钙钛矿界面缺陷电子性质的第一性原理研究

发布时间：2018-01-05 14:30

本文关键词：钙钛矿界面缺陷电子性质的第一性原理研究　出处：《中国科学院大学(中国科学院物理研究所)》2017年博士论文　论文类型：学位论文

【摘要】：化学通式为ABX_3的钙钛矿材料具有许多非常有趣的性质,因此在很多领域具有广泛的应用。钙钛矿材料界面处的电子性质往往是决定钙钛矿在实际应用中的性能和效率的关键因素。另外,钙钛矿材料中通常含有一定量的X位空位,空位的存在影响了钙钛矿材料器件的性能,但有时候也可以用于调控新奇物理现象的出现。我们分别研究了 MAPbI3钙钛矿太阳能电池中碘空位的界面电子性质,以及钙钛矿铁电PbTiO_3/SrTiO_3异质结中氧空位诱导的界面磁电耦合效应。有机—无机杂化的MAPbI_3钙钛矿太阳能电池已经成为了光伏器件的前沿研究领域,在全世界范围内引起了高度的关注。在高性能的MAPbI_3太阳能电池中,MAPbI_3料主要是以薄膜的形式存在,决定太阳能电池性能的关键的电荷传输过程以及其他的电子动力学过程均发生在电池的界面处。同时,碘空位VI普遍被认为是引起扫描光电流—光电压时严重的回滞现象的主要原因,而回滞现象的存在限制了钙钛矿太阳能电池的工业化。然而,目前还没有关于V:在钙钛矿太阳能电池的表面和界面的性质的系统研究。我们利用第一性原理的方法和非绝热电子动力学模拟,研究了 V:在MAPbI_3薄膜以及MAPbI_3/TiO_2异质结的不同位置处的结构和电子性质。我们发现VI位于表面和界面时比位于体相中更稳定,这也与实验上观测到的V:倾向于在晶界处聚集相吻合。当外加扫描电场时,VI在MAPbI_3中迁移并导致了回滞现象。VI位于Pb-I层时和位于MA-I层时的性质不一样:V:位于MA-I层时更稳定,而VI位于Pb-I层时的缺陷态更局域且为弱共价键态。V:的存在加快了MAPbI_3太阳能电池的电子注入率,但也加快了电子—空穴复合率,所以总体上来说会减小电池的效率。但是我们发现与随机分布在块体中的VI相比,界面处的VI对电池的效率影响最小,使得MAPbI_3太阳能电池能拥有如此高的效率。接着,我们利用第一性原理计算,预测了在PbTiO_3/SrTiO_3异质结界面上出现由二维电子气(2DEG)调制的磁电耦合效应。由SrTiO_3中本征的氧空位提供的自由电子在铁电PbTiO_3的极化电场的作用下,被拽到了异质结的界面处,并在界面处聚集,形成2DEG。这些电子被界面的Ti原子所捕获,当界面的电荷密度达到一个很小的临界值σ～15.5 μC/cm2时,界面Ti原子将体现出铁磁性,同时当界面电荷密度达到σ～80μC/cm2时,铁磁居里温度TC能达到室温,此时PbTiO_3/SrTiO_3异质结如果用作磁电材料的话,将可以在室温下进行电控磁的操作。由界面2DEG调制的磁电耦合效应很强,因此可以很方便地实现电控磁和磁控电。另外,由于PbTiO_3/SrTiO_3异质结很廉价、很容易生长、可控性强,它在自旋器件和信息存储方面具有巨大的潜在应用价值。
[Abstract]:Perovskite materials with chemical formula ABX_3 have many interesting properties. The electronic properties at the interface of perovskite materials are often the key factors to determine the performance and efficiency of perovskite in practical applications. There is usually a certain amount of X vacancy in perovskite materials, which affects the performance of perovskite devices. But sometimes it can be used to regulate the emergence of novel physical phenomena. We have studied the interface electronic properties of iodine vacancies in MAPbI3 perovskite solar cells. The interface magnetoelectric coupling effect induced by oxygen vacancy in perovskite ferroelectric PbTiO_3/SrTiO_3 heterojunction. The organic-inorganic hybrid MAPbI_3 perovskite solar cell has become light. The frontier research field of volt devices. In high performance MAPbI_3 solar cells, MAPbI3 is mainly in the form of thin films. The key charge transfer processes and other electronic dynamics processes that determine the performance of solar cells occur at the interface of the cell. Iodine vacancy VI is generally considered to be the main cause of severe hysteresis in scanning photocurrent-photovoltage, which limits the industrialization of perovskite solar cells. There is no systematic study on the properties of V: surfaces and interfaces of perovskite solar cells. We use first-principles methods and non-adiabatic electronic dynamics simulations. The study of V:. The structure and electronic properties of MAPbI_3 films and MAPbI_3/TiO_2 heterostructures at different locations. We found that VI is more stable at the surface and interface than in the bulk phase. It also coincides with the observed V: it tends to gather at grain boundaries, when an external scanning electric field is applied. VI migrates in MAPbI_3 and results in hysteresis. VI is more stable when located in the Pb-I layer than in the MA-I layer. The defect state of VI in Pb-I layer is more localized and weak covalent bond state. V: increases the electron injection rate of MAPbI_3 solar cells, but also accelerates the electron-hole recombination rate. So the overall efficiency of the battery will be reduced, but we found that compared with the random distribution of VI in the block, VI at the interface has the least impact on the efficiency of the cell. So that MAPbI_3 solar cells can be so efficient. Then, we use first-principles calculations. It is predicted that 2DEG will occur at the interface of PbTiO_3/SrTiO_3 heterojunction. Modulated magnetoelectric coupling effect. Free electrons supplied by oxygen vacancies in SrTiO_3 are subjected to the polarizing electric field of ferroelectric PbTiO_3. They are dragged to the interface of the heterojunction and gathered at the interface to form a 2DEG. These electrons are trapped by the Ti atoms at the interface. When the interface charge density reaches a very small critical value 蟽 = 15.5 渭 C / cm ~ 2, the interface Ti atom will exhibit ferromagnetism, and when the interface charge density reaches 蟽 ~ (80 渭 C / cm ~ 2), the interface Ti atom will exhibit ferromagnetism. The ferromagnetic Curie temperature TC can reach room temperature, and the PbTiO_3/SrTiO_3 heterojunction can be used as magnetoelectric material. The magnetoelectric coupling effect modulated by 2DEG interface is very strong, so it is easy to realize electrically controlled magnetic and magnetically controlled electricity. Because PbTiO_3/SrTiO_3 heterojunction is cheap, easy to grow and controllable, it has great potential applications in spin devices and information storage.
【学位授予单位】：中国科学院大学(中国科学院物理研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O469

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