The Theoretical Influence of the Difference Between the LUMO Energy Levels of Donor and Acceptor in Organic Photovoltaic Triplejunction Solar Cells

In organic photovoltaic solar cells, light absorption does not immediately lead to free charge carriers. Instead, an exciton is created. The highest efficiency is reached when the lowest unoccupied molecular orbital (LUMO) of the donor is as close as possible to the LUMO of the acceptor. However, a necessary condition for efficient dissociation of the created excitons is that the difference between the LUMOs of donor and acceptor is higher than the exciton binding energy. The value of the exciton binding energy in different materials is a subject of discussion. The excess of this necessary minimum of the LUMO-difference corresponds with an energy loss. Moreover, it is often not possible to optimize suitable material combinations for organic photovoltaic cells to an ideal (low) LUMO difference. Another energy loss in organic solar cells is caused by their narrow absorption windows, compared to the absorption band of inorganic solar cells. A way to capture a wider band of the solar radiation is using more solar cells with different bandgaps in a row. In this article, we study three organic cells in a row, i.e. a triple-junction. More specifically, we study the theoretical influence of the difference between the LUMO energy levels of donor and acceptor for an organic triple-junction solar cell. We study as well the monolithic as the stacked configuration.