Current density-voltage (J-V) and capacitance-voltage (C-V) characteristics of P3HT/n⁻-silicon heterojunction diodes were investigated to clarify the carrier conduction mechanism at the organic/inorganic heterojunction. The J-V characteristics of the P3HT/n⁻-Si junctions can be explained by a Schottky diode model with an interfacial layer. Diode parameters such as Schottky barrier height and ideality factor were estimated to be 0.78eV and 3.2, respectively. The C-V analysis suggests that the depletion layer appears in the n⁻-Si layer with a thickness of 1.2 µm from the junction with zero bias and the diffusion potential was estimated at 0.40eV at the open-circuit condition. The present heterojunction allows a photovoltaic operation with power conversion efficiencies up to 0.38% with a simulated solar light exposure of 100mW/cm². The forward bias current was enhanced by coating the Si surface with a SiC layer, where the ideality factor was improved to be the level of 1.45∼1.50.