  In this work, perovskite solar cells (PSCs) with different transport layers
were fabricated to understand the hysteresis phenomenon under a series of scan
rates. The experimental results show that the hysteresis phenomenon would be
affected by the dielectric constant of transport layers and scan rate
significantly. To explain this, a modified Poisson and drift-diffusion solver
coupled with a fully time-dependent ion migration model is developed to analyze
how the ion migration affects the performance and hysteresis of PSCs. The
simulation model was optimized for carrier transportation of organic materials,
which can simulate the organic transport layer correctly without using heavy
doping in simulating the organic transport layer. The modeling results show
that the most crucial factor in the hysteresis behavior is the built-in
electric field of the perovskite. The non-linear hysteresis curves are
demonstrated under different scan rates, and the mechanism of the hysteresis
behavior is explained. The findings reveal why the change in hysteresis degree
with scan rate is Gaussian shaped rather than monotonic. Additionally, other
factors contributing to the degree of hysteresis are determined to be the
degree of degradation in the perovskite material, the quality of the perovskite
crystal, and the materials of the transport layer, which corresponds to the
total ion density, carrier lifetime of perovskite, and the dielectric constant
of the transport layer, respectively. Finally, it was found that the dielectric
constant of the transport layer is a key factor affecting hysteresis in
perovskite solar cells; a lower dielectric constant corresponds to a higher
electric field of the transport layer. Hence, if the electric field of the
perovskite material is small, the degree of hysteresis is small and vice versa.
