Unconventional Temperature Dependence of Exciton Diamagnetism in 2D Ruddlesden-Popper Lead Halide Perovskites

TL;DR

This study reveals an unconventional increase in exciton binding energy with temperature in 2D Ruddlesden-Popper lead halide perovskites, challenging previous assumptions about exciton behavior in such materials.

Contribution

It provides the first evidence of a smaller excitonic radius at higher temperatures in layered perovskites and measures the temperature dependence of exciton diamagnetic shifts up to 40 T.

Findings

Exciton binding energy increases at higher temperatures.

Diamagnetic shift coefficient decreases at room temperature.

Contrasts with behavior observed in 3D perovskites.

Abstract

Layered hybrid perovskites containing larger organic cations have demonstrated superior environmental stability, but the presence of these insulating spacers also strengthens the exciton binding energy, which contributes to reduced carrier separation. The consequences of increased binding energy on device efficiency are still not fully documented, and binding energy measurements are often conducted at cryogenic temperatures where linewidths are decreased and a series of hydrogen-like bound states can be identified, but not under ambient conditions where devices are expected to operate. In contrast to the quenching observed in 3D perovskites such as methylammonium lead iodide, where exciton binding energies are thought to decrease at higher temperatures, we present evidence for a smaller excitonic radius at higher temperatures in the $n=5$ member of butylammonium-spaced methylammonium lead iodide, (BA)$_2$(MA)$_{n-1}$Pb$_n$I$_{3n+1}$. We measured the temperature-dependent diamagnetic shift coefficient in magnetic fields up to 40\,T, which is one-third as large at room temperature as those at cryogenic temperatures. In both the ideal 2D and 3D hydrogen models, this trend would indicate that the exciton binding energy more than triples at room temperature.