Spatially segregated free-carrier and exciton populations in individual lead halide perovskite grains

The nature of charge carriers in methylammonium lead iodide perovskites (MAPbI 3) at room temperature is still a matter of considerable debate. Here, we demonstrate that within single grains of MAPbI 3, strong spatial heterogeneities on the nanometre length scale are present and associated with simultaneous free-carrier and exciton populations. These heterogeneous populations, hidden in ensemble measurements, have a signature of spatially resolved relaxation dynamics for above-bandgap photoexcitation. Using spectrally resolved transient absorption microscopy, we directly observe both red- and blueshifts of the band-edge absorption across individual grains due to a dynamic Stark shift and screening of excitonic transitions by hot carriers. These observations help address a long-standing debate on the identity of the charge carriers, showing that both excitons and free carriers coexist, but are spatially segregated on the length scale of hundreds of nanometres.