Direct Evidence of Exciton-Exciton Annihilation in Single-Crystalline Organic Metal Halide Nanotube Assemblies

Excitons in low-dimensional organic-inorganic metal halide hybrid structures are commonly thought to undergo rapid self-trapping following creation due to strong quantum confinement and exciton-phonon interaction. Here we report an experimental study probing the dynamics of these self-trapped excitons in the single-crystalline bulk assemblies of 1D organic metal halide nanotubes, (C₆H₁₃N₄)₃Pb₂Br₇. Through time-resolved photoluminescence (PL) measurements at different excitation intensities, we observed a marked variation in the PL decay behavior that is manifested by an accelerated decay rate with increasing excitation fluence. Our results offer direct evidence of the occurrence of an exciton-exciton annihilation process, a nonlinear relaxation phenomenon that takes place only when some of the self-trapped excitons become mobile and can approach either each other or those trapped excitons. We further identify a fast and dominant PL decay component with a lifetime of ∼2 ns with a nearly invariant relative area for all acquired PL kinetics, suggesting that this rapid relaxation process is intrinsic.