Examining the Links Between Organic Photovoltaic Operation and Complex Morphological Structures

Abstract

This thesis utilises computational simulations to investigate the relationship between morphological structure of the active layer within organic photovoltaic devices, and its impact on the device performance. Specifically, the effects of hot charge transfer states, the mixed molecular phase and fullerene aggregation on organic photovoltaic performance, and polymer crystallinity on carrier mobility, are explored using kinetic Monte Carlo simulations. These investigations agree with experimental results and shed new light on the processes of recombination, ultrafast charge separation and the utility of the amorphous phase within the context of tie-chains.

A more accurate charge separation kinetic model is proposed in order to correctly describe the biexponential carrier recombination determined from Monte Carlo investigations. The model incorporates a `quasi-free' state where charges are still Coulombically bound but sufficiently separated to prevent recombination. This is conceptually similar to the cooled remains of hot charge transfer states, the effects of which are investigated on device operation and shown to provide a benefit that is strongly dependent on the aggregation of the fullerene phase, the limitation of the molecularly mixed phase and the relative charge carrier mobilities. Crystallisation within the polymer medium is then comprehensively explored using a combination of molecular dynamics and Monte Carlo simulations, along with quantum chemical calculations to help elucidate the observed annealing temperature and molecular weight dependencies of the carrier mobility for a poly(3-hexylthiophene-2,5-diyl) test system. The annealing temperature trend can be explained by increased crystallite size and order, but the molecular weight dependence is not satisfactorily explained by the crystalline regions. Instead, mobility is shown to be limited by the availability of tie-chains in the amorphous phase of the morphology, linking together crystals and providing regions of high mobility through the amorphous material.