We report on the synthesis of two naphthalene diimide-based organic derivatives: POANT containing electronically isolated 1,4,5,8-naphthalenetetracarboxylic diimide units and FCAND - a low-molar-mass compound with two different (fluorenone and 1,4,5,8-naphthalenetetracarboxylic diimide) electron-accepting units. Initial decomposition temperatures of 305 degrees C and 390 degrees C were indicated for FCAND and POANT, respectively. Organic semiconductor/Si heterostructures have been prepared by using spin coating (POANT, PEPK) and thermal evaporation in vacuum (FCAND). All heterojunctions demonstrated nonlinear I-U dependences and defined rectifying properties. The Schottky thermoionic emission model has been applied to explain the carrier transport and to estimate the barrier height and the ideality factor n of heterojunctions.