Cation effect on excitons in perovskite nanocrystals from single-dot photoluminescence of C H3 N H3Pb I3

The single-dot photoluminescence properties of ∼7nm perovskite MAPbI3(MA=CH3NH3+) nanocrystals (NCs) were investigated in the 5-295K temperature range both in spectral and time domains. Repeatable single-dot measurements were facilitated by the use of a protective polymer, which stabilized the NCs. Temperature-induced phase transition and exciton-phonon interactions were revealed as well as the exciton fine structure. A pronounced spectral jump of the emission peak at 140-160K, indicating a tetragonal-orthorhombic phase transition, was observed. In addition, the emission linewidth of ∼0.6meV was measured, which is the narrowest ever recorded for this perovskite material system. A ∼4.0meV phonon mode was identified for the NCs at 5 K, defining the linewidth thermal broadening. In general, the presence of MA+ leads to broader spectra than for Cs+ or FA+ containing perovskite NCs. It is attributed to higher polarity of this cation, thus it is more susceptible to spectral diffusion, which is clearly observed here. Photoluminescence decay measurements indicated that the recombination from the lowest energy state of the emission level manifold is partially forbidden. This is opposite to Cs+ cation NCs, highlighting the central role of the positive ion in the exchange interaction in perovskites. Finally, delayed luminescence was found to govern the recombination dynamics below room temperature, suggesting an involvement of trap sites for the orthorhombic phase. The reported photophysics of a quantum-confined exciton in this material, which is of interest for various light-converting applications, clarifies the role of the cation in perovskite nanocrystal optical properties.