Modeling charge-carrier transport and generation–recombination mechanisms in p+n+ a-Si tunnel junctions

Electrical contacts between subcells in a-Si tandem solar cells are n+p+ junctions in which a highly conductive path is provided by charge-carrier tunneling transitions between extended states and the states in the gap of a-Si. A tunneling-assisted capture-emission model of carriers is developed in n+p+ a-Si junctions with a high built-in electrical field in the depletion region. The pure thermal and the tunneling-assisted thermal capture and emission of carriers determine the occupancy function in the mobility gap and generation–recombination rates in the space-charge region. The developed model is applied to calculate the current–voltage characteristics of an n+p+ a-Si junction, and a good agreement with measured characteristics is obtained.