Ultrafast Energy Transfer from Local Exciton to Intermolecular CT States in a Supramolecular Model of the Donor–Acceptor Interfaces
Loïc Tanguy, Alexandre Fleury, Paul‐Ludovic Karsenti, Gessie Brisard, Armand Soldera, Pierre D. Harvey
Abstract
The donor–acceptor contacts in bulk heterojunctions (BHJ) is the birthplace of primary energy leaks in organic solar cells (OSCs): Voc (voltage at open circuit) losses. These interfaces are the least understood and the least accessible experimentally. A bottom up approach is being used to mimic this interface via supramolecular interactions of two graphene-based materials: reduced graphene oxide, RGO, and, graphene nanoribbon, GNR, with six different pyrene-linked dyes used as probes to assess the photophysical changes observed upon interface formation in a donor–acceptor bulk heterojunction device. These assemblies are secured through strong π–π interactions between pyrene anchors and GNR, and the formation of these probe–GNR assemblies were studied by absorption spectroscopy. A total of 11 static charge transfer (CT) complexes were identified in 6 different donor–acceptor interfaces, and their properties and dynamics were studied by steady state, time-resolved fluorescence and transient absorption spectroscopy. Three distinct cases were identified: (1) nonharvesting CT assemblies, (2) harvesting and emissive CT assemblies, and (3) harvesting but nonemissive CT complexes. The excitons of the GNR chains are rapidly and fully channeled to the CT complex (<1 ps). Depending on the driving force, the formation of long-lived CT states (302–483 ps) is observed. A three-state model with a coupling between a CT state and a local exciton (LE) is used to describe the behavior of this interface. A HOMOGNR–HOMO2PyrPBI energy gap of 0.16 eV is found to be large enough to efficiently promote a pure charge transfer state at the interface.