Organic photovoltaics (OPVs) provide an exciting prospect for solution processed, low-cost, large area, integrated, and tuneable energy production. Despite rapid improvements in device efficiencies, driven by non-fullerene acceptor (NFA) adoption, OPVs are yet to achieve mass commercialisation, mainly due to limited operational stabilities of high-performance materials and a congested energy market with many competing technologies, such as perovskite photovoltaics. To overcome these barriers and realise the potential of OPVs it is essential to improve their stability and find niche applications to facilitate market entry. In this thesis, both problems are addressed.
Firstly, the photostability and degradation mechanisms of high-performance NFAs are investigated using a range of techniques, particularly Raman spectroscopy. This allows for a molecular level understanding of the degradation mechanisms of a range of NFAs. The importance of molecular conformation and acceptor crystallinity are highlighted, and acceptor photostability is found to be critical for device stability. From these findings, several molecular design rules are suggested for improving NFA stability, with particular emphasis on using methods to lock molecular conformation and increase intermolecular interactions. Additionally, filtration of the photoactive layer solution prior to deposition is demonstrated to be an easy and effective way of improving device stability.
Secondly, the applicability of OPVs for low-light applications is investigated using low-cost materials, and scalable techniques. The performance of OPVs is shown to be superior to silicon at low-light illumination, highlighting the potential of OPVs for low-light applications. The effect of parasitic resistances on low-light performance are investigated, and optimisation considerations discussed. Additionally, a light-soaking effect critical to low-light performance is observed and characterised as originating from poor charge extraction. Finally, this effect is mitigated by utilizing a different electron transport layer to fabricate a large area module that sets a realistic commercial benchmark for the low-light performance of OPVs.