Charge mobility in π-conjugated polymer systems

Thesis

Conjugated polymers represent a class of materials which exhibit excellent potential as versatile and cheap semiconductors in microelectronic devices. Their use in OLED displays is already wide-spread, and increasingly efficient solar cells are reported continuously. A key theoretical challenge in π-conjugated polymer systems is the incomplete understanding of charge transport mechanisms, and how competing microscopic and macroscopic effects influence it. In this thesis, charge transport and charge mobility are modelled at various length scales and varying levels of computational accuracy to better understand these processes. The models are based on phenyl-containing polymeric systems, but are generic in their implementation and would be adaptable to a range of comparable systems.

A novel atomistic model of charge mobility in bulk poly(p-phenylene-vinylene) (PPV) is presented. The incorporated structural and energetic disorder results in Anderson localised and quasi-extended donor and acceptor states. Charge migration through the system is facilitated by a kinetic Monte Carlo algorithm, aided by transition rates of separate inter- and intra-chain events defined within the framework of Marcus theory. The effects of disorder, temperature, field strength bias are studied. The log µ ∝ √F field dependence of mobility, which has already been widely reported in literature, naturally emerges at high electric field values. Crucially, the relative importance of inter-chain and intra-chain is highlighted by this model – at low fields, charge transport occurs almost entirely intermolecularly, between highly localised states, while at high fields, intramolecular transitions with the aid of quasi-extended states become most important.

The nature of intramolecular charge dynamics is further examined using a mixed quantum-classical model of charge-polaron transport. The dynamics of the charge follows the time-dependent Schrödinger equation, while the rotational motion of monomers is modelled via the Langevin equation, encapsulating the effect of Brownian fluctuations in rotational displacements. The two are coupled using the Ehrenfest equations of motion. The emergence of two distinct temperature regimes are observed. At the lowest temperatures, the polaron `crawls' along the polymer chain stochastically, while remaining fully localised and in the lowest quasidiabatic state. As the temperature increases, an activated, Landau-Zener type of dynamics is observed, leading to hopping transport between diabatic states. This additional pathway provides a link to the bulk charge transport mechanism.

Authors: Berencei, L

• DOI: 10.5287/ora-mqgnv6anr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: π-Conjugated Polymers; Charge Transfer; Quantum Mechanics
• Subjects: Chemistry, Physical and theoretical