Modeling the influence of doping on the performance of bulk heterojunction organic solar cells

This work reports on two different approaches to simulate bulk-heterojunction organic solar cells (BHJ OSCs). The first is a one-dimensional (1D) description which is based on one effective semiconductor and the second is a two-dimensional model (2D) explicitly taking into account both the donor and the acceptor phase. A comparison between the two models is drawn and the influence of either p or n doping on the photovoltaic performance is investigated in detail. Slight differences in the results of the two models are explained with the underlying geometry and it is shown that many features can already be well described with the 1D effective semiconductor model. The influence of doping donor and acceptor phase independently can only be described with the 2D donor/acceptor model. The results indicate that this can be very beneficial for BHJ OSCs. The reason is that upon doping both phases the depletion zone is centered around the donor/acceptor interface and regions of high conductivity are formed in the bulk of the phases, i.e., farthest away from the donor/acceptor interface where charge carriers can recombine. This leads to a reduction of the transport resistance and thus to a significant increase in fill factor. This effect does not occur in a solar cell made of one material only. It is based on the fact that a BHJ OSC is a majority carrier device.