Simulation of the influence of the absorption window for stacked and monolithic organic tandem solar cells

In order for organic bulk heterojunction solar cells to compete with the traditional inorganic cells, higher power conversion efficiencies are desirable. A way to improve the efficiency is to use a tandem configuration. In this article, we study the theoretical efficiency potential of organic bulk heterojunction tandem solar cells. We study the influence of the energy levels of donor and acceptor, as well as different absorption windows of the subcells for both a stacked and a monolithic configuration. Ideal material characteristics are obtained from these calculations, giving an idea of how the ideal organic tandem cell should look like. An interesting result shows that it would not pay off to develop photovoltaic organic materials with an absorption window broader than 400 nm, because hardly any efficiency gain can be achieved by a broader absorption window. The optimal bandgaps with a sufficient absorption window of 400 nm are Eg1=1.8 eV and Eg2=1.1 eV for both configurations. Furthermore, for a stacked organic tandem cell, both subcells need not necessarily have a large absorption window. This does not apply for the monolithic cell. As soon as one subcell has a low absorption window, the efficiency decreases rapidly.