  The main problem in theoretical analysis of structures with strong
confinement is the fact that standard mathematical tools: differential
equations and Fourier's transformations are no longer applicable. In this paper
we have demonstrated that method of Green's functions can be successfully used
on low-dimension crystal samples, as a consequence of quantum size effects. We
can illustrate modified model through the prime cubic structure molecular
crystal: bulk and ultrathin film. Our analysis starts with standard exciton
Hamiltonian with definition of commutative Green's function and equation of
motion. We have presented detailed procedure of calculations of Green's
functions, and further dispersion law, distribution of states and relative
permittivity for bulk samples. After this, we have followed the same procedures
for obtaining the properties of excitons in ultra-thin films. The results have
been presented graphically. Besides modified method of Green's functions we
have shown that the exciton energy spectrum is discrete in film structures
(with number of energy levels equal to the number of atomic planes of the
film). Compared to the bulk structures, with continual absorption zone, in film
structures exist resonant absorption peaks. With increased film thickness
differences between bulk and film vanish.
