Due to its simplicity and cost-effectiveness, single solution casting of organic optoelectronics has grown in popularity for device fabrication to produce technologies such as organic photovoltaics and thin film transistors. In order to explore the structural evolution that occurs in the film formation of a blend composed of polystyrene and the benchmark conjugated polymer MEH-PPV, we have performed a series of neutron reflectivity experiments focused on studying the film structure as it changes through the thermal annealing process both in the presence and absence of white light. Results indicate the formation of a nonhomogeneous blend upon casting, which becomes stratified with thermal annealing. More importantly, the extent of stratification varies with illumination, where exposure to white light increases stratification. This data suggests in situ illumination is a potential novel tool to manipulate device-relevant morphologies of optoelectronic active layers throughout the fabrication process, offering a cheap nondestructive tool to effectively tune desired structural parameters. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1142-1149