Characterization of triplet separation and diffusion in amorphous pentacene films via ultrafast infrared spectroscopy

The performance of photovoltaic and light-emitting devices that utilize singlet fission sensitization has been modest to date, despite spectroscopic measurements reporting high triplet exciton multiplication yields in the singlet fission active layer of these devices. This contrast highlights the need to characterize the factors that underpin device performance, such as triplet separation and diffusion. Here, we use ultrafast infrared spectroscopy to probe the dynamics and separation of correlated triplet pair intermediates following singlet fission in amorphous and crystalline pentacene films. The ultrafast vibrational measurements reveal that triplet-pair separation occurs on similar timescales in both types of films, despite differences in intermolecular coupling strength. Conversely, ultrafast electronic spectroscopy measurements of diffusion-controlled triplet-Triplet annihilation reveal that triplet diffusion in the amorphous film is an order of magnitude lower than the crystalline analog. Together, these results suggest that sparse triplet traps limit the transport of triplet excitons in the amorphous film. Therefore, device developers should seek to identify the structural origins of these states to identify molecular structures that self-Assemble in patterns that avoid triplet trap state formation.