Femtosecond Microscopy of Charge Transport in Perovskite Thin Films

非技术描述：半导体中的电荷传输是决定太阳能电池和发光二极管等设备效率的重要过程。钙钛矿材料是一种新型的半导体材料，由于其成本极低，原料丰富，易于制备，是一种非常有前途的硅替代物。在过去的5年里，钙钛矿型太阳能电池的效率接近20%，超过了包括有机和非晶硅太阳能电池在内的其他技术。一个主要的研究挑战是对电荷输运性质和薄膜结构之间的关系的不完全理解，这阻碍了材料设计的合理方法。这项研究通过直接成像电荷在空间和时间上的移动来揭示钙钛矿薄膜中电荷传输的限制因素，从而解决了这一挑战，从而实现了实现有效电荷传输的设计原则。该项目的跨学科性质为培训K-12、本科生和研究生在纳米技术和可再生能源研究的前沿获得经验提供了一个完美的平台。技术描述：钙钛矿型薄膜在下一代太阳能电池中具有很高的应用前景。揭示与器件效率相关的电荷传输控制机制的一个主要困难在于这些钙钛矿型薄膜的复杂而不均匀的形貌。为了设计最优的器件，需要全面地了解电荷载流子动力学和输运是如何受到形貌的影响的。应对这一挑战需要能够以同时的空间和时间分辨率直接绘制依赖于形态的动力学和传输的实验工具。在这个项目中，飞秒瞬时吸收显微镜首次提供了钙钛矿薄膜中电荷在空间和时间以及跨晶界传输的测量。以~200fs的时间分辨率和~50 nm的空间精度对光激发后的电荷布居和动力学进行了成像。为了了解结晶度、微区大小和晶界等形貌对输运的影响，瞬时吸收显微镜与原子力显微镜和X射线散射测量相关联。本研究揭示了电荷输运与形貌之间的关系，为高效器件的设计提供了合理的依据。

Non-technical description:Charge transport in semiconductors is an important process that determines efficiency for devices such as solar cells and light emitting diodes. Perovskite materials, a new class of semiconductors, are very promising alternatives to silicon because of their extreme low cost and ease of fabrication with abundant starting materials. In the past 5 years, perovskite solar cells have demonstrated efficiency approaching 20%, surpassing other technologies including organic and amorphous silicon solar cells. A major research challenge is an incomplete understanding of the relationship between charge transport properties and film structure, which prevents a rational approach in material design. This research addresses this challenge by unraveling limiting factors for charge transport in perovskite thin films by directly imaging how charges move in space and in time to enable design principles for achieving efficient charge transport. The interdisciplinary nature of the project provides a perfect platform for training K-12, undergraduate, and graduate students to gain experience at the frontiers of nanotechnology and renewable energy research. Technical description:Perovskite thin films are highly promising for next generation solar cell applications. A major difficulty in unraveling mechanisms controlling charge transport relevant for device efficiency lies in the complex and heterogeneous morphology of these perovskite thin films. A comprehensive understanding of how charge carrier dynamics and transport are affected by morphology is required for the design of optimal devices. Addressing this challenge requires experimental tools that are capable of mapping morphology-dependent dynamics and transport directly with simultaneous spatial and temporal resolutions. In this project, femtosecond transient absorption microscopy provides first-of-a-kind measurements of charge transport in space and time and across grain boundaries in perovskite thin films. Charge populations and dynamics following photoexcitation are imaged with simultaneous ~200 fs temporal resolution and ~50 nm spatial precision. To gain understanding of how morphology such as crystallinity, domain size, and grain boundary affect transport, transient absorption microscopy is correlated with atomic force microscopy and X-Ray scattering measurements. This research unravels the relationship between charge transport and morphology to provide rational design principles for efficient devices.