Electrochemical studies of semiconducting polymers: correlation of energetics and stability

In order to make organic solar cells commercially viable, it is necessary to design devices with higher efficiency and longer lifetimes than is currently accessible. Energetics of materials used in organic solar cells are already known to play a key role in various aspects of device performance, including charge generation and ambient stability. In order to make progress on either of these fronts, a clear understanding of frontier molecular orbital energy levels in semiconducting polymers is key, which is one issue that this thesis aims to address. Less understood is what role energetics plays in operational stability, which is another issue that this thesis aims to address. Cyclic voltammetry (CV) was employed to study the effect of relative semiconducting polymer crystallinity on highest occupied molecular orbital (HOMO) energy levels in neat films. Two HOMO energy levels were found, with a difference in energy of ca 60 meV, corresponding to relatively ordered and disordered phases in the film. The effect of various thermal treatments on the relative ratio of ordered:disordered phases is investigated. During solar cell operation, the semiconducting polymers are partially oxidised, with bulk hole polaron densities of ca 10^15-17 cm-3. Chronoamperometry and absorption spectroscopy were combined to develop a methodology for accelerating degradation caused by the presence of hole polarons in neat films. The methodology was initially employed to investigate the relative stabilities of the hole polarons formed when P3HT and two structurally analogous semiconducting polymers with deeper HOMO energy levels are partially oxidised. The study was extended to include a variety of donor- acceptor polymers. It is shown that stabilising the HOMO results in a less stable hole polaron.Open Acces

Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells

We present the synthesis and characterization of two novel thiazole-containing conjugated polymers (<b>PTTTz</b> and <b>PTTz</b>) that are isostructural to poly­(3-hexylthiophene) (P3HT). The novel materials demonstrate optical and morphological properties almost identical to those of P3HT but with HOMO and LUMO levels that are up to 0.45 eV deeper. An intramolecular planarizing nitrogen–sulfur nonbonding interaction is observed, and its magnitude and origin are discussed. Both materials demonstrate significantly greater open circuit voltages than P3HT in bulk heterojunction solar cells. <b>PTTTz</b> is shown to be an extremely versatile donor polymer that can be used with a wide variety of fullerene acceptors with device efficiencies of up to 4.5%. It is anticipated that this material could be used as a high-open circuit voltage alternative to P3HT in organic solar cells