Bright Triplet Emission from Lead Halide Perovskite Nanocrystals

The emission of fully inorganic cesium lead halide (CsPbX3, where X = I,Br,Cl) perovskite-type nanocrystals is tunable over a wide energy range with ultrahigh photoluminescence quantum yields of up to 90%[1] and exhibits narrow emission lines. Due to their facile solution processability and their potential for high-efficiency photovoltaics and light sources they have gained enormous interest.Experiments on single perovskite quantum dots reveal a unique energetic level structure with a lowest bright triplet state[2], thus enabling photon emission rates ~20 and ~1000 times higher compared to any other conventional semiconductor nanocrystals at room and cryogenic temperatures, respectively. We investigate the nature of this exceptionally fast photon emission by temperature dependent quantum yield measurements. Furthermore we discriminate it from composition dependent “A-type” blinking behaviour in intensity-decay time correlation measurements and demonstrate stable, narrowband emission, with suppressed blinking and small spectral diffusion[3] for single CsPbBr2Cl nanocrystals. By means of polarization dependent high resolution spectroscopy, the complex nature of the exciton fine structure splitting and charged exciton emission has been characterized.Based on these measurements, supported by effective-mass models and group theory calculations, we conclude that the triplet exciton state is responsible for the extraordinary photon emission properties of lead halide perovskites. Our results can assist to identify other semiconductors that exhibit bright triplet excitons, with potential implications for improved optoelectronic devices