Development of processing conditions for organic photovoltaic devices

Organic photovoltaic (OPV) devices are attracting significant attention as an alternative renewable energy source. Recent advancements have led to an increase in device efficiency and stability; however a greater focus on reducing material and processing costs is needed. This thesis sets out to consider these issues. Indium-tin oxide (ITO) is widely believed to be a major contributor towards the material cost of the device. Highly conductive poly(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was investigated as an alternative and inexpensive transparent conducting electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by dimethyl sulfoxide (DMSO) treatments. Ultra-violet absorption spectroscopy (UV-Vis), X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) were used to gain understanding as to how the different treatments improved the film conductivity and how this influenced OPV device performance. This study highlighted the possibility of using PEDOT:PSS as an alternative electrode and emphasised the importance of optimising and tailoring the electrode to the specific photoactive layer. Careful control over processing conditions has widely contributed to the development of OPV devices. However, although inert atmospheres and casting from halogenated solvents results in high efficiencies, they also further add to the processing costs. A method to process the poly(3-hexylthiophene):phenyl-C61- butyric acid methyl ester (P3HT:PCBM) photoactive layer under ambient conditions was developed, which was found to be applicable on both ITO and PEDOT:PSS electrodes. This methodology was then used to process inverted P3HT:PCBM devices spin cast out of non-halogenated solvents which gave comparable overall device performance to those cast out of dichlorobenzene (DCB). This indicates the potential of producing low-cost OPV devices processed using more environmentally friendly solvents and under an ambient environment which can be more easily incorporated in a roll-to-roll process