46906-AC6
Exciton Fission in Solid Tetracene and Related Materials: A Possible Strategy for High Efficiency Organic Solar Cells
Exciton fission, where an initially excited singlet exciton spontaneously splits into a pair of triplet excitons, may provide a novel way to extract electron-hole pairs from high energy solar photons. The goal of this research is to investigate the phenomenon of exciton fission in solid tetracene and its derivatives. We began by measuring luminescence decays and femtosecond transient absorption for a series of evaporated tetracene polycrystalline films. We have made the following observations:
1) The lifetime of the delayed fluorescence, and thus the triplet excitons, is much shorter in polycrystalline films than in single crystals, 200 nanoseconds versus >10 microseconds.
2) Solid tetracene exhibits an induced excited state absorption that is characteristic of exciton-exciton transitions, and that has a lifetime >1 nanosecond. This may correspond to a dark exciton state that is a precursor to the triplet pair state generated by fission. This state is different from the luminescent exciton which decays on the order of 200 picoseconds.
Unfortunately, we were unable to continue these experiments after my post-doc left for a staff scientist position at Caltech. After three offers, I am still trying to hire a qualified post-doc to continue this project. In the meantime, I have a graduate student working on it, who will continue the transient absorption experiments in collaboration with the Center for Nanoscale Materials at Argonne National Lab. I also plan to collaborate with a group at NREL in the coming year to measure the generation of free carriers via fission using time-resolved microwave conductivity. In the event that last offer I have made does not result in hiring a qualified post-doc, I plan to request a rebudget to support students and pay for travel to collaborating institutions.
