(K,Na)NbO3(KNN)基无铅压电材料的带隙宽度调制和光伏效应研究

ABO3 peroskite functional oxides have performed various interesting properties, especially those ferroelectric oxides based photovoltaics have gained a renaissance in research due to the ever-incereasig gloal concern for clean and sustainable energy. The coupling between photon and ferroic orderings provides us with multiple degrees of freedoms for controlling the photovoltaic effect, and this can be used to endow future solar cells with much new functionality. The tunability of the output signal of a solar cell made of ferroelectric matrials is related to mechanical, electrical and magnetic means. However, the band gaps of the matrials and Schottky barrier heights at the ferroelectric/electrode interfaces play a crucial role in enhancing the power conversion efficiency, stabilizing the ferroelectric domain and establishing highly reliable performance of the devices. This project will concern on (K,Na)NbO3 (KNN) based lead-free piezoelectrics instead of Pb contained traditional PZT ferroelectrics. At first, the modulation of the KNN band gap will be explored through site specific doping during the process of solid state solution. Further, the controlling of the gaps and photovoltaic outputs in KNN thin films will be intensively investigated, by adjusting the strain, substrate symmetry, electrode materials, device configurations, fabrication parameters etc, in order to optimize the devices performance. At last, the surfaces and interfaces study of KNN films will be carried out by means of x-ray photoelectron spectroscopy (XPS), and we try to give a systematical, direct and quantitive determination results of the Schottky barriers at the KNN interfaces with different electrodes. The effects of the magnitude and orientation of polarization, and the interface Schottky barrier heights on the ferroelectric photovoltaics will be deduced. This study is important for clarifying the mechanism of the tunablity in ferroelectric band gaps and the photovoltaics in ferroelectric thin films.The results definitely will be helpful for the experimental and theoretical research on solar cells and other new functional devices, also the photoelectron spectroscopy route will provide another possibility to investigate the polar interfaces in oxide heterojunctions.

以铁电材料为基础的光伏器件，其光、电和机械应力之间的耦合丰富了光电作用的物理图像，赋予了器件许多新颖的功能和特点，引起人们极大的研究兴趣。铁电光伏特性很大程度上决定于其带隙宽度和界面的肖特基势垒，本项目将以钙钛矿结构的(K,Na)NbO3（KNN）基无铅压电材料为研究对象，主要通过特定序位离子掺杂，以掺杂离子类型、浓度、有序性等为调制参量，结合衬底外延应力和对称性进行相结构调控，致力于在保持铁电性的基础上减小其带隙宽度，探索钙钛矿结构材料的能带调制机制；采用光电子谱（XPS）方法，对KNN与金属（氧化物）电极之间的肖特基势垒、界面反应和能带排列，进行系统研究和定量表征，以期优化铁电光伏器件界面，提升电极捕获载流子的效率，并厘清界面势垒、极化强度和方向与光伏效应之间的关系。本项目将对阐明钙钛矿结构铁电材料的带隙调制机制和推动新型光伏器件的实验与理论研究，提供可靠的实验依据。

(K,Na)NbO3 (KNN)基无铅铁电材料在现代电子元件中的应用非常广泛，并且长期以来一直被认为有望替代铅基材料，引起了人们极大的研究兴趣。然而，到目前为止，由于K，Na的挥发性，高质量的KNN基无铅薄膜难以制备，且其铁电性和温度稳定性都存在很大的不足。本项目以(K,Na)NbO3-LiTaO3-CaZrO3（KNNLT-CZ）无铅体系为研究对象，制备了高质量的外延薄膜及纳米复合薄膜，利用外延应力对其进行了物性调控和复杂环境下的物性研究，发现：（1）外延应力能够明显的调控薄膜的正交-四方相变，压应变能够提升薄膜的剩余极化，压电系数和居里温度；（2）生长在La0.07Ba0.93SnO3上的KNNLT-CZ外延薄膜具有很高的光学透过率，居里温度超过420 ˚C且在室温至250˚C具有很好的铁电性；（3）生长在La0.7Sr0.3MnO3上的薄膜形成独特的纳米复合结构，有效提升了垂直方向的应力作用，使得薄膜余极化(2Pr)高达64.91 μC/cm2，且在-196 ˚C至300 ˚C范围内具有出色的铁电热稳定性，其居里温度高达400 ˚C；（4）导电氧化物La0.67Ca0.33MnO3上生长的KNNLT-CZ薄膜具有良好的抗辐照性能，在经过50 Mrad(Si)剂量的伽玛射线辐照之后，仅表现出10％的介电常数的降低；（5）以导电氧化物为上下电极的KNNLT-CZ薄膜具有优异的抗中子辐照特性。本项目对阐明KNN基无铅压电薄膜的物性调控机制，揭示体系的自发纳米结构的形成机理提供了实验结果，为无铅压电厚膜的制备及新型压电器件的实验与理论研究奠定了基础，也为以此类材料制备的器件在复杂环境中的应用提供了实验证据。受益于本项目资助，共发表SCI论文9篇，培养博士研究生1人。