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Xiaoyang Zhu and Antoine Kahn Abstract We present our understanding of the electronic energy landscape and dynamics of charge separation at organic donor/acceptor interfaces. The organic/organic interface serves as a valuable point of reference and plays an important role in emerging electronic and optoelectronic applications, particularly organic photovoltaics (OPVs). The key issue on electronic structure at organic donor/acceptor interfaces is the difference in the lowest unoccupied molecular orbitals or that in the highest occupied molecular orbitals. This difference represents an energy gain needed to overcome the exciton binding energy in a charge-separation process in OPV. A sufficiently large energy gain favors the formation of charge transfer (CT) states that are energetically close to the charge-separation state. At an organic donor/acceptor interface in an OPV device, these high-energy CT states, also called hot CT excitons, are necessary intermediates in a successful charge-separation process.

Introduction Organic photovoltaics (OPVs) based on donor/acceptor (D/A) materials1 are subjects of intense research efforts.2–4 The mechanisms for photocurrent generation in OPVs are complex, but the most critical steps are believed to occur at D/A interfaces. Photoexcitation of organic (including polymeric) semiconductors results in bound electron-hole (e-h) pairs, called Frenkel or molecular excitons, with binding energy in the range of 0.1–1 eV. A common consensus is that overcoming such a high binding energy requires an energetic driving force provided by differences in molecular orbital energies at the D/A interface. However, after interfacial charge transfer, the resulting electrons and holes are not immediately free. The low dielectric constants of organic semiconductors ensure that an e-h pair across the D/A interface is bound by the Coulomb potential. From typical sizes of conjugated molecules or

repeating units, we can estimate that the binding energy of the e-h pair across a D/A interface is slightly lower (e.g., by a factor of two) than that of the Frenkel exciton, but still approximately an order of magnitude higher than thermal energy at room temperature. A successful charge separation event also must overcome the Coulomb potential across the D/A interface. This description calls for answers to two fundamental questions: (1) What determines the energetic landscape at the organic D/A interface? (2) What are the dynamic processes responsible for the success or failure of charge carriers in escaping the Coulomb trap at the interface? The first question has been addressed extensively from both experimental and theoretical perspectives, but significant confusion and misconceptions remain in the OPV community. For example, it is common practice to use ionization energies (IEs) and electron affini-

MRS BULLETIN • VOLUME 35 • JUNE 2010 • www.mrs.org/bulletin

ties (EAs) of the organic materials in determining alignment of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular o