(253a) Exciton Annihilation and Power-Dependent Photoluminescence Quantum Yields of 2D Manganese-Doped Perovskite Nanoplatelets

Colloidal lead halide perovskite nanoplatelets have recently emerged as a new class of semiconductors. Their anisotropic nature, strong quantum- and dielectric-confinement, strong absorption and high-quality emission characteristics make them one of the most promising candidates for the next-generation devices. However, reports on the synthesis of metal-doped perovskite nanoplatelets and in-depth studies on their properties are still limited. Considering that metal doping is one of the most widely used technique to expand the functionalities of semiconductor crystals, more studies on metal-doped perovskite nanoplatelets are needed in order to utilize the material in a wider range of applications.

In this work, we start with demonstrating that colloidal manganese-doped two-dimensional organic-inorganic hybrid perovskite nanoplatelets (Chemical formula: L₂[APb_1-xMn_xBr₃]_n-1Pb_1-xMn_xBr₄, L: butylammonium, A: methylammonium or formamidinium, n(=1 or 2): number of PbBr₆^4-octahedral layers in out-of-plane direction) can be synthesized at room temperature using ligand-assisted reprecipitation method without requiring any post-treatment. We further show that the concentration of the dopants (Mn²⁺) can be controlled by varying the ratio between lead and manganese ions in the precursor solution mixture. Substitutional doping of Mn²⁺ in perovskite nanoplatelets introduce Mn²⁺ atomic states into the system that can act as an additional photoactive site along with the nanoplatelet band edge. Using time-resolved photoluminescence spectroscopy, it is shown that the dopant emission exhibits millisecond-scale lifetime, which can be attributed to spin-forbidden nature of the transition, while band edge emission exhibits nanosecond-scale lifetime.

And since the emissions from the nanoplatelet band edge and the dopant are spectrally well-resolved, photoluminescence quantum yields (PLQYs) of band edge and dopant emissions can be calculated separately. We demonstrate that, as the dopant concentration increases, band edge PLQY decreases while dopant PLQY, and thus the total PLQY, significantly increases. We believe that this can be attributed to the higher intrinsic quantum efficiency of the dopant state reaching up to 100%. More interestingly, band edge PLQY and dopant PLQY exhibit distinct excitation power-dependence: Band edge PLQY is constant while dopant PLQY significantly decreases with increasing excitation power.

Considering that the excitons can be transferred from the band edge to the dopant state that exhibits orders-of-magnitude longer decay lifetime, we first investigate the possibility of the dopant states being saturated at higher excitation power, potentially leading to the observed power dependence. However, using a kinetic model with parameters obtained from time-resolved spectroscopy, we show that the exciton transfer combined with the dopant state saturation effect alone cannot explain the observed power dependence. We then demonstrate that exciton-exciton annihilation must be considered in order to rationalize experimental observations. Furthermore, we compare band edge-to-dopant exciton transfer time constants obtained by several independent techniques including time-resolved spectroscopy as well as power-dependent PLQY combined with the kinetic model analysis. Results show that methylammonium-based nanoplatelets exhibit faster exciton transfer compared to formamidinium-based nanoplatelets.

To summarize, this work can provide fundamental insights on the behavior of excitons in Mn-doped perovskite nanoplatelets. We would like to emphasize that not only band edge-to-dopant exciton transfer, but also exciton-exciton annihilation has to be considered to accurately predict the exciton dynamics in the system. We believe that Mn²⁺ doping in colloidal perovskite nanoplatelets accompanied by strongly power-dependent PLQY and organic-cation-dependent (methylammonium vs. formamidinium) exciton transfer rate can open up new possibilities in further manipulating material properties.