Beyond the Shockley-Queisser Limit: Understanding and Controlling Carrier Multiplication in Carbon Nanotube PN Junctions

非技术描述：由传统材料制成的太阳能电池将太阳光转化为电能和不需要的热能的混合物。热能的比例很重要，因为阳光激发的电子将它们多余的能量转移到材料晶体的振动中。为了提高太阳能电池的性能，研究人员正在寻找一种新的材料，在这种材料中，阳光激发的电子将多余的能量传递给其他电子，而不是传递给晶格中的振动。半导体碳纳米管是新一代太阳能电池设计中很有前途的材料。碳纳米管中的电子彼此之间的相互作用非常强烈，这表明能量可以有效地从一个电子转移到另一个电子。该研究项目解决了优化光伏设备的挑战，以了解这些材料中强相互作用电子的物理特性。该项目有一个公众参与的组成部分，PI作为俄勒冈科学与工业博物馆的科学传播研究员，领导俄勒冈州立大学的物理推广团队。该项目包括为来自未被充分代表的少数民族的年轻科学家提供暑期研究经验。技术描述：在低维材料中，电荷载流子之间的库仑相互作用增强，从而增强载流子倍增现象。载流子倍增可以提高太阳能电池器件的能量转换效率，因为当光子能量超过带隙的两倍时，单个光子可以激发多个电子-空穴对。先前对零维量子点的实验表明，载流子提取是利用载流子倍增的主要障碍。这个项目的重点是在半导体碳纳米管中载流子的倍增，这是一个一维的系统，载流子可以有效地提取。实验利用单个碳纳米管pn结来揭示温度、电场和介电屏蔽对载流子增殖过程的影响。研究了半导体碳纳米管中激子解离机制、载流子倍增效率和光电流量子产率的基本限制。通过进一步加深我们对强相互作用电子系统中载流子倍增的理解，该项目支持开发超越Schockley-Queisser极限的太阳能电池设备。

Non-technical Description: Solar cells built from traditional materials convert sunlight into a mixture of electrical energy and unwanted thermal energy. The fraction of thermal energy is significant, because sunlight-excited electrons transfer some of their excess energy into vibrations in the material's crystal. To improve the performance of solar cells, researchers seek a new class of materials in which sunlight-excited electrons transfer excess energy to other electrons, rather than to vibrations in the crystal lattice. Semiconducting carbon nanotubes are a promising material for this new generation of solar cell design. The electrons in carbon nanotubes interact very strongly with each other, suggesting that energy can be efficiently transferred from one electron to another. This research project addresses the challenge of optimizing a photovoltaic device to understand the physics of strongly interacting electrons in these materials. The project has a public-engagement component, with the PI working as a Science Communication Fellow associated with the Oregon Museum of Science and Industry and leading the physics outreach team at Oregon State University. The project includes summer research experience for young scientists from under-represented minorities.Technical Description: In low-dimensional materials, Coulomb interactions between charge carriers are enhanced, thereby enhancing the phenomena of carrier multiplication. Carrier multiplication can increase the energy conversion efficiency of solar cell devices because multiple electron-hole pairs can be excited by a single photon when photon energy exceeds twice the band gap. Previous experiments on zero-dimensional quantum dots have shown that carrier extraction is a major obstacle to harnessing carrier multiplication. This project focuses on carrier multiplication in a semiconducting carbon nanotube, a one-dimensional system in which carriers can be efficiently extracted. The experiments utilize individual carbon nanotube pn junctions to reveal the effect of temperature, electric field, and dielectric screening on the carrier multiplication process. The research addresses the exciton dissociation mechanism, carrier multiplication efficiency, and the fundamental limits of photocurrent quantum yield in semiconducting carbon nanotubes. By furthering our understanding of carrier multiplication in a strongly-interacting electronic system, the project supports the development of solar cell devices that beat the Schockley-Queisser limit.