Speaker
Description
Herein, polycrystalline of cesium (Cs) doped in formamidinium lead triiodide (HC(NH$_2$)$_{2}$PbI$_{3}$ or FAPbI$_{3}$) films were successfully prepared by spin-casting from solutions. Changes in structural, optical and morphological properties of the obtained films upon varied Cs concentrations were systematically investigated. A phase stability and the grain sizes of the films are very strongly dependent on the amounts of Cs dopants. XRD results reveal that a phase transition from an unstable $\delta$-phase to the stable $\alpha$-phase of the perovskite films with an increase in the grain size up to ca. 1 $\mu m$ was clearly observed when the film was doped with 0.1$\%$ at Cs. A decrease in the lattice parameters of the film due to the substitution of Cs$^{+}$ ions into FA$^{+}$ sites in the FAPbI$_3$ structure was quantified. An optical band gap of the films exhibits a blue shift from 1.52 eV for an undoped FAPbI$_3$ film to 1.56 eV for the film doped with 0.1$\%$ at due to a quantum-size effect. Effects of Cs doping in the FAPbI$_3$ films on the charge separation and charge transport behaviors as probed by surface photovoltage (SPV) spectroscopy will be presented and discussed.
