Organic materials offer a range of low-cost options for large area fabrication of solar (photovoltaic) cells. Unfortunately, organic photovoltaics suffer from low efficiencies. The interface between two different types of organic semiconductor (a heterojunction) lies at the heart of organic solar cells, as this is the site of charge generation within the device. A major obstacle to increasing efficiency is the lack of a full understanding of the structure of the interfaces within these devices. This proposal is the first step on the way to performing an in depth investigation of the link between structure, photophysics and device performance at photovoltaic heterojunctions. In this experiment we will perform specular and off-specular reflectivity measurements on P3HT/PCBM bilayers annealed at a range of temperatures, as a means of characterising the nanoscale interface composition.