  Layered halide perovskites are solution-processed natural heterostructures
where quantum and dielectric confinement effects down to the nanoscale strongly
influence the optical properties, leading to stabilization of bound excitons.
Achieving a detailed understanding of the exciton properties is crucial to
boost the exploitation of these materials in energy conversion and light
emission applications, with current on-going debate related to the energy order
of the four components of the most stable exciton. To provide theoretical
feedback and solve among contrasting literature reports, we perform here
ab-initio solution of the Bethe Salpeter Equation (BSE), with detailed
interpretation of the spectroscopic observables based on symmetry-analysis. We
confirm the Edark < Ein plane < Eout-of-plane fine-structure assignment, as
from recent magneto-absorption experiments. We further suggest that polar
distortions may lead to stabilization of the in-plane component and ultimately
end-up in a bright lowest exciton component. Also, we discuss the exciton
landscape over a broad energy range and clarify the exciton spin-character,
when large spin-orbit coupling is in play, to rationalize the potential of
halide perovskites as triplet sensitizers in combination with organic dyes. In
addition to contributing to the current understanding of the exciton properties
of layered halide perovskites, the work further evidence the in-depth knowledge
gained by combining advanced ab-initio simulations and group theory.
