The efficiency of solar energy conversion in single-junction photovoltaic devices limited by the Shockley-Queisser limit to values of ~34% can be improved to ~45% through singlet fission (SF). SF is a process where an excited state chromophore couples with a ground state chromophore, generating a correlated triplet pair, which eventually separates into two triplets. Thus, SF generates a pair of charge carriers from the absorption of a single photon, thereby increasing the energy conversion efficiency of photovoltaic devices. Understanding the molecular structural dynamics during the SF process will help to fabricate materials with optimal charge separation and transport properties for highly efficient organic photovoltaics. Here, we use femtosecond stimulated Raman spectroscopy (FSRS) to provide new insights to the structural evolution during the SF and triplet separation processes in rubrene and structurally similar derivatives. FSRS is an ultrafast vibrational technique capable of probing the nuclear dynamics within a system on a femtosecond timescale, thus providing information about the real time structural changes that the system undergoes after photoexcitation. In crystalline rubrene, we observed frequency shifts of two vibrational modes due to structural rearrangement during triplet separation. These results, accompanied by DFT calculations and spontaneous Raman measurements on a fluorinated derivative of rubrene (F<sub>20</sub>-rubrene), proved that the loss of electron density from the tetracene backbone is associated with the triplet separation from the correlated triplet pair in rubrene. I will discuss how deliberately functionalizing the molecular structure to promote decreased ground state electron density in the tetracene core affects the SF dynamics. These results, based on our initial studies, enable us to intelligently design and propose new synthetic molecules for efficient SF and prove that this rational design gives valuable insights into the structure-function relationship during SF.

Exploring structural dynamics during singlet fission using femtosecond stimulated Raman spectroscopy

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.