Organic photovoltaic devices have attracted great interest of researchers due to the wide range of materials, ease in processing, mechanical flexibility and so on. One of the major challenges in organic solar cells is the low exciton diffusion length, which is a limiting factor for its photocurrent, and thus the power conversion efficiency. Therefore, the determination of the exciton diffusion length can help the researchers to modify the device architecture in a beneficial way. Here, I want to determine the exciton diffusion length in organic semiconducting materials. Processing organic diodes by solution-processing, I will investigate different material systems of donors and acceptors. The devices will be studied using spectroscopic and electrical investigation methods such as current-voltage measurement and photovoltaic quantum efficiency measurements. The major objective of this work is to find out an approach to determine the exciton diffusion length in organic semiconductors, and to get further insights into the fundamental processes in organic solar cells.