High quality CH₃NH₃PbI₃ perovskite films without residual PbI₂ are fabricated from the Lewis adduct of PbI₂·xDMF through molecular exchange. The photovoltaic performances of the perovskite solar cells are thus improved significantly.

In situ observations of the domain evolution and the polarization rotation under bipolar cyclic and static electric fields are carried out for [001]-oriented $68\text{Pb}\left({\text{Mg}}_{1/3}{\text{Nb}}_{2/3}\right){\text{O}}_3\text{−}32{\text{PbTiO}}_3$ (PMN-32PT) single crystal which is located at the morphotropic phase boundary (MPB). The results demonstrate that after being poled in [001] direction, the original rhombohedral $\left(R\right)$ phase is transformed into a monoclinic $M_C$ phase with $Pm$ space group. When the single crystal is subjected to bipolar cyclic electric field, distinct domain-boundary structures are revealed which are unlikely to be detected in static electric loadings. The existence of $R$ phase is also detected during the polarization reversal. Under static electric field, polarization rotation from $M_C$ to $R$ occurs and, as the field increases, phase transition from $M_C$ to tetragonal $\left(T\right)$ phase takes place. In the vicinity of regions where $M_C\text{−}T$ phase transition takes place, polarization reversal of $M_C$ phase is observed, which is attributed to the local stress field from the strain compatibility between the transitioned area and the nontransitioned area. With the removal of the field, $T$ and $M_C$ phases retain while $R$ phase reverses to $M_C$ phase. The results demonstrate that the energy differences among $M_C$, $R$, and $T$ phases are rather slim. Accordingly, the multiphase coexistence and polarization rotation under both cyclic and static electric loadings might be responsible for the ultrahigh piezoelectric response of the [001]-oriented rhombohedral $\text{Pb}\left({\text{Mg}}_{1/3}{\text{Nb}}_{2/3}\right){\text{O}}_3{\text{-PbTiO}}_3$ single crystals near the MPB.