1. Existing Proposal: (1) SF in pentacene-C 60 (PEN:C 60 ) solar cells generates two PEN triplets at energy E T 1 from a single optical singlet at energy E S (E S ≥ 2E T 1, see Fig. 1). (2) Each triplet donates an electron to a C 60 molecule, thus generating two electron-hole pairs from one photon! Our Work: Calculations for the idealized PEN:C 60 heterostructure of Fig. 2(a): E S ≥ 2E T 1, sufficient for (1), but not for (2). Necessary conditions for (2): (i) E(PEN + C 60 − ) ≤ E T 1, (ii) Binding energy of PEN + C 60 − small in spite of low E(PEN + C 60 − ). Fig. 2(b): Gas-phase molecular parameters give E(PEN + C 60 − ) > E T 1. Larger HOMO- LUMO offsets between PEN and C 60 in the solid state may lead to lower E(PEN + C 60 − ). Fig. 2(c): binding energy of PEN + C 60 − versus ε, the magnitude of the reduction in energy of PEN + C 60 −, for different Coulomb interaction parameters. Binding energy independent of absolute energy of PEN + C 60 −, which is good news! Overall Conclusions: (1) Energy difference between LUMO of C 60 and HOMO of PEN must be less than 1.6 eV for SF-driven enhanced QE of PEN:C 60 solar cell. Not clear whether this is satisfied in real systems. (2) SF in arbitrary donor molecules will not necessarily lead to higher QE of solar cells. Does Singlet Fission (SF) Enhance the Quantum Efficiency (QE) of Organic Solar Cells? Sumit Mazumdar, University of Arizona, DMR 0705163 Figure 2: κ ∥ and κ ⊥ in panel (c) are the intra-unit and inter-unit dielectric constants respectively.. Figure 1: SF of an exciton into two triplets

2. Organic Photovoltaics: External QE of ~ 10% has been reached in organic solar cells. Properly implemented, SF can in principle enhance performance of organic solar cells further, thus reaching the break-even point necessary for commercialization. Our work gives the first systematic study of the conditions that need to be satisfied for SF to give enhanced efficiency, which will depend strongly on the electronic structure of the donor-acceptor interface, and not on the singlet-triplet energy difference alone in any single component Educational Impact: Two undergraduates, Jorge Muñoz from the University of Arizona and Armando Garcia from the University of Texas at El Paso were trained. A publication with Munoz as a co-author is in preparation. Dose Singlet Fission Enhance the Performance of Organic Solar Cells? K. Aryanpour, J. A. Muñoz and S. Mazumdar, in preparation. Dose Singlet Fission (SF) Enhance the Quantum Efficiency (QE) of Organic Solar Cells? Sumit Mazumdar, University of Arizona, DMR 0705163 Figure 2 Broader Impacts Organic Photovoltaics: External QE of ~ 10% has been reached in organic solar cells. Properly implemented, SF can in principle enhance performance of organic solar cells further, thus reaching the break-even point necessary for commercialization. Our work gives the first systematic study of the conditions that need to be satisfied for SF to give enhanced efficiency, which will depend strongly on the electronic structure of the donor-acceptor interface, and not on the singlet-triplet energy difference alone in any single component Educational Impacts: K. Aryanpour, J. A. Muñoz and S. Mazumdar, in preparation. UTEP undergraduate student Armando Garcia explains his poster to postdoctoral fellow Karan Aryanpour in a poster session at the University of Arizona.