Collaborative Research: Ultrafast Carrier Dynamics in Semiconductor Nanocrystal Solar Cells

主要研究者：巴克斯特，杰森/默里，克里斯托弗提案编号：1333649/1335821机构：德雷克塞尔大学/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（NC）或量子点的紧密堆积阵列是相互作用的量子限制材料中光致电荷和能量转移的基础研究的理想系统。NC的材料、直径和布置可用于调节NC间耦合以利用单个NC的性质和固体的长程效应。 NC超晶格的光学、电学和热学性质可能会在光电子学、光子学和热电学等应用领域带来变革性的改进，本研究的主要目标是：（1）理解半导体超晶格中超快载流子的产生、分离、复合和输运现象;以及（2）控制这些基本的电子物理过程以改善太阳能电池性能。具体而言，我们将研究薄膜的CdSe，CdTe，和Cu2ZnSnS4（CZTS）NC。CdSe和CdTe NC是优秀的模型系统，因为它们的合成和光学性质是很好的理解，使基本的超快研究的载体动力学在玻璃阵列和有序超晶格的一个单一的单分散NC物种，以及二元NC超晶格。CZTS NC为由无毒、地球丰富的元素制成的高效光致发光材料提供了一个令人兴奋的新方向。 PI将改进单分散CZTS纳米碳的合成，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。太赫兹光谱是电子材料的理想的非接触式探针，因为太赫兹频率范围（0.1 - 3THz）包含半导体中典型的载流子散射率。THz光谱在区分激子和自由载流子以及在亚皮秒至纳秒时间尺度上测量其动态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。泵浦-探测TRTS和TA是研究自然时间和能量尺度上光激发载流子的界面电荷转移、复合和NC间输运动力学的理想技术，这一工作将促进我们对NC和NC超晶格中超快激子和自由载流子动力学物理现象的理解。 具体研究将包括：（1）确定NC超晶格中的电荷传输机制，例如通过扩展态或通过活化跳跃;（2）测量单个单分散NC物质的超晶格中的NC间耦合、界面电荷传输和长程电荷传输的动力学;（3）确定动力学和传输机制对NC尺寸、封端配体、NC间间距和长程有序的依赖性;（4）理解二元NC超晶格中的电荷分离和传输;（5）将良好的候选材料应用于太阳能电池，以证明精心设计的NC架构带来的效率提高。这项工作将解决维持量子限制NC电子物理学的挑战，同时也使设备所需的长距离电荷传输。PI巴克斯特？在超快光谱和太阳能电池和PI默里的专业知识？我们在纳米碳和超晶格合成方面的专业知识使我们的团队能够很好地完成这项工作。我们对纳米碳超晶格中的界面电荷转移、复合和纳米碳间输运等基本物理过程的理解可以应用于制造高效率的纳米碳太阳能电池。由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的可用性将改变美国的能源组合。该项目将教育和培训两名博士。学生和多名本科生。此外，PI巴克斯特正在为两所大学的学生开发新的"太阳能电池基础"和基于实验室的"能源应用纳米制造"课程。外展将扩大到K-12学生的PI？继续参与NanoDay@Penn，Drexel的费城材料日，并通过NSF RET和大学计划指导当地高中教师。 这些计划对代表性不足的群体特别有益，因为他们的目标是来自费城学区的学生和教师，该学区的学生超过80%是少数民族。

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.