4-bromoanisole is used as a very versatile processing additive to control the phase separation and phase purity of organic photovoltaic devices. Polymer-polymer blends based on P3HT as donor and a wide range of acceptor materials (F8TBT, PCDTBT,.) are investigated. The additive promotes the aggregation of P3HT which improves the morphology for both initially mixed and demixed blends.