Matsoha, Teboho C.2023-10-102023-10-102021https://hdl.handle.net/10539/36630A dissertation submitted in fulfilment of the requirements for the degree of Master of Science to the Faculty of Science, School of Physics, University of the Witwatersrand, 2021Organic solar cells show a remarkable combination of rapid improvement in performance and low cost which has led to much attention as a potential renewable energy competitor to traditional silicon cells. When metal oxides such as ZnO are introduced to these devices either as stand-alone electron transport layers or mixed with the active layer to facilitate electron transport, improvements in the performance arises through enhanced exciton dissociation and improved charge transport capabilities. In this study, to further improve the intrinsic properties of ZnO, magnesium and gallium dopants were incorporated into ZnO and their effects on the structure, particle morphology, optical and photoluminescence properties were analyzed. Nanoparticles of ZnO were successfully synthesized using the sol-gel technique and then annealed at 500°C in a furnace. X-ray diffraction of the nanoparticles matched the peak positions of the samples to the hexagonal wurtzite structure of ZnO as indexed using ICSD card no. 180052. SEM and TEM have shown that the particle morphology comprises of rod-like structures which agglomerated to form nanoflowers. However, the Mg doped ZnO exhibited a deviant agglomerated nanosphere morphology. Energy Dispersive Spectroscopy revealed the weight percentages of the individual atoms in the nanoparticle and EDS mapping showing a non-homogeneous atomic composition distribution over the nanoparticles independent of their morphology. Raman spectroscopy measurements corroborated the structural phase identified by XRD having exhibited the vibrational modes associated with a wurtzite phase of ZnO. Nanoparticles sizes were found to increase upon annealing from room temperature to 500°C while the band gap decreased as the annealing temperature was increased from room temperature to 500°C. Optical studies by diffuse reflectance confirmed band gap energies in the range of 3.11 – 3.18 eV for the annealed samples. An overall decrease in the band gap energies was observed upon annealing the samples from room temperature to 500°C and was attributed to the increase in crystallinity as observed from the sharpening of the X-ray diffraction peaks. Photoluminescence studies displayed both the characteristic near band emission (NBE) and broad band emission (yellow emission) from ZnO. The de-convoluted peaks of the broad emission band were associated with defect transitions such as zinc vacancies and oxygen interstitials. At room temperature ZnO is diamagnetic as seen from the magnetic moment dependence on the applied field where the magnetic susceptibility is negative and can be estimated to have a magnetic permeability slightly less than 1. Temperature dependence of the induced moment in the presence of a magnetic field (20000 Oe) was evaluated. It was evident that upon annealing at higher temperature co-doped ZnO behaves like a diamagnetic material up to very low temperatures for which the incidence of super paramagnetism becomes apparent. This was seen when the experiment was repeated at a higher field, 2T, in which transition temperature shifted to higher values (4K). The ZnO nanoparticles were then incorporated into organic solar cell as a constituent of the electron affinity layer as an acceptor. This was done by adding the nanoparticles to the active layer blend of P3HT:PCBM in the ratio 1:1 dissolved in chlorobenzene. In this way, the role of ZnO nanoparticles was to help in the electron extraction from the donor in the photoactive layer of the solar cell devices. The devices were then annealed to allow for interconnected network to form percolated layers. Raman spectroscopy analysis on the fabricated devices showed all modes associated with P3HT and micrographs depicted the distribution of nanoparticles over the surface of the devices. Absorption measurements gave spectra that were characterized by a broad absorption peak ranging from 438 – 570 nm with a maximum at 513 nm. A large decrease in the power conversion efficiencies was observed upon the introduction of the nanoparticles into the active layer blend. This drop can be attributed to the nanoparticles causing cracks probably due to differing particle sizes and non-homogeneous distribution which form alternative pathways for current to flow through the device, and they can also affect the contact between the active layer and the top aluminium electrode.enDissertation