Stochastic Charge-Transfer Excitons in 2D Metal-Halide Perovskites

2d metal-halide perovskites (MHPs) have much larger excitonic binding energies than 3d MHPs, typically around 100-500 meV. Despite the large binding energies, much larger than kT, the excitons in 2d MHPs are still able to dissociate on picosecond time scales [1]. Two mechanisms have been proposed for this, namely edge states at grain boundaries and the formation of polarons. By analyzing the exciton's linear and quadratic Stark effect as measured through temperature-dependent electroabsorption, we propose a third mechanism. We find substantial evidence that excitons are photoexcited into spatially-separated electron and hole states with large dipole moments, resembling charge-transfer excitons, which are created through dynamic disorder from thermal fluctuations in the polar lattice. We find many similarities with the static disorder seen in some 2d MHP materials. Molecular dynamics simulations [2] predict dipole moments which reasonably agree with our measured values, changing on the proper time scale, with energies that are comparable to the excitonic binding energies and thus could be sufficient to ionize the electron-hole system. We therefore propose that these "stochastic charge transfer excitons" are at least partly responsible for the fast excitonic dissociation in 2d MHPs.

[1] Sun et al., Am. Chem. Soc. 2021, 143 (45), 19128.

[2] Kang and Wang, J. Phys. Chem. Lett. 2017, 8 (16), 3875−3880.