Theoretical and spectroscopic studies of energy and charge transport in organic semiconductors

Abstract

The performance of organic semiconductor devices is heavily dependent on the precise molecular-level arrangement and overall morphology of the functional layers. In organic photovoltaic applications, exciton mobility, fission/fusion or dissociation, as well as charge transport and separation are some of the morphology-dependent processes that are of interest for efficient device design. In this work a combination of experimental and computational techniques are used to elucidate the behaviour of excitons in conjugated polymer and small-molecule semiconductor systems. While there is an emphasis on photovoltaic applications, many concepts are generally applicable to other organic electronic applications such organic light emitting diodes and photodetectors. In Chapter 3, a pump-push-probe transient absorption technique is used to observe high-energy “hot” excitons formed by photoexcitation of the conjugated polymer poly(3-hexylthiophene) (P3HT). The work demonstrates the ability to clearly isolate the transient signal of the hot exciton decay processes from the thermalised exciton population, where picosecond and sub-picosecond relaxation of hot excitons through torsional motion in the disordered polymer chain is observed. In addition, the push-induced dissociation of high-energy excitons into free charge carriers is able to be quantified and an upper bound on the exciton binding energy determined. Spectroscopic experiments on P3HT are accompanied by a hybrid quantum-classical exciton hopping model in Chapter 4. Coarse-grained molecular dynamics are used to obtain realistic structures of P3HT free chains and nanofibre aggregates, to which a Frenkel–Holstein exciton model and Monte Carlo hopping simulation is applied. This novel approach captures exciton transport properties of polymer systems with a monomer-level of detail unachievable with continuum or lattice style models, but at a large scale infeasible with fully quantum calculations. Reasonable quantitative agreement with experimental observables is obtained, offering insight into the morphology-dependence of exciton transport in conjugated polymers. In particular, the observed tendency for exciton migration to the core of the polymer aggregate can explain the relatively poor performance of highly crystalline or nanofibre-based polymer solar cells, as well as the unusually high fluorescence yield of aqueous P3HT nanoparticles. The effect of disorder in small molecule semiconductor films is investigated in Chapter 5 in the context of singlet exciton fission and triplet fusion under the influence of applied magnetic fields. A model is presented that extends the historical theory of molecular spin interactions in crystalline materials and corrects the current understanding in the literature regarding such disordered solid-phase systems. The possibility of using the fluorescence response to magnetic fields to probe the morphology and degree disorder in the films is demonstrated. Extending the model to solution-phase behaviour is then discussed in Chapter 6, where the potential of improving the light-harvesting ability of solar cells through a molecular triplet–triplet annihilation upconversion process is considered. Molecular dynamics simulations are used to obtain physical parameters and collision geometry of the emitter molecules in solution. The complications of applying a static model of triplet fusion to the dynamic solution-phase behaviour are elucidated and the potential of synthesising an ideal upconversion emitter molecule is discussed.