The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells

Abstract

Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH 3 NH 3 + ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH 3 NH 3 PbI 3 crystals with a room temperature residence time of â 1/414â €‰ps. Free rotation, Ï €-flips and ionic diffusion are ruled out within a 1-200-ps time window. Monte Carlo simulations of interacting CH 3 NH 3 + dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH 3 NH 3 + in either antiferroelectric or ferroelectric domains. Collective realignment of CH 3 NH 3 + 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 â 1/40.1-1â €‰ms, faster than most observed hysteresis.