Low temperature synthesis of graphene as an alternative transparent electrode for large area organic photovoltaics

This thesis presents a systematic study of the fabrication and optimisation of graphene films as an alternative electrode for large area organic photovoltaics (OPVs). It is mainly focused on the growth of graphene layers at low temperatures (below 700 °C) using chemical vapour deposition (CVD) method. A routine procedure was developed to produce large-area graphene films of centimetre size. Firstly, we demonstrated that we could fabricate multi-layers of graphene films utilising organic solvent residual in a polymer film matrix as the carbon source. The polymer matrix is poly (methyl methacrylate) (PMMA), which can be dissolved in a polar solvent, such as, chlorobenzene. When PMMA is dissolved in chlorobenzene and drop-cast as a film into a quartz slide, a small amount of chlorobenzene is trapped in the PMMA. When heating up the quartz slide to 180 °C, chlorobenzene molecules evaporate and land on copper foils, which is maintained at much high temperature in the growth zone in the CVD system. Copper (Cu) catalytically promotes chlorobenzene dissociation and formation of micron-sized graphene domains at the different growth temperature. After a parametric study, we found that at 75 sccm (standard cubic centimeters per minute) of H2 flow during the growth while maintaining the Cu foil at 600 °C, produced the optimal graphene growth conditions. We also compared PMMA dissolved in other organic solvents and as carbon sources at low-temperature growth ~450 °C for deposition of the graphene layers onto a Cu catalyst. An optimisation process was carried out to see the effects of other carbon sources on the quality of graphene films. The carbon sources studied were both aliphatic solvents (dichloromethane, chloroform, acetone) and aromatic solvents (p-xylene, toluene, o-xylene, chlorobenzene, dichlorobenzene), to probe the growth mechanism of graphene formation. However, none of the other solvents produced a better quality of graphene than chlorobenzene. Lastly, graphene films were used to replace indium tin oxide (ITO) in the OPV device fabrication. The results showed that working devices were successfully made for both small and large areas OPVs.