Nonthermal Site Occupation at the Donor-Acceptor Interface of Organic Solar Cells

We explore the nature of occupation and relaxation within the charge transfer density of states (CT DOS) for bulk heterojunction organic solar cells consisting of the donor boron subphthalocyanine chloride and the acceptor C60. We observe relaxation of geminate CT states on a sub-ns timescale via an approximately 70-meV dynamic redshift in their photoluminescence, whereas free carrier relaxation at longer times leads to the formation of nongeminate CT states at even lower energy. In steady state, we find that thermalization within the DOS is incomplete, resulting in a Boltzmann-like CT state distribution characterized by an effective temperature above that of the ambient. These results confirm that electron and hole populations can be far from equilibrium in organic solar cells and may prompt a reassessment of analyses that assume the same temperature for their charge-carrier distributions in the dark and under illumination.