2, Argonne National Laboratory, Lemont, Illinois, United States
The most widely used electro-optic devices today employ inorganic semiconductors because of their high dielectric constants, but organic light emitting diodes (OLEDs) and organic photovoltaic devices (OPVs) are beginning to claim market share because organic materials are cheap, manufacturable on a large scale, non-toxic, and plentiful. The fundamental chemical challenge with organic optical materials, however, arises from their low dielectric constants and localized excitons. Excitations in these materials couple to vibrational modes which substantially hinders migration of the exciton to a heterojunction, thereby fundamentally limiting device efficiency. By exploiting long-range transport in dark states of organic H-Aggregates, the limitations of exciton diffusion length can be circumvented altogether---an approach which is fundamentally different from current state-of-the-art materials for organic photovoltaics. Today, bulk heterojunction OPVs create topologies that eliminate the need for long diffusion lengths, but the blended nature of the donor and acceptor layers significantly increases the probability of charge carrier recombination and trapped excitons. My approach couples individual molecular dipoles to allow coherent energy transfer through a dark state that is delocalized across many molecular units. Phthalocyanines, a close cousin to the biologically relevant solar-harvesting porphyrins, are being synthesized and utilized as a model system. Energies and couplings between electronic states will be captured in real time while imaging both coherent and incoherent transport mechanisms utilizing ultrafast Gradient Assisted Photon Echo Spectroscopy (GRAPES). Further utilizing nanosecond transient absorption in conjunction with GRAPES will provide spectral information that spans twelve orders of magnitude in time and can inform how the dark states in H-Aggregates participate in exciton relaxation and diffusion.