超高时空分辨研究钙钛矿基太阳能电池材料中的微纳结构、界面工程及转换效率调控方法

This proposal is to investigate one of the key scientific challenges: the structure and interfaces in optoelectronics and devices in the subfield of perovskite solar cells, addressing photoelectric conversion materials for energy applications, as one of the critical challenges outlined in 2014 Natural Science Foundation research guidelines. More specifically, we propose to carry out systematic investigation on the design, fabrication, and characterization of interfaces and nanostructures and their impacts on the photoelectric conversion, in combination with our unique expertise of in-situ high time (picosecond) and high spatial (sub-nanometer) resolution spectrum and microscopic characterization. The objectives of the proposed research are twofold: (a) achieving a better fundamental understanding of the relationship between the interface chemistry and nanostructures and the photoelectric power conversion and (b) developing and establishing the in-situ picosecond and atomic resolution characterization technology. It is anticipated that the proposed research are going to accomplish the following: (1) understanding the molecular mechanism of exceptionally high optical density of and efficient transport for both electrons and holes in light absorption perovskites through in-situ characterization by means of a combination of pico-second time resolved cathodoluminescence and other techniques, (2) understanding the mechanism and impacts of morphology, chemical composition, defect density, and thickness of the interface layer on the transport, trapping/detrapping, reflection, and recombination of charge carriers based on in-situ characterization by combining atomic resolution microscopy and nanosecond/nanometer spectroscopy, and (3) design, synthesis and fabrication of new types of perovskite materials with higher optical density and broader absorption spectrum, better electron and hole transport materials with less recombination, and electrodes with better transparency and less interface resistance. The proposed research is anticipated to advance the perovskite solar cells in three aspects: better fundamental understanding in molecular level and atomic resolution that would assist the chemists to better design and synthesize new materials, establishment of unique in-situ high spatial and time resolution characterization techniques, and synthesis of new materials for better solar cell devices.

本项目紧密围绕自然科学基金2014年度重大研究计划 “面向能源的光电转换材料”项目指南 “钙钛矿型太阳电池领域” 中的核心科学问题“光电材料与器件中的结构和表面界面”关于新型钙钛矿材料中表面界面结构、微纳结构对光-电/电-光转换性能的影响规律，建立和发展界面结构及聚集态结构的原位、实时表征方法，分析实现高效稳定的光伏以及与能源利用相关的光电器件新原理和新结构,努力实现预期目标：1）通过皮秒时间分辨光谱结合其它研究技术在分子轨道能级水平钙钛矿吸光材料极高的吸光系数和双极电荷传输能力的微观机理；2）通过原子尺度空间分辨技术结合原位电镜+纳米光谱研究技术揭示界面结构、界面成份、界面厚度和缺陷密度对载流子的反射、输运路径、复合几率、界面能耗等的影响规律与调控途径；3）优化设计合成出吸光性更强、光谱范围更宽的新型钙钛矿吸光层、性能更优的电子/空穴传输接触层、透明顶电极和低接触电阻的金属底电极材料。

有机无机杂化的CH3NH3PbI3钙钛矿材料，由于其合适的禁带宽度、超强的光吸收能力、优良的双极型载流子输运性质成为新一代光伏器件的极佳材料，光电转换效率在短短几年时间已超过20%。但其自身遇水易水解和光照下的不稳定性使得器件的稳定性无法长久维持，成为阻碍该种材料发展的瓶颈所在。本项目将长链分子PEG引入到钙钛矿材料中，形成的三维网络可有力的支撑CH3NH3PbI3晶粒，显著改善了材料的成膜质量、大面积覆盖度和可重复性。构成的钙钛矿太阳能电池器件的光电转换效率比不含PEG的器件有了显著提高（从原来的8%提高到16%），而且器件在潮湿环境中稳定性也得到大幅提升，在70%的相对湿度下可稳定工作300小时。不仅如此，由于PEG分子的吸湿性和其与钙钛矿材料之间很强的分子间作用，使钙钛矿材料薄膜和光伏器件均展现出自修复功能：在被水汽破坏后，1分钟内能够迅速恢复至原来的状态和效率。使钙钛矿电池对水破坏性和潮湿环境的耐受度大大增强，极大提高了电池在湿度环境下的稳定性，解决了钙钛矿电池害怕水汽和电池效率在潮湿环境下会迅速减退的应用难题，为今后钙钛矿电池商业化提供了一条新思路，具有重要的科学意义和应用价值。通过温度，光强，电场三场共同调制的集成测试系统，对CH3NH3PbI3薄膜形成的横向结构进行了变温（17-295 K）和不同光强（0-20 mW/cm2）下的恒电流测试。通过系统和定量分析，得到了CH3NH3PbI3在不同光强下的离子迁移的活化能数据。随着光强的增强，活化能降低了五倍(0.82 to 0.15 eV)。这强有力的证明了离子迁移在光照下得到了显著增强，也很好的揭示了钙钛矿电池在光照下稳定性变差的根本原因。