Predictive Design of Nanocrystal Photovoltaic Materials Based on the Phonon Bottleneck Effect

摘要本研究的目的是利用纳米晶体中的声子瓶颈效应来了解和减少太阳能电池材料的热化损失，从而提高能量转换效率。由于太阳光谱的宽频，能量高于带隙的光子可以在比晶格高得多的温度下产生热电子。通常情况下，这些热电子通过电子-声子散射过程迅速将其多余的势能传递给晶格，将多余的能量转化为热量，导致太阳能转换效率降低。在纳米晶体中，连续的能带变成离散的能级，间隔可以被设计成比单个声子的能量更大，使电子通过声子的弛豫成为一个缓慢的过程。这种“声子瓶颈效应”可以显著降低电子-声子弛豫率，提高太阳能电池效率。然而，目前对这种现象的理解非常有限-实验数据往往不一致，理论模型只是定性的，这阻碍了最大限度地发挥声子瓶颈效应的最佳纳米晶体的预测设计。智力优势：在这项研究中，pi将整合理论，模拟，合成和表征，以最大限度地减少纳米晶体太阳能材料中的热电子弛豫。将开发一种非绝热分子动力学方法来模拟声子辅助的热电子弛豫速率，并将用于确定最慢热电子弛豫的最佳尺寸，形状和表面末端。基于数值结果，pi期望对纳米材料的原子结构如何影响其电子-声子耦合有一个深刻的理解。通过精确的尺寸和形状控制，可以合成具有最佳电子-声子耦合的计算纳米结构。然后，这些材料将使用飞秒激光来表征其减慢的弛豫速率。基于这些优化的量子点的太阳能电池将被制造和测试，并且它们的效率将与它们的实体相比较。计算、合成和表征相结合将使纳米晶体的优化实现声子瓶颈效应和更高的太阳能电池效率。更广泛的影响：这项研究解决了美国面临的重大能源挑战之一。该项目是pi努力将基础物理学纳入能源传输和转换的综合研究教育工作的一部分。从这个项目中获得的新知识将大大丰富pi教授的课程。私人学院一直在积极招募代表性不足的群体参与他们的研究项目。该团队将通过讲习班、研讨会和示范项目，广泛参与K-12和当地社区的能源教育和推广活动。pi还将通过nanoHUB和thermalHUB与传热和纳米技术研究界开展外展活动

0933559RuanSummaryThe objective of the proposed research is to understand and reduce thermalization loss in solar cell materials by using the phonon bottleneck effect in nanocrystals, and therefore to increase the energy conversion efficiency. Due to the broadband of solar spectrum, photons with energy higher than the bandgap can generate hot electrons at a temperature much higher than the lattice. Normally these hot electrons rapidly pass their excess potential energy to the lattice through electron-phonon scattering processes, losing their excess energy to heat and causing lower solar energy conversion efficiency. In nanocrystals the continuous bands become discrete energy levels and the spacing can be engineered to be larger than the energy of a single phonon, making the electron relaxation through phonons a slow process. This "phonon bottleneck effect" can lead to significantly reduced electron-phonon relaxation rates and enhanced solar cell efficiency. However, the current understanding of this phenomenon is very limited - the experimental data are often inconsistent, and the theoretical models are only qualitative, preventing the predictive design of optimum nanocrystals that maximize the phonon bottleneck effect. Intellectual Merits: In this study, the PIs will integrate theory, simulation, synthesis, and characterizations to minimize the hot electron relaxation in nanocrystal solar materials. A non-adiabatic molecular dynamics method will be developed to simulate the phonon-assisted hot electron relaxation rates, and will be used to determine the optimum size, shape, and surface terminations that give the slowest hot electron relaxation. Based on the numerical results the PIs anticipate to gain a profound understanding of how atomic structures of nanomaterials affect their electron-phonon coupling. The computed nanostructures with optimum electron-phonon coupling will be synthesized with precise size and shape control. These materials will then be characterized using femtosecond lasers for the slowed relaxation rates. Solar cells based on these optimized quantum dots will be fabricated and tested and their efficiencies will be compared with their bulk counterpart. The combined computation, synthesis, and characterization will allow optimization of the nanocrystals to achieve the phonon bottleneck effect and higher solar cell efficiency. Broader impact: The research addresses one of the grand energy challenges for the nation. The project is part of the PIs' efforts to include fundamental physics into an integrated research-education effort in energy transport and conversion. The new knowledge acquired from this project will significantly enrich the courses taught by the PIs. The PIs have been actively recruiting underrepresented groups in their research programs. The team will extensively engage in energy education and outreach activities for K-12 and local community through workshops, seminars, and demonstration projects. The PIs will also engage in the outreach activities with the heat transfer and nanotechnology research communities via nanoHUB and thermalHUB