Revisiting the Optical Response of Two-Dimensional Perovskites: Beyond Excitons

Two-dimensional (2D) Ruddlesden-Popper metal halide perovskites exhibit an unusually rich optical response, characterized by multiple sidebands, broad quasi-plateaus, and pronounced thickness-dependent spectral features. In this Review, we reassess their optical properties by examining the interplay between electronic structure, exciton fine structure, exciton-phonon coupling, and photonic effects. We show that the exceptionally large excitonic oscillator strength and high refractive index naturally give rise to excitonic stop bands and interference effects that can dominate reflection, transmission, and absorption spectra, even in nominally free-standing crystals. We further discuss the complexity of the photoluminescence response, including evidence for exciton-polaron formation, self-trapping, and defect-assisted recombination. By placing electronic, vibrational, and photonic effects on equal footing, this Review provides a unified framework for interpreting optical spectra in 2D perovskites.