Lead halide perovskite nanocrystals
(NCs) have superior photoluminescence
(PL) properties, such as high PL quantum yields and wide PL wavelength
tunability, for optoelectronic applications. Here, we report the PL
spectra of single formamidinium lead halide perovskite FAPbX3 (X = Br, I) NCs examined by single-dot spectroscopy at low temperature.
We found four PL peaks in the low-energy region below the strong exciton
PL peak that originate from two longitudinal-optical (LO) phonon replicas
of the exciton PL, biexcitons, and charged excitons (trions). The
binding energies of the biexcitons and trions become larger as the
NCs decrease in size. The LO phonon energies show no size dependence,
but the Huang–Rhys factors, which reflect the strength of the
exciton–phonon coupling, become larger for smaller NCs. Our
findings provide important insights into the exciton properties of
perovskite NCs.