Understanding singlet fission (SF) is an important strategy for extending the power conversion efficiency of next-generation solar cells beyond the Shockley-Queisser limit. Thus far, most studies of SF have focused on polycrystalline films, whose heterogeneity often makes it difficult to probe the electronic states involved in the formation of the correlated triplet biexciton state,1(T1T1), and its decorrelation into free triplet excitons. Polarization-resolved, femtosecond transient absorption microscopy of a 1,6,9,14-tetraphenylterrylene-3,4:11,13-bis(dicarboximide) (Ph4TDI) single crystal reveals the formation of transient spectra in <200 fs having features characteristic of1(T1T1),1(S1S0), and Ph4TDI•+-Ph4TDI•-charge-transfer (CT) states. This indicates that either an ultrafast equilibrium between these states occurs or that the initially formed biexciton state is a mixture of these states. This spectrum evolves in time to give the Tn← T1spectrum of the free triplet excitons. We show further that this initially formed state has its optical transition polarized nearly perpendicular to that of the Tn← T1transition in the free triplet excitons. This indicates that the observed free triplet excitons have migrated from the π-stackedPh4TDIcolumns where the1(T1T1) state is produced to neighboring, nearly orthogonal π-stackedPh4TDIcolumns, resulting in a weaker electronic interaction between the free triplet excitons essential to extending their lifetimes. However, the mobility of the1(T1T1) state in thePh4TDIπ-stacking direction results in significant annihilation of biexciton pairs limiting the yield of free triplet excitons to about 60%.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
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Schlesinger, I. (Contributor), Zhao, X. (Contributor), Powers-Riggs, N. E. (Contributor) & Wasielewski, M. R. (Contributor), Cambridge Crystallographic Data Centre, 2021