Surface functionalization of Graphene Oxide for application in organic solar cells

Abstract

The enormous increase in global energy demand concomitant the environmental impact of non-renewable fossil fuels led to exploration of energy sources that are safe, cheaper and sustainable. Within a range of renewable technologies, photovoltaics (PVs) are an emerging way of effectively generating sustainable energy. In photovoltaic devices, the most important aspect is the power conversion efficiency (PCE) which is driven by the crystal structure of the organic polymers, the surface morphology of the thin film and the choice of electron donor and acceptor materials. Multiple routes have been taken to achieve efficient charge transfer between poly(3-hexylthiophene) (P3HT) and graphene, but to the best that we can tell, not much has been done on the use of graphene oxide tailored with rare earth ions and other semiconductor materials. The conducted research work is devoted to preparation of nanomaterials, growth of thin films and fabrication of devices based on P3HT, holmium (Ho), zinc oxide (ZnO) and graphene oxide (GO). These materials were chosen owing to their superior properties and abundance and they were prepared and deposited using simple synthesis routes and deposition techniques. The primary focus was to investigate the structural, morphological, spectroscopic and solar device properties of heterostructure films for application in organic photovoltaic cells. The properties of the thin films were analysed using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), UV-visible near-infrared spectroscopy (UV/VIS/NIR), Photoluminescence spectroscopy (PL), Time-correlated single photon counting (TCSPC) and Keithley 2400 technique, respectively. This work began with the successful growth of P3HT and GO/P3HT thin films using the drop coating method. Properties of these thin films were investigated and found to have amorphous and crystalline domains and they revealed the interaction through a decrease in lattice spacing, an indication of strain on the thin films. The presence of GO changed the surface morphology to flaky-like nanostructures, as seen from SEM images. The FTIR spectra presented various vibrational frequencies symbolic of the interactions at the interface between GO/P3HT thin films. Further ionic interactions are revealed by the increase in absorbance and broadening of the absorption spectra of GO/P3HT thin films. The doping of GO with Ho was also found to impact absorption properties of P3HT positively. The quenching of P3HT emission, ideal for photovoltaic applications was observed when grown with GO which indicates an efficient donor-accepter charge transfer process. In an attempt to attain an efficient donor regioregular P3HT as it is a vital component for better performance of organic solar cells (OSC), we investigated the effect of deposition methods. The evaluated deposition methods for the growth of P3HT thin films operate in the absence of a vacuum and they produced thin films of different thicknesses. The crystallinity of P3HT revealed strong thickness dependence where 30 nm thin film had a low diffraction intensity and 148 nm thin film revealed the highest crystallinity. Raman analysis confirmed the high structural ordering of P3HT in terms of thicker thin films. A uniform P3HT thin film morphology was produced using the spin coating method. The increase in thin film thickness led to variations in absorption spectra due to change in defect states within the bandgap of P3HT. The luminescence properties revealed an increase in emission intensity of P3HT with the increment in the thin film thickness due to the increase in defect density. We successfully applied the GO/P3HT, ZnO/P3HT and GO/ZnO/P3HT design architecture using spin coating method towards device fabrication. The crystalline structure of P3HT and GO were confirmed using XRD analytical technique where ZnO crystallized into hexagonal wurtzite structure. The chemical stoichiometry from EDS suggested the absence of impurities in the GO/ZnO/P3HT thin films. SEM results presented GO sheets intercalated in P3HT upon interaction. The bonding interactions from FTIR confirmed alteration of P3HT structure upon interaction with GO through a decrease in average conjugation length from 1.20 to 1.12. We witnessed a reduction in band gap energy which caused a decrease in energetic driving force for GO/P3HT. These observations correlated very well with the J-V characteristics whereby the incorporation of GO into GO/P3HT and GO/ZnO/P3HT devices yielded performance deterioration with a reduction of ~ 38 % in energy conversion efficiency. Thus, this makes ZnO/P3HT the most efficient device compared to GO/P3HT and GO/ZnO/P3HT devices. The fluorescent decay curves revealed a decline in exciton lifetime depicting a quicker charge transfer from P3HT to GO which resulted into a decrease in exciton diffusion length.