Strain-induced structural phase transition, electric polarization and unusual electric properties in photovoltaic materials CsMI (M = Pb, Sn).

The structural phase transition, ferroelectric polarization, and electric properties have been investigated for photovoltaic films CsMI (M = Pb, Sn) epitaxially grown along (001) direction based on the density functional theory. The calculated results indicate that the phase diagrams of two epitaxial CsPbI and CsSnI films are almost identical, except critical transition strains varying slightly. The epitaxial tensile strains induce two ferroelectric phases 2, and 2, while the compressive strains drive two paraelectric phases 222, 222. The larger compressive strain enhances the ferroelectric instability in these two films, eventually rendering them another ferroelectric state . Whether CsPbI or CsSnI, the total polarization of 2 phase comes from the main contribution of B-position cations (Pb or Sn), whereas, for 2 phase, the main contributor is the I ion. Moreover, the epitaxial strain effects on antiferrodistortive vector, polarization and band gap of CsMI (M = Pb, Sn) are further discussed. Unusual electronic properties under epitaxial strains are also revealed and interpreted.