Dendritic structural inhomogeneities in thin Cs$_{0.2}$FA$_{0.8}$PbI$_{2.93}$Cl$_{0.07}$ layers for perovskite solar cells

In this work, the Kelvin probe microscopy (KPM) study of thin films Cs$_{0.2}$(CH(NH$_2$)$_2$)$_{0.8}$PbI$_{2.93}$Cl$_{0.07}$ (0.5 mkm) with a frontal passivation coating of Al$_2$O$_3$ (10 nm) was performed. When using liquid-phase growth methods of Cs$_{0.2}$(CH(NH$_2$)$_2$)$_{0.8}$PbI$_{2.93}$Cl$_{0.07}$, the surface morphology is characterized by the presence of shagreen and surface profile fluctuations in the range of tens of nanometers due to the presence of stresses in the crystal lattice. Dendritic structural inhomogeneities with lateral dimensions of 10 $\mu$m and occupying about 25% of the surface were detected. A comprehensive KPM analysis allowed us to determine that the composition of the dendrites corresponds to $\delta$-CsPbI$_3$. This indicates phase segregation in multicationic films with a non-passivated frontal surface. It is shown that in the absence of the upper protective layer, the initial Cs$_{0.2}$(CH(NH$_2$)$_2$)$_{0.8}$PbI$_{2.93}$Cl$_{0.07}$ film is transformed into dendritic regions of $\delta$-CsPbI$_3$ and surrounding regions of FAPbI$_{2.93}$Cl$_{0.07}$. In the presence of the front passivation layer of Al$_2$O$_3$, the Cs$_{0.2}$(CH(NH$_2$)$_2$)$_{0.8}$PbI$_{2.93}$Cl$_{0.07}$ film is stable. The photopotential of the films and dendritic inhomogeneities under illumination was studied. It was found that under illumination with diffused sunlight, the dendritic structures are charged with a negative charge and a residual potential of $U_{\mathrm{res}}$ = -100 mV arises. When the illumination is turned off, a small residual potential arises at the boundaries of the dendritic structures, which weakens with a characteristic time of 20–30 minutes.