The modulation of threshold voltage () of organic thin-film transistors (OTFTs) was investigated by embedding a thin CuO layer between the two semiconductor layers. The results showed that the of OTFTs with a CuO layer can be effectively tuned by controlling the positive gate-to-source voltage () under stress of gate-to-source voltages. The shifts from -3.67 to -0.82 V when the positive varies from 30 to 50 V. This can be explained by the mechanism of trapping electrons at the interface between the CuO charge-separation layer and the active layer. To confirm the role of the CuO layer acting as the charge-separation source, two organic thin-film diodes, indium-tin oxide(ITO)/tris (8-quinolinolato) aluminum(III) (Alq)/pentacene/Al (inverted-stack diode) and ITO/Alq/CuO/pentacene/Al (inverted-stack diode with a CuO layer), were fabricated and their diode current characteristics were measured. For the second device, a large current flow was observed at positive bias on the ITO electrodes, which is ascribed to the internal bipolar charge separation within the added CuO zone.