In this paper we focus upon the role of interfacial energetics in influencing the separation of charge transfer states into dissociated charge carriers in organic donor/acceptor blend films. In particular, we undertake transient optical studies of films comprising regioregular poly(3-hexylthiophene) (P3HT) blended with a series of perylene-3,4:9,10-tetracarboxydiimide (PDI) electron acceptors. For this film series, we observe a close correlation between the PDI electron affinity and the efficiency of charge separation. This correlation is discussed in the context of studies of charge photogeneration for other organic donor/acceptor blend films, including other polymers, blend compositions, and the widely used electron acceptor 3'-phenyl-3'H-cyclopropa[1,9](C(60)-I(h))[5,6]fullerene-3'-butanoic acid methyl ester (PCBM).