Nicotinamide/nicotinic acid complexes with centered Co metal (called Co-complexes) were synthesized by chemically reactions and characterized by thermogravimetric analysis (TGA), UV–Vis spectrometer and atomic force microscopy (AFM) techniques. While the composition of the Co-complexes was confirmed by TGA, the compatibilities of the Co-complexes for optoelectronic devices were revealed by UV–Vis spectrometer and AFM techniques. The Co-complexes were dissolved in water for various weight amounts of 0.5 mg, 1.0 mg, 2.0 mg and 3.0 mg, and solutions were coated onto Si wafer pieces to obtain Co-complex interlayered film in the Al/p-Si metal semiconductor devices. I–V and I–t measurements were performed to investigate photodiode and photodetector behaviors of the Al/Co-complex/p-Si devices for various light power illumination intensities. The result revealed that Al/Co-complex/p-Si devices can be used for optoelectronic applications.