${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is an ideal three-dimensional topological insulator in which the chemical potential can be brought into the bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize the photoexcited carrier decay in ${\mathrm{Bi}}_{2-x}{\mathrm{Sb}}_x{\mathrm{Se}}_3$ nanoplatelets. We report a substantial slowing of the bulk carrier relaxation time in bulk-insulating ${\mathrm{Bi}}_{2-x}{\mathrm{Sb}}_x{\mathrm{Se}}_3$ as compared to $n$-type bulk-metallic ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ at low temperatures, which approaches $3.3\mathrm{ns}$ in the zero pump fluence limit. This long-lived decay is correlated across different fluences and antimony concentrations, revealing unique decay dynamics not present in $n$-type ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, namely the slow bimolecular recombination of bulk carriers.

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• Topological insulators
• Reflectivity
• Ultrafast pump-probe spectroscopy