Singlet Fission in Terrylenediimide Single Crystals: Competition between Biexciton Annihilation and Free Triplet Exciton Formation

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 of (1)(T1T1), 1(S1S0), and Ph4TDI center dot+-Ph4TDI center dot- 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 T-n <- T-1 spectrum of the free triplet excitons. We show further that this initially formed state has its optical transition polarized nearly perpendicular to that of the T-n <- T-1 transition in the free triplet excitons. This indicates that the observed free triplet excitons have migrated from the pi-stacked Ph4TDI columns where the (1)(T1T1) state is produced to neighboring, nearly orthogonal pi-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 pi-stacking direction results in significant annihilation of biexciton pairs limiting the yield of free triplet excitons to about 60%.