Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
Methylammonium lead iodide perovskite (MAPI) can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Within the perovskite crystal structure the methylammonium (MA) ions are caged between lead halide octahedra. The MA ions are electrical dipoles which have the potential to contribute to ferroelectric properties of the material. These ions are also speculated to play a critical role in the stability and hysteresis of MAPI photovoltaic devices.
Here we report quasielastic neutron scattering measurements showing that MA ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ~14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window.
The inferred active fraction of rotating MA is analysed. The proportion of CH3-rotors undergoing reorientation around the C-N axis increases linearly with temperature, which could be consistent with the reported H-bonds between MA and the halides of the inorganic moiety. The fraction experiencing reorientations of the C-N axis itself is independent of temperature, thus pointing at steric hindrance due to the extreme softness of the material at atomic level.
Monte Carlo simulations of interacting CH3NH3+ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3+ in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3+ to screen a device’s built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ~0.1–1 ms, faster than most observed hysteresis.
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