Observation of positive and negative trions in organic-inorganic hybrid perovskite nanocrystals

Lead halide perovskite nanocrystals are promising light emitting materials with high photoluminescence quantum yields and widely tunable emission wavelengths. Their optical responses are significantly influenced by efficient nonradiative Auger recombination of trions (charged excitons) and biexcitons. Here, we report the observation of positive and negative trions in formamidinium lead bromide nanocrystals and discuss their role in the photoluminescence dynamics. It is found that the addition of copper thiocyanate causes an additional intermediate state by photochemical doping. We clarify that as-prepared nanocrystals show a two-state blinking between neutral excitons and positive trions, while the postsynthesis treated nanocrystals exhibit blinking between all three states, including negative trions. We confirmed that the biexciton Auger rate evaluated from femtosecond transient absorption measurements can be expressed as the sum of the Auger rates of positive and negative trions obtained by single-dot spectroscopy.