半导体纳米材料的多激子产生研究

The development of new structures, materials and methods to increase the photon-to-electron conversion efficiency of low-cost solar cells has become one of the most active areas in both fundamental research and device applications. The multiple exciton generation (MEG) process of semiconductor nano-materials can make full use of the ultraviolet radiation in the solar spectrum by creating multiple electron-hole pairs upon absorption of a single photon. Thus, the researh of MEG effect has the potential to greatly increase the photon-to-electron conversion efficiency in solar cell devices and will play important roles in solving the current energy crisis and decreasing the environmental pollution. In this project, we plan to use single molecule spectroscopy and ultrafast optical techniques, including pump-probe, stimulated Raman, resonant energy transfer and integrated spectrum measurments, to systematically study the MEG process and its underlying physical mechanisms in various semiconductor nano-materals. The main research directions of this poject include: (1)Investigating novel optical techniques for the measurement of MEG effect in semiconductor nano-materials; (2)Elucidating the current controversy on MEG effect in colloidal quantum dots; (3) Demonstrating MEG effect in new material systems such as self-assembled quantum dots and single-walled carbon nanotubes; (4) Proposing practical plans for the optimization of photovoltaic devices using MEG effect of semiconductor nano-materials.

发展基于太阳能全光谱利用的低成本太阳能电池，探索大幅度提高其光电转换效率的新结构、新材料、新方法，已经成为当前最为前沿的基础和应用研究领域之一。半导体纳米材料体系中的多激子产生过程能够充分利用太阳光光谱中的紫外波段，解除一个吸收的单光子最多只能产生一个电子-空穴对的限制，从而革命性地提高太阳能电池的光电转换效率，对于解决能源危机和减少环境污染具有战略性的意义。本项目计划结合单分子光谱和超快光谱技术，通过泵浦-探测、受激拉曼、共振能量转移和稳态光谱测量等方法系统地研究各种半导体纳米材料体系中的多激子产生过程和物理机制。项目的主要研究目标包括：探索半导体纳米材料多激子产生测量的新方法，澄清胶体量子点体系中多激子产生效率的争议，展现自组装量子点和碳纳米结构等新体系中的多激子产生过程，并提出利用多激子产生效应优化光伏器件的应用方案。

单个半导体纳米结构在吸收两个以上低能光子时由于带填充效应会形成多个电子-空穴对，而这个多激子态也可能在吸收一个高能光子后通过载流子倍增效应而产生。研究半导体纳米结构的多激子产生过程对其在激光、光探测器、发光二极管、太阳电池和量子信息处理方面的应用具有极大的指导意义。在本项目中，我们首先表征了外延生长自组织量子点、胶体金属硫化物纳米晶、石墨烯纳米片和单壁碳纳米管的基本光学特性，从中筛选出胶体金属硫化物CdSe纳米晶作为适合多激子产生研究的量子受限材料体系。之后，我们首次在单个CdSe纳米晶中实现了单激子和双激子能量转移过程的测量，并利用所掌握的技术首次将该过程用于探测系综CdSe纳米晶中由于载流子倍增效应所产生的双激子工作中。接下来，我们首次实现了紫外激发下对单个CdSe纳米晶的荧光特性测量，并在单粒子的精度确认了载流子倍增效应的存在并对其双激子产生效率进行了精确评估。最后，我们首次在胶体钙钛矿纳米晶中观察到单光子发射现象并揭示了其不同于传统胶体CdSe纳米晶的新奇光学特性，从而为下一步研究该新型纳米材料体系的多激子产生过程提供了坚实的基础。基于上述工作共发表13篇项目标注文章，包扩Nature Communications 1篇、Physical Review Letters 1篇、Nano Letters 2篇、ACS Nano 3篇、Scientific Reports 2篇、Applied Physics Letters 2篇、ACS Photonics 1篇和Optics Express 1篇。