Collaborative Research: Ultrafast Carrier Dynamics in Semiconductor Nanocrystal Solar Cells

项目负责人：Baxter， Jason / Murray， christopher提案编号：1333649 / 1335821机构：Drexel大学/宾夕法尼亚大学标题：合作研究：半导体纳米晶体太阳能电池中的超快载流子动力学半导体纳米晶体（NCs）或量子点的紧密排列阵列是量子限制材料相互作用中光诱导电荷和能量转移基础研究的理想系统。nc的材料、直径和排列可以用来调整nc之间的耦合，以利用单个nc的特性和固体的长期效应。NC超晶格的新兴光学、电子和热学特性可能导致包括光伏、光子学和热电学在内的应用的转型改进。本提案的主要目标是：(1)了解半导体纳米晶体超晶格中超快电荷载流子的产生、分离、重组和输运现象，以及(2)控制这些基本的光物理过程以提高太阳能电池的性能。具体来说，我们将研究CdSe， CdTe和Cu2ZnSnS4 (CZTS) NCs的薄膜。CdSe和CdTe NC是很好的模型系统，因为它们的合成和光学性质很好理解，使得在玻璃阵列和单分散NC物种的有序超晶格以及二元NC超晶格中进行载流子动力学的基本超快研究成为可能。CZTS NCs为无毒、丰富的地球元素制成的高效光伏发电提供了一个令人兴奋的新方向。pi将改进单分散CZTS NCs的合成，以实现有意义的超快光谱表征。该方法以时间分辨太赫兹光谱（TRTS）和飞秒可见/红外瞬态吸收（TA）为中心，分别探测带内和带间跃迁。太赫兹光谱是一种理想的、非接触的电子材料探针，因为太赫兹频率范围（0.1 - 3thz）符合半导体中典型的载流子散射率。太赫兹光谱学在区分激子和自由载流子以及在亚皮秒到纳秒时间尺度上测量它们的动力学方面具有独特的能力，为我们的稳态场效应晶体管（FET）测量提供了极好的补充。泵-探针TRTS和TA是研究光激发载流子在自然时间和能量尺度上的界面电荷转移、重组和nc间输运动力学的理想技术。这项工作将促进我们对NC和NC超晶格中控制超快激子和自由载流子动力学的物理现象的理解。具体的研究将包括：(1)确定NC超晶格中的电荷输运机制，例如通过扩展状态或激活跳变；(2)测量单分散纳米粒子超晶格中纳米粒子间耦合、界面电荷转移和远程电荷输运的动力学；(3)确定NC尺寸、盖盖配体、NC间距和远程顺序对动力学和传递机制的依赖；(4)理解二元NC超晶格中的电荷分离和输运；(5)将良好的候选材料整合到太阳能电池中，以证明精心设计的NC架构提高了效率。这项工作将解决维持量子限制的NC光物理的挑战，同时也实现了设备所需的长距离电荷传输。π巴克斯特?他在超快光谱学和太阳能电池方面的专业知识，以及PI Murray？他在合成纳米碳和超晶格方面的专业知识使他的团队有能力开展这项工作。对NC超晶格中界面电荷转移、重组和NC间输运等基本光物理过程的理解可以应用于制造高效的NC太阳能电池。利用地球上丰富的无毒材料制成的高效、低成本、清洁和可持续的太阳能电池，将改变美国的能源组合。本项目将培养2名博士生和多名本科生。此外，PI Baxter正在为两所大学的学生开发“太阳能电池基础知识”和基于实验室的“能源应用纳米制造”的新课程。通过pi将服务范围扩大到K-12学生。继续参加NanoDay@Penn，德雷克塞尔的费城材料日，并通过NSF RET和大学项目指导当地高中教师。这些项目对代表性不足的群体尤其有利，因为它们针对的是费城学区的学生和教师，该学区的学生中80%以上是少数族裔。

PI: Baxter, Jason / Murray, ChristopherProposal Number: 1333649 / 1335821Institution: Drexel University / University of PennsylvaniaTitle: Collaborative Research: Ultrafast Carrier Dynamics in Semiconductor Nanocrystal Solar CellsClose-packed arrays of semiconductor nanocrystals (NCs), or quantum dots, are ideal systems for fundamental investigations of photo-induced charge and energy transfer in interacting quantum-confined materials. The materials, diameters, and arrangement of the NCs can be used to tune the inter-NC coupling to exploit both the properties of the individual NCs and the long-range effects of the solid. The emergent optical, electronic, and thermal properties of NC superlattices may lead to transformational improvements in applications including photovoltaics, photonics, and thermoelectrics.The broad objectives of this proposal are (1) to understand ultrafast charge carrier generation, separation, recombination, and transport phenomena in semiconductor nanocrystal superlattices, and (2) to control these fundamental photophysical processes to improve solar cell performance. Specifically, we will investigate films of CdSe, CdTe, and Cu2ZnSnS4 (CZTS) NCs. CdSe and CdTe NCs are excellent model systems because their synthesis and optical properties are well-understood, enabling fundamental ultrafast studies of carrier dynamics in glassy arrays and ordered superlattices of a single monodisperse NC species, as well as binary NC superlattices. CZTS NCs provide an exciting new direction for high efficiency photovoltaics made from non-toxic, earth-abundant elements. The PIs will refine the synthesis of