Abstract
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. Here we present ultrafast time-resolved photoluminescence measurements on formamidinum 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. 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.
Highlights
I n the last 10 years, solar cells using hybrid metal halide perovskites (ABX3) for light absorption have shown continuing increases in power conversion efficiency (PCE)
Several possible causes of this phenomenon have been suggested,[13,17,18] and greater understanding of how the presence of tin affects the lattice structure and optoelectronic properties is likely to lead to further improvements in device performance in the future. Another photovoltaic device concept offering the potential for higher power conversion efficiency is hot charge-carrier extraction, which theoretically offers up to 66% PCE for a single-junction cell.[19]
We further present an experimental study of charge-carrier cooling in formamidinium (FA) tin triiodide, FASnI3, prepared with the addition of SnF2 to moderate doping levels
Summary
Letter involves large excess energies, and so these materials are a natural choice for an investigation of charge-carrier cooling. Spectral broadening is accounted for to avoid errors at low carrier temperatures; the assumption of a constant density of states which leads to increasing overestimation of temperatures for hotter distributions is replaced by an expression appropriate for direct transitions between parabolic bands, and the dependence on choice of spectral region to fit is eliminated The adoption of this full line shape model offers a path to more accurate and readily comparable results in future studies of carrier cooling, so that the true potential for hot-carrier solar cells can be accurately assessed. Experimental details, film characterization (SEM images, thickness measurement, and XRD spectra), absorbance spectrum, further discussion of full-spectrum model for carrier temperature determination, details of fitting procedure, additional PL decay curves, and further details of polarization anisotropy measurements including error analysis (PDF)
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