Competition between the Charge Transfer State and the Singlet States of Donor or Acceptor Limiting the Efficiency in Polymer:Fullerene Solar Cells - Faist, MA; Kirchartz, T; Gong, W; Ashraf, RS; McCulloch, I; de, Mello JC; Ekins-Daukes, NJ; Bradley, DDC; Nelson, J (JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 2012)