基于液晶剥离石墨构筑柔性透明导电薄膜及其光伏器件

Flexible transparent conductors are an extremely important component of modern electronic technology, and are generally fabricated from an inorganic conductive layer and an organic flexible substrate. They, however, have not been commercially used in the flexible electrics due to low transparence, electrical conductivity and interface stability in thin layers. The present project attempts to find an alternative way to solve this problem using a combined strategy. We try to prepare highly conductive materials based on graphene sheets. In the present work, graphene will be obtained by liquid-phase exfoliation of graphite in the isotropic liquid state of thermotropic liquid crystals upon heating up to over clear point, at which liquid crystals change into a clear, transparent liquid with low viscosity. At this point, graphite is easily exfoliated to give stable dispersions of mono- and few-layer graphene sheets due to the fact that the inter-graphene layers in graphite are intercalated by liquid molecules under microwave treatment followed by ultrasonication. However, liquid crystals will be recovered at low temperature to give an increased viscosity. The newly exfoliated graphene sheets are well stabilized and separated by the viscous resistance and steric hindrance arising from the local molecular alignment of liquid crystals. As a result, liquid crystals assisted exfoliation of graphite will produce high quality, stale dispersion, large area, defect-free graphene sheets. Moreover, the flexible substrate of PET will be modified by two methods using imidazolium ionic liquids to improve the interface adhesion with graphene layer through their π-π and/or cation-π stacking interactions. One involves that imidazolium molecules are chemically grafted to PET sheets, and another is performed by electrospinning the solution of PET and imidazolium-based poly(ionic liquid)s to afford the composite nanofiber films, which can be further used as the filtration membrane and flexible substrate. Graphene suspensions are filtered under vacuum to produce flexible transparent conducting films, which are expected to possess highly electrical conductivity while maintaining good optical transparency and flexibility. Several fundamental questions will be systematically addressed, including the optimal processing and molecular mechanism of exfoliating graphite in liquid crystals, and the structure-dependent properties of electrospinning nanofibers as well as the intrinsic relationship between electrical conductivity, optical transparency and thickness of graphene films made by vacuum filtration. Finally, polymer photovoltaic cells and dye-sensitized solar cells will be constructed by using graphene films as the working electrode and the counter electrode, respectively. The proposed project will create a new methodology of producing high quality, defect-free graphene, and fabricating highly conductive, transparent and flexible conductors for photovoltaic applications.

针对当前柔性透明导电薄膜面临电导率、透光率和稳定性偏低等问题，本项目拟结合液晶剥离石墨和功能化柔性衬底探求一条解决途径。利用液晶升温至清亮点形成低粘度液体，借助微波、超声"外场"作用，结合液晶与石墨之间的"内场"共轭作用，内外合力促进分子插层剥离石墨得到石墨烯；而降温至粘度增加的液晶态时，石墨烯片之间因受粘滞阻力和分子取向的空间限制而稳定分散。通过溶液法加工，分别以咪唑分子接枝PET基片、咪唑盐聚离子液体/PET电纺纤维为柔性衬底，构筑界面结合力强、抗挠曲性好的石墨烯透明导电薄膜，并以其为电极构建光伏电池。本项目将系统研究液晶分子结构、石墨剥离条件与石墨烯形态之间的相互关系；揭示纺丝条件、溶液参数与纳米纤维结构之间的调控规律；调节石墨烯溶液参数以调控薄膜电阻和透光率；阐明光伏电池的能量转换机制。本项目的实施将为制备高质量石墨烯、优化构筑透明导电薄膜和高性能光伏器件提供新的方法学和理论依据。

本项目以液相剥离石墨制备高质量石墨烯为核心，继而系统研究石墨烯及其纳米复合材料的微观结构与器件性能。其工作主要包括：(1)结合液晶相转变和咪唑盐分子结构特征，建立了分子结构、剥离条件与石墨烯参数之间的相互关系，揭示了石墨烯/离子液晶复合体系的流变学特征和离子液晶高效剥离石墨、高浓度稳定石墨烯的新机理，继而将实验室方法向工业化推进，发明了低成本、规模化、绿色高效制备石墨烯的新技术；(2)以电纺咪唑盐基聚离子液体/PET纳米复合纤维为柔性衬底构筑石墨烯透明导电薄膜，研究了石墨烯参数、薄膜表面电阻、透光率的相互关系及其光伏器件性能；(3)借助光还原诱导组装、流场定向和静电相互作用，构造了具有仿生层状纳米结构的自支撑石墨烯基复合薄膜；通过室温催化原位生长、离位复合和原位聚合等方法，可控制备了石墨烯/MnO2、石墨烯/Co3O4、石墨烯/聚离子液体和石墨烯/聚苯胺等纳米复合材料；(4)深入研究石墨烯基复合材料的电化学储能特性，揭示了石墨烯提高电极比容量、倍率性能和循环稳定性能的内在机理和调控规律；(5)以难自聚、易共聚和双功能分子修饰石墨烯，继而原位聚合以提高石墨烯在基体中的分散性和界面结合力，拓展了石墨烯在高性能聚合物材料领域的应用；(6)结合原位光聚合、力场诱导和液晶定向作用，制备了光响应快、可逆致动、形变量和输出力可控的石墨烯/液晶弹性体取向复合纤维，开创性结合石墨烯的光热转换效应和液晶弹性体的热机械响应特征，为构建远程化、集成化、智能化的微机械驱动与传感器件提供了新材料和新论据。本项目的实施将为加快石墨烯产业创新发展、带动材料产业升级换代、助推传统产业改造提质和支撑新兴产业培育壮大奠定理论和实践基础。