Excitons and biexcitons bound to a positive ion in a bismuth-doped inorganic-organic layered lead iodide semiconductor

We have studied optical properties of an inorganic-organic layered lead iodide semiconductor doped with bismuth ions (Bi³⁺). A new absorption band was observed at 2.150.3em0exeV that is by 0.210.3em0exeV lower than two-dimensional $1s$ excitons (2.360.3em0exeV) and is assigned to excitons bound to the bismuth ions. In the photoluminescence spectra with high excitation density (~0.40.3em0exMW∕cm²), a novel emission band was observed at the energy of 2.510.3em0exeV that is by 0.150.3em0exeV higher than the exciton resonance, and is attributed to biexcitons bound to the bismuth ions. On the basis of the experimental results, the binding energies of the bismuth-bound exciton and biexciton were estimated approximately as 210 and 100.3em0exmeV, respectively. It is found that the binding energy of the positive-ion (Bi³⁺) bound biexciton is by far smaller than that of the corresponding bound exciton. The much lower stability of the positive-ion bound biexciton is discussed.