First principles study on polarization and piezoelectric properties of group substitution regulated lead-free organic perovskite ferroelectrics

Abstract

Organic ferroelectrics are desirable for the applications in the field of wearable electronics due to their eco-friendly process-ability, mechanical flexibility, low processing temperatures, and lightweight. In this work, we use five organic groups as substitution for organic cation and study the effects of organic cations on the structural stability, electronic structure, mechanical properties and spontaneous polarization of metal-free perovskite <i>A</i>-NH₄-(PF₆)₃ (<i>A</i> = MDABCO, CNDABCO, ODABCO, NODABCO, SHDABCO) through first-principles calculations. Firstly, the stabilities of the five materials are calculated by molecular dynamics simulations, and the energy values of all systems are negative and stable after 500 fs, which demonstrates the stabilities of the five materials at 300 K. The electronic structure calculation shows that the organic perovskite materials have wide band gap with a value of about 7.05 eV. The valence band maximum (VBM) and Cconduction band minimum (CBM) are occupied by different elements, which is conductive to the separation of electrons and holes. We find that organic cations have an important contribution to the spontaneous polarization of materials, with a contribution rate over 50%. The presence of hydrogen atoms in the substituting groups (MDABCO, ODABCO) enhances the hydrogen bond interaction between the organic cations and <inline-formula><tex-math id="Z-20240616143151">\begin{document}${\rm PF}_6^- $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20240385_Z-20240616143151.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20240385_Z-20240616143151.png"/></alternatives></inline-formula> 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 observe large piezoelectric strain components, the calculated value of <i>d</i>₃₃ is 36.5 pC/N for CNDABCO-NH₄-(PF₆)₃, 32.3 pC/N for SHNDABCO-NH₄-(PF₆)₃, which is larger than the known value of <i>d</i>₃₃ of MDABCO-NH₄-I₃(14pC/N). The calculated value of <i>d</i>₁₄ 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 <i>d</i>₁₄ is found to be closely related to the large value of elastic compliance tensor <i>s</i>₄₄. The analysis of Young’s modulus and bulk’s modulus shows that these organic perovskite materials have good ductility. These results indicate that these organic materials are excellent candidates for future environmentally friendly piezoelectric materials.