In this work, the optoelectronic performance of organic/inorganic heterojunction photodiode based on alpha-sexithiophene (alpha-6T/n-Si) is introduced. A thin film of alpha-6T was deposited on the n-type silicon substrate by a thermal evaporation technique. The topographical properties of the alpha-6T thin film grown on the n-Si substrate were investigated using a field emission scanning electron microscope (FESEM) technique. A network of nanocrystalline needles over the film surface was observed which give rise to an improvement in the electric charge transport. The optical properties of the prepared thin film were investigated using a spectrophotometric technique. The high absorption of alpha-6T in UV and visible region suggested the ability of this architecture for UV and visible light detection. The I-V characteristics of the fabricated photodiode were investigated in dark and under different illumination intensities and different wavelengths. The present architecture showed a good response to halogen lamb light, where the estimated values of rising and falling time at 160 mW/cm(2) were about 400 ms and 450 ms, respectively. The results show the possibility of using Au/alpha-6T/n-Si/Al structure as a photodetector for a wide range of the solar spectrum (UV-Visible).