  2D 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 example 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.
