Femtosecond Microscopy of Charge Transport in Perovskite Thin Films

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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.