Kinetics of charge transfer processes in organic solar cells: Implications for the design of acceptor molecules

We report the electronic properties of a new class of non-fullerene electron acceptor molecules with electron affinities tunable over an approximately 1 eV range. This tunability allows us to vary the thermodynamic driving force for electron transfer (Delta G degrees) such that it is equal-and-opposite-to the reorganization energy for the ionized states (lambda). We utilize this design principle, derived from Marcus-Hush theory, to optimize the rate of charge transfer in blends of these acceptors with poly(3-n-hexylthiophene-2,5-diyl) (P3HT) - a standard organic solar cell donor material. We show that computationally inexpensive calculations can be used to parameterize Marcus-Hush theory so as to correctly predict whether quenching will occur. Arguments based solely on energetics are common in the literature and we show that such theories do not predict the trends observed in our photoluminescence quenching experiments. This is the case whether the energies determined from experiments [cyclic voltammetry (CV) and the optical gap) or calculated from density functional theory for the solid state. We predict essentially barrier-less photoelectron transfer (PET) from P3HT to the acceptor 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c] [1,2,5] thiadiazol-4-yl}methylene]malononitrile (or K12), consistent with the experimental photoluminescence quenching efficiencies found for P3HT:K12 blends. Our results clearly show that energetics alone is not sufficient to predict PET between the acceptor-donor pair, and that kinetics are an important determining factor. (C) 2012 Elsevier B.V. All rights reserved.