We analyze the thermoelectric properties of p-type tetrathiotetracene-iodide crystals, TTT₂I₃ and of n-type tetrathiotetracene-tetracyanoquinodimethane crystals, TTT(TCNQ)₂ in the frame of a more complete 3D physical model. It is shown that in order to increase the thermoelectric efficiency it is necessary to decrease the carrier concentration in crystals of p-type and to increase in those of n-type in comparison with the stoichiometric ones. Considered crystals admit nonstoichiometric composition. Optimal parameters are determined in order to achieve as high as possible values of thermoelectric figure of merit ZT. The thermoelectric efficiency of a p-n-module is modelled as function of crystal parameters.