Band Structure and Exciton Dynamics in Quasi-2D Dodecylammonium Halide Perovskites

Femtosecond transient absorption spectroscopy (FTAS) is an important tool to investigate the physics of halide perovskites having different dimensionality, morphology, and architectures, giving insights into the electronic and excitonic optical transitions. Here, FTAS on monolayer (n = 1) and multilayer (n = 2, 3) quasi-2D perovskites in the Ruddlesden-Popper phase: DA(2)MA(n-1)Pb(n)I(3n+1) (DAMAPI) is presented, with the dodecylammonium (DA = CH3-(CH2)(11)-NH3+) as the spacer and methylammonium (MA = CH3NH3+) as the organic cation for samples with n > 1. The measurements, performed at 77 K and room temperature using several pump energies and excitation densities, allow the observation of different absorption bleaching energies. Those energies are compared with the results of first-principles theoretical simulations based on density functional theory, the GW method, and the Bethe-Salpeter equation and assigned to transitions involving excitons with principal quantum numbers 1s and 2s. The temporal analysis of the absorption bleaching indicates the exciton-exciton annihilation as the main relaxation mechanism in the first picoseconds after excitation, while exciton radiative recombination is observed at longer time delays (>100 ps). Therefore, FTAS allows the study of the carrier dynamics and, given its high sensitivity to carrier density changes, the observation of spectral features not observable with steady-state measurements.