannot exclude other phenomena, such as interplay between differently charged defects or interfaces with contacts.50, 71 Tress et al.52 measured J-V curves at different scan voltages. They found that the J-V curve is dependent on the voltage sweep direction in the MAPbI3 perovskite. Li et al.72 deduced different energy barriers for ion migration by measuring the decay time during forward and reverse voltage sweeping. Our results agree well with these experiments. As for structure A, iodine defects always require a long time to reach the electrodes to form a compensating built-in field, effectively avoiding the J-V hysteresis. Moreover, since structure A is weakly sensitive to the external field, the difference in its J-V curve under forward and reverse scanning is rather small, suppressing the hysteresis. Our results establish that the MA+ orientation not only affects the ion diffusion, but also can induce a strong J-V hysteresis curve at a slow scanning rate. Meanwhile, the J-V hysteresis can be avoided when the MA+ molecules or other organic cations can be assembled in an appropriate (anti-ferroelectric) manner. In summary, using first-principles calculations, we have systematically studied the migration of the I-/MA+/Pb2+ ions in the MAPbI3 perovskite. A strong effect of the MA+ molecular orientation on the ion migration is identified. Owing to the inner dipole polarization field induced by the MA+ molecule, the positively/negatively charged ions prefer to diffuse along/against the dipole direction. Meanwhile, the ion migration does not exhibit any favorite direction in the parallel plane, since the neighboring MA+ dipoles are compensated. Notably, not only I- is the most mobile ion, but also its migration is most strongly affected by the MA+ orientation, creating conditions for the current-voltage hysteresis. Finally, the arrangements of MA+ molecules can modulate the


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current-voltage hysteresis by inducing different built-in fields, depending the MA+ dipole arrangement. This work both establishes the mechanism of ion migration in MAPbI3, and identifies that the hysteresis effect can be suppressed through adjusting the arrangements of the organic cations, which can be achieved by structural and electric phase transitions in appropriately selected HOIPs. The reported results are particularly important for improving photovoltaic performance of the perovskite materials.

ASSOCIATED CONTENT Supporting Information Description of the computational details and first-principle molecular dynamics (FPMD) on perovskite MAPbI3.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program of China Grant No. 2016YFB0700700 and National Natural Science Foundation of China (No. 51572016 and U1530401). We acknowledge the computational support from a Tianhe-2JK computing time award at the Beijing Computational Science Research Center (CSRC) and the Special Program for Applied Research on Super Computation of the NSFC- Guangdong Joint Fund (the second phase). Chuan-Jia Tong acknowledges the financial support from the program (201604890017) of China Scholarship Council. O.V.P. acknowledges support of the U.S. National Science Foundation, Grant no. CHE-1565704.

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Role of Methylammonium Orientation in Ion Diffusion and Current

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