In a bilateral resarch project on "Magnetic Resonance Tools for Solar Materials" (MARS), which is funded by the Deutsche Forschungsgemeinschaft and the Agence Nationale de la Recherche, we will investigate the influence of electron spins on the photo-physical properties of organic semiconductors and thin perovskite films. For this purpose we are going to develop new spin-sensitive techniques. These will allow us to probe the properties of photo-excited states that appear in exciton fission systems and novel materials with broad impact on fundamental optoelectronics and its applications. The MARS project is a collaborative effort involving scientists from Paris (Université Paris-Sud), Versailles (Université Versailles Saint-Quentin), Grenoble (Institut Néel), Bayreuth (Universität Bayreuth) and Berlin (Freie Universität Berlin).