The increased atmospheric concentration of greenhouse gases due to the combustion of fossil fuels and its impact on the temperature of the planet has led to the development of alternative energy generation methods. Organic solar cells (OSCs) are a promising technology that is simple to process, flexible, customizable, and potentially low-cost. However, relatively low efficiencies, and short lifetimes compared with other photovoltaic technologies are as the main challenges of organic solar cells. This thesis aims to characterise and optimise the hole transporting interface to improve the efficiency and long-term stability of OSCs. To achieve this, first, the device fabrication process of P3HT:PCBM based devices was optimised to produce devices that perform at a standard commensurate with those reported in literature with a similar architecture and fabricated under similar conditions. In particular, two different evaporation techniques for the deposition of the electron conducting electrode were compared. Electron-beam evaporation significantly decreased the crystallinity of P3HT while thermal evaporation proved to be effective for producing P3HT and PCDTBT based devices with an efficiency comparable to the values reported in literature for a similar device architecture.
In an attempt to improve the efficiency of devices, the electrical conductivity of the PEDOT:PSS hole transporting layer was increased by almost two orders of magnitude using a zwitterionic additive (DYMAP) to dope the PEDOT:PSS dispersion. The liquid and solid phase structural modifications of the conductivity enhanced PEDOT:PSS were studied to understand the effects of conductivity enhancing additives on the morphology of PEDOT:PSS. Small angle neutron scattering revealed that the interchain distance between PSS backbone chains, and the screening length of neutralised PSS segments increase as the concentration of DYMAP increases from 0 to 25 mM. However, at 30 mM doping concentration, DYMAP induces gelation in the PEDOT:PSS dispersion resulting in a decreased interchain distance similar to that of the undoped PEDOT:PSS, and a significantly increased screening length compared to that of the 25 mM doped dispersion. The vertical structure of DYMAP doped PEDOT:PSS films was studied with neutron reflectivity which revealed that at low doping concentration, the film separates into a quasi-bilayer film in which the dopant segregates at the bottom of the film. However, at higher DYMAP doping concentration, DYMAP is evenly distributed throughout the film which results in a homogeneous single structure. The DYMAP doped PEDOT:PSS films were then incorporated as the hole transporting layer in OSCs which resulted in the decreased photovoltaic performance of devices compared to the control devices. This was found to be mainly due to the poor contact between the doped HTL and the active layer as a result of the increased phobicity of the doped PEDOT:PSS films to the solvent of the active layer.
Another approach to improving the device efficiency and stability was to incorporate three variants of a P3HT50-b-PSSx block co-polymer as an interfacial layer between PEDOT:PSS and P3HT:PCBM to improve the hole transport and stability between such layers. The incorporation of a 10 nm P3HT50-b-PSS16 and a 13 nm P3HT50-b-PSS23 interfacial layer resulted in a 9% and 12% increased device efficiency respectively compared to the reference devices. This was mainly due to a 9% increase of the open circuit voltage caused by the more energetically favourable alignment of the HOMO of the block co-polymers with the HOMO of P3HT. The fill factor of the 10 nm P3HT50-b-PSS16 and 13 nm P3HT50-b-PSS23 incorporated devices also increased by 2.8% and 6.2% respectively due to a smoother surface than PEDOT:PSS and the more compatible contact between the P3HT block of the block co-polymer and the P3HT, and the PSS block with the PEDOT:PSS. Moreover, the devices with the interfacial block co-polymer had a higher normalised efficiency than the control devices after 2200 hours of storage, demonstrating that the block co-polymer not only improves device efficiency, but crucially prevents degradation by stabilising the interface between PEDOT:PSS and P3HT.
