Cation effect on excitons in perovskite nanocrystals from single-dot photoluminescence of CH3NH3PbI3

Abstract

The single-dot photoluminescence properties of $\ensuremath{\sim}7\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ perovskite $\mathrm{MAPb}{\mathrm{I}}_{3}(\mathrm{MA}=\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{{\mathrm{H}}_{3}}^{+})$ nanocrystals (NCs) were investigated in the $5\text{--}295\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ 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 $140\text{--}160\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, indicating a tetragonal-orthorhombic phase transition, was observed. In addition, the emission linewidth of $\ensuremath{\sim}0.6\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ was measured, which is the narrowest ever recorded for this perovskite material system. A $\ensuremath{\sim}4.0\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ phonon mode was identified for the NCs at 5 K, defining the linewidth thermal broadening. In general, the presence of $\mathrm{M}{\mathrm{A}}^{+}$ leads to broader spectra than for $\mathrm{C}{\mathrm{s}}^{+}$ or $\mathrm{F}{\mathrm{A}}^{+}$ 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 $\mathrm{C}{\mathrm{s}}^{+}$ 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.