Physics Colloquium

We have taken a materials intensive approach to developing an understanding of the mechanism of photocurrent and photovoltage generation in organic photovoltaic devices (OPVs). We have explored the use of metal porphyrin complexes as donor materials in OPVs. The complexes we have chosen have high nonplanar structures in the ground state and excited state. Using a ring fusion process we have generated mono-porphyrin complexes that absorb light efficiently to ca. 1000 nm. The exciton is a critical part of each of these processes, and being able to control the location, lifetime and energy of the exciton is essential to achieving high efficiency. We have investigated methods for tuning exciton energies and controlling their migration paths, both intramolecularly and within a thin film. I will discuss our most recent work with porphyrinic materials for OPVs. This involves a careful materials design study that leads to both low energy absorption (into the nearIR) and the use of substituted porphyrins to efficiently harvest photons through the entire visible spectrum. To that end we have used transient absorption spectroscopy and measured the rates of singlet and triplet energy transfers between organic dyes (BODIPY and tetracenes). Both intra- and inter-molecular energy transfers take place on the picoseconds time scale. Thus, the systems are fully equilibrated into the lowest energy triplet state(s) before nonradiative decay. Using this approach we can efficiently harvest energy across the visible and into the NIR. Both long wavelength and broad absorption are achieved with high extinction. I will discuss the photophysical properties of these materials and their use in OPVs.