Atomistic Approach To Simulate Processes Relevant for the Efficiencies of Organic Solar Cells as a Function of Molecular Properties. II. Kinetic Aspects Charlotte Brü ckner, † Frank Wü rthner, ‡ Klaus Meerholz, § and Bernd Engels* ,† † Institut fü r Theoretische Chemie, Universitä t Wü rzburg, Emil-Fischer-Straße 42, 97074 Wü rzburg, Germany ‡ Institut fü r Organische Chemie, Universitä t Wü rzburg, Am Hubland, 97074 Wü rzburg, Germany § Department Chemie, Universitä t zu Kö ln, Luxemburgerstr. 116, 50939 Kö ln, Germany * S Supporting Information ABSTRACT: The individual steps of the light-to-energy conversion process in the vicinity of the interfaces of organic solar cells are investigated with kinetic Monte Carlo simulations employing Marcus hopping rates obtained from quantum-chemical calculations. A chemically diverse set of p- type semiconducting molecules in heterojunction with full- erene C 60 is used. Starting with exciton diffusion, exciton dissociation, charge generation, and charge separation are modeled on an atomistic level. Numerous aspects were already analyzed, but comprehensive simulations including all three processes in amorphous model interface systems and a comparison of various different molecular p-type semi- conductors seem to be missing. Our investigation identifies several important kinetic effects that could limit device efficiencies, such as the strong reduction of charge transport rates in the vicinity of the interface due to Coulomb interactions between the charges, the importance of adjusting the relative rates of exciton transfer and dissociation, and the impact of morphology. Charge drift velocities and hole mobilities obtained from the simulations compare well with experimental values indicating that the main effects are covered by the simulations. A correlation between experimental short-circuit currents and simulated charge drift velocities suggests that slow charge-transfer processes could represent a major efficiency-limiting parameter in organic solar cells. I n recent years, organic solar cells (OSCs) have attracted much research interest, and promising device efficiencies were observed, especially for the bulk heterojunction (BHJ) cell architecture where p-type and n-type semiconducting layers are intermixed. 1,2 Assuming the so-called “cold exciton breakup”, 3 the light-to-energy conversion in these OSCs can be described as a five-step process. 4,5 By light absorption, an exciton is created in one of the semiconducting layers (Step 1: light absorption) and diffuses within the respective bulk phases (Step 2: exciton diffusion/transport). If it reaches the interface between the p-type and the n-type semiconductor, it dissociates into a charge-transfer state across the interface (Step 3: photoinduced charge transfer). At first, depending on the exact energies, the charge-transfer state can still be bound due to significant Coulomb attraction between the geminately formed electron and hole. Despite this Coulomb attraction, the electron and the hole can overcome their mutual binding energy so that they migrate independently through the respective semiconducting layer (Step 4: hole/electron separation and transport). This step can be further subdivided into charge separation taking place near the interface and charge transport occurring as soon as the mutual attraction has diminished. Finally the charges are recollected at the electrodes (Step 5: charge recollection). 6 Intense research in the field has brought about a plethora of polymers 7 and small organic molecules 8,9 for OSCs, all with advantages and drawbacks. In order to further optimize device efficiencies, structure-property relationships would be ex- tremely helpful to correlate molecular and aggregate proper- ties 10 with the efficiencies of the individual steps of the light-to- energy conversion process. Simulations of these processes are a field of intense research so that a complete review of previous investigations is beyond the scope of this work. Hence we can only focus on some examples. Further information can be taken from reviews on charge transport, 11 on exciton transport, 12 and on the charge-transfer and recombination processes. 13 Exciton and charge transport in the disordered semi- conducting layers are usually considered to be incoherent; i.e., they are viewed as successive individual hopping processes of excitons and charges between localized states. 14-16 The Received: November 13, 2016 Revised: December 13, 2016 Published: December 19, 2016 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.6b11340 J. Phys. Chem. C XXXX, XXX, XXX-XXX