Charge-Carrier Cooling and Polarization Memory Loss in Formamidinium Tin Triiodide
Kimberley Savill a, Matthew Klug a, Rebecca Milot a b, Henry Snaith a, Laura Herz a
a University of Oxford, Department of Physics, Clarendon Laboratory, UK, Parks Road, United Kingdom
b Department of Physics, University of Warwick, Coventry, CV47AL, United Kingdom
Oral, Kimberley Savill, presentation 007
Publication date: 25th November 2019

Reports of slow charge-carrier cooling in hybrid metal halide perovskites have prompted hopes of achieving higher photovoltaic cell voltages through hot-carrier extraction. However, observations of long-lived hot charge carriers even at low photoexcitation densities and an orders-of-magnitude spread in reported cooling times have been challenging to explain.

In this work we present ultrafast time-resolved photoluminescence measurements on formamidinium tin triiodide, showing fast initial cooling over tens of picoseconds and demonstrating that a perceived secondary regime of slower cooling instead derives from electronic relaxation, state-filling, and recombination in the presence of energetic disorder. We identify limitations of some widely used approaches to determine charge-carrier temperature and make use of an improved model which accounts for the full photoluminescence line shape. The adoption of this model offers a path to more accurate and readily comparable determination of charge-carrier temperatures across perovskite compositions, to assess the true potential for hot-carrier solar cells. Further, we do not find any persistent polarization anisotropy in FASnI3 within 270 fs after excitation, indicating that excited carriers rapidly lose both polarization memory and excess energy through interactions with the perovskite lattice.

We acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC), UK. K. Savill thanks the Rhodes Trust for financial support through a Rhodes Scholarship. L. Herz thanks the Humboldt Foundation for support through a Friedrich-Wilhelm-Bessel Research Award.

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