Low-intensity illumination induced relaxation and charge transport behavior of single crystal halide perovskites

We have investigated impedance, electric modulus, and dielectric spectroscopies, along with AC conductivity, on single crystals of methylammonium lead iodide (MAPI) and formamidinium lead iodide (FAPI) in the frequency range $50\phantom{\rule{0.16em}{0ex}}\mathrm{Hz}\ensuremath{\le}f\ensuremath{\le}1\phantom{\rule{0.16em}{0ex}}\mathrm{MHz}$ in the dark and under low-intensity illumination ($\ensuremath{\le}80\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{W}/{\mathrm{cm}}^{2}$). It is demonstrated that the relaxation observed in this frequency range in these single crystals can be attributed to space charge effects in the bulk of the crystals, which are caused by the finite time scale associated with charge relaxation, which can occur in this frequency range due to the large static dielectric constant and low conductivity of these solids. The relaxation was found to be faster in FAPI (with higher conductivity) compared to that in MAPI (with lower conductivity). The electron-hole pair generated by illumination enhances electronic conductivity and accelerates ionic migration by lowering the barrier; this, in turn, decreases the charge relaxation time and enhances the relaxation process. The barrier lowering inferred from the reduction in relaxation times by illumination is proposed to be associated with changes in the chemical potential attributed to carrier generation.