Abstract
A study of single grains of lead halide perovskite reveals the presence of both excitons and free charge carriers. The nature of charge carriers in methylammonium lead iodide perovskites (MAPbI3) at room temperature is still a matter of considerable debate1,2,3. Here, we demonstrate that within single grains of MAPbI3, 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.
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