Fabrication of photoelectrodes with enhanced light utilization for dye-sensitized solar cells

Abstract

An increase in global energy demand, environmental concerns and the finite nature of fossil fuels have led to a great interest and development in the field of renewable energy sources. Solar energy is one of the most promising renewable energy sources because of its cleanliness and abundance. Among photovoltaic devices, dye-sensitized solar cells (DSSCs) have attracted substantial attention as a renewable energy conversion device because of their low production cost, easy fabrication process, and aesthetically appealing design. However, in order for DSSCs to realistically be considered an alternative to conventional solid-state photovoltaic devices, improvements in power conversion efficiency must be made. For improving the efficiency of DSSCs, the light absorption performance of photoanode, a key component of DSSCs, should be enhanced as much as possible to increase the production of photocurrent. Thus far, various attempts have been made to enhance the light harvesting efficiency of photoanodes. These efforts include developing new sensitizers with a broader range of absorption wavelength and a higher extinction coefficient, construction of new semiconducting electrode structures with light scattering properties, mixing different dyes for co-sensitization, and introducing non-radiative resonance energy transfer concepts. Nevertheless, it is still difficult to attain the desired level of light efficiency that would lead to significantly improved efficiency. In this study, new approaches for obtaining enhanced light utilization in photoanodes of DSSCs are demonstrated by incorporating optical-active inorganic materials such as quantum dots and precious metal nanoparticles into the photoanode of DSSCs. The first part presents dye-sensitized solar cells with silica-coated quantum dot embedded nanoparticles (SiO2/QD@SiO2 NPs). QDs have been considered promising materials with potential to be applied to photovoltaic devices owing to their powerful light absorption property. However, it is hard to apply QDs to DSSCs because of their instability in iodide pair electrolyte system which is most commonly used in DSSCs. To overcome this problem, QDs were embedded in SiO2 nanoparticle and coated with thin SiO2 layer. SiO2/QD@SiO2 NPs were incorporated into the photoanode of DSSCs. The enhanced performance of the SiO2/QD@SiO2 NP containing DSSC was believed to be mainly due to the improved short-circuit current density (JSC), which was a direct result of enhanced light utilization in the photoanode. Rather than working as a sensitizer, QDs in DSSCs act as a light reservoir that absorb the extra light and re-emit the absorbed light. The second part discusses plasmon-enhanced dye-sensitized solar cells using SiO2 spheres decorated with tightly assembled silver nanoparticles (Ag NPs). The plasmonic enhancement effects of the photoanode in DSSCs were investigated. To activate the strong plasmon coupling, new structure of Ag NPs assembly was designed through electromagnetic wave simulation. Tightly assembled Ag NPs on a SiO2 core showed broadband plasmonic absorption developed by coupled plasmon modes, which was not limited to a specific wavelength. By incorporating SiO2-t-Ag@SiO2 into the photoanodes of DSSCs, light absorption by the photoanode thin films definitely increased and overall power conversion efficiencies of DSSCs were improved.