Developing Nonlinear Time-of-Flight Spectroscopies for Studies of Transport Mechanisms in Layered Perovskite Solar Cells

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With support from the Chemical Measurement and Imaging (CMI) Program in the Division of Chemistry, Professor Andrew Moran of the University of North Carolina, Chapel Hill is developing spectroscopic tools and time-of-flight techniques to measure electron movement in perovskites. Single pulse, time-of-flight techniques have been used to measure electron velocities in two-dimensional materials such as solar cells for many decades; however, the time required to collect the signal is traditionally limited by the electronics employed for signal detection. Professor Moran and his group will construct an instrument to reduce this time by detecting the photocurrent induced with multiple laser pulses. Both drift and diffusion of photoexcited species within perovskite solar cells will then be simultaneously measured. Their discoveries could lead to a better understanding of electron transport mechanisms in layered perovskite materials. Students involved in this project will develop technical skills involving lasers, optics, and electronics. Multidimensional action spectroscopies capable of disentangling parallel energy and charge funneling processes in photovoltaic cells will be used to measure spontaneous processes such as the photocurrent and fluorescence produced by a device. Direct detection of such spontaneous processes clarifies the relevance of a measurement to the function of a device and minimizes the ambiguities inherent in conventional spectroscopic approaches. To fully characterize the parallel processes occurring in the active layers of photovoltaic cells, Professor Moran and his group will develop a transient grating photocurrent spectrometer in which drift and diffusion are simultaneously measured in orthogonal directions within the active layers of the devices. In addition, separate detection channels will be incorporated to distinguish transport processes involving excitons (fluorescence) and free charge carriers (photocurrent). Drift velocity dispersion will be characterized with time-of-flight measurements in which one pulse initiates drift and the color of a second pulse is tuned to probe photoexcited carriers at various depths in the active layer.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.