According to embodiments of the present invention, a method of manufacturing an organic semiconductor device includes forming a first organic semiconductor layer on a substrate, and forming a self-assembled monolayer by providing a self-assembly precursor onto the first organic semiconductor layer. The first organic semiconductor layer has a reactive group on a top surface of the first organic semiconductor layer. The forming of the self-assembled monolayer includes forming a chemical bond between the self-assembly precursor and the reactive group of the first organic semiconductor layer.