npj: 杂化钙钛矿光伏特性—铁电畴的调控

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带电的铁电畴壁是令很多科学家着迷的电拓扑缺陷，常具有非凡的特性。近年来，杂化钙钛矿因具有较高的光伏效率而被广泛关注，但其带电畴壁可分离铁电畴，其中是否蕴含着全新的现象，是否可用来影响材料的性质，尚未有研究探索。

阿肯色大学的Yurong Yang及其同事，采用第一性原理计算，全面研究了带电畴壁分离的铁电畴的畴宽，对材料性能的影响。他们分析了以甲基铵碘化铅杂化钙钛矿构成的光伏材料，研究了畴宽对其中具有带电畴壁的畴性质的影响。结果发现，这样的畴非常稳定，任何畴宽（即多达13个晶格常数）都具有相当低的畴壁能量。增加畴宽首先是将电子带隙线性地从≃1.4eV 减小到大约0 eV（从而提供了有效的带隙工程），然后体系从绝缘体过渡到金属、并在畴宽最大时保持金属特性。所有这些结果都可从以下方面得到解释：（i）沿畴壁法线的极化分量在数量上很小; （ii）内部电场与畴宽基本无关; （iii）畴壁之间的电荷转移可忽略不计。概括地说，改变杂合钙钛矿中铁电畴的宽度会影响带隙宽度，并出现沿畴壁的金属导电性，使畴壁转变为金属。他们的发现加深了人们对带电铁电畴壁的认识，并进一步扩大其应用潜力，特别是在卤化物钙钛矿光伏材料领域。

该文近期发表于npj Computational Materials 4: 75 (2018)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Tailoring properties of hybrid perovskites by domain-width engineering with charged walls 

Lan Chen,Charles Paillard,Hong Jian Zhao,Jorge Íñiguez,Yurong Yang & Laurent Bellaiche 

Charged ferroelectric domain walls are fascinatingelectrical topological defects that can exhibit unusual properties. Here, inthe search for novel phenomena, we perform and analyze first-principlescalculations to investigate the effect of domain width on properties of domainswith charged walls in the photovoltaic material consisting of methylammoniumlead iodide hybrid perovskite. We report that such domains are stable and haverather low domain wall energy for any investigated width (that is, up to 13lattice constants). Increasing the domain width first linearly decreases theelectronic band gap from ≃1.4eV to about zero (which therefore provides an efficient band-gapengineering), before the system undergoes an insulator-to-metal transition andthen remains metallic (with both the tail-to-tail and head-to-head domain wallsbeing conductive) for the largest widths. All these results can be understoodin terms of: (i) components of polarization along the normal of the domainwalls being small in magnitude; (ii) an internal electric field that isbasically independent of the domain width; and (iii) rather negligible chargetransfer between walls. These findings deepen the knowledge of chargedferroelectric domain walls and can further broaden their potential forapplications, particularly in the context of halide perovskites forphotovoltaics.

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