表面等离激元增强应变BiFeO3外延薄膜异常光伏效应研究

Anomalous photovoltaic effect of BiFeO3 is an intriguing phenomenon cosely associated with its peroidic stripe-like ferroelectric domains, which is electrically switchable and can yield open-circuit voltage far above the bandgap.This effect bodes well for applications in photovoltaic areas and opens up the perspective of combining optical, electric, mechanical and magnetic functionalities in future generations of wireless devices. However, due to the rather narrow absorption spectrum and the low quantum efficiency of BiFeO3 system, the total energy conversion efficiency turns out to be very limited which restricts its possible applications. Here we will focus on the above problems and try to find out ways to enhance the anomalous photovoltaic effect of BiFeO3. On one hand, we plan to design and fabricate strained BiFeO3 epitaxial thin films. By imposing compressive biaxial-stress on the films the bandgap is reduced and therefore the absorption is extended. On the other hand，we propose to confine the absorption in the vicinity of the surface by using plasmonic light-trapping structures on purpose of increasing the internal quantum efficiency. The width of the domain walls are increased near the surface, which allows for better charge blocking across the wall and reduces the carrier recombination rate. Here we choose Ag/Au core-shell nanoparticle arrays derived from micellar method to construct the plasmonic light-trapping layer, since the optical properties are sensitive to the relative size of the core and shell layers, and have the advantage of broad-band enhancement of the total absorption. This research is helpful for gaining better insight into the microscopic mechanism of surface plasmon-enhanced anomalous photovoltaic effect of strained BiFeO3 and can provide a feasible way to commercialize it.

铁酸铋的异常光伏效应是与其周期性条状畴密切相关的物理现象，具有可由外场开关、开路电压远大于带隙等独特性质，在光伏及未来光、电、力、弹多功能无源器件领域具有巨大的应用前景。然而，由于铁酸铋光谱响应范围较窄及内量子效率较低，造成光电转换效率不理想，严重制约了其可能应用。本项目将着眼于上述问题展开，探寻增强异常光伏效应的可行途径。首先，设计、制备应变铁酸铋外延薄膜，利用面内双轴压应力减小其带隙，从而拓宽其光谱吸收范围。其次，通过金属纳米颗粒表面等离激元陷光结构将光吸收限制在铁酸铋表面附近，充分利用表面处畴壁展宽使电荷阻挡作用得以增强的特点，有效抑制载流子复合，提高体系内量子效率。较单金属纳米颗粒而言，本项目利用胶束法制备的Ag/Au芯-壳结构纳米颗粒具有易于精确调控，可实现广谱吸收增强的优点。基于项目研究，探明表面等离激元增强应变铁酸铋薄膜异常光伏效应的微观机理，为其实用化奠定理论和实验基础。

铁酸铋光伏效应在新型光伏器件及未来光、电、力、弹多功能无源器件领域具有巨大的应用前景。然而，由于铁酸铋光谱响应范围较窄及内量子效率较低，造成光电转换效率不理想，严重制约了其可能应用。立足于解决该问题，我们开展了如下研究：.1) 优化BiFeO3薄膜的异质外延生长工艺，并基于此研究了BiFeO3外延薄膜结构随应力和厚度的演化。我们发现，BiFeO3表现出晶畴旋转这一新型的应力弛豫路径，与经典的位错弛豫机理不同。此外，我们阐明了BiFeO3外延薄膜在应力作用下的不同于传统认识的相变路径，丰富了对BiFeO3相图的认识。.2）我们通过两种途径，即衬底错切和各向异性衬底，成功制备了周期条状畴结构。特别是，我们在PMN-PT（001）表面得到的鳞片状有序畴结构，该结构此前在BiFeO3体系中尚未见报道。.3）借助于面内双轴应力，我们有效降低了BiFeO3的带隙并使其光吸收拓展至红光区域。结果显示，Cr掺杂同样是调节带隙、增强BiFeO3光吸收的有力途径。.4）利用反胶束法，制得了间距、大小可控的金属纳米颗粒有序阵列；探索了利用反浸润法制备金属纳米颗粒的可行性；通过金属纳米颗粒尺寸、周围介质环境及核-壳构型对其光学特性进行了调控。.5）分别制备了平面型及垂直型BiFeO3光伏器件构型。将等离激元陷光层与器件集成，通过金属纳米颗粒的近场增强及光散射效应，器件性能获得了大幅的提升。.6）在对称性失配的衬底上得到了亚稳的四方相BiFeO3，并将之与半导体平台成功集成于一体，为高效BiFeO3光伏器件的开发提供了可能性。我们将金属纳米颗粒陷光这一研究思路成功拓展至其它光伏器件，并取得了显著的效果。. 通过以上工作，我们发展了一条增强光伏器件性能的可行方法，加深了对于BiFeO3结构、相调控、光电性质和畴形成及演化等一些列问题的认识。通过课题资助，我们在国际权威期刊发表文章12篇，申请并获授权国家发明专利2项。