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 is used to study both perylene and terrylene diimide single crystals. Bis(3′-aminopentyl)-perylene-(3,4:9,10)-bis(dicarboximide) (C₅PDI) crystallizes into a structure having π-stacked dimers, wherein the PDI pairs are twisted ~51° relative to one another. Formation of the correlated triplet pair state ¹(T₁T₁) in the C₅PDI single crystals occurs in τ = 57 ± 4 ps mediated by an excimer-like intermediate having charge transfer character, while in a solvent-vapor-annealed C₅PDI polycrystalline thin film, ¹(T₁T₁) formation occurs in τ = 169 ± 6 ps. While the quantum yield of the ¹(T₁T₁) state formation in each case is nearly quantitative, the free triplet exciton quantum yield in the single crystals is 70%, while that in the annealed polycrystalline films is only 29%. Steady-state and time-resolved emission measurements indicate that the disorder in the polycrystalline film hinders SF performance via long-lived excimer trap states at sites. In contrast, single crystals of 1,6,9,14-tetraphenylterrylene-3,4:11,13-bis(dicarboximide) (Ph₄TDI) reveal the formation of transient spectra in < 200 fs having features characteristic of ¹(T₁T₁), ¹(S₁S₀), and Ph₄TDI⁺-Ph₄TDI^- charge transfer (CT) states. This spectrum evolves in time to give the T_n ← T₁ spectrum of the free triplet excitons. We show that this initially formed state has its optical transition polarized nearly perpendicular to that of the T_n ← T₁ transition in the free triplet excitons. This indicates that the free triplet excitons have migrated from the p-stacked Ph₄TDI columns, where the ¹(T₁T₁) state is produced, to the neighboring orthogonal π-stacked Ph₄TDI columns, resulting in a weaker electronic interaction between the free triplet excitons essential to extending their lifetimes. However, the mobility of the ¹(T₁T₁) state in the Ph₄TDI π-stacking direction results in significant annihilation of biexciton pairs limiting the yield of free triplet excitons to about 60%.

Malik Williams

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