Influence of the A-site cation on hysteresis and ion migration in lead-free perovskite single crystals

Lead-free halide perovskites have recently emerged as promising photoactive materials due to their nontoxicity, stability, and unique photophysical properties. However, similar to lead-based perovskites, ion migration plays a significant role in determining their device potential and needs an in-depth understanding. Here, we study the role of A-site cation size and its nature on the ion transport dynamics in the ${A}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}$ [A= methylamine (MA) and formamidinium (FA)] perovskite single crystals (SCs) using the temperature-dependent electrochemical impedance spectroscopy. The activation energy was estimated as 0.54 and 0.66 eV with a corresponding trap density of $13\ifmmode\times\else\texttimes\fi{}{10}^{9}$ and $6.7\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}3}$ for $\mathrm{M}{\mathrm{A}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}$ and $\mathrm{F}{\mathrm{A}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}\mathrm{SC}$, respectively. The higher activation energy in $\mathrm{F}{\mathrm{A}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}\mathrm{SCs}$ indicates suppressed ion migration which can be attributed to its large cation size leading to lattice enlargement and reduced rotational motion. Additionally, the less acidic and less polar character of the FA cation and strong hydrogen bond plays an important role to reduce the defect states. Furthermore, current-voltage (I-V) measurements also confirm the lower degree of hysteresis in $\mathrm{F}{\mathrm{A}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}\mathrm{SC}$ than that of $\mathrm{M}{\mathrm{A}}_{3}{\mathrm{Bi}}_{2}{\mathrm{I}}_{9}$ SC, demonstrating lower ionic conductivity in these materials. Our paper provides a fundamental understanding of the role of the A-cation effect on the ionic and electrical responses in these lead-free SCs, which will be beneficial for designing the perovskite devices to improving their performance and overall stability.