The development of field-effect transistors (FETs) based on graphene and organic semiconductor (OSC) boosted the flourishing multi-functional optoelectronic devices, such as photodetectors, light-emitting transistors, photo-controlled memories etc. The charge transport properties of these new types of optoelectronic devices could be manipulated not only by applying gate bias, but also by changing the intensity of incident light. Photodetectors based on graphene and OSC combine light detection and signal amplification in a single device and have many advantages, such as mechanical flexibility, low-cost fabrication process, and large-scale production. However, graphene and OSC typically have low light absorption for high performance of photodetectors. Ru-complex was applied to the graphene and N,N'-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI, as OSC) FETs via a simple solution method. By way of the functionalization of a Ru-complex on those materials, the performance of these photodetectors was greatly improved due to the metal-ligand charge transfer (MLCT). Our study paves a simple way to improve photoresponsivity of carbon-based material such as graphene and OSC.