  Two-dimensional (2D) perovskite materials have recently re-attracted intense
research interest for applications in photovoltaics and optoelectronics. As a
consequence of the dielectric and quantum confinement effect, they show
strongly bound and stable excitons at room temperature. In this report, the
band-edge exciton fine structure and in particular its exciton and biexciton
dynamics in high quality crystals of (PEA)2PbI4 are investigated. A comparison
of bulk and surface exciton lifetimes yields a room temperature surface
recombination velocity of 2x10^3cm/s and an intrinsic lifetime of 185ns.
Biexciton emission is evidenced at room temperature, with binding energy of
about 45meV and a lifetime of 80ps. At low temperature, exciton state splitting
is observed, which is caused by the electron-hole exchange interaction.
Transient photoluminescence resolves the low-lying dark exciton state, with a
bright/dark splitting energy estimated to be 10meV. This work contributes to
understand the complex scenario of the elementary photoexcitations in 2D
perovskites.
