Beyond Bloch: a theoretical blueprint for conjugated polymer optoelectronics

Abstract Conjugated polymers have recently regained strong interest because of their potential in organic electronic devices such as organic light-emitting diodes (OLEDs), field-effect transistors (FETs), and organic photovoltaics (OPVs). Unlike inorganic semiconductors where electrons are commonly treated as delocalized or quasi-free Bloch states,electrons in conjugated polymers occupy strongly coupled, localized σ and π orbitals and interact intensely with the ionic lattice. These materials therefore deviate markedly from the assumptions underlying Bloch-based approaches, motivating the development of alternative theoretical descriptions of charge transport. Here we present a many-body model that explicitly includes electron-electron interactions, both on-site and between π states on alternating chain sites. The model naturally explains semiconducting behavior in the undoped polymer and predicts two novel flat bands of excited bonding states. These excited states exhibit bosonic character and enable an unconventional charge-transport channel. The model also quantitatively reproduces experimental observations, yielding excellent fits to measured UV-Vis absorption and electro-luminescence spectra.