铁电偶极子电场优化敏化太阳能电池电子输运性能机理研究

Because there is no internal electric field in sensitized solar cells, interface charge recombination competes with forward electron transfer. In preliminary studies, the applicant found that by using a LiNbO3 crystal as a substrate of photoanode, polarization-generated electric field greatly increased short-circuit current and fill factor, which in turn played a positive role on improvement of photoelectric conversion performance in sensitized solar cells. It was speculated that the polarization-generated electric field can efficiently help photo-induced electron-hole separation and shorten electron transfer path in the photoanode film. In this project, we plan to introduce LiNbO3 nanocrystallines into photoanode in order to create an electric field inside the film based on optimization of composite films. Photoconductive atomic force microscopy and time-resolved electrostatic force microscopy are employed to measure and analysis local transient photoelectron behavior and I-V curves and their relation to the film local microstructure, and intensity modulated photocurrent/photovoltage spectroscopy and electrochemical impedance spectroscopy are used to obtain dynamic parameters that reflect electron transfer and recombination in sensitized solar cells. Combined with simulation, it is able to clarify the influence of intensity and direction of electric field of ferroelectric dipoles on electron transport properties and its work mechanism, and thus to make a scientific assessment of ferroelectric materials for improvement of photoelectric conversion performance in sensitized solar cells.

敏化太阳能电池内部不存在内建电场，其界面电荷复合与正向电子传输呈竞争趋势。申请者前期发现：用LiNbO3作光阳极衬底，其极化产生的电场能够明显增大短路光电流、提高填充因子，对改善敏化太阳能电池光电转换性能起到积极作用。经分析推测：作用在光阳极的电场能够帮助有效分离光生电子-空穴，缩短电子在薄膜中的传输路径。本项目拟在光阳极中引入LiNbO3纳米晶，通过优化制备复合薄膜，创建和强化薄膜内部电场；运用光导原子力显微镜和瞬间分辨静电力显微镜观测分析光阳极原位瞬态电子行为和I-V特性，探索其与薄膜微观结构之间的关系；运用强度调制光电流（压）谱和电化学阻抗谱等技术获得反映敏化太阳能电池内部电子传输和复合的动力学参数；结合模拟计算, 明确铁电偶极子电场强度和方向对电子传输性能的影响规律，阐明偶极子电场改善电子输运性能的机理，对铁电材料改善敏化太阳能电池光电转换性能的作用地位进行科学评估。

敏化太阳能电池内部不存在内建电场，其界面电荷复合与正向电子传输呈竞争趋势。铁电体晶格中的正负电荷中心不重合而产生电偶极距，在居里点附近电偶极子自发地沿着晶轴进行取向排列形成自发极化。自发极化在极化矢量两端的表面形成高密度极化束缚电荷。处于电荷非平衡系统中的铁电体表面的电荷补偿作用小，能够产生较大的退极化场。本项目利用铁电退极化场在敏化太阳能电池光电极薄膜中创建内建电场，改善电子输运性能。在水热法可控合成TiO2, LiNbO3, BaTiO3纳米晶颗粒的基础上，实现了优化制备TiO2/铁电纳米晶复合薄膜。采用TiO2/铁电纳米晶单层复合薄膜和TiO2/铁电-TiO2层状结构复合薄膜为光电极材料，开展了复合薄膜微结构及其电流-电压特性的研究。构建了TiO2/ITO/铁电的结构体系，分别以相反极化方向的LiNbO3单晶片做光电极薄膜衬底，以及不同极化强度的LiNbO3和BaTiO3单晶片做衬底，研究并揭示了铁电极化方向和强度对光电极薄膜中电子传输与复合的影响机制。结果显示：TiO2/BaTiO3纳米晶复合薄膜颗粒分布均匀, 添加9 wt%乙基纤维素制成的薄膜无裂缝且表面平整，染料吸附性能好；高温烧结后复合薄膜中的铁电纳米晶的铁电性能保持完好；铁电极化方向指向光电极薄膜时形成的内建电场有利于光生电子正向传输，铁电极化强度越大光电极薄膜中电子的传输性能越好；TiO2/ITO/LiNbO3结构体系的光电转换效率提升了31.5%， TiO2/BaTiO3（1.0 wt%）单层复合薄膜电池的光电转换效率提升了11%，两者均归因于光电流（Jsc）和填充因子（FF）的提高。通过建立结构模型和界面电荷/电势分布解析实验结果，阐明了铁电极化产生的内建电场改善电子输运性能的作用机制，对铁电材料改善敏化太阳能电池光电转换性能的作用地位进行了科学评估。