Strain-induced structural phase transition, electric polarization and unusual electric properties in photovoltaic materials CsMI 3 (M = Pb, Sn).

The structural phase transition, ferroelectric polarization, and electric properties have been investigated for photovoltaic films CsMI 3 (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 3 and CsSnI 3 films are almost identical, except critical transition strains varying slightly. The epitaxial tensile strains induce two ferroelectric phases Pmc 2 1 , and Pmn 2 1 , while the compressive strains drive two paraelectric phases P 2 1 2 1 2 1 , P 2 1 2 1 2. The larger compressive strain enhances the ferroelectric instability in these two films, eventually rendering them another ferroelectric state Pc . Whether CsPbI 3 or CsSnI 3 , the total polarization of Pmn 2 1 phase comes from the main contribution of B-position cations (Pb or Sn), whereas, for Pmc 2 1 phase, the main contributor is the I ion. Moreover, the epitaxial strain effects on antiferrodistortive vector, polarization and band gap of CsMI 3 (M = Pb, Sn) are further discussed. Unusual electronic properties under epitaxial strains are also revealed and interpreted.