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Two-dimensional organic–inorganic hybrid perovskites (2DHPs) consist of alternating anionic metal-halide and cationic organic layers. They have widely tunable structural and optical properties. We review the role of the organic cation in defining the structural and optical properties of 2DHPs through the example of lead iodide 2DHPs. Even though excitons reside in the metal-halide layers, the organic and inorganic frameworks cannot be separated—they must be considered as a single unit to fully understand the photophysics of 2DHPs. We correlate cation-induced distortion and disorder in the inorganic lattice with the resulting optical properties. We also discuss the role of the cation in creating and altering the discrete excitonic structure that appears at cryogenic temperatures in some 2DHPs, including the cation-dependent presence of hot-exciton photoluminescence. We conclude our review with an outlook for 2DHPs, highlighting existing gaps in fundamental knowledge as well as potential future applications. [ABSTRACT FROM AUTHOR]

Quantum and dielectric confinement effects in Ruddlesden-Popper 2D hybrid perovskites create excitons with a binding energy exceeding 150 meV. We exploit the large exciton binding energy to study exciton and carrier dynamics as well as electron–phonon coupling (EPC) in hybrid perovskites using absorption and photoluminescence (PL) spectroscopies. At temperatures <75 K, we resolve splitting of the excitonic absorption and PL into multiple regularly spaced resonances every 40–46 meV, consistent with EPC to phonons located on the organic cation. We also resolve resonances with a 14 meV spacing, in accord with coupling to phonons with mixed organic and inorganic character. These assignments are supported by density-functional theory calculations. Hot exciton PL and time-resolved PL measurements show that vibrational relaxation occurs on a picosecond time scale competitive with that for PL. At temperatures >75 K, excitonic absorption and PL exhibit homogeneous broadening. While absorption remains homogeneous, PL becomes inhomogeneous at temperatures <75K, which we speculate is caused by the formation and subsequent dynamics of a polaronic exciton. [ABSTRACT FROM AUTHOR]