Spectroscopic method to determine exciton densities, trap interactions, and exciton decay pathways in organic semiconductors

The unique optoelectronic properties of organic semiconductors make them attractive candidates for functional electronic devices. Organic molecules exhibiting singlet fission---a process in which a photoexcited singlet exciton converts into two triplet excitons---hold particular promise for enhancing the efficiency of organic optoelectronic devices. Once generated, triplet excitons decay either through triplet-triplet annihilation (TTA) or interaction with trap states. Identifying these decay pathways is essential for understanding and controlling the performance of organic semiconductors. Modulated photocurrent (MPC) spectroscopy is a useful technique for extracting information about various parameters of the trap states of inorganic and organic semiconductors. Recently, it was shown that in organic semiconductors the MPC is highly sensitive to exciton-trap interactions, where excitons dissociate into mobile holes. Here, using this approach, we present a new spectroscopic method based on MPC for extracting both the triplet exciton density and the density of traps involved in their dissociation in two-terminal organic devices: rubrene single crystals and pentacene thin films. Furthermore, we show that the characteristic dependence of triplet exciton density on the light generation rate enables the separation of different decay pathways. Our results reveal that triplet exciton decay is dominated by TTA in rubrene crystals, while in pentacene films it primarily proceeds via triplet-trap interactions.