  We report the observation of exciton formation from the cooled band-edge
carriers in mixed two-dimensional hybrid organic-inorganic perovskites using
femtosecond transient absorption spectroscopy. By monitoring the changes of
bleach signal upon excitations with various photon energy, we are able to
extract the values of exciton binding energy and the occupancies of carriers of
free and bound states for each two-dimensional phase. We also confirm the
existence of Mahan exciton when injected carrier density is above the Mott
criterion.
