Photophysics of pentacene thin films: The role of exciton fission and heating effects

There is evidence that the photo-generated singlet exciton in polycrystalline pentacene films undergoes rapid and efficient fission to form two triplet excitons. However, the role of exciton fission in pentacene has been controversial, with previous studies putting forward alternate relaxation pathways for the singlet exciton, such as excimer or charge formation, or internal conversion to a doubly excited exciton. We report temperature- and angular-dependent ultrafast transient optical absorption measurements in a wide spectral and temporal window. Angular-dependent transient spectra identify a common origin to photo-induced absorptions at 530 and 860 nm, which we associate with triplet excitons. These constitute the dominant relaxation channel for singlet excitons. Other features, particularly near 620 nm, previously assigned to excimers or electronic charges, are shown to be caused by thermal modulation from the optical pump.