Modulation of organic groups substitution on the polarization and piezoelectric properties of lead-free organic perovskite ferroelectrics via first principles study

Organic ferroelectrics are desirable for the application in the field of wearable electronics due to their eco-friendly process-ability, mechanical flexibility, low processing temperatures, and lightweight. In this work, we used five organic groups as substitution for organic cation and studied the effects of organic cations on the structural stability, electronic structure, mechanical properties and spontaneous polarization of metal-free perovskite A-NH₄-(PF₆)₃(A=MDABCO, CNDABCO, ODABCO, NODABCO, SHDABCO) through first-principles calculations. Firstly, the stability of the five materials was calculated by molecular dynamics simulations, and the energy of all systems is negative and stable after 500 fs, which demonstrated the stability of the five materials in 300 K. The electronic structure calculation shows that the organic perovskite materials have wide band gap with the value of about 7.05 eV. The VBM(Valence Band Maximum) and CBM (Conduction Band Minimum) are occupied by different elements, which is conductive to the separation of electrons and holes. We found that organic cations have an important contribution to the spontaneous polarization of materials, with the contributing over 50%. The presence of hydrogen atoms in the substituting groups (MDABCO, ODABCO) enhances the hydrogen bond interaction between the organic cations and PF₆^-and increases the displacement of the organic cation, resulting in an increase in the contribution of the polarization of the organic cation to the total polarization. In addition, we observed large piezoelectric strain components, the calculated d₃₃ is 36.5 pC/N for CNDABCO-NH₄-(PF₆)₃, 32.3pC/N for SHNDABCO-NH₄-(PF₆)₃, which is larger than the known d₃₃ of MDABCO-NH₄-I₃(14pC/N). The calculated d₁₄ is 57.5 pC/N for ODABCO-NH₄-(PF₆)₃, 27.5 pC/N for NODABCO-NH₄-(PF₆)₃. These components are at a high level among known organic perovskite materials and comparable to many known inorganic crystals. The large value of d₁₄ is found to be closely related with the large value of elastic compliance tensor s₄₄. The analysis of Young’s modulus and bulk’s modulus found that these organic perovskite materials have good ductility. These results show that these organic materials are excellent candidates for future environmentally friendly piezoelectric materials.