Bi5Ti3FeO15/CuO异质结薄膜的制备、光伏特性与载流子输运机理研究

Enviroment friendly lead-free ferroelectric films have petential applications in the areas of photoelectric sensor and multifunctional passive devices due to polarization-dependent tunability and above-bandgap photovoltage. Aiming at the key problems of low photoelectric conversion efficiency and unclear mechanism of photovoltaic effect of the lead-free ferroelectric films, this project will explore a feasible way to solve the problems. Firstly, some series of undoped and doped (Co, Mn, La) Bi5Ti3FeO15 (BFTO) films will be fabricated on different substrates (Pt, FTO, Nb doped SrTiO3) by PLD technique. Surface energy band and diffusion effect of the interface in films/ electrodes would be measured. Microscopic structure of ferroelectric domain, macroscopic and microscopic photovoltaic effect would be investigated. The effect of ferroelectric polarization, interface barrier and domain structure on the photovoltaic properties would be systematically investigated. Based on above investigations, BFTO/CuO heterojunction multilayer films will be prepared by the same technique. For the series of fims, electronic structure will be calculated and the non-equilibrium minority carrier transport under illumination will be depicted theoretically. The impact of heterojunction interface defects on the transport of charge carriers was analyzed. The influence of conduction and valence band misfit in the hetero-interface on the transport and tunneling of charge carriers would be systematically investigated. To elucidate the origin of photovoltaic effect and improve the efficiency for the lead-free ferroelectric films, intensive study about the separation mechanism of photo-induced carriers will be implemented from two aspects of expriment and theory. The research results of the project will enrich and develop the basic theory and technology of the photovoltaic effect for the ferroelectric thin films and will also lay a solid theoretical and experimental ground for its application.

因具有可由外场调控、开路电压远大于带隙等独特性质，环境友好型无铅铁电薄膜在光电传感和多功能无源器件领域具有广阔的应用前景。针对光电转换效率低和尚未澄清的光伏效应机制等关键问题，本项目将探寻可行的解决途径。首先，在不同衬底上制备系列未掺杂及掺杂(Co、Mn和La)的Bi5Ti3FeO15 (BFTO)薄膜。表征其表面能带和薄膜-电极界面的扩散效应。研究薄膜的电畴结构,宏观、微区光伏效应，分析铁电极化、势垒和电畴结构等因素与光伏效应的关系。在此基础上，制备BFTO/CuO异质结双层膜,计算系列薄膜的电子结构，模拟光照下非平衡少数载流子的输运。分析异质结界面缺陷对载流子输运的影响，探索界面能级失配对载流子输运及隧穿的影响规律。从实验和理论两方面深入研究光生载流子的分离机制以阐明光伏效应的起源并提高效率。项目研究成果可以丰富和发展无铅铁电薄膜光伏效应的基础理论和技术，为其实用化提供理论和实验基础。

因具有可由外电场调控等独特性质，无铅铁电薄膜在光电传感和信息存储等领域有广泛的应用前景。针对光电转换效率较低和光伏效应物理机制有待澄清等问题，本项目做了系列探索，主要研究内容可概括为：(1) 脉冲激光沉积法制备铋基铁电、半导体薄膜的微结构及其光伏性能，(2) 化学溶液法制备铋基铁电、半导体薄膜及其光伏性能，(3) 铋基铁电氧化物纳米材料在染料敏化太阳能电池中的应用研究。围绕这些研究内容，本项目取得了如下代表性研究成果：(a) 采用PLD法制备BiFeO3外延薄膜，构筑Pt/ BiFeO3/ La0.7Sr0.3MnO3/PMN-PT器件，未极化时开路电压、短路电流为-0.15 V、0.395 µA。当BFO薄膜正极化后，相应地数值为-0.2 V、0.7 µA。极化反转后，开路电压和短路电流随之反向。表明BFO薄膜显示出强烈的极化态相关的光伏效应，该器件在原位动态应变调控下光电转换效率可以增加218%。此工作提供了一种通过逆压电效应提高铁电薄膜光伏效应的方法。(b) 制备CuO/Bi5Ti3FeO15(BFTO)薄膜，构筑Au/CuO/ BFTO/FTO器件，探讨了铁电极化对BFTO和CuO/BFTO异质结光伏特性的影响，BFTO和CuO/BFTO薄膜在反向极化调控状态下均实现开路电压和短路电流的方向反转。引入CuO缓冲层，大大提高了BFTO的光伏效应的同时又表现出良好的铁电性，短路电流密度从7.64 µA /cm2增加到76.20 µA/cm2。此项工作首次通过引入半导体缓冲层以提高BFTO铁电薄膜的光伏效应。(c) 制备La掺杂BFO(BLFO)薄膜，具有宏观铁电性质的BLFO薄膜展示了极化可调的光伏效应。在BLFO薄膜表面上镀一层金纳米颗粒(AuNPs)可以显著增强其光伏效应，在铁电极化和表面等离激元效应共同作用下，短路电流密度和开路电压由5.3 A/cm2、0.2 V增加到20 A/cm2、0.4 V。本工作不仅提供了一种改善铁电薄膜光伏效应的方法，还为理解铁电光伏效应提供了重要的视角。总之，本项目在动态应变和铁电极化对铋基铁电薄膜光伏效应的调控，BFTO/CuO光伏效应增强的物理机制，表面等离激元改善铁电薄膜的光伏效应和BFTO纳米材料在染料敏化太阳能电池对电极中的应用等方面均取得了有意义的研究进展。