基于电脉冲法的铁电薄膜光伏效应调控机制研究

Ferroelectrics, which should have higher conversion efficiency theoretically due to its abnormal photovoltaic effect, has become a new focus of attention. However, the conversion efficiency is low because of its narrow spectrum response range, poor intrinsic conductivity and low internal quantum efficiency. In our previous research it is found that the conversion efficiency can be effectively tuned by a simple approach of applying proper electric pulse, yet this physical mechanism is not clear at present. For this purpose, the regulatory mechanism of the photovoltaic effect in ferroelectric films based on electric pulse is taken as the mainly research purpose in this project. BiFeO3 films with high quality and with different microstructures (domain, oxygen vacancies and interface) are prepared by controlling preparation conditions and by choosing metal electrode with different work functions. The laws of photovoltaic effect as a function of microstructures and electric pulse parameter are found according to measure and analysis the photovoltaic effects of BiFeO3 films before and after applying electric pulses. The laws of microstructure as a function of electric pulse parameter are found according to measure and analysis the variation of microstructures before and after applying electric pulses, to probe the produce and separate process of photo induced carriers in the films combing with the laws of photovoltaic effect as a function of microstructures and electric pulse parameter, to reveal the physical rules and mechanisms of photovoltaic effects, and establish theoretical and technical basis for enhancing the conversion efficiency effectively in ferroelectrics.

铁电材料由于具有反常的光伏特性，理论上应有较高的光电转换效率而成为关注的新焦点，但是其光谱响应范围窄、导电性差和内量子效率低导致光电转换效率比较低。申请者在前期的研究中发现，采用施加电脉冲这一简便的方式可以有效调控铁电材料的光伏特性，然而其物理机制还不清楚。为此，本项目以基于电脉冲法的铁电薄膜光伏效应调控机制为研究对象，通过控制制备条件以及选择不同功函数的金属电极制备出具有不同微结构的高质量的铁酸铋铁电薄膜；通过研究施加脉冲前后上述铁酸铋薄膜的光伏特性，找出光伏特性随微结构以及光伏特性随电脉冲参数的变化规律；通过对施加电脉冲前后薄膜的微结构进行表征，找出微结构随电脉冲参数的变化规律，结合光伏特性随微结构和电脉冲参数的变化规律，探索光生载流子在薄膜中的产生、分离及输运过程，揭示电脉冲法调控铁电薄膜光伏特性的机制，为有效提高铁电材料光电转换效率奠定理论和技术基础。

铁电材料由于具有反常的光生伏特效应而受到广泛关注，但是其光电转换效率还有待进一步提升。除了可以通过降低禁带宽度来提高其光伏效率的因素外，我们还发现通过对铁电材料施加电脉冲的方法在一定程度上可以提高其光伏效应，当时其物理机制不是很清楚。这是因为在施加电脉冲的过程中，氧空位的分布、电畴的类型、极化方向、界面势垒的大小、退极化场的大小等因素都会发生变化，而这些因素对铁电光伏效应都具有明显的作用。究竟这些因素如何影响光伏效应呢？这是必须弄清楚的问题。本项目主要做了如下几个方面的工作：（1）通过改变退火工艺、组成控制铁酸铋薄膜的电畴结构、光学特性和铁电性，从而实现了宏观光伏特性的调控，找出了铁电性与光伏特性的关系，进一步明确了窄带隙和大极化强度是铁电材料获得较佳光伏特性的重要条件，这为改善铁电材料光伏特性提供了依据。（2）掌握了铁酸铋薄膜电畴结构的控制方法，对电畴结构与铁酸铋薄膜光伏效应之间的关系进行了研究，找出了铁酸铋薄膜光伏效应增强的电畴类型，为通过电畴类型调控铁电薄膜光伏效应奠定了基础。（3）研究了氧空位的分布对光伏效应的影响，发现氧空位是影响界面势垒和光生载流子迁移率的重要因素。（4）采用电脉冲对底电极进行作用，探究电场作用下底电极中氧空位的分布情况，发现底电极中的氧空位会在电场作用下发生迁移，不但间接证明了铁电薄膜中电场对氧空位的影响，也表明电场作用下底电极与铁电薄膜之间的界面势垒可能发生变化，为通过界面调控铁电光伏效应提供了依据。（5）除了铁电性、电畴、界面势垒外，退极化场的分布也是影响铁电薄膜光伏特性的另一重要因素，通过改变电极的结构、获得发散退极化场的铁电薄膜，可以改变光生载流子在退极化场下的运动过程，从而提高铁电薄膜中光剩载流子的分离效果，这为增强铁电材料光伏特性提供了新的思路。