Molecular Insight Into the Energy Levels at the Organic Donor/Acceptor Interface: A Quantum Mechanics/Molecular Mechanics Study

We present an investigation of the band levels and charge transfer (CT) states at the interface between two organic semiconductors, metal-free phthalocyanine (H2Pc) and 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI), using a combined quantum mechanics/molecular mechanics (QM/MM) technique. Near the organic organic interface, significant changes from the bulk, as large as 0.2 eV, are found in the excited state energies, ionization potentials, and electron affinities, due to differences in molecular packing and polarizabilities of the two molecules. The changes in the ionization potential and electron affinity cause the CT states at the interface to be on average higher in energy than fully separated charges in the bulk materials despite having a typical local binding energy of 0.15 eV. Furthermore, we find that thermal fluctuations can induce variations of up to 0.1 eV in the CT binding energy. These results suggest that it is possible for bound interfacial CT states to dissociate in a barrierless fashion without involving "hot" CT states. This observation has direct relevance to the design of more efficient organic photovoltaics.