Origin of the open circuit voltage of donor-acceptor solar cells: Do polaronic energy levels play a role?

This letter analyzes the influence of the energy of the highest-energy occupied molecular orbital (HOMO) and the single occupied molecular orbital (SOMO) (i.e., the low-energy polaronic level) of different electron donors (D) on the open circuit voltage (VOC) value delivered by bilayer and bulk-heterojunction solar cells containing fullerenes as acceptor (A). The analysis further confirms that the VOC of these devices scales with the energy difference between the HOMO of the D and the lowest-energy unoccupied molecular orbital of the A. As observed, a better relationship is obtained if the SOMO energy of the D—extracted from photoinduced absorption spectra—is considered.