基于原子力显微术的钙钛矿光伏薄膜微区光电特性研究

Perovskite based solar cell has been considered as one of the most promising photovoltaic technologies due to its high conversion efficiency, simple preparation process, and potential low production cost. However, the understanding of some fundamental questions on perovskite photovoltaic materials including microscopic I-V characteristics, ionic migration, and grain interior/boundary characteristics, is far from satisfactory. What is the relationship between the microstructure, ionic migration of perovskite films and the I-V hysteresis of the devices? How do the photoelectric characteristics near the grain interior/boundary region effect the charge transport properties? All those questions need to be further answered. Using Atomic Force Microscope conductive probe techniques, this project will characterize the local conductivity, variations in surface potential, work functions and field induced property change of perovskite photovoltaic materials at the microscopic levels, and explain how the charge transport layers, element doping, crystallographic properties effect the microscopic photoelectric properties of perovskite films. This project will also directly correlate the local morphology with local electronic properties, show the electric potential and the local I-V characteristics near grain boundaries compared with that within grain interior, reveal the role of the grain boundaries in perovskite photovoltaic materials, and achieve high performance through rational modulation. The proposed work could provides important scientific significance for understanding the fundamental device physics and improving the photovoltaic performance.

基于钙钛矿结构的光伏电池因具有高效率、制备过程简单，潜在的低成本等优点已被认为是一种极具应用前景的光伏技术。然而，目前对钙钛矿光伏材料微区伏安特性、离子迁移、晶粒晶界特性等基础问题的研究还不够充分。钙钛矿薄膜的微观结构、离子迁移跟电池的I-V迟滞存在什么样的对应关系，晶粒晶界微区光电响应特性如何影响器件电荷传输性能等问题都是需要进一步回答的。本项目提出以导电探针原子力显微术为手段，从微观尺度上对钙钛矿材料的微区导电性、功函数、电势差以及场致性能变化等进行表征，解释电荷传输层、元素掺杂以及晶粒特性如何影响钙钛矿薄膜的微区光电性能；将钙钛矿薄膜的微观形貌与电学性能直接地联系起来，分析晶界和晶体表面电势差以及电流电压曲线变化，揭示钙钛矿光伏材料中晶界的影响与作用机制，实现对钙钛矿光伏材料的调控。本项目的研究，将对深入认识钙钛矿光伏器件的物理性质和进一步改善电池光电性能具有重要的科学意义。

铅卤素钙钛矿材料由于具有非常良好的电荷传输特性和光电转换特性，是近几年光伏和发光研究领域的热点。越来越多的研究表明，不同钙钛矿的微区光电特性呈现显著的差异，通过对钙钛矿材料的界面修饰、表界面钝化等方式可以消除钙钛矿材料的微观缺陷。本项目利用扫描探针手段开展了钙钛矿在微观尺度上的光电特性及其光伏特性的研究。针对平面异质结钙钛矿薄膜电池结构，本项目分别采用了几种电子传输材料和缓冲层对电池界面进行了修饰，获得了界面层对钙钛矿电池器件性能的影响规律；利用扫描探针研究了Li元素和Co元素掺杂对空穴传输层性能的微区导电性及表面电势的影响；探索了各工艺参数（包含退火气氛、后处理等）对薄膜光电性能的影响。另外，本项目在微观尺度上分析了MACl对薄膜的电学特性的影响，特别是在薄膜晶粒晶界处的差异，解释了其中的物理过程；扫描开尔文探针显微镜（SKPM）结果发现底层电荷传输材料的导电类型会显著改变钙钛矿薄膜的表面电势，这一结果证实了钙钛矿薄膜的弱掺杂属性。本项目研究了钙钛矿薄膜的热稳定性与表面光电压之间的联系，总结出表面光电压的变化能一定程度反映薄膜的热稳定性，并得出溴离子（Br -），甲基氯化胺（MACl）和铯离子（Cs +）对钙钛矿薄膜的热稳定性具有一定提升作用。