Vertical stratification and its impact on device performance in a polycarbazole based copolymer solar cells

Using neutron-reflectivity, we study vertical stratification and device performance in bulk hetero-junction organic photovoltaic (OPV) cells consisting of a blend of PC71BM with a carbazole-based donor–acceptor copolymer PCDTBT1. We find that when the blend is cast on a PEDOT:PSS/ITO anode, a PC71BM-depleted (polymer-rich) layer is formed at the PEDOT:PSS interface, whilst a PC71BM-depleted layer is instead located at the air-interface when the same blend is cast on a solution processed MoOx thin film. OPV device characterization measurements indicate that unfavourable vertical segregation can have a profound effect on OPV device characteristics via increased charge recombination.