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Room-temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An ab initio molecular dynamics perspective

Jonathan Lahnsteiner, Georg Kresse, Abhinav Kumar, D. D. Sarma, Cesare Franchini, and Menno Bokdam
Phys. Rev. B 94, 214114 – Published 28 December 2016
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  • INTRODUCTION
  • COMPUTATIONAL METHOD
  • MOLECULAR DYNAMICS: UNIT AND SUPERCELLS
  • DYNAMICS OF INDIVIDUAL MOLECULES
  • CORRELATION BETWEEN MOLECULES
  • DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    The high efficiency of lead organo-metal-halide perovskite solar cells has raised many questions about the role of the methylammonium (MA) molecules in the Pb-I framework. Experiments indicate that the MA molecules are able to “freely” spin around at room temperature even though they carry an intrinsic dipole moment. We have performed large supercell (2592 atoms) finite-temperature ab initio molecular dynamics calculations to study the correlation between the molecules in the framework. An underlying long-range antiferroelectric ordering of the molecular dipoles is observed. The dynamical correlation between neighboring molecules shows a maximum around room temperature in the mid-temperature phase. In this phase, the rotations are slow enough to (partially) couple to neighbors via the Pb-I cage. This results in a collective motion of neighboring molecules in which the cage acts as the mediator. At lower and higher temperatures, the motions are less correlated.

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    • Received 17 August 2016
    • Revised 30 November 2016

    DOI:https://doi.org/10.1103/PhysRevB.94.214114

    ©2016 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Crystal structureFirst-principles calculationsLattice dynamics
    1. Physical Systems
    PerovskitesPhotovoltaic absorbers
    1. Techniques
    Density functional theoryDensity functional theory developmentMolecular dynamics
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Jonathan Lahnsteiner1, Georg Kresse1, Abhinav Kumar2, D. D. Sarma2, Cesare Franchini1, and Menno Bokdam1,*

    • 1University of Vienna, Faculty of Physics and Center for Computational Materials Science, Sensengasse 8/12, A-1090 Vienna, Austria
    • 2Solid State and Structural Chemistry Unit, Indian Institute of Science, 560012 Bangaluru, India

    • *menno.bokdam@univie.ac.at

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    Vol. 94, Iss. 21 — 1 December 2016

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    Images

    • Figure 1
      Figure 1

      The three different MAPbI3 crystal phases. The unit cells (48 atoms, 4 molecules, solid red lines) are indicated, except for the cubic phase where a 2×2×2 supercell of the pseudocubic unit cell is shown. The dashed orange lines show the hydrogen bonds between the NH3 groups and the iodines. The left/right column shows a top/side view, respectively. The polar coordinates reference frame (ϕ,θ) to describe the orientation of the C-N bond of the MA molecule is drawn on the top. vesta [60] was used for this figure.

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    • Figure 2
      Figure 2

      Molecular polarization Pmol(t) in the 2-cell, 4-cell, and 6-cell at 300 K starting from an unpolarized-random (R) structure. The polarization resulting from a polarized-aligned (A) starting structure in the 4-cell is given for comparison.

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    • Figure 3
      Figure 3

      The polar plot of ten randomly chosen dipoles in the 6-cell shows the rotational mobility (ϕi(t),θi(t)) in a time frame of 8 ps at 150 and 300 K.

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    • Figure 4
      Figure 4

      Autocorrelation function rmol(t) of MA molecules in a semilogarithmic plot. (a) At 300 K in the 2-, 4-, and 6-cell structures starting from a random (R) and aligned (A) configuration of the molecules. (b) At various temperatures between 100–400 K in the 4-cell. The dashed lines indicate exponential fits.

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    • Figure 5
      Figure 5

      Polar angle distribution of the MA molecules at 300 K in the 2-cell (top), 4-cell (middle), and 6-cell (bottom). The plots show the full rotational space (left) and the downfolded plot (right) into 1/8 of the full space.

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    • Figure 6
      Figure 6

      From left to right, downfolded polar angle distribution of the MA molecules at 400, 300, 250, 200, 150, and 100 K in the 4-cell.

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    • Figure 7
      Figure 7

      Top: Polar angle distribution of the MA molecules at 100 K in the 4-cell. Bottom: Ideal molecular ordering of the orthorhombic structure with the molecules in the xy plane (black circles), the xz plane (red squares), and the yz plane (green triangles) with a π/3 relative orientation. The downfolded polar angle distribution is included for comparison.

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    • Figure 8
      Figure 8

      Static correlation between molecules at 300 K in the (a) first, (b) second, and (c) third NN shells expressed by the distribution of di,j, going from AFE (↑↓) to FE (↑↑) alignment. The red/blue/green lines correspond to the 2-cell/4-cell/6-cell, respectively. The three different directions for the (d) first (cubic axis) and the (e) second (face-diagonal) NN in the 4-cell at 300 K. (The integral of the di,j distributions are normalized to one.)

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    • Figure 9
      Figure 9

      Static correlation between molecules in the first (left) and second (right) NN shell of the 4-cell at temperatures between 100 and 300 K expressed by the distribution of di,j. The dot product values corresponding to the ideal molecular ordering pattern in the orthorhombic/tetragonal phase are shown (in arb. units) by the filled/striped histograms, respectively.

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    • Figure 10
      Figure 10

      Dynamical correlation between molecules in the first four NN shells of the 4-cell at temperatures between 100 and 300 K expressed by square Pearson correlation coefficient (rc2) for θiθj,ϕiθj, and ϕiϕj. The filled symbols correspond to the θiθj correlation in the 6-cell. Experimental phase-transition temperatures have been indicated by the dashed lines.

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