  Recently, the celebrated Keldysh potential has been widely used to describe
the Coulomb interaction of few-body complexes in monolayer transition-metal
dichalcogenides. Using this potential to model charged excitons (trions), one
finds a strong dependence of the binding energy on whether the monolayer is
suspended in air, supported on SiO$_2$, or encapsulated in hexagonal
boron-nitride. However, empirical values of the trion binding energies show
weak dependence on the monolayer configuration. This deficiency indicates that
the description of the Coulomb potential is still lacking in this important
class of materials. We address this problem and derive a new potential form,
which takes into account the three atomic sheets that compose a monolayer of
transition-metal dichalcogenides. The new potential self-consistently supports
(i) the non-hydrogenic Rydberg series of neutral excitons, and (ii) the weak
dependence of the trion binding energy on the environment. Furthermore, we
identify an important trion-lattice coupling due to the phonon cloud in the
vicinity of charged complexes. Neutral excitons, on the other hand, have weaker
coupling to the lattice due to the confluence of their charge neutrality and
small Bohr radius.
