Spatial Heterogeneity of Biexcitons in Two-Dimensional Ruddlesden-Popper Lead Iodide Perovskites

Quantum confinement in two-dimensional (2D) Ruddlesden-Popper(RP) perovskites leads to the formation of stable quasi-particles,including excitons and biexcitons, the latter of which may enablelasing in these materials. Due to their hybrid organic-inorganicstructures and the solution phase synthesis, microcrystals of 2D RPperovskites can be quite heterogeneous, with variations in excitonicand biexcitonic properties between crystals from the same synthesisand even within individual crystals. Here, we employ one- and two-quantumtwo-dimensional white-light microscopy to systematically study thespatial variations of excitons and biexcitons in microcrystals ofa series of 2D RP perovskites BA(2)MA( n-1)Pb (n) I3n+1 (n = 2-4, BA= butylammonium, MA= methylammonium). We find that the average biexciton binding energyof around 60 meV is essentially independent of the perovskite layerthickness (n). We also resolve spatial variationsof the exciton and biexciton energies on micron length scales withinindividual crystals. By comparing the one-quantum and two-quantumspectra at each pixel, we conclude that biexcitons are more sensitiveto their environments than excitons. These results shed new lighton the ways disorder can modify the energetic landscape of excitonsand biexcitons in RP perovskites and how biexcitons can be used asa sensitive probe of the microscopic environment of a semiconductor.