Enhanced conduction and charge-selectivity by N-doped graphene flakes in the active layer of bulk-heterojunction organic solar cellsElectronic supplementary information (ESI) available: See DOI: 10.1039/c3ee40963e

Polymeric organic photovoltaic (OPV) cells are promising candidates for low-cost, high-performance energy sources due to their low material and processing costs, flexibility, and ease of manufacturing by solution processes. However, low power-conversion efficiency (PCE) has impeded the development of OPV cells. The low PCE in OPV solar cells has been attributed to low carrier mobility, which is related to the transport length of the charge carriers within active layers. Graphene can be an ideal material for assisting the charge transport in the active layer of OPV cells due to its excellent charger carrier mobility, thermal and chemical stability, and compatibility with the solution process. In this work, we demonstrated for the first time an improvement of the PCE (up to 40%) in OPV bulk-heterojunction (BHJ) cells by incorporating charge-selective graphene flakes into the BHJ active layer. The charge selectivity of graphene flakes was achieved by nitrogen doping (N-doped graphene). The N-doped graphene, when mixed in the active layer (N-doped graphene/polymer:fullerene composites), provided transport pathways exclusively to specific charge carriers through the modulation of band-gap structures. We discuss further the enhancement of the PCE in OPV cells with respect to charge-carrier mobility. A promising improvement of power conversion efficiency (up to 40%) can be achieved in bulk-heterojunction organic solar cells by incorporating charge-selectively N-doped graphene flakes into the active layer.