The frontier in solar energy research now lies in learning how to integrate functional entities across multiple length scales to create optimal devices. Advancing the field requires transformative experimental tools that probe energy and charge transfer processes from the nano to the meso lengthscales. To address this challenge, we have been developing ultrafast microscopy as a new means to image multi-scale energy transport across both length and time scales, coupling simultaneous high spatial, structural, and temporal resolutions. In my talk, I will focus on our recent progress on visualization of exciton and charge transport in solar cell materials.
With simultaneous femtosecond temporal resolution and nanoscale spatial precision, we have recently revealed a singlet-mediated triplet transport mechanism in certain singlet fission materials . Such new triplet exciton transport mechanism leads to favorable long-range triplet exciton diffusion on the picosecond and nanosecond timescales for solar cell applications.
We have also directly visualized hot-carrier migration in methylammonium lead iodide (CH3NH3PbI3) thin films using ultrafast microscopy approaches, demonstrating three distinct transport regimes . Quasiballistic transport was observed to correlate with excess kinetic energy, resulting in up to 230 nanometers transport distance that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~ 600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot-carrier devices based on hybrid perovskites.
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