Theoretical investigations of the interaction between electron and hole polarons in thiophene oligomers

Interaction between negatively charged (electron) and positively charged (hole) polarons in polythiophene are studied theoretically. Two basic systems are used in order to model polarons residing on a single oligothiophene chain as well as on two separate oligothiophene chains. The purpose of the work is to contribute to the understanding of electroluminescence in polythiophene as the phenomenon appears in light-emitting diode devices. The calculations are performed at a semiempirical level using the Austin model-1 Hamiltonian to optimize the polaron geometries, and the intermediate neglect of differential overlap Hamiltonian in combination with configuration interaction to calculate excited states. The calculations show that when the electron polaron and the hole polaron residing on different oligomers are allowed to interact, an excitonic state delocalized to both oligomers is formed. This state corresponds to the second excited state (S-2) and has a lifetime of 1.6 x 10(-9) s and an energy of 2.37 eV for radiative transition to the ground state. The first excited state (S-1) has an energy of similar to 1.86 eV, and the transition from this state to the ground state is forbidden by symmetry. Triplet states are calculated to appear very close in energy to the singlet excited states, which indicates that spin-orbit coupling might play an important role in this system. When the electron polaron and the hole polaron reside on the same oligomer the results show that the picture of separate polarons breaks down even for the case where the polaron geometries are clearly separated, and that electroluminescence for this case is best described as deexcitation from the first excited state S-1. As opposed to the case with two separate oligomers the transition from SI to the ground state is symmetry allowed, with an energy and lifetime of 2.08 eV and 8.9 x 10(-10) s, respectively.