Current density-voltage (J-V) characteristics of poly(3-hexylthiophene) (P3HT)/n-silicon heterojunctions are investigated to improve the electrical characteristics of organic/inorganic heterojunction diodes. The diodes are fabricated by spin coating P3HT with SiC or post-HF treatment. These treatments are effective in suppressing the interfacial oxide between the P3HT and Si layers, where the forward current density is improved effectively. The thickness of the interfacial oxide layer correlates with improvements of not only the forward-bias current but also the ideality factor. The interfacial oxide layer effect on the carrier transport properties is discussed using a metal-insulator-semiconductor junction model.