Ultrafast Spectroscopic Study For Singlet Fission In Crystalline Tetracene

Singlet fission is a spin-allowed process that creates two triplet excitons from one photo-excited singlet exciton in organic semiconductors. This process of carrier multiplication holds the great potential to break the theoretical efficiency limit in single-junction solar cells by making better use of high-energy photons, while capturing lower-energy photons in the usual style. Though discovered for decades, the phenomenon of singlet fission has caused great attention owing to its significant application in photo electrical conversion devices recently. However, the underlying physics about the singlet fission mechanism is still unclear.Due to the endothermic nature of crystalline tetracene, the theoretical and experimental studies of singlet fission are under great debate. Both the direct and the indirect mechanism is used to describe the fission process. Moreover, the mediation of multi-exciton state, the interference of charge transfer state, the molecular interaction and the quantum coherence are all introduced for considering the physical map of singlet fission. To test these outstanding controversies, we systematically examine the dynamics of SF process in tetracene films and tetracene single crystals with broadband ultrafast TA spectroscopy.1, The singlet-fission rate is assumed to be linearly dependent on the singlet exciton density. We have found that the rate of singlet fission has a nonlinear dependence on the singlet exciton density in tetracene films with small crystalline grains. We disentangle the spectrotemporal features of singlet and triplet dynamics from the raw data using the method of singular value decomposition. The correlation between the singlet and triplet dynamics indicates a superlinear dependence of fission rate on the density of singlet excitons, which may be caused from exciton delocalization as the same as the phenomenon of superradiance. The quantum coherence effect could play an important role in singlet fission.2, We conduct polarization-dependent ultrafast spectroscopic experiment to study the dynamics of singlet fission in tetracene single crystals. The spectrotemporal features of singlet and triplet excitons in transient absorption spectra are found to be strongly dependent on the probe polarization. The signals consist of different transitions from ground-state bleaching, stimulated emission and excited-state absorption related to singlet and triplet excitons are decomposed by analyzing the polarization dependence. Finally, we obtain the accurate singlet and triplet dynamics. The anisotropy of exciton dynamics reflects how the dipoles arrangement affects the process of singlet fission. The techniques of polarized ultrafast spectroscopy provides an alternative approach to understand the mechanism of singlet fission in organic semiconductors.