Charge separation, triplets and photochemical stability in polymer / fullerene solar cells

Abstract

Charge generation and stability are key issues that have great impact upon the commercial viability of organic solar cells. In this thesis, a range of donor polymers, mainly of donor-acceptor class, were employed. Various materials characterisation, photophysical and photostability studies were performed on neat polymer films and polymer/fullerene blend films with the aim of establishing relationships between material structure and device function/stability. Transient absorption spectroscopy (TAS) was employed for the photophysical studies on neat films. The photophysics of triplet excitons are found to strongly correlate with relative polymer crystallinity as determined from wide-angled x-ray diffraction (WAXD), with the more amorphous polymers exhibiting longer triplet lifetimes. The rate constant and yield of oxygen quenching of these triplet states also showed clear correlations with material crystallinity. Charge generation in polymer/fullerene blend films was investigated using TAS and steady state optical spectroscopies. Compositional dependence studies with varying fullerene loadings were conducted on two polymers of different crystallinity, with a stronger dependence being observed in the more amorphous blend films. A comparison of charge generation pathways via electron or hole transfer suggests that the energetics between donor and acceptor can affect the efficiency of these pathways. This is consistent with the observation of a correlation between polaron yield and the energy offset driving charge separation for a series of blend films. Furthermore, the polaron yield estimated from TAS was correlated with device photocurrent. Photochemical stability is of significant concern in organic solar cells, as organic materials are susceptible towards photo-oxidation. Accelerated photodegradation in neat and blend films was monitored using steady state absorption spectroscopy under oxygen atmosphere. More crystalline polymers with shorter triplet lifetimes are found to be more stable. The mechanism of photodegradation involving triplet-mediated singlet oxygen generation was investigated with a molecular fluorescent probe, and found to be a significant photodegradation pathway.