Neutron reflection will be used to investigate the normal composition profile in thin films of blends of electron-donor poly(alkyl thiophene) and the electron-acceptor fullerenes (C60, PCBM and bis-PCBM). These compounds are currently under investigation as model bulk heterojunction solar cells. The phase behavior of the film as a function of nucleating agent composition up to 2 wt% will be evaluated using NR and compared to device performance measurements. The nucleating agents are known to directly affect the degree of crystallinity and therefore the amorphous-crystalline surface area to volume ratio. This will have an effect on the interaction area between the P3AT-fullerenes which is known to have dramatic effect on electronic performance of these materials.