Multi-wall carbon nanotubes: A versatile electronic material for organic photovoltaics.

The work presented in this thesis is concerned with organic photovoltaic devices (OPVs), in particular with the use of multi-wall carbon nanotubes (MWCNT) to improve device performance. The initial chapters introduce the subject areas of OPVs, carbon nanotubes (CNT) and experimental techniques. Within the thesis, it is demonstrated that MWCNT can be utilised as an interpenetrating, work function tuneable, electrode in bi-layer OPVs. A remarkably low concentration of MWCNT (~1 wt.%) uniformly distributed within the donor layer is shown to greatly increase the cell fill factor (FF) without complicating the process of device fabrication. Furthermore, functionalising the MWCNT with polar surface groups is shown to be an effective means of modifying the open circuit voltage (Voc). This study clearly demonstrates the potential of this approach to significantly increase the power conversion efficiency of bi-layer OPVs. We also introduce, a water soluble acid oxidised multi-wall carbon nanotube (o-MWCNT)-polythiophene composite for bi-layer OPVs. OPVs utilising this nanocomposite material as the donor layer exhibit a ~ 20 % increase in FF and commensurate increase in power conversion efficiency as compared to cells without o-MWCNT. Crucially o-MWCNT are incorporated into the cell structure using an environmentally compatible solvent without complicating the process of device fabrication. In this context the MWCNT enhances the conductivity of the donor layer, reducing cell series resistance and increasing FF. The growth of MWCNT directly onto indium-tin oxide (ITO) coated aluminosilicate glass via chemical vapour deposition as a large area semi-transparent electrodes bulk heterojunction OPV is also described. The rate of nanotube growth on this ternary oxide is shown to be greatly reduced as compared to that on silicon dioxide and soda lime glass enabling a high degree of control over CNT height. The strong potential of this nano-structured semi-transparent substrate as an interpenetrating hole-extracting electrode in bulk heterojunction OPVs is also demonstrated. Combining the work contained in the thesis, a hybrid OPV prototype is designed and fabricated, wherein MWCNT grown directly onto ITO coated glass function as an air stable cathode which contributes to exciton dissociation