The diffusion of fullerenes such as C-60 and PCBM in organic semiconductors is a key factor in controlling the efficiency of organic solar cells, though it is challenging to measure and to control. A simple optical method based on photoluminescence quenching is developed to assess the diffusion of a quencher molecule such as C-60 through a semiconducting polymer film, in this case made with the polymer polyfluorene. When the mobility of the polymer chains is reduced by chemical crosslinking, the diffusion coefficient of C-60 can be reduced by up to three orders of magnitude.