  A theoretical study of the exciton binding energy in the two-dimensional
hexagonal boron nitride monolayer is presented within the tight-binding
approximation (TBA). A self-consistent equation for the interband electron-hole
propagators is derived and in the long wavelength limit reduced to the standard
hydrogen atom like Schrodinger equation. It is shown that inclusion of
dynamically screened Coulomb interaction in ladder term is of crucial
importance for proper description of exciton binding energy. This leads to the
self-consistent eigenvalue problem with dynamical screening. The dependence of
the exciton energy on the orbital quantum number is studied. It is predicted
that for the fixed principal quantum number the states with higher orbital
momentum have lower energy than the states with lower orbital momentum. Using
the developed formulas and the experimental optical gap the quasiparticle gap
is estimated. In the limit of high polarizability, a semiclassical procedure
was used to obtain the exciton binding energy. The TBA parametrization is
supported by ab initio calculations.
