Inter layers between the active layer and the electrodes in organic devices are known to modify the electrode work function and enhance carrier extraction/injection, consequently improving device performance. It was recently demonstrated that chemical interactions between the evaporated electrode and interlayer additive can induce additive migration toward the metal/organic interface to spontaneously form the interlayer. In this work we used P3HT:PEG blends as a research platform to investigate the driving force for additive migration to the organic/metal interface and the source of the work function modification in OPVs. For this purpose PEG derivatives with different end groups were blended with P3HT or deposited on top of P3HT layer, topped with Al or Au evaporated electrodes. The correlation between the additive chemical structure, the V-oc of corresponding devices, and the metal/organic interface composition determined by XPS revealed that the driving force for additive migration toward the blend/metal interface is the chemical interaction between the additives' end group and the deposited metal atoms. Replacing the PEG additives with alkyl additives bearing the same end groups has shown that the Al work function is actually modulated by the PEG backbone. Hence, in this work we have identified and separated between structural features controlling the migration of the interlayer additive to the organic/metal interface and those responsible for the modification of the metal work function.