Miranda, RP; (2015) Efficient multiconfigurational time-dependent simulation of conjugated polymers. Doctoral thesis , UCL (University College London).

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Abstract

Conjugated polymers have become an important class of functional materials for a wide range of optoelectronic applications, from which polymer-based solar cells stand out as one of the most promising new devices. While experimental progress has been made at a good pace over the last couple of decades, the fundamental processes governing the photophysics of conjugated polymers are not yet completely understood. A theoretical description is challenging, since these systems exhibit both strong electron-electron and electron-nuclear interactions. A detailed understanding of the photoexcitation process, and of the steps following photoexcitation, requires a nonadiabatic treatment of the electron-nuclear dynamics, and a proper description of the excited electronic states and interchain interactions, for which many-body effects are important. Some of these ingredients have often been neglected in dynamical calculations. In particular, most studies which include electron-electron interactions have ignored the singlet character of the photoexcited state, by restricting the wavefunction to the form of a single Slater determinant. In this thesis, we develop a nonadiabatic molecular dynamics method which allows for the coupled evolution of the nuclear degrees of freedom and of multiconfigurational electronic wavefunctions. The proposed scheme effectively establishes a compromise between efficiency and accuracy, which enables the study of large systems. Furthermore, it is designed to take into account the appropriate spin symmetry of the electronic wavefunction, thus allowing us to distinguish between singlet and triplet excited states, which exhibit quite different properties. The formalism is applied to semiempirical single- and double-strand models of a prototypical conjugated polymer, in order to investigate the effects of Coulomb interactions and interchain coupling on the dynamics of low-lying excitations. The nature of the photoexcited states and the issue of charge photogeneration in conjugated polymers are also addressed, as well as the charge transfer process at donor/acceptor interfaces.

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