铁电光伏器件的载流子输运过程和能量转换微观机理研究

The ferroelectric photovoltaic (FPV) effect has recently drawn attentions due to its above-bandgap voltages and increasing photocurrent output. However, the mechanism behind the FPV effect of ferroelectric materials is not very clear. In order to explain the anomalous photovoltage phenomenon, several models/theories/effects have been proposed in early years, including bulk photovoltaic effect, depolarization field model, grain boundary theory, domain wall theory, Schottky-junction effect. Based on these theories, the ultrahigh photovoltage output has gradually been associated to the potential steps at grain boundary/domain wall interfaces. However, almost all of these theories, except for the domain wall theory, are mainly focused on the photovoltage output, and the energy conversion mechanism for FPV device is not very clear up to now. So far, the electron transition from valence band to conduction band is defaulted to be the basic energy conversion process of FPV device, but this common sense is conflict with above-bandgap voltages of FPV device in essence, and we still don’t know how carriers move from one side of the device to another under the light excitation. . In order to figure out the photocurrent output process and build up a reasonable energy conversion mechanism for FPV device, the following work will be employed in our project.. 1) Considering the unreasonable boundary conditions of the primitive potential model, we plan to optimize it by combining the ANSYS simulation technique and TEM microstructure analysis technique. The potential distribution of FPV device system at different boundary conditions (open-circuit and short-circuit) will be simulated.. 2) The the FPV device will be equivalent to a microcircuit, which is composed of bulk of ferroelectric gains, grain boundary/domain wall interfaces and FE-electrode interfaces. Then the different roles of these local elements will be analyzed by black box testing of microcircuit through SPM characterization at different experiment conditions.. 3) Due to the potential/electric field distribution and the determined roles of different local elements are two of the most important part to build the framework of FPV mechanism, we will combine the results of the first two parts, and systematically analyze the carriers’ transportation process and energy conversion mechanism by further consider the important phenomena of photocurrent output.

铁电光伏效应是光伏领域一个极具发展潜力的研究方向，然而目前为止尚未形成一个系统的铁电光伏理论。现有理论通过在微观尺度上构建铁电体的电势分布，将铁电体超越禁带宽度的光伏输出归结为界面处的电势台阶。在此基础上，如何系统协调载流子在晶粒-晶界、电畴-畴界中不同的运动行为，并寻找一个与电流、电压输出过程自洽的光-电能量转换基本过程，是目前铁电光伏理论研究亟待攻克的关键问题。本课题拟通过ANSYS仿真模拟，结合系统的TEM界面微结构分析，构建铁电体数字化的电势分布图像；与此同时，采用传统电路分析方法，通过SPM测试开路、闭路、光照等条件下的微观电流-电压特性，解析晶粒-晶界、电畴-畴界在光-电转换过程中的基本功能；最终结合两部分结果，以电势分布的基础，微观要素的基本功能为框架，形成一套融合电压输出、电流输出、能量转换的铁电光伏理论，为铁电光伏器件的优化设计以及转换效率的提升奠定理论基础。

铁电体具有非对称晶体结构，其晶胞中的正负电荷重心不重合，能够在材料内诱导出内建电场。基于这一特征，可在铁电材料中建立与传统半导体光伏电池完全不同的铁电光伏/反常光伏效应，获得超禁带光伏输出，开路电压可达1000 V/cm以上；同时，由于其特殊的光电转换特征，有望挣脱半导体光伏电池效率的Shockley-Queisser极限，因此备受关注。然而，目前为止其光电转换效率仍十分微弱，究其根源在于领域内尚没有形成完善的铁电光伏理论以支持应用研究，此外相关研究还受制于有限的材料体系。本项目研究的开展，首次在领域内构建起统一的铁电体内电势分布方程，发现其zigzag形特征；基于此，本研究创造性的提出铁电体光电能量转换的界面隧穿模型，成功将现有多个体系的反常光伏效应统一到一种机制中，解释了现有理论无法解释的诸多特异现象，明确了影响器件性能的三大因素，在其指导下将BaTiO3基光伏器件的短路电流和开路电压分别提高了40倍和10倍。此外，本研究工作通过应力工程，首次在非极性BiVO4材料中发现弯电耦合效应诱导的反常光伏效应，开路电压达40 V以上，颠覆了反常光伏效应只存在于极性材料的常规认识，为半导体光伏器件设计提供新的调控维度。于此同时，提出利用界面吸收调控材料光吸收特性的思路，在铋层状结构材料中通过超晶格结构设计，将材料的光吸收边从紫外光区拓展到近红外光区，界面吸收边为0.88 eV，对推动铁电材料的光伏、光催化应用有重要价值。本项目研究有很好可拓展性，相关成果为高性能铁电光伏器件的设计奠定了坚实的理论基础，也极大的拓展了铁电光伏材料体系。同时，其价值不仅局限于铁电光伏领域，铁电体内电势方程的建立对极性材料的多功能耦合效应研究有参考价值。