A thin film transistor comprising a substrate; a gate electrode (20; 20') on the substrate (10;10'); an organic semiconductor layer (60; 60'; 60"; 60"') on the substrate (10; 10'); a gate insulating layer (30; 30'; 30") between the gate electrode (20; 20') and the organic semiconductor layer (60; 60'; 60"); source and drain electrodes (40, 50; 40', 50'; 40", 50"; 40"', 50"') connected to the organic semiconductor layer (60; 60'; 60"; 60'"); and an organic passivation layer covering at least the organic semiconductor layer (60; 60'; 60"; 60"'), wherein the organic passivation layer is the cross-linked product of: a cross-linking agent, the cross-linking agent being reacted in an amount such that the mole ratio of the cross-linking functional group(s) of the crosslinking agent to the hydroxyl group(s) of the structural unit of Chemical Formula is 1:0.75 to 1:1.25; and an oligomer or a polymer including structural units represented by the following Chemical Formulae 1 and 2, wherein, in Chemical Formula 1, Cy1 is a functional group including one of a substituted or unsubstituted C4 to C20 carbocyclic group, a substituted or unsubstituted C6 to C20 monocyclic aromatic group, a substituted or unsubstituted C2 to C20 condensed polycyclic aromatic group, and a C2 to C20 non-condensed polycyclic aromatic group, and wherein, in Chemical Formula 2, Cy2 is one of a substituted or unsubstituted C6 to C40 arylene group, a substituted or unsubstituted C3 to C40 heteroarylene group, a substituted or unsubstituted C5 to C40 cycloalkylene group, and a substituted or unsubstituted C5 to C40 heterocycloalkylene group, and k is an integer of 2 or more. a cross-linking agent, the cross-linking agent being reacted in an amount such that the mole ratio of the cross-linking functional group(s) of the crosslinking agent to the hydroxyl group(s) of the structural unit of Chemical Formula is 1:0.75 to 1:1.25; and an oligomer or a polymer including structural units represented by the following Chemical Formulae 1 and 2, wherein, in Chemical Formula 1, Cy1 is a functional group including one of a substituted or unsubstituted C4 to C20 carbocyclic group, a substituted or unsubstituted C6 to C20 monocyclic aromatic group, a substituted or unsubstituted C2 to C20 condensed polycyclic aromatic group, and a C2 to C20 non-condensed polycyclic aromatic group, and wherein, in Chemical Formula 2, Cy2 is one of a substituted or unsubstituted C6 to C40 arylene group, a substituted or unsubstituted C3 to C40 heteroarylene group, a substituted or unsubstituted C5 to C40 cycloalkylene group, and a substituted or unsubstituted C5 to C40 heterocycloalkylene group, and k is an integer of 2 or more. Cy1 is a functional group including one of a substituted or unsubstituted C4 to C20 carbocyclic group, a substituted or unsubstituted C6 to C20 monocyclic aromatic group, a substituted or unsubstituted C2 to C20 condensed polycyclic aromatic group, and a C2 to C20 non-condensed polycyclic aromatic group, and wherein, in Chemical Formula 2, Cy2 is one of a substituted or unsubstituted C6 to C40 arylene group, a substituted or unsubstituted C3 to C40 heteroarylene group, a substituted or unsubstituted C5 to C40 cycloalkylene group, and a substituted or unsubstituted C5 to C40 heterocycloalkylene group, and k is an integer of 2 or more. Cy2 is one of a substituted or unsubstituted C6 to C40 arylene group, a substituted or unsubstituted C3 to C40 heteroarylene group, a substituted or unsubstituted C5 to C40 cycloalkylene group, and a substituted or unsubstituted C5 to C40 heterocycloalkylene group, and k is an integer of 2 or more.