Charge and energy transport in organic semiconductors

This work describes the application of Kinetic Monte Carlo (KMC) modelling technique to organic photovoltaic (OPV) devices. Such devices are an exciting and relatively new form of photovoltaic (PV) technology, which can help bring solar power to the mass market using low energy processing methods, and materials that are cheap, and have several novel characteristics, such as being lightweight, flexible, and potentially even translucent. The modelling technique and many of the results found here are also applicable to other organic devices, such as organic light-emitting displays (OLEDs), as the underlying device physics is very similar. Following an introduction and discussions on the theoretical basis of the work and its computational implementation, the work described in the thesis falls into three main sections: Firstly, an evaluation is performed as to the accuracy of the First Reaction Method (FRM), a means of reducing the computational complexity of KMC simulations. Although this method is widely used, its accuracy when used to model OPV devices has never been satisfactorily evaluated, leading it to be questioned by some authors. Hence, its accuracy under a range of scenarios relevant for OPV simulations was tested and quantified. The findings presented here confirm its validity within the field and disorder ranges that are applicable to OPV device operation, and also give some insight into low-field geminate separation dynamics. Secondly, the KMC methodology, with the FRM approximation, is applied to the investigation of the role of device morphology in determining OPV efficiency. Morphology optimisation has frequently been identified as being key to future device design, and the KMC methodology is unique in its ability to examine this. Furthermore, as the popularity of using self-assembled bicontinuous nanostructures in OPVs grows, it is useful to evaluate their potential impact on OPV efficiency, using the insight gained from investigating morphology in general. Among the main conclusions reached from this work, it was determined that one of the key limiting factors in the efficiency of devices is the angle of the heterojunctions to the field, which is a feature of the device morphology. It was also found that, because of this, bicontinuous structures are unlikely to greatly improve OPV efficiency. Thirdly, modelling was performed in an attempt to reproduce the quantitative experimental characteristics of PFB:F8BT devices. This was achieved through first modelling individual charge mobility in the two polymers in question, and quantifying the effects of different forms of disorder. Having found disorder descriptions that could reproduce the single carrier mobility of both PFB and F8BT, as deduced by Blakesley et. al. using drift-diffusion modelling, this disorder description was applied to single layer devices, in order to deduce the injection barrier. Finally, the disorder and injection barriers deduced were combined with optical modelling to reproduce full photovoltaic behaviour. This was generally found to be successful, and therefore potentially gives some real insight into the nature of polymer disorder, whilst also validating the KMC model used in this thesis. An additional implication of this work is that the KMC model can, in the future, be applied to experimental data which cannot be satisfactorily modelled using drift-diffusion simulations.