Non langevin recombination in fullerene and non fullerene acceptor solar cells Organic solar cells (OSCs) in recent years have shown high efficiencies through the development of novel non fullerene acceptors (NFAs). Fullerene derivatives have been the centerpiece of the accepting materials used throughout organic photovoltaic (OPV) research. However since 2015 novel NFAs have been a game changer and have overtaken fullerenes. However the current understanding of the properties of NFAs for OPV is still relatively limited and critical mechanisms defining the performance of OPVs are still topics of debate. In this thesis attention is paid to understanding reduced Langevin recombination with respect to the device physics properties of fullerene and non fullerene systems. The work is comprised of four closely linked studies. The first is a detailed exploration of the fill factor (FF) expressed in terms of transport and recombination properties in a comparison of fullerene and non fullerene acceptors. We investigated the key reason behind the reduced FF in the NFA (ITIC based) devices which is faster non geminate recombination relative to the fullerene (PCBM[70] based) devices. [...]