Charge transport in organic solar cells

The process of charge transport is fundamental to the operation of all electronic devices. In organic photovoltaics, high efficiencies can only be achieved if charge transport is able to extract charge carriers from the active layer with minimal recombination losses. This work presents new insights into the measurement of charge transport, the underlying physics, as well as new approaches for modelling. Numeric simulation software using a drift-diffusion-recombination model is developed and applied to organic photovoltaic devices. Specifically, this model is used to design and interpret charge transport experiments that are applicable to operational organic solar cells. Charge carrier mobility is studied using photogenerated charge extraction by linearly increasing voltage (photo-CELIV) and the novel technique of resistance-dependent photovoltage (RPV). These experiments demonstrate the absence of "hot carrier" relaxation effects on the timescales of charge transport in several organic photovoltaic polymer:fullerene blends. This is surprising because it has previously been argued that such relaxation is the cause of the deterimental dispersive transport that affects many organic semiconductor devices. It is argued instead that dispersive transport arises from the loss of carriers to trap states. Next, the techniques are extended to recombination measurements, where the recombination coefficient in a benchmark polymer:fullerene system is found to depend upon the polymer's molecular weight. Modelling of the steady-state photocurrent produced by a solar cell demonstrates the conditions under which non-geminate recombination may be avoided, and presents a design rule for avoiding non-geminate recombination. Experimental measurements on devices of varying thickness support the conclusion that the space-charge limited current is a fundamental threshold for high-efficiency photocurrent extraction. Finally, fractional kinetics and generalised diffusion equations are explored. We show that the Poisson summation theorem permits the analytic solution of a fractional diffusion equation to be collapsed into closed form. Subsequently, these techniques are applied to a new type of kinetic model that is capable of unifying normal and dispersive transport within a single framework