  Impurities play an important role during recombination processes in
semiconductors. Their important role is sharpened in atomically-thin
transition-metal dichalcogenides whose two-dimensional character renders
electrons and holes highly susceptible to localization caused by remote charged
impurities. We study a multitude of phenomena that arise from the interaction
of localized electrons with excitonic complexes. Emphasis is given to the
amplification of the phonon-assisted recombination of biexcitons when it is
mediated by localized electrons, showing that this mechanism can explain recent
photoluminescence experiments in ML-WSe$_2$. In addition, the magnetic-field
dependence of this mechanism is analyzed. The results of this work point to (i)
an intriguing coupling between the longitudinal-optical and homopolar phonon
modes that can further elucidate various experimental results, (ii) the physics
behind a series of localization-induced optical transitions in tungsten-based
materials, and (iii) the importance of localization centers in facilitating the
creation of biexcitons and exciton-exciton annihilation processes.
