Charge carrier recombination and open circuit voltage in organic solar cells : from bilayer model systems to hybrid multi junctions Tremendous progress in the development of thin film solar cell techniques has been made over the last decade. The field of organic solar cells is constantly developing new material classes like Perowskite solar cells are emerging and different types of hybrid organic inorganic material combinations are being investigated for their physical properties and their applicability in thin film electronics. Besides typical single junction architectures for solar cells multi junction concepts are also being investigated as they enable the overcoming of theoretical limitations of a single junction. In multi junction devices each sub cell operates in different wavelength regimes and should exhibit optimized band gap energies. It is exactly this tunability of the band gap energy that renders organic solar cell materials interesting candidates for multi junction applications. Nevertheless only few attempts have been made to combine inorganic and organic solar cells in series connected multi junction architectures. Even though a great diversity…