基团替代调控无铅有机钙钛矿铁电体的极化和压电特性的第一性原理研究

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

随着可穿戴电子产品要求提升,无毒的有机钙钛矿铁电体成为潜在候选材料.本工作应用第一性原理计算系统研究了无铅有机钙钛矿A-NH_(4)-(PF_(6))_(3)(A=MDABCO,CNDABCO,ODABCO,NODABCO,SHDABCO)的电子态密度、自发极化、弹性特性和压电效应.通过分子动力学和结合能计算发现,有机钙钛矿在室温下具有稳定性且预测其在实验上易于合成.对电子态密度研究发现,A-NH4-(PF6)3的价带主要来自F元素的贡献,价带顶和导带底分别来自取代基团中的元素和N元素的贡献,因此有利于电子-空穴对的分离.依据Born稳定性判据,有机钙钛矿具有稳定的机械性质.除此之外,A位有机阳离子的取代基团可以改变材料中氢键的数量,对总铁电极化的贡献有着明显影响.最后通过压电性能计算,揭示了有机钙钛矿具有良好的压电效果,该效应源于材料引入的有机阳离子增加的材料的柔性.计算结果为后续实验提供了理论基础.

Organic ferroelectrics are desirable for the applications in the field of wearable electronics due to their ecofriendly 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 A-NH_(4)-(PF_(6))_(3)(A=MDABCO,CNDABCO,ODABCO,NODABCO,SHDABCO)through firstprinciples 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 PF_(6)^(-) 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 d_(33) is 36.5 pC/N for CNDABCO-NH_(4)-(PF_(6))3,32.3 pC/N for SHNDABCONH_(4)-(PF_(6))_(3),which is larger than the known value of d_(33) of MDABCO-NH_(4)-I_(3)(14pC/N).The calculated value of d_(14) is 57.5 pC/N for ODABCO-NH_(4)-(PF_(6))_(3),27.5 pC/N for NODABCO-NH_(4)-(PF_(6))_(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 to the large value of elastic compliance tensor s_(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.