论文标题：Ferroic domains regulate photocurrent in single-crystalline CH3NH3PbI3 films self-grown on FTO/TiO2 substrate

期刊：npj Quantum Materials

作者：Boyuan Huang, Guoli Kong, Ehsan Nasr Esfahani, Shulin Chen, Qian Li, Junxi Yu, Ningan Xu, Ying Zhang, Shuhong Xie, Haidan Wen, Peng Gao, Jinjin Zhao & Jiangyu Li

发表时间：2018/06/22

数字识别码：10.1038/s41535-018-0104-5

原文链接：https://www.nature.com/articles/s41535-018-0104-5?utm_source=Other_website&utm_medium=Website_links&utm_content=JesGuo-Nature-npj_Quantum_Materials-Energy-China&utm_campaign=NPJ_USG_JRCN_JG_npj_Ferroic_Aug_sciencenet

最近十年，钙钛矿结构最耀眼的明星，无疑是以CH3NH3PbI3为代表的有机无机杂化钙钛矿。自2009 年日本Miyasaka 研究组首次报道以来，钙钛矿太阳能电池的转换效率，在短短十年不到的时间从约3 % 大幅度增长至22 % 以上，刺激了全球范围内激烈竞争。

究其优异光电性能背后的物理根源，材料结构-性能关系则是回答这一问题更基本、关键的基础。目前普遍接受的是CH3NH3PbI3在高温下是立方结构、室温下是四方结构。四方结构的空间群又有两种可能性：一种是属中心对称的 I4/mcm 结构；另一种是属非中心对称 I4cm 结构。遗憾的是，这两种晶格结构差别细微，非常难以分辨。CH3NH3PbI3的铁电是非也一直是国际争论的焦点。

最近黄博远等人对CH3NH3PbI3单晶畴结构进行了深入分析，他们通过极性光学显微镜、扫描电镜、AFM形貌、和PFM，都可以观察到畴结构。这些畴很可能是立方-四方相变所产生的变体所形成的结构。

其次，他们观测到：对电致正应变而言，存在高响应畴和低响应畴，如图1 所示。有意思的是，高响应畴中一阶线性效应占主导，而低响应畴中二阶非线性畴占主导。进一步分析揭示极性与非极性畴共存。如图2所示，高正应变响应的畴在合适取向下具有高的剪切应变，与极性畴匹配；低正应变响应的畴剪切应变响应低，与非极性畴匹配。

图1：高低响应畴一阶二阶应变比较。

此外，他们还发现极性畴光生电流低，非极性畴光生电流高。更有意思的是，无论是畴结构、压电响应、还是光电流，在升温降温过程中，都随相变产生明显的记忆效应，如图 3 所示。这进一步证明了光电流与畴结构之间的紧密关联。为此类体系的畴控制研究找到了夹缝中成长与扩张的理由！

图2：高低响应、剪切应变与品质因子，表明极性与非极性畴共存。

图3： AFM 形貌(上)，PFM 响应(中)，光电流(下) 相变过程中的记忆效应。

摘要：Photovoltaic conversion efficiency (PCE) of halide perovskite solar cells has risen spectacularly, yet the very crystalline structure of CH3NH3PbI3 remains ambiguous after extensive researches, and its polar nature remains hotly debated. Here we present compelling evidences that CH3NH3PbI3 crystals self-grown on FTO/TiO2 substrate consist of ferroic domains with alternating polar and nonpolar orders, in contrast to previous experimental and theoretical expectations, and polar domains possess reduced photocurrent. It is found that polar and nonpolar orders of CH3NH3PbI3 can be distinguished from their distinct lateral piezoresponse, energy dissipation, first and second harmonic electromechanical couplings, and temperature variation, even though their difference in crystalline lattice is very subtle, and they possess two-way memory effect through cubic-tetragonal phase transition. We hope these findings resolve key questions regarding polar nature of CH3NH3PbI3 and its implication on photovoltaics, reconcile contradictory data widely reported, and point a direction toward engineering ferroic domains for enhanced PCE.

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期刊介绍：

npj Quantum Materials is an online-only, open access journal, publishing original research results and reviews on broad coverage of quantum materials, their fundamental properties, fabrication and applications.

来源：科学网