Solar energy is one of the most promising alternative sources of energy due to its widespread availability. However, the efficiency of single junction photovoltaic devices is limited by the Shockley-Queisser limit to ~34%. This efficiency can be improved to ~45% through singlet fission (SF). SF is a process where an excited singlet state couples with a neighboring ground state molecule, forming 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 design 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. FSRS is an ultrafast technique capable of probing the vibrational dynamics within a system on a femtosecond timescale, thus providing information on the structural changes that the system undergoes during the triplet pair generation and separation processes. We observe frequency shifts of two vibrational modes due to structural rearrangement during triplet separation in rubrene. These results, accompanied by DFT calculations and spontaneous Raman measurements on a fluorinated derivative of rubrene (F<sub>20</sub>-rubrene), suggest that the loss of electron density from the tetracene backbone drives the triplet separation from the correlated triplet pair in rubrene. Hence, materials that will promote similar changes in the electron density can be synthesized and monitored to investigate and improve the SF yield in organic materials, making SF a viable means to achieve higher efficiency photovoltaic devices.

Probing structural rearrangements during rubrene singlet fission using femtosecond stimulated Raman spectroscopy

255th National Meeting (2018)

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