Solvent additives such as diiodooctane (DIO) are becoming ubiquitous in processing high performance organic photovoltaic (OPV) active layers. Here, we investigate the effects of DIO on the long-term stability of the active layer by studying the photodegradation under ambient white light illumination of the polymer PTB7-Th in pure polymer thin films and in blend films with PC71BM. Using X-ray fluorescence, we directly detect iodine in the active layer films, indicating the presence of residual DIO after casting from solution. Additionally, we show that this residual DIO dramatically decreases the photostability of the active layer. Structural changes in the films upon illumination are probed with grazing-incidence wide-angle X-ray scattering (GIWAXS). FTIR spectroscopy is used to monitor chemical changes in the polymer structure during irradiation in the presence of DIO. Furthermore, we demonstrate that film treatment either with high vacuum (10-8 Torr) for 60 min or with a high-temperature thermal anneal at 175 °C for 30 min removes residual DIO from the film and delays photodegradation. Therefore, when processing polymer solar cells with DIO-containing solutions, it is imperative to remove any trace amounts of DIO from deposited films.