monodisperse CZTS NCs to enable meaningful ultrafast spectroscopic characterization.This approach centers on time-resolved terahertz spectroscopy (TRTS) and femtosecond visible/infrared transient absorption (TA) to probe intraband and interband transitions, respectively. THz spectroscopy is an ideal, non-contact probe of electronic materials because the THz frequency regime (0.1 - 3 THz) brackets typical carrier scattering rates in semiconductors. THz spectroscopy is unique in its abilities to distinguish between excitons and free carriers and to measure their dynamics on sub-picosecond to nanosecond time scales, providing an excellent complement to our steady-state field effect transistor (FET) measurements. Pump-probe TRTS and TA are ideal techniques to investigate the dynamics of interfacial charge transfer, recombination, and inter-NC transport of photoexcited carriers on their natural time and energy scales.This work will advance our understanding of the physical phenomena that govern ultrafast exciton and free carrier dynamics in NCs and NC superlattices. Specific studies will include: (1) Determining mechanisms of charge transport in NC superlattices, e.g. by extended states or by activated hopping; (2) Measuring dynamics of inter-NC coupling, interfacial charge transfer, and long-range charge transport in superlattices of a single monodisperse NC species; (3) Determining the dependence of dynamics and transport mechanisms on NC size, capping ligand, inter-NC spacing, and long range order; (4) Understanding charge separation and transport in binary NC superlattices; and (5) Incorporating good candidate materials into solar cells to demonstrate improvements in efficiency that result from carefully designed NC architectures. This work will address the challenge of maintaining quantum-confined NC photophysics while also enabling long range charge transport necessary for devices. PI Baxter?s expertise in ultrafast spectroscopy and solar cells and PI Murray?s expertise in synthesis of NCs and superlattices make the team well-equipped to carry out this work.The understanding of fundamental photophysical processes such as interfacial charge transfer, recombination, and inter-NC transport in NC superlattices developed here can be applied to create high-efficiency NC solar cells. Availability of efficient, low-cost, clean, and sustainable solar cells made from earth-abundant, non-toxic materials would transform the US energy portfolio. This project will result in the education and training of two Ph.D. students and multiple undergraduates. Additionally, PI Baxter is developing new courses on "Fundamentals of Solar Cells" and lab-based "Nanomanufacturing for Energy Applications" for students from both universities. Outreach will extend to K-12 students by the PIs? continued participation in NanoDay@Penn, Philly Materials Day at Drexel, and mentoring local high school teachers through NSF RET and university programs. These programs are particularly beneficial for underrepresented groups since they target students and teachers from the School District of Philadelphia, whose student body is over 80% minorities.