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)