  With the rapid evolution of lead halide perovskite solar cells, there are
considerable evidences that the mobile ions strongly impact their properties.
However, except the activation energy, the other properties of ionic migration
are mostly explored with first principle calculations. Here we investigate the
migration of halide ion (vacancy) in lead bromide perovskite single crystalline
microplates. In contrast to conventional studies with external bias voltages,
we introduce built-in potentials into the microplates via low-energy electron
beams and study the corresponding ionic migrations. By aligning the potentials
along particular crystal planes, the reduction of Pb2+ to Pb0 has been observed
at the un-irradiated center of each cell, demonstrating that the migration of
halide ion has preferred directions for the first time. Our observations are
consistent with the recent theoretical models and shall open up a new route to
improve the performances of perovskite solar cells.
