The Effect of Tin Substitution on the Excitonic Properties of Two Dimensional Metal Halide Perovskites

With growing interest in the lead-free derivatives of the metal-halide perovskites (MHP), it is imperative to fully understand the contribution of the metal cation to their desirable excitonic characteristics. Here, we explore this question by performing an in-depth spectroscopic and theoretical analysis of phenethylammonium tin iodide ((PEA)₂SnI₄), a prototypical tin based MHP, and rigorously compare it with its lead counterpart. We elaborate on the origin of multiple excitonic resonances uniquely observed in the linear absorption spectrum of (PEA)₂SnI₄ at energies about 200-300meV above the primary exciton. By performing calculations based on density functional theory and many- body perturbation theory, we suggest that the excitonic series at these higher energies are composed of electronic transitions from a lower lying valence band. Importantly, the valence band splitting is driven by the octahedral conformations that follow subtle variations in the organic-inorganic interactions within the crystal lattice. We experimentally show that the presence of the higher energy excitonic resonance results in a relatively slow nanosecond component in the formation dynamics of the primary exciton, in addition to the ultrafast phonon-driven hot carrier thermalization. While the presence of such slow relaxation channel for the excitons might be beneficial to many optoelectronic applications, our work suggests its possible control via systematic design of the organic cation. Moreover, our observations indicate that spin-orbit coupling does not play a primary role in the intricate yet crucial changes in the excitonic characteristics imparted by the tin substitution.