Electrode architectural engineering for enhancing solar conversion efficiency

Abstract

Population explosion and significant advancement in technology have dramatically increased global energy demand. Currently, fossil fuel-based power generators remain as our primary power supply. However, fossil fuels are nonrenewable energy resources and its combustion will result in severe environmental problems. Therefore, there is an urgent obligation for our generation to develop practical sustainable energy resources that will address the issue of dwindling fossil fuels and reduce environmental degradation. The energy influx from the sun to earth per hour is roughly equal to the yearly energy consumption across the world. There is hence a general belief that photovoltaic (PV) devices are the most promising in satisfying the drastically inflating energy demand. The fast-expanding knowledge in nanotechnology and material science have contributed to the development of nanomaterials with fascinating features. Integrating materials nanotechnology in electrode design may bring us new opportunities to craft groundbreaking next-generation devices. Electrode architectural engineering is particularly important in fabricating high-performance PV cells to attain excellent charge transport/transfer properties and desirable electrochemical reaction kinetics. The mission of this interdisciplinary PhD program is to design novel electrode architectures with favorable materials nanostructures and functional properties for improving the solar conversion efficiency of low-cost PV devices, specifically polymer PV devices and dye sensitized solar cells (DSSC), as well as to explore their efficiency enhancement mechanisms, so as to push our current knowledge in electrode design to a new frontier. In order to improve the fill factor of polymer PV devices, uniformly distributed gold nanoparticles (Au NPs) were inserted at the interface between vanadium pentoxide (V2O5) anodic buffer layer and ITO electrode to enhance the charge extraction within the cell. The resultant power conversion efficiency (PCE) of the modified device exhibited a ~16 % enhancement compared to the device without Au NP. Theoretical impedance analysis revealed that a lower charge transport resistance and higher charge recombination resistance, which are key factors leading to an improved fill factor, of the modified OPV device. Moreover, the incorporation of Au NPs have induced a better crystallinity of poly(3-hexylthiophene-2,5-diyl) (P3HT) within the bulk heterojunction networks, hence resulting in enhanced charge transportation process. This study provides new insights into the roles of Au NPs in improving the performance of polymer solar cells. Size-tunable TiO2 mesocrystals with high crystallinity were prepared using a simple solvothermal approach and utilized as the photoanode material of DSSC. The size of the TiO2 mesocrystals was controlled through tuning the hydrolysis rate of titanium alkoxide precursor. The unique well-aligned mesocrystal structure enabled efficient charge transportation pathway within the photoanode, while suppressing the charge recombination at the TiO2/electrolyte interface. Furthermore, the submicronsize mesoporous structure provided a large surface area for dye adsorption and effective light scattering capability. The conversion efficiency of DSSCs was significantly enhanced (~36 %) through the utilization of a mesocrystal TiO2-based photoanode compared to that of a P25 controlled cell. A novel interconnected NiCo2S4 nanosheets network was successfully grown on fluorine-doped tin oxide (FTO) and applied as the counter electrode (CE) of DSSC. Detailed studies revealed that the compositional ratio of NiCo2S4 could significantly affect its catalytic activity in redox mediator regeneration. Furthermore, the development of interconnected nanosheets on electrodes could substantially increase the electrochemically active surface area, thus leading to the improved kinetics of Iregeneration reactions. DSSC assembled using the optimal interconnected NiCo2S4 nanosheets CE exhibited a higher power conversion efficiency (7.22 %) compared to that of a conventional device (6.87 %) employing sputtered Pt CE.