Small Number of Defects per Nanostructure Leads to "Digital" Quenching of Photoluminescence: The Case of Metal Halide Perovskites

Long charge carrier diffusion length and large grain size are commonly believed to be inherent properties of highly luminescent polycrystalline thin-film semiconductors. However, exactly these two properties make luminescence very susceptible to quenching by just one strongly quenching defect state if present in each grain. Moreover, when the number of quenchers per grain is small (say 1-10), it varies greatly from grain to grain, purely for statistical reasons. These fluctuations, which resemble digital signal switching, can be one of the reasons for large differences between the luminescence brightness of different grains in polycrystalline films. This and other peculiarities of photoluminescence in systems where the number of strong quenchers per grain/crystallite is small is discussed in detail using metal halide perovskites as examples